Surround sound and control rooms
Cinema surround – its origins. TV surround – the differences. Music-only surround – its needs. Stereo and surround – the compromises. Perception and surround. Fold-down complications. Rear channels – the different concepts. Low frequency options. Close-field options. Study of an actual design.
One of the greatest problems facing designers of surround sound rooms is the lack of a clear consensus amongst the music business as a whole as to exactly what they want from surround sound. Perhaps, therefore, it would be expedient to first look at the systems that have become virtual standards in the multimedia use of surround, and then move on to the implications for music only surround. Historically, cinema led the way in the serious use of surround sound, because Walt Disney’s animation film ‘Fantasia’ featured multi-channel surround audio, 30 years before the record companies made their big push with quadrophonics in the early 1970s.
Dolby did the industry a great service many years ago by clearly specifying their intentions for their cinema systems and issuing adequate guidelines to mixing personnel. They did not try to establish a system that would do all things for all people. Instead, they chose systems that would, within a restricted range of use and in reasonably controlled acoustics, give a good representation to the cinema-goers of the film directors’ creative wishes. They defined the reasonable limits of surround reproduction at an early stage, and history has shown them to be remarkably intelligent given the chaos of quadrophonics from which they developed their early systems.
A few important facts regarding mixing for Dolby cinema surround are given below.
1 The most important signals will usually be coming from the front, because the action is on the screen, and the directors do not want people to be distracted from it by important sounds coming from rear locations. This would tend to excite the human reflex response of turning to face the sound, which may be warning of danger in real life, but it is not much use in a cinema, except in IMAX, perhaps.
2 The mixes will be played back in theatres that comply with a reasonably tight range of electro-acoustic and acoustic requirements.
3 Due to the sheer cost of film production, mixing will usually be carried out by skilled, experienced, and knowledgeable personnel, who will almost invariably limit their creative exuberance to getting the very best out of what they have to work with. They will not get too carried away creating wonderful effects in the mixing room which will be detrimental to the experience in the theatres. The success of a film tends to ride on how many people see the first screenings in the cinemas and recommend the experience to their friends. Creating nonsense does not help this situation.
4 The objective is usually to create a one-off, big performance, and it generally means that high quality programme material and equipment are available to achieve the high standards that are normally expected. However, mixing room size can demand different solutions to the problems of achieving these high standards if the mixes are to transfer to the cinemas with the required degree of accuracy. In all cases, though, it is desirable to keep the quality of the mixing room a good jump ahead of the likely playback room performance. This needs to be especially so in terms of perception of detail whilst maintaining the overall frequency responses within the accepted ranges.
5 The whole concept of the various Dolby surround formats is to deliver a balanced programme to a group of people, with no particular ‘sweet spots’. This they have proved themselves very capable of doing, over many years and around the world.
TV and video, at first consideration, seem to need the same general requirements as cinema because, once again, the final audience will be sat in front of a screen with moving pictures, and will be surrounded by (usually) five loudspeaker channels. However, there are other things to consider with regard to domestic ‘viewing’.
1 Reproduction will usually not take place with SPLs of over 90 dB.
2 Background noises in the listening environment will usually be higher than in cinemas.
3 Because of the two above points, dynamic ranges will be much more restricted.
4 Many broadcasting companies have their own set of standards, and sometimes are forced by international agreements to follow standards that are not always beneficial for the sound quality.
5 Reproduction will be presumed to be largely on poor-to-medium quality domestic loudspeaker systems, and getting a reasonably good sound for all is fundamental if the revenue of advertisers is what keeps the TV channels in business. The high impact of cinema audio is neither a requirement nor a practicable concept for TV mixing.
The above, and many other points as well, generally mean that rooms designed for TV or domestic video production will be much more compromised in terms of overall sound quality. It is no doubt that it is the existence of so many restrictions which often leads to a lamentable attitude about sound quality from many people in the world’s TV industries.
One therefore cannot easily mix for surround TV in a room designed for big-screen, high dynamic-range productions. Neither would it seem wise to try to mix audiophile quality music recordings in rooms designed for TV/video sound mixing, where many of the necessary subtleties for music mixing would almost certainly not have been considered during their design. This does highlight that surround room design becomes very specific to the goals of the reproduction circumstances, and gives some insight into why the variations shown in Figures 14.10 to 14.23 have come into existence.
Now we come to the big question; rooms for mixing music-only, high fidelity surround. Unlike in the cinema, TV, and video worlds, there are no set standards. In rooms for high quality stereo music mixing there are several schools of thought. There are the ‘Live-End, Dead End’ types of rooms (see Chapter 17), and earlier designs by people such as Jensen, which were intended to give a clean first-pass to the sound from the loudspeakers, with a diffuse room decay from the rear, and to varying degrees from the sides. The ‘Non-Environment’ concept (see Chapter 16) attempts to make the room as anechoic as possible to the loudspeakers. Only the floor and front wall are reflective, to give life to speech and actions in the room, thus avoiding the creation of an uncomfortably dead ambience in which to work. Sam Toyoshima and Eastlake Audio both opt for something that is the same as the Non-Environment concept, but with a careful distribution of reflective surfaces (principally for the higher frequencies) scattered about the room to add a touch of extra ambience. The rear walls are highly absorbent in both cases.
It has to be said that well-designed and well-constructed types of all of the above rooms, and others besides, in the hands of skilled recording staff can produce stereo recordings of the very highest quality. None of them are perfect though. As designers have their own individual hierarchies of priorities, each type of room has its own special strengths, such as the resolution of ultra-fine detail, the ambient ‘feel’ of the room, the low frequency behaviour of transient signals, or other aspects of performance. Nevertheless, all of the better rooms of each type, when the recording staffs know the rooms, can produce results that travel well to domestic situations.
However, what they all have in common is that they are bi-directional: the front halves differ from the rear halves. In no case can one simply transfer the monitor loudspeakers to the rear wall and still have a room that sounds as good. Yet, this is exactly where rear loudspeakers are needed in a surround room that is intended to have fully symmetrical five-channel monitoring. Therefore, if all of the very best stereo mixing rooms need bi-directional acoustics, this would seem to lead to only two possible conclusions.
1 In surround rooms with bi-directional acoustics (like the good stereo rooms), the rear loudspeakers responses will not be as good as the front loudspeaker responses.
2 In surround rooms that do have fully symmetrical acoustics and monitoring, the frontal stereo cannot be as good as in the best bi-directional stereo rooms.
It would seem that the first choice would be the better compromise for high quality music mixing, because even in a surround mix the frontal sound stage is the most important in 95%, or more, of recordings. To compromise our current high quality stereo for a more enveloping sound is to trade quality for quantity. The second option described above is reminiscent of an old comedy show about the textile industry, on British television many years ago, called ‘Never Mind the Quality, Feel the Width’. Some surround room designs seem to be saying, ‘Never mind the quality, feel the space’. Whether or not, that is a backward step is perhaps a subjective issue.
In the classical recording world, the orchestral layout is a fundamental part of the music, which is designed to deliver the composers’ emotions to the audience. Nobody within the orchestra hears the true, intended balance of instruments, so having the orchestra wrapped around the listener does not seem to be a worthwhile goal. This suggests that for most classical recordings, the surround channels will be delivering purely ambience, and only rarely for special effects will they be reproducing the direct signals from musical instruments. In most classical works where off-stage sounds are used, they are usually intended to be ethereal in quality, so will be of an ambient nature, which is well suited to surround in the bi-directional rooms. These rooms may not be appropriate for a close-mic’d wrap-around orchestral recording though, because of the lack of ‘symmetrical’ monitoring.
Based on all the above, it would appear that the design of surround mixing rooms for music-only recordings should not differ fundamentally from the design of rooms for Dolby cinema mixing. The object should be to try to achieve the best that can be achieved from the rear channels without compromising the response of the frontal stereo, be it two, three, or five channels. The only significant difference in design may be in the choice of the front wall materials in the absence of a screen. In surround rooms, the front walls still need to be solid to act as baffle extensions for the loudspeakers, but they should also be irregular to break up any specular (discrete) reflexions of the sound striking them from the rear loudspeakers.
The most high fidelity (i.e. highly faithful to the original) surround sound reproduction can only really take place in anechoic surroundings, but even the craziest of surround audiophiles are unlikely to have anechoic chambers for domestic listening. Nevertheless, it would seem to be a professional attitude for the music industry to try to generally monitor in conditions that are better than could be expected in domestic reproduction because, eccentric as many audiophiles are widely deemed to be, it is hard to deny their right to expect that the recording standards should live up to their equipment standards. There is no justification for the widespread television attitude in a professional music recording industry, but there now seems to be ominous pressure (such as from MP3) to lower many of the general quality aims. This is a risky situation though, because if compromise is forced on the end-result, enthusiasm will be lost at the production end of the chain, and that would be ruinous for creativity. Standards erosion must be resisted!
To maintain the quality levels of stereo hi-fi in commercial surround recordings requires vigilance and discipline. Producers and engineers are likely to be disappointed if they expect to be able to do whatever they want with the distribution of instruments whilst hoping to produce results of a quality equal to the current quality of the best stereo. (And, to be even more disappointed if they expect equally balanced reproduction in most people’s homes.)
There is a huge carrot dangling in front of studio designers to produce ‘the ultimate quality’ in symmetrical surround mixing rooms. Time will surely find the rooms lacking, though. This is because achieving fully symmetrical five-channel monitoring with no compromises to the level of quality of current two-channel stereo; and to have rooms which are also usable for multi-format mixing (cinema/music/TV, etc.), basically cannot be done. Control room design for surround mixing is therefore a very format-based concept. Until we get more rationale than the situation shown in Figures 14.10 to 14.23, the difference in the concepts of surround control rooms will lead to great confusion, with people being tempted to do the wrong things in the wrong rooms; or even the wrong things in any room.
From many points of view, it would seem that the best answer for the future of surround is for everybody to follow the basic five-channel cinema format. It has many things going for it.
1 It is proven.
2 It is widespread.
3 It was created by professionals after a great deal of knowledgeable and rational thought.
4 It can do (or is capable of doing with little adaptation) almost all that surround can reasonably be expected to do.
5 Much of what it will not do is not possible anyway, which becomes clearer when the finer points of surround are better understood.
6 It requires no separate systems for music and cinema.
7 It is more room-tolerant than many other systems.
8 It does allow some flexibility in the choice and location of the rear/side loudspeakers.
It is worth remembering there are many things that one can dream of doing on other systems, but there is not much that one can actually do well which cannot be supported by the current cinema formats.
21.5 The psychoacoustics of surround sound
The relatively stable phantom images of conventional stereo only function well when they subtend an angle of about 60° centred on our noses. This fact was grossly under-appreciated during the quadrophonic era of the 1970s. An enormous number of people (the author included) attempted to pan musical signals to all points around the room on quadrophonic pan-pot ‘joysticks’. It was a great source of mystery to many people why the front-left/rear-left panning only stabilised when at fully front or fully rear positions, with all points in-between yielding images which flip-flopped from one extreme to another at the slightest movement of the head. In fact, it is only in the horizontal frontal listening arc where we generally have good localisation. Over this arc, it can be very good, with many people being able to resolve differences in position to an accuracy of one degree for some sounds. When a loudspeaker is placed behind the head, the accuracy of localisation is reduced, and phantom images between two or more loudspeakers cannot be stable. The perceived frequency response will also be different to that perceived from similar loudspeakers in front of the listener.
In surround listening, whether to live instruments or recordings via loudspeakers, it is a fact that perceived tonality will change according to source position. However, in many peoples’ heads, there seems to be an idea that if symmetrical monitoring conditions can be achieved, then this will lead to more ideal surround sound monitoring. A big problem arises when surround mixes monitored under such circumstances are heard in stereo or mono. What was perceived as correct when heard from behind may be inappropriate in both level and frequency balance when folded down to fewer channels and heard from a frontal direction.1 In such cases, it is hard to see why distributed rear loudspeakers, such as used in the cinema world, would be at any great disadvantage to discrete loudspeakers, because neither relate perfectly to the perception of a folded-down mix.
It has been recognised that the ambient or discrete-instrument type of surround mixes are better served by correspondingly distributed or discrete loudspeaker sources at the rear, but the existence of these conflicting concepts is largely due to the fact that it has also been widely recognised that five channels are not enough to provide the best of both worlds. Ten channels are seen to be a minimum for truly flexible surround sound systems, but it is deemed too unwieldy to be commercially practicable.
When used for ambient mixes, the two-discrete-loudspeaker option for the rear channels often does not work as well as some of the other rear channel loudspeaker options (see also next section). Therefore, the whole concept of discrete rear loudspeakers does not appear to have much in its favour (except for the fact that market pressures are pushing for it), but as we shall see later, it can sometimes win by default.
Dolby cleverly recognised these conceptual weaknesses in the 1970s when they introduced ‘Dolby Stereo’, which was really a four-channel surround format (see Figure 14.11). In those days of analogue-only technology, the quadrophonic vinyl-disc systems used rather poor phase encoding techniques to try to put four channels on a stereo disc. Dolby saw the nonsense of it all, but then effectively used a similar phase encoding technology to produce not the left-front/right-front/left-rear/right-rear of quadrophonics, but a left-front/centre-front/right-front/single-channel-rear format. They put one of the loudspeaker channels at the centre-front location, which stabilised things immensely, and split the mono rear signal between several loudspeakers, widely distributed, which could give quite a spacious ambience. For what it was, the system worked well. Once again, therefore, the cinema people moved one jump ahead of the music industry on the subject of surround. The advent of this system could have saved quadrophonics, but the music business was already largely fed up with it, and a generation was to pass before they again attempted to re-invent the rather useless ‘square wheel’ under a new name – surround. With digital technology, the cinema world soon moved on to three front channels and distributed stereo surround, which represented yet another march forward.
It has been shown that multi-channel surround is more realistic than two channel surround (surround here meaning the channels other than the front channels), although two surround channels can be made more effective as ambience channels by using diffuse source techniques rather than two, simple, discrete loudspeakers. Tomlinson Holman proposed another concept for domestic use; single dipole sources to each side and slightly to the rear of the listener (see Figure 14.17).2 These sources presented their nulls to the listener, who would receive only reflected energy from the room loudspeakers, and hence a more diffuse sense of spaciousness.
David Bell successfully used single discrete loudspeakers as the rear channels in small post-production rooms by hanging them from the ceiling, facing away from the listening position, but pointing directly at proprietary diffuser panels to reflect diffuse energy into the listening area.3 The author successfully used distributed mode panel loudspeakers, which are naturally diffuse sources, in a film laboratory screening room that met Dolby specifications.4
These things are all ultimately subjective, but a large body of opinion believes that discrete instruments occasionally played through the above diffuse surround systems suffer less loss of realism than two-channel ambience played through a pair of discrete rear loudspeakers. The consumer market may ultimately dictate the de facto ‘standards’ to be used, but the diffuse sources referred to in the previous paragraph could well be more tolerant of listening room acoustics than the fully discrete, five identical loudspeaker option; which seeks to achieve ostensibly identical monitoring from five locations, whether the sources are perceived to sound identical, or not.
Clearly, the five different surround channel systems so far discussed will all tend to produce different responses, both measured and perceived, at the listening position(s). So, let us now consider the responses of the various approaches in different acoustical conditions.
21.7.1 The simple discrete source
Taking the simple, single loudspeaker first, the response at the listening position will be dependent upon the mounting conditions, especially at the rear of the room, and the nature of the surfaces that face the loudspeaker. Let us assume that we have put a full, five-channel system of identical loudspeakers in an anechoic chamber, mounted at the recommended points on the perimeter of a circle. An omni-directional measuring microphone at the listening position would pick up the same frequency response from each source. On the other hand, a listener at the same position as the microphone would perceive less high frequencies from the rear loudspeakers than from the front loudspeakers, due to the pinnae (outer ears) being more responsive to high frequencies from a frontal direction. As there would be no reflexion of sound, the rear loudspeakers could be perceived as single discrete sources. Nevertheless, these conditions would be the ones under which the overall flattest response could be expected from all the loudspeakers.
In a stereophonic type of control room with a dead front end and a live rear end, the situation would be somewhat different. At high frequencies, the frontal loudspeakers would project directly into the ears of the listener, and excite the reflected field from the rear, but the rear loudspeakers would do neither. The perception of the rear loudspeakers would tend to be dull and lifeless. The situation at low frequencies would depend on the mounting conditions. If all the loudspeakers were flush mounted, the front loudspeakers would, hopefully, face a rear wall with enough low frequency diffusion to discourage standing wave resonances, but the rear loudspeakers would face a front wall which would typically be quite solid and reflective at low frequencies. Response disturbance could be expected due to the reflected wave interfering with the direct wave.
If the loudspeakers were free standing, then many of the same results would obtain, except that a less uniform bass response could be expected. The omni-directional low frequency radiation would travel behind the front loudspeakers and reflect off the front wall, again causing response irregularities due to the interference of the direct and reflected waves. The rear radiation from the rear loudspeakers would hopefully have a less disturbing effect due to the presence of the rear wall diffusers, but the forward moving radiation would still suffer interference from the front wall reflexions.
In a room with a wide-band reflecting front wall and absorbent rear wall, such as described in Chapter 16, the rear loudspeakers would subjectively sound brighter than in either the anechoic or the dead front-end conditions, due to the high frequencies reflecting from the front wall and back to the listener from a forward direction. A measurement microphone at the listening position would show a less flat mid-range response than in a room with an absorbent front end, due to the interference of the direct and reflective waves. At low frequencies, surface mounting would yield different responses from front and rear loudspeakers, due to the absorbent rear and side walls providing a less effective baffle extension to the loudspeakers than that enjoyed by the front loudspeakers (see Chapter 11). This could be legitimately equalised to some degree, but not precisely. For free standing loudspeakers, the rear channels may be perceived to be flatter than in a room with a reflective/diffusive rear-wall, due to the relatively non-reflective, adjacent rear surface.
Tomlinson Holman, and others, have proposed that rooms for surround should be of generally lower decay time than conventional listening rooms, both to avoid colouration of the recorded surround and to help to ameliorate the variable low frequency response problem. Less ambient ‘help’ from the room is needed for surround sound because the ambience is usually already recorded in the surround channels. Holman and others have also suggested that the room should be made more acoustically uniform by the careful and appropriate distribution of reflective/diffuse materials on the surfaces of the walls and ceilings. These are eminently sensible suggestions for a more uniform perceived response from all of the loudspeakers, but they cannot avoid the criticism that the frontal loudspeaker response, which is usually of prime importance in the vast majority of music recordings, cannot be as good as in the finest, acoustically bi-directional stereo rooms. However, the proponents of the ideas were talking about surround listening rooms, and not about stereo compatible control rooms. The compromising of the frontal responses to the benefit of rear responses may well be valid in the context of some surround-only rooms, but it must be accepted that a trade-off exists. The degree to which this trade-off is beneficial or otherwise may be heavily dependent upon programme material and personal tastes.
21.7.2 The multiple distributed source
This is the Dolby cinema approach. In anechoic conditions, the main difference between this and the discrete source concept is that it would still be perceived as a distributed source, because that is exactly what it is, (although the precedence effect will have a tendency to pull the image towards the nearest loudspeaker of a group). Depending upon the precise distribution of the loudspeakers, it could be the case that the distributed sources would sound brighter than the single discrete rear sources, because some of the loudspeakers could be expected to be more directly pointing towards the ear canal, as can be seen from Figure 14.12. The response of a measuring microphone at the listening position in anechoic conditions would be less flat than from the single discreet source, because of the constructive and destructive interference due to even minor path length differences from the different sources. Subjectively, however, this may not be a problem at all; and when reflective boundaries are added, the situation would again change.
In the case of either extreme of stereo control room design, whether the front walls be reflective at high frequencies or not, the response from multiple distributed sound sources would tend to be less different from that which could be expected from a discrete rear source in the different rooms. Surface mounting of the multiple distributed sources is normal, because flush mounting tends to be rather structurally complicated, and multiple free-standing sources tend to be rather an obstruction to everyday work and general activities.
Subjectively, the distributed system seems to work very well, especially for ambient and special effect surround. When the rear channels are fed via a signal delay, as in the Dolby system, the effects can be very lifelike, because the precedence effect will tend to ensure that front-originating sounds cannot be pulled back into the surrounds. This can be effective even when a listener is closer to one of the surround loudspeakers than to the most distant frontal loudspeaker, as can be the case in many cinema and home theatre installations, especially where an audience of more than one person can often be the norm. For home use, though, the multiple distributed surround loudspeaker concept is cumbersome.
21.7.3 Dipole surround loudspeakers
This option is shown diagrammatically in Figure 14.17. Clearly, in anechoic conditions this choice would be somewhat of a nonsense, because with the nulls facing the listener (see Figure 11.3[a]) and no refexions returning, not much would be heard. In a ‘stereo’ style of control room, the side of the dipole facing the hardest surface would give rise to most of the high frequencies. Obviously, therefore, the most suitable type of room for using such a system would be one with relatively evenly spread reflective surfaces, such as tends to be found in many domestic rooms. This is not too surprising, because Tom Holman initially proposed this technique for domestic use, where it does have a lot of potential, but it is hard to see how a flat frequency response could be expected at the listening position from such an arrangement using the purely reflected energy from arbitrary boundary conditions.
For the purpose of this discussion, the true diffuse sources such as the DML and the approach of a discrete loudspeaker pointing at a wall-mounted diffuser can be lumped together (see Figure 14.21). They could both be expected to deliver a relatively flat response to the listening position, almost despite the nature of the room acoustics. The DML behaves rather differently from other loudspeakers in that the initial SPL drop with doubling of distance tends to be more like 3 dB rather than the conventional 6 dB.
Diffuse sources have wide radiation patterns over an extended range of frequencies, and tend to suffer less from the effects of room resonance and standing wave interference. The DMLs suffer from a rather curtailed low frequency response, but they can be so advantageous as ambient sources that the extra effort involved in adding a common sub-woofer to them would seem to be well worthwhile.5 Essentially, however, they consist of a radiating surface made from a material that exhibits a very dense modal activity, spread throughout its surface. They are energised by moving coils, but not in the sense of a conventional magnet and chassis system. Despite being a mass of resonances, the early part of the impulse response is remarkably rapid. Since the late 1990s these loudspeakers have been causing some reassessment of conventional thinking, and in many areas they have been well received.
Obviously, the discrete loudspeaker pointing at a wall-mounted diffuser will behave more like a conventional source at low frequencies, where the radiation pattern tends to be that of an omni-directional compact source. Its response in this region may differ from that of the DML, but the low frequency responses of surround systems in general are something of a minefield, so perhaps that is what we should now look at in more detail.
21.8 Low frequencies and surround
Figure 21.1 shows a typical layout for a Dolby Digital theatre. This is the archetypal 5.1 system, where the ‘point-one’ (0.1), or low frequency effects channel, is fed to a dedicated sub-woofer system. In cinema mixing for Dolby Digital, DTS (Digital Theatre Systems) and SDDS (Sony Dynamic Digital Sound), what goes to this channel is determined by the mixing engineer. However, in the Dolby Stereo analogue system (which despite its name is a matrix surround format), the sub-woofer is fed from a low frequency management system in the processors, somewhat like in domestic ‘home theatre’ systems.
Figure 21.1 3-D conceptualisation of a Dolby Digital theatre
It will be noted from Figure 21.1 that the sub-woofer is set off-centre. This is done to try to avoid driving the room symmetrically, where the tendency would be to drive fewer modes more strongly, due to the equal distance from a centre loudspeaker to the two side walls. The off-centre arrangement tends to produce more response peaks and dips but of lesser magnitude than would be the case for a symmetrical drive. The degree to which the off-centre location of a sub-woofer can be detected by ear is usually a function of the upper frequency limit. Below 80 Hz it is generally very difficult to detect the source position, but as the cut-off frequency rises, the low frequency source position can become more noticeable.
To eliminate noticing the off-centre source location whilst maintaining an asymmetrical room drive, Dolby now recommend the use of two subwoofers, fed from the same electrical signal. One should be placed one-third of the distance across the room from one side wall, and the other placed onefifth of the distance across the room from the opposite side wall, as shown in Figure 21.2. The use of one or two large sub-woofers in such cinema installations takes into account the fact that cinemas, whilst being designed to decent acoustic criteria, are not so heavily controlled as music control rooms. Fewer low frequency sources are therefore more practical than three or five full-range sources. They also address the need for a large area, relative to the size of the room, to be covered by a respectably even sound field, so that the paying customers all receive their money’s worth. The existence of sweet seats and poor seats, acoustically speaking, would not be commercially viable. Large sub-woofers also offer the power required for explosions, and other sound effects that would tax full-range loudspeakers to their limits in larger theatres.
Figure 21.2 Dolby recommendation for siting two sub-woofers; one of them one-fifth of the room width from one side wall, the other one-third of the room width from the other side wall. This not only avoids both the localisation of a single sub-woofer towards one side of the room, but also avoids the symmetrical driving of room modes by the central placement of the sub-woofer(s)
There is thus little in common between the dedicated ‘low frequency effects’ (LFE) channels of digital cinema and the low frequency extension (LFE?) sub-woofers used in Dolby Stereo systems. The latter are fed via bass management processing in the playback systems, and are normally provided to allow the use of smaller ‘satellite’ loudspeakers on the main five channels. This is a commercial necessity resulting from the great reluctance of many households to accept the presence of five large full-range loudspeakers in domestic living rooms. (Even if they could find a sonically good place to put them that did not block a doorway or cut half the light from a window.)
21.8.1 Music-only low frequencies
In the mixing of surround sound for music only, there is no such luxury as specifying the means of playback and the environment in which it will be heard, let alone any means of enforcing compliance with any specifications. Sub-woofers, therefore, tend to add a complication to the mixing environment if traditional, full range monitors are not used. They add another crossover point, and mono sub-woofers lack the undoubted benefit which stereo bass can add to certain music, especially in terms of the spaciousness, which is, somewhat perversely, ostensibly surround sound’s raison d’être.
Nevertheless, the situation still exists that a mix done on one concept of system is likely to sound quite different when played back in a different control room, when the number of sub-woofers can vary from zero to two, and the full range loudspeakers (if used) can vary (normally) between two, three and five. In fact, the four-channel (no centre-front) option is another possibility, and does still see use, albeit in a slightly modified geometry from the old quadrophonic (square) layout. (See Figures 14.10 and 14.11.) At the time of writing (2003) some people are mixing to a four-channel format which resembles the five-channel format of Figures 14.19 and 14.20, but without the centre front loudspeaker.
It would seem that the optimum arrangement for a music-only control room for the highest quality monitoring would use five full-range loudspeakers and no sub-woofer. However, Chapter 14 discussed the behaviour of multiple loudspeakers in rooms, and from that discussion it can be understood that one must be careful not to route individual low frequency signals to any more channels than absolutely necessary if playback compatibility problems in other rooms and on other systems are to be minimised. Moreover, only in rooms with well-controlled acoustics can the five full-range channel option be heard to be superior to the single (or double) sub-woofer option. In rooms with less ideal low frequency control, the single sub-woofer option can reduce the variability of a sound as it is routed to different locations (which at low frequencies would tend to drive different modes to different degrees). Whether such poorly controlled rooms should be in use for serious surround mixing is a moot point, but the fact is that many of them are used for such purposes. It should also be added, here, that attempting to mix on a subwoofer/satellite system, using bass-management, can be very risky. These systems can often be so far from the reality of what is on the recording medium that they could hardly be considered to be ‘monitoring’ anything other than their own ideosyncratic sound.
21.9 Close-field surround monitoring
Not unlike the way that many people resorted to the use of small, close-field loudspeakers in an attempt to escape from the problems of stereo monitoring variability from studio to studio, the use of satellite loudspeakers, on stands in the close field, and a common sub-woofer, has found widespread use for multi-channel mixing. In this case, though, one of the driving forces behind the choice was the lack of purpose-designed surround control rooms with adequate full-range monitoring. The reasons for this dearth of facilities have been:
1 Lack of clear guidelines/standards for the design of music only surround rooms.
2 Because of 1 (above), there has been a corresponding lack of people willing to invest in the building of dedicated surround rooms, which may be short-lived in use if the ‘wrong’ layout is chosen.
3 As the ideal needs of surround rooms and stereo rooms are not entirely compatible if the highest performance is required, people have not been willing to compromise their stereo rooms whilst surround sound has remained only a challenger to the market supremacy of stereo.
4 Good dedicated surround rooms require the commitment to a considerable and long-term investment, and the recording market has shown no clear intention of being prepared to pay significantly more for surround mixing than for stereo facilities. For many studio owners, only earning the same rate as for stereo recording does not warrant taking the risk of investing on such a shaky basis.
Thus many commercial surround mixing rooms are, in fact, nothing more than stereo mixing rooms in which a satellite/sub-woofer system has been installed, perhaps with the addition of a few acoustic contrivances to help to (or at least to appear to) control a few acoustic irregularities. The fact that this can be passed off as professional mixing is due partly to the fact that the mixes will be expected to pass through a surround mastering facility, in order to make them sound like the perceived, accepted, current norm. However, it can also be got away with because of the fact that the situation in the domestic playback circumstances is variable to the point of absurdity. If the mixes are not up to the highest standard, then who is going to know? If nobody knows, then nobody is likely to complain, but is this a professional attitude? The impression given is more that it is all a bit shoddy.
Manufacturers of domestic equipment and programme material have done little to help the situation. Ludicrous situations have arisen whereby the promotion of audiophile quality DVD ‘A’ discs have only been found to be viably marketable by making relatively cheap DVD video players read them digitally, whilst only passing the output through D to A converters of the lower resolution used for the DVD videos. In many cases it has been almost impossible to tell apart the compressed audio of the ordinary DVD audio channels and the high sampling rate/high bit rate of the potentially vastly superior DVD A when passed through the cheap converters.
It may well be that this is not too different from using common master recordings/mixes for Compact Cassette and CD release. Those who choose to buy the appropriate playback equipment get the appropriate results … hopefully. On the other hand, Compact Cassettes never claimed to be superior to CDs, but the marketing of the surround formats certainly contained many implicit suggestions that a whole step forward was to be expected from DVDs vis-à-vis stereo CDs. This is certainly not the case for DVD A when played on DVD video systems. In fact, it would take a whole step forward in the world economy before people would be able to afford five loudspeakers, five amplifier channels and all the associated processors and converters which were of equal quality to the two of everything required for stereo. The development and specification of professional music surround facilities has been greatly hampered by the badly conceived marketing hype that has tried to force a new medium on a public who were not exactly crying out for it.
21.10 Practical design solutions
Figure 21.3 shows a control room which was designed by the author for high quality, music-only surround use. By discussing some of the design options and choices it will be possible to highlight some of the concepts and compromises that have been touched on in the previous sections of this chapter.
Figure 21.3 The main monitoring installation at Producciones Silvestras, Catalonia, Spain. A music-only surround room. The finished room is shown on the front cover of the book
The room was required to be principally a stereo room that could be used for high quality surround recordings and mixing. The first change to the initially proposed design was to turn the control room through 90°, so that the monitor wall could be free of windows. It was considered to be very important for surround use to have three, full-range, flush mounted monitor loudspeakers, all at the same height. The reasons for setting the mid-range loudspeaker drivers of all three loudspeakers at a height of about 147 cm above the ground was discussed in the previous chapter, so the option of elevating all the loudspeakers to a height above the window was rejected, and the room was duely rotated by 90°.
Looking sideways into the studio was no great price to pay, and in this instance it was balanced by looking sideways, the other way, into the forest. As the stereo was of great importance, no effort was made to change the bi-directionality of the room acoustics, and so the rear wall was made maximally absorptive, in accordance with the chosen control room philosophy. The side walls were made relatively absorbent, except for the two windows and the glass door. The glass was all of the 12 mm laminated variety, and the windows were angled quite steeply upwards, to attempt to persuade any reflected energy to head in the direction of the absorbent ceiling. It must be remembered that even plane surfaces give rise to a certain degree of scattering at high frequencies, but from these positions it was considered improbable that too much energy would return to the listening position from any of the loudspeakers. Had the rear loudspeakers not been needed, the windows would also have been angled backwards, to tend to direct the reflexions from the front loudspeakers into the rear trap. This was therefore another small compromise that was made to the room design for the benefit of the surround performance (the 90° re-orientation being the first). The front wall was made with an irregular surface of stone, to help to reduce any tendency for specular reflexions with the rear loudspeakers, although this in no way compromises the frontal stereo performance.
The centre loudspeaker, it should be noted, should be connected to poweredup (switched-on) amplifiers, even when not in use during stereo recording and mixing, to avoid loudspeaker resonances from affecting the stereo response. If the amplifier is not connected to the low frequency drivers, or is connected but switched off, the loudspeaker cones and the tuning ports would be free to resonate at their natural frequencies. With power to the amplifier, the very low output impedance acts as a brake on the movement of the loudspeaker cones, holding them rigidly. The port resonance is less of a problem because the receiving/radiating area is smaller, and the excitation tendency (at the sub 20 Hz tuning frequency) is less likely. In general, loudspeakers should never be left in control rooms if the amplifiers to which they are connected are not switched on, because they can affect the sound from nearby loudspeakers, both by absorption and coloration, due to their resonant tendencies.
Anyhow, what we have described so far is really a ‘3-channel stereo’ room with full range monitoring from 20 Hz to 20 kHz, built to the principles discussed in Chapter 16. To convert this into a surround room we therefore need to add a system of suitably chosen and mounted rear loudspeakers, and, in this case, the general consensus was to set the rear loudspeakers at around 120° either side of centre-front.
One hundred and ten degrees, or less, would have obstructed doors and windows to an unacceptable degree, which again highlights the fact that ideal surround mixing rooms are best built as such, and should not be compromised by access to studio rooms or views of the forest. Again, the cinema people have a better approach – dedicated mixing rooms – but the tight budgets of the music industry tend to require rooms of more flexibility in use. Nonetheless, in this case, nobody really believed that siting surround loudspeakers 10° aft of ‘normal’ would significantly alter the perception of the sound in such a room.
21.10.1 The choice of rear loudspeakers
Here, the concepts discussed in Section 12.6 can be considered again in the context of a specific room. The overall room design was as shown in Figure 21.3. It should be obvious that symmetrical monitoring would not be possible. The front loudspeakers are set into a solid, very rigid front wall, whereas the rear loudspeakers, even if identical to the front cabinets, would perform differently because they would be set in absorbent surroundings (see Figures 16.1 and 16.2). They would not enjoy the low-frequency loading provided by the front wall acting as a baffle extension. The effects of such loading differences were discussed in Chapter 11, and whilst it is true that the reduced low frequency loading could be equalised in the feeds to the rear loudspeakers, this could require up to four times the amplifier power to do so. This may or may not be a great problem, but the intransigent problem is that of the first reflexions from the opposing wall.
The rear absorbent trap is designed to minimise the effect of the reflected energy from the front loudspeakers from interfering with the direct signal. With the rear-mounted loudspeakers facing a solid front wall, nothing can effectively be done to prevent the response irregularities caused by the reflected wave, and no conventional equalisation could flatten the response. The mid and high frequencies would also face different terminations at the front and rear of the room. Therefore, even notwithstanding the differences in perception in terms of the frequency balance of signals arriving at the ears from the front or from the rear, the loudspeaker/room combination itself could not deliver identically balanced signals to a measuring microphone at the listening position.
To enable such a symmetrical monitoring condition to exist would, in the opinion of all the people concerned with the design of this room, have required unacceptable compromises to be made to the stereo performance. This also applied to the frontal sound-stage performance of a surround mix, which was also considered to take precedence over the rear channel performance. The option of identical loudspeakers all round was therefore abandoned.
The dipole option, in this case, would result in virtually zero sound coming from the rear because of the highly absorbent rear wall. Little would arrive directly from the loudspeaker to the listening position because of the null in the plane of the baffle. Almost all of the audible output from the loudspeakers would therefore be by reflexion from the front wall, which would therefore produce no surround sound at all, only confused stereo. Not surprisingly, this option was also rejected.
The multiple loudspeaker (cinema) choice was rejected because this was a music only studio. Despite the fact that the cinema technique has much to offer to music-only mixing, the music industry in general has not woken up to this fact. The option was therefore rejected on the basis of lack of acceptance by the clients, but not from a system-engineering viewpoint.
The single diffuse arrangements, such as the DMLs or loudspeakers pointing towards wall-mounted diffusers, were considered carefully. The DML option was finally rejected due to the lack of low frequency response unless very large panels were used. The option of using smaller DMLs with a common sub-woofer was rejected on the grounds of unnecessary complexity. The much larger panels were rejected because of worries about large reflecting surfaces at the rear of the room creating problems with the frontal stereo, although their hemispherical directivity over a wide frequency range could have allowed the panels to be angled such as to minimise this effect. Unfortunately, this would also have taken up valuable space in a smallish control room of just under 50m2, but this option remained under discussion until the final choice was made. It was certainly a serious contender.
As the rear of the room was relatively dead, acoustically, and the designated mixing/listening area was so small (6m2 or thereabouts) not much benefit was seen from the option of pointing a loudspeaker at a diffuser panel on each sidewall. Ultimately, the decision was made to use a pair of single, conventional, discrete loudspeakers, but effectively only by the rejection of all the other options.
The owner then consulted his clients, and nobody seemed to be intending to use the rear channels for bass guitar or bass drums. Therefore, in order to minimise any interference with the pure stereo use of the room, it was convenient to use relatively small loudspeakers that could be mounted on stands at the rear of the room. The actual model of loudspeaker was chosen for its sonic compatibility with the front monitors. Their ability to produce around 108 dB SPL at 1m, down to 70 Hz, was considered sufficient in a room where nobody listening seriously would be more than four metres from them, hence around 100 dB at the listening position from each surround loudspeaker would be guaranteed. The peak response was about 6 dB higher.
It was also acknowledged that certain clients might wish to use their own choice of satellite and sub-woofer systems; and that the studio may in the future purchase its own such system. In this case, the rear loudspeakers could be moved on their stands to a closer position in order to serve as the rear loudspeakers for the satellite system. The thinking behind the two choices of monitor system was that, as in much stereo recording, the large, full range monitors could be used during the recording process, to track down any distortions or noises and to check the low frequency balances. The satellite systems could be used for the ‘domestic reference’ and for those who desired to mix on them. The large system could also be used to ‘vibe’ the musicians (or other personnel) when necessary.
Although no claims are being made that the system described above is definitive, the description and the discussion about the thinking process that led to its final design can perhaps be useful to help to outline the options and typical compromises that go into the design of surround sound control rooms. Hopefully, from this, readers will be able to get a grasp of the general feel of the strengths and weaknesses of the various surround concepts.
21.11 Other compromises, other results
Figure 21.4 shows a small screening room in a film laboratory using DML panels for the surround. In this instance the compromises produced different results, because the needs were different. The room had to meet the specifications for Dolby Digital, and hence the surround sound field over the area of 26 seats could not be allowed to vary by more than 3 dB. The 3 dB drop per doubling of distance in the close field of the DMLs, together with their wideband hemispherical directivity pattern, made them an excellent engineering choice. Their diffuse nature, and the relative inability of the audience to localise them audibly, made them a good psychoacoustic choice. The fact that they were readily received for the overall natural impression of the surround tracks also made them a good subjective choice.
Figure 21.4 Front and surround monitoring in the Dolby Digital screening room at the Tobis film laboratories, Lisbon, Portugal. (a) Front monitor distribution. (b) The DML panels, high on the side walls, used as diffuse surround sources
In the case of this screening room, the typical choice of multiple surround loudspeakers, as normally used in cinemas, was rejected because it was considered that it would be difficult to prevent the audience from localising the surround sound on the nearest loudspeaker. This was due to the narrower directivity angles of typical cinema surround loudspeakers and their extremely close proximity to the seats in this room. It was also considered problematical to get the required evenness of coverage unless a very great number of loudspeakers were used, again because of the directivity of the typical surround loudspeakers.
Of course, in this room, the seating was all much closer to the surround loudspeakers than to the front loudspeakers, and the precedence effect suggests that the source of any sound routed to the front and the surround loudspeakers (which in fact is a relatively rare thing in cinema mixes) would be localised in the surrounds. However, as mentioned earlier, in the Dolby processors there is always a 100 ms delay to the surround feeds. This ensures that in any theatre where the difference in the distance to the listener from the surround and front loudspeakers is less than about 30 m, the sound will always be localised in the front loudspeakers.
Once again, the cinema people did their homework and came up with a carefully conceived standard. However, the delay, which works well in the cinema, could limit the options for music-only surround if lead instruments were to be put in the rear channels. With ambience in the surrounds, the effect can be advantageous, but obvious timing difficulties may be encountered if an ensemble were to be split across front and rear sources. Nevertheless, a room for ambient surround mixing could be extremely effective if built according to Figure 21.5. The only precaution necessary would be to ensure that any reverberation fed to the surround loudspeakers was fed via a short delay, but as this is relatively standard practice, it should cause no extra complications.
In the design of a surround-sound control room, therefore, many things need to be taken into account; it is not a simple exercise. However, the process is exacerbated by the lack of consensus or standards in the music industry about precisely what the goal is intended to be. The quality of the frontal stereo, be it two or three channels, is still of prime importance in most cases, so it is likely that most rooms used for surround mixing will be unlikely to compromise this, merely to benefit the rear channels, from which our hearing is anyhow much less discriminating. The question of ‘how to surround’ then becomes a function of the likely type of programme and the basic philosophy of the room acoustics. It is a very complex subject, which is made no easier by the electro-acoustic complications created by multiple sources, as described in Chapter 14.
The fact is that the psycho-electro-acoustics of surround is a far more complicated subject than is the case with stereo, and not least because of the much increased number of possibilities of making poor choices based on lack of a thorough understanding of the problems. Furthermore, the choice of ‘only’ five channels does not make for the optimum ability to compromise to multiple and often conflicting requirements without significant loss of performance. An interesting development of the concept is shown in Figure 20.5, which can take the clarity of perception to an entirely new level. Very significant reductions in the level of intermodulation distortions is also reported, which is hardly surprising.
Figure 21.5 Non-Environment concept using multiple rear sources. Versatile 5.1 channel, fullrange surround monitoring system, with multi-format monitoring capability. Left and right surround channels split between two groups of four loudspeakers, or, alternatively, perhaps with diffusive radiators, such as distributed mode, loudspeakers. All loudspeakers full-range, except the optional ‘effects’ sub-woofer
The lack of consensus about what format music-only surround should take has not been a help to the design of appropriate studios.
The most practical solutions for surround sound came from the cinema industry.
TV and video surround formats take into account their own sets of requirements.
Surround for music-only recordings would seem to serve little purpose if the frontal stereo channels are compromised.
The need for monitoring systems using five identical loudspeakers seems to be questionable, because the application and perception problems do not lead to symmetrical performance.
Five channels is a commercially imposed limit, which is only part way between two-channel stereo and what surround should be. Things get much better with ten channels, but it seems to be commercially impossible to implement.
Many surround (rear/side) formats have been proposed and tested. All tend to give different results, with their strengths and weaknesses suiting different music and circumstances. No outright winner has emerged.
In rooms where the acoustics have been designed to be as uniform as possible for the surround monitoring, the frontal stereo performance invariably seems to be compromised.
Conversely, in the better performing stereo rooms, the different concepts yield different responses and perception of the surround (rear) loudspeakers, and so provide no standard reference.
It is generally accepted that surround sound control rooms should have short decay times – perhaps the shorter the better until a point of ambient discomfort is reached.
The choice of separate low frequency sources or a single sub-woofer can be very dependent on the room acoustics. Perhaps the greatest degree of reality can be achieved by separate sources in highly controlled room acoustics.
‘Three-channel stereo’ rooms can be built with realistic rear loudspeaker responses which do not compromise the stereo performance. For ‘stereo and ambience’ surround mixes, they can give excellent results.
1 Newell, Philip, ‘A Load of Old Bells’, Acoustics Bulletin (Journal of the UK Institute of Acoustics), Vol. 26, No. 3, pp. 33–7 (2001)
2 Holman, Tom, ‘New Factors in Sound for Cinema and Television’, Journal of the Audio Engineering Society, Vol. 39, No. 7–8, pp. 529–39 (July/August 1991)
3 Chase, Jason, ‘Hi-Fi or Surround. Part Two’, Audio Media (European Edition), Issue 92, pp. 122–6 (July 1998)
4 Newell, P. R., Holland, K. R. and Castro, S. V., ‘An Experimental Screening Room for Dolby 5.1’, Proceedings of the Institute of Acoustics, Reproduced Sound 15, Vol. 21, Part 8, pp. 157–66 (1999)
5 Bank, Graham, ‘The Distributed Mode Loudspeaker (DML)’ in Borwick, John (ed.) The Loudspeaker and Headphone Handbook, 3rd Edn, Chapter 4, Focal Press, Oxford, UK (2001)
Holman, Tomlinson, 5.1 Surround Sound – Up and Running, Focal Press, Boston, USA and Oxford, UK (2001)