The drive unit for a horn loudspeaker is essentially a small direct-radiator loudspeaker that couples to the throat of a flaring horn as shown in
Fig. 9.1. In the next part we shall discuss the characteristics of the horn itself. In this section we restrict ourselves to that part of the frequency range where the complex mechanical impedance
Z
MT
looking into the throat of a horn is a pure resistance:
A cross-sectional drawing of a compression drive unit for a horn loudspeaker is shown in
Fig. 9.2. It has a diaphragm and voice coil with a total mass
M
MD
, a mechanical compliance
C
MS
, and a mechanical resistance
R
MS
=
1/
G
MS
. The quantity
G
MS
is the mechanical conductance of the diaphragm in m/N
s.
Behind the diaphragm is a back cavity that is usually filled with a soft acoustical material. At low frequencies this space acts as a compliance
C
MB
, which can be lumped in with the compliance of the diaphragm. At high frequencies the reactance of this space
becomes small so that the space behind the diaphragm becomes a mechanical radiation resistance
R
MB
=
1/
G
MB
with a magnitude equal to that given in
Eq. (9.1). This resistance combines with the mechanical radiation resistance of the throat, and the diaphragm must develop power both to its front and its back. Obviously, any power developed behind the diaphragm is wasted, and at high frequencies this sometimes becomes as much as one-half of the total generated acoustic power.
By inspection, we draw the admittance-type analogous circuit shown in
Fig. 9.3. In this circuit, forces “flow”
through the elements, and the velocity “drops”
across them. The generator open-circuit voltage and resistance are
e˜g
and
R
g
. The electric current is
i˜
; the linear velocity of the voice coil and diaphragm is
u˜c
; the linear velocity of the air at the throat of the horn is
u˜T
; and the force at the throat of the horn is
f˜T
. As before, the area of the diaphragm is
S
D
and that of the throat is
S
T
.