A horn is in effect an acoustic transformer. It transforms a small-area diaphragm into a large-area diaphragm without the difficulties of cone resonances discussed in Section 6.14. A large-area diaphragm has a radiation impedance that is more nearly resistive over the desired frequency range than is the radiation impedance for a small-area diaphragm (see Fig. 4.35). As a result, more power is radiated at low frequencies for a given volume velocity of air. A horn is also a directivity controlling device, which radiates over an angle defined by the flare angle of the mouth.