Figure 7-5.--Direct-radiator loudspeaker.

diameter of the baffle. If the loudspeaker is mounted in

a wall or is completely enclosed, the baffle is called an

infinite baffle. When a cabinet is used as a baffle, it is

desirable to line the inside with a sound-absorbing

material to minimize the effect of cabinet resonances

produced by standing waves within the enclosure.

Horn Loudspeaker

The use of the direct radiator loudspeaker is limited

because of its low radiation efficiency. When it is

necessary to produce high sound intensities or to cover

large areas with sound, the radiation efficiency of the

loudspeaker must be increased to keep the size of the

amplifier within reasonable limits. Horns with

appropriate driver units provide a practical solution to

the problem. A horn maybe considered as an impedance

matching device for coupling a relatively heavy

vibrating surface at the horn throat to a relatively light

medium (the air) at the mouth of the horn. Figure 7-6 is

an illustration of a straight-horn loudspeaker.

For a horn to operate effectively, the mouth must be

Figure 7-5.--Direct-radiator loudspeaker.

sufficiently large in comparison with the longest

wavelength (lowest frequency) of sound that is to be

transmitted. Low-frequency horns often are considered

to be useful at frequencies above that for which the

mouth diameter is about one-third wavelength. The

At low frequencies, where the wavelength is large

performance of a horn loudspeaker near the

compared with the dimensions of the loudspeaker, the

low-frequency cutoff point depends to a great extent

rarefaction wave from the back of the diaphragm meets

upon the flare or shape of the horn. The function of the

the compression wave from the front of the diaphragm

horn contour is to produce a smooth and continuous

and neutralizes it because the waves are in opposite

increase in cross-sectional area in progressing from the

phase relation. Thus, low frequencies are not

small throat to the large mouth. The shape most

reproduced from this type of direct radiator.

commonly employed is the exponential horn in which

the diameter increases progressively by a fixed

At higher frequencies, where the wavelength of the

percentage for each equal-distance increment along the

sound is small compared with the dimensions of the

horn axis. For the horn to be of a practical size and shape,

loudspeaker, the sound waves from the front of the

diaphragm have time to travel an appreciable distance

away from the loudspeaker (in terms of wavelength),

and the phase of vibration of the diaphragm changes

before the interfering wave from behind can traverse the

distance around the diaphragm. Hence, a baffle is

necessary only to reproduce low frequencies from a

direct radiator. The purpose of the baffle is to delay the

meeting of the front and back waves by artificially

increasing the distance of the sound-wave path from the

front to the back of the diaphragm.

The simplest form of baffle is a flat board with a

hole in the center to accommodate the loudspeaker. This

type of baffle is effective down to a frequency the

Figure 7-6.--Straight-horn loudspeaker.

wavelength of which is approximately four times the

7-7