Figure 7-7.--Folded-horn loudspeaker.

in cross-sectional areas at folds or joints in the horn.

a folded-horn loudspeaker (fig. 7-7) is employed in

preference to a straight horn. There is a practical limit

Vibrations of the horn walls must be sufficiently damped

to the power that can be handled by a conventional

to avoid introducing irregularities into the response, as

driver unit. When extremely high sound intensities must

well as transient effects.

be produced, multiunit loudspeakers are employed in

DIRECTIVITY.-- The directivity of a loud-

which the units are coupled to individual horn sections

speaker is an important factor in determining the

that are combined mechanically into a common

loudspeaker assembly.

efficiency of the sound radiation over the listening area.

All practical forms of sound radiators exhibit some

Characteristics of Loudspeakers

directional effects. If a radiator is placed in free space

where the results are not affected by interfering

Loudspeakers are rated according to their (1)

reflections, the sound pressure at a given distance is not

frequency response, (2) directivity, (3) capacity, (4)

the same in all directions. The directivity of a

efficiency, and (5) impedance.

loudspeaker is a function of both frequency and the size

FREQUENCY RESPONSE.-- In the majority of

of the horn mouth of the loudspeaker. Thus, a

cases, the frequency response of the loudspeaker is the

loudspeaker becomes more directional with increasing

limiting factor in the overall response of a sound system.

frequency because of the shorter wavelength, and a

For direct radiators, the low-frequency response is

influenced by the (1) baffle or enclosure, (2) diameter

direct radiator or horn mouth of large size is more

of the cone, (3) ability of the cone and voice coil to

directional than one of smaller size. These factors of

execute large amplitudes of vibration, and (4) strength

frequency and size are interrelated in that the size

of the magnetic field in the air gap. The high-frequency

becomes a factor relative to the wavelength of the sound

response is limited by the mass of the voice coil and

being transmitted. Thus, the directional pattern of a

diaphragm.

small loudspeaker transmitting a high-frequency signal

For horn loudspeakers, the low-frequency response

(short wavelength) is similar to that of a large

is influenced principal y by the (1) basic horn formula

loudspeaker transmitting a low-frequency signal (long

employed, (2) flare, and (3) mouth dimensions. The

wavelength). In general, a horn loudspeaker of a given

high-frequency response is limited by the (1) mass of

mouth diameter is more directional than a direct radiator

the voice coil and the diaphragm, (2) phase effects

caused by differences in path lengths due to bends, and

of the same diameter, particularly at the low frequencies.

(3) impedance irregularities caused by sudden changes

The directivity of a horn loudspeaker is also

dependent upon the rate of flare; that is, the directivity

increases as the flare is made more gradual (longer

horn). If a rectangular horn having a long narrow mouth

(in terms of wavelength) is mounted with the long

dimension of the mouth vertical, the radiation in the

horizontal plane corresponds to that of a small radiator

with a broad distribution pattern. The radiation in the

vertical plane acts as a large radiator with a relatively

narrow beam. In other words, the horn is made relatively

much less directional in the horizontal plane than in the

vertical plane, It is obvious that the reverse is true if the

horn is turned so that the long dimension of the mouth

is horizontal. Thus, the sound energy is flattened out in

a plane at right angles to the long dimension of the

loudspeaker mouth. This principle is used to obtain the

required directional characteristics for efficient

high-intensity reproduction on the flight decks of

Figure 7-7.--Folded-horn loudspeaker.

aircraft carriers.

7-8