User Guide

Sound System Design Reference Manual
Isobars have become popular in recent years.
They give the angular contours in spherical
coordinates about the principal axis along which the
response is -3, -6, and -9 dB, relative to the on-axis
maximum. It is relatively easy to interpolate visually
between adjacent isobars to arrive at a reasonable
estimate of relative response over the useful frontal
solid radiation angle of the horn. Isobars are useful in
advanced computer layout techniques for
determining sound coverage over entire seating
areas. The normal method of isobar presentation is
shown in Figure 3-4D.
Still another way to show the directional
characteristics of radiators is by means of a family of
off-axis frequency response curves, as shown in
Figure 3-5. At A , note that the off-axis response
curves of the JBL model 2360 Bi-Radial horn run
almost parallel to the on-axis response curve. What
this means is that a listener seated off the main axis
will perceive smooth response when a Bi-Radial
constant coverage horn is used. Contrast this with
the off-axis response curves of the older (and
obsolete) JBL model 2350 radial horn shown at B . If
this device is equalized for flat on-axis response,
then listeners off-axis will perceive rolled-off HF
response.
Directivity of Circular Radiators
Any radiator has little directional control for
frequencies whose wavelengths are large compared
with the radiating area. Even when the radiating area
is large compared to the wavelength, constant
pattern control will not result unless the device has
been specifically designed to maintain a constant
pattern. Nothing demonstrates this better than a
simple radiating piston. Figure 3-6 shows the
sharpening of on-axis response of a piston mounted
in a flat baffle. The wavelength varies over a 24-to-1
range. If the piston were, say a 300 mm (12”)
loudspeaker, then the wavelength illustrated in the
figure would correspond to frequencies spanning the
range from about 350 Hz to 8 kHz.
Among other things, this illustration points out
why “full range,” single-cone loudspeakers are of
little use in sound reinforcement engineering. While
the on-axis response can be maintained through
equalization, off-axis response falls off drastically
above the frequency whose wavelength is about
equal to the diameter of the piston. Note that when
the diameter equals the wavelength, the radiation
pattern is approximately a 90° cone with - 6 dB
response at ±45°.
3-4
Figure 3-5. Families of off-axis frequency response curves