Handbook for Sound Engineers

Sound System Design 1311

box resonant frequency fb to help protect the loud-

speaker from overexcursion.

A 12 or 18 dB/octave low-pass filter at 12.5–20 kHz

will reduce unwanted RF signals and will help prevent

system electronic oscillation. In a music system, a low

pass at 16–20 kHz will improve the system for the same

reasons. Any system with a high-pass filter should also

have a low-pass filter for frequency-response balance

(the sound quality will be improved).

High-pass and low-pass filters are often included as

part of the system equalizer and are available modules

in almost every DSP device.

34.5.2.2.11 Equalization Using TEF or Other Equipment

That Measures Direct Sound Response

Some users of Time-Energy-Frequency (TEF) test

equipment, and other test equipment that can measure

direct sound response, have reported very good success

in equalizing the direct sound from the loudspeaker sys-

tem, ignoring (at least temporarily) the frequency

response of the reverberant sound. This tends to sup-

port the idea that the primary usefulness of equalization

may be to smooth the response of the loudspeakers, not

the room.

When equalizing the direct sound using TEF-type

equipment, do not follow the high-frequency roll-off

curves indicated in Fig. 34-76. Instead, equalize for a

relatively flat response. Then, begin a gradual

high-frequency roll-off at 10–12 kHz. Finally, adjust the

overall response for a subjectively pleasing sound while

remembering the goals of smooth response and

intelligibility.

34.5.2.2.12 Use of Narrow-Band Filters in Equalization

Very narrow-band filters are sometimes added to an

equalized system for the purpose of controlling feed-

back and the ringing that occurs in a system that is near

feedback. Used in a well-designed and carefully equal-

ized system, narrow-band filters can be successful for

these purposes. Common filter types include parametric

filters tuned to a very narrow bandwidth and active and

passive narrow-band or notch filters.

Narrow-band techniques for controlling feedback

work best in fixed systems in rooms with constant

microphone and loudspeaker positions. Even in a

portable system, a skilled operator may be able to read-

just a set of narrow-band filters to make them useful for

feedback control. As in any attempt at feedback control

by filtering or equalization, only two or three feedback

Figure 34-76. Various recommended response curves for
equalization. Courtesy Bosch/Electro-Voice.

D. Recommended response curve for high-level

rock music reinforcement systems.

A. Recommended response curve for speech

reinforcement systems.

B. International standard response curve for

cinema playback systems.

C. Recommended response curve for studio

control room monitoring systems.

±2 dB 200 Hz to 1 kHz

±2 dB above 80 Hz

Rolloff 3 dB/octave

above 1 kHz

Rolloff 3 dB/octave

Select rolloffappropriate to

loudspeakers user

Select rolloffappropriate to

loudspeakers user

Select rolloff

appropriate toloudspeakers user

±2 dB 100 Hz to 8 kHz

Rolloff above 5 kHz 3 dB/octave

Rolloff 3 dB to 6 dB/octave

above 8 kHz

Variations below 100 Hz

sometimes not equalizable

High pass below 40 Hz12 dB/octave

Maintain curve to 12.5 kHz

±2 dB 200 Hz to 1 kHz

50 100 200 500 1 k 2 k 5 k 10 k 20 k

50 100 200 500 1 k 2 k 5 k 10 k 20 k

50 100 200 500 1 k 2 k 5 k 10 k 20 k

50 100 200 500 1 k 2 k 5 k 10 k 20 k