HOW DO WE EQUALIZE?: Let me count the ways.
Aug 1, 1998 12:00 PM, Glen Ballou
Equalization is not the solution to all problems. Before thinking about equalizing a sound-reinforcement system, be aware that EQ will never make an improperly designed sound system good. It will probably make matters worse. Design the system with proper loudspeaker coverage and equal (+/-3 dB) sound pressure level (SPL) in the listening area. Only then can EQ improve sound quality and reduce feedback.
Equalization goes back a long way. It appears Dr. Wayne Rudmose was the first to perform meaningful EQ (Sound System Engineering, Don and Carolyn Davis), but Dr. C. P. Boner was the first to build a user-definable EQ. Dr. Boner equalized systems by making each filter by hand and retesting, changing the filter and testing again. If the system didn't work, he sent for loudspeakers with higher directivity until intelligibility was satisfactory.
Don Davis apprenticed with Dr. Boner, and when Altec introduced Acousta-Voice, Don and Carolyn Davis traveled the country demonstrating and teaching Acousta-Voicing. The filters were passive and required precise impedance matching, and instrumentation was designed for lab work rather than for equalizing in the field.
Impedance was measured with a General Radio impedance bridge. The test signal was pink noise (constant power per octave) produced by a General Radio 1382 random noise generator. There were no RTAs (real time analyzers); the sound from the loudspeaker/room was measured with a General Radio 1564A sound and vibration analyzer, 1/3 octave at a time. Each 1/3 octave was recorded by hand and displayed on graph paper. The filters were adjusted, and the process repeated until EQ was complete. Another method was connecting the pink noise generator to the 1/3 octave filter so only 1/3 octave noise was produced. The output of the loudspeaker and room was recorded from a sound level meter. The sound system now had to produce only a narrow frequency range at a time. Adjusting the filter 24 times and recording the SPL took time. Equipment was AC powered, and EQ was then often measured in days or weeks.
I was invited to the first east coast Acousta-Voice demonstration in New York in 1968. The evening before the session, I walked into the room just as Don and Carolyn Davis were doing an A-B test with Classical music. Typical of the period, hotel conference room acoustics were poor, and the loudspeakers were not particularly smooth. As Don switched back and forth between equalized and unequalized, Carolyn, who has a platinum ear, said "The demonstration is wonderful; everyone will be impressed." I thought, "No wonder it doesn't feed back. They removed the highs and lows and gave it the response of a telephone." Don switched back to the smooth wide-band response. Carolyn turned to me and said, "Isn't it wonderful how EQ can make the sound so smooth and extended?" I had assumed the narrow, poor response was the equalized one, but wide frequency response doesn't cause feedback; it is peaks in the systems, even those with a very narrow bandwidth. Remove the peaks, and feedback is reduced.
Soon, an inexpensive metered RTA was introduced. It consisted of three rows of eight VU meters laid out exactly like Altec's Acoustavoice filters. With this system, the meters could be seen at one time, and the effects of moving the mics or adjusting the filters could be immediately observed. For the record, we had to transfer the meter readings onto graph paper. Measurements of 1/3 octave have not changed much even though test equipment has become more sophisticated.
Eventually, portable RTAs were produced; the most noticeable was the IVIE IE-30A, which was battery operated and compact. It had two memories, allowing comparison of before with after EQ or one position in the room with a second position. The output LEDs gave bar graph output with a display range from 15 dB to 45 dB and selectable resolution of 1, 2 or 3 dB. The unit could also make octave analysis and was a sound level meter. Portable signal generators were produced, and soon active filters were being made.
New instruments can measure 1/6 octave and 1/12 octaves, measure RT60, interface with PCs, have multi-channels, store as many curves as required, compare curves, and measure frequency response at a particular distance, eliminating room effects.
RTAs can measure the characteristics of a room with all of the reflections, but they cannot measure the sound emanating from the loudspeaker without the effects of the room. Fortunately, Dick Heyser invented TDS and subsequently TEF (time energy frequency), all before we had ICs and DSPs. TEF is still the instrument to which all others are compared. With the TEF machine, we can measure to a higher frequency resolution than any other way.
In the meantime, Meyer Sound Laboratories came out with the SIM System II analyzer. SIM (source independent measurement) allows equalization during performances by using the program material as a reference to compute the transfer function of the hall or speaker system. The system can measure various areas in the room by electronically switching between judiciously placed mics.
Gold Line manufactures an MX4 four channel mic multiplexer that averages the information from four mics with the resulting curve called "spatially averaged". This allows you to set up four mics in different locations and measure and record the results of each position separately or multiplex all four together. This may be able to give a better overall EQ to a room that has less than perfect sound system/room interface. Spatial averaging is recommended by major film cinema/home theater organizations, including LucasFilm and Dolby.
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