CHAPTER 18 Sound System Equalization distortion is print through on tape recordings where we hear the echo first followed by the desired sound. This never occurs in nature, thus our brain is extremely sensitive to its occurrence. So it is with "boost filters." In nature any two signals can combine and go to various depths depending upon the relative phase angle between them. Under ideal conditions, they can only add to 3 dB greater levels and psycho- logically we notice the presence of something far more than its absence. Subjectively, over more than 40 years of system equalization in the field, qualified investigators report slope rates in excess of 18 dB/ octave as audible. Therefore, filter sets (equalizers) should be designed to avoid steep slope rates, be combining, and not introduce unnatural distortions (boosting). A REAL-TIME REGENERATIVE-RESPONSE METHOD OF EQUALIZING A SOUND SYSTEM One of the earliest demonstrations of the effect of operation near regeneration on the measured fre- quency response of a sound reinforcement system was by William B. Snow before the February 1954 meeting of the Audio Engineering Society in Los Angeles. Snow recorded on a high-speed graphic level recorder the dramatic amplitude changes that occurred in the overall amplitude-versus-frequency response as the reinforcement system was brought nearer and nearer regeneration. This same method is still used with a manually operated oscillator to identify those frequencies whose amplitude responded unduly to the approach of the regeneration point of the sound system. Shock excitation was employed to observe the increased decay periods of frequencies that otherwise would not feed back upon being increased in gain but were unduly affected by the approach of the system regeneration point. Snow's paper had also demonstrated this point, proving that such frequencies,