PART VII Acoustics and Sound Reinforcement Active 1/3 octave band -- boost BBF Zero degrees Passive 1/3 octave band -- rejection BRF Vertical: 6 dB/division Horizontal: Auto 0.00--2000.24 Hz Resolution: 1.0010 E + 01 Hz (a) EFC Vertical: 45 degrees/division Horizontal: Auto 0.00--2000.24 Hz Resolution: 1.0010 E + 01 Hz (b) Phasefrequencycurve,PFC FIGURE 18.28 Notch filter in opposition to boost filter. Figure 18.28a shows an active 1 / 3 octave band-boost filter (BBF) raised 9 dB, a passive 1 / 3 octave band-rejection filter (BRF) lowered 9 dB, and the resultant smooth amplitude response. Figure 18.28b shows the phase response of the BBF and the BRF as well as the resultant smooth phase response. 542 The distinction between a bandpass filter (BPF) and a band-boost filter (BBF) is that the "skirts" of a BPF continue on to minus infinity ( ). The skirts on a BBF return to zero reference after passing through the peak. Such filters are found in active parametric filter sets and extreme caution is advised in their use in live system stability adjustment. One must exercise caution when it comes to program effects' adjustment. TEF ANALYSIS IN EQUALIZATION With TEF analysis in use for almost all serious equalization work today, the engineer can view L D , L R , and L RE (early reflection levels), separately or together as desired. The level, direction, and time of travel for each reflective interference can be observed. An aberration in L T can be segregated and if it is caused by L D equalized, if caused by L RE blocked, and if due to L R , it can be treated in the statistical manner. Formerly well hidden transducer aberrations, particularly in phase and in time behavior, are now strik- ingly evident and consequently easily prevented. As a consequence of this enhanced ability to actually "see" what's going in the total system, both rap- idly and accurately, the design procedures are modified to incorporate this new knowledge and we are finding that less and less equalization is required in the newer systems. Where and when equalization is needed, it is invaluable. Misapplied, it can create harsh sounding high frequency distortion, instabil- ity in the system, and, worst of all, a belief that equalization has solved a problem that in actual fact is still unaddressed. Figure 18.29a is the Envelope Time Curve, ETC, of a packaged system with a separation between the low frequency unit and the high frequency of 0.17 ft, 0.15 ms. Figure 18.29b is the magnitude and phase of the same loudspeaker with the phase response made as smooth as possible. Figure 18.29c is the Nyquist display of the same loudspeaker with the cursor set at a high frequency that has encircled the origin of the display indicating a non-minimum phase system. Failure of a cursor in a Nyquist plot to rotate clockwise as frequency increases indicates a mis-selected signal arrival time at the measurement microphone. Harry Nyquist of the Bell Telephone Labs in the old days was truly a genius.