Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!linus!security!genrad!wjh12!n44a!ima!inmet!muller From: muller@inmet.UUCP Newsgroups: net.audio Subject: Re: Orphaned Response - (nf) Message-ID: <795@inmet.UUCP> Date: Fri, 27-Jan-84 05:48:42 EST Article-I.D.: inmet.795 Posted: Fri Jan 27 05:48:42 1984 Date-Received: Tue, 31-Jan-84 03:24:06 EST Lines: 91 #R:decwrl:-490100:inmet:2600031:177600:5972 inmet!muller Jan 26 16:45:00 1984 There are several possible reasons for your experience, all related to sound reinforcement phemonena, not signal processing. Without more info tho', we can't say zactly what caused it. You said there was a monitor at each end of the stage...were they really monitors, i.e., speakers for the performer's benefit, or were they actual reinforcement speakers for the audience? (I suspect the latter.) This might make a difference... The first possibility is that you were hearing feedback effects. Yes, feedback can produce significant frequency coloration well below the self-sustain level, e.g., +3 to -6 (?) dB at 3 dB below self-sustain. (We can call this amplitude response rather than frequency response, if you prefer, tho' some relative phase shifts also occur. These numbers were recalled from my fuzzy memory of having actually plugged munbers into the equations once...and we don't know how close the engineers were actually running it anyway.) These levels are well within your and others' discernibility. This is caused by the interference between the coherent sounds FROM 4 sources (2 speakers, 1 piano, 1 voice), and INTO 2 mics. As a recording engineer, you probably have never run into the FROM problem, but may have had to fight the INTO problem. I would guess that the vox mic was mixed hotter than the piano mic, and thus was the real offender. This problem would be more likely if the speakers on the stage were true monitors. Other contributing factors include the off- axis rejection characteristics of the mics and the angle at which they were pointing. Unfortunately there are many so-called engineers doing reinforcement who don't understand wave phenomena at all. The second possibility involves your seating position. The relative distances to the speakers was probably such that you were hearing both at similar amplitudes (assuming the speakers were pointed at you, not at Mr. Winston) if you were near center. Similar amplitudes is, of course necessary for significant interference to occur. If, then, one speaker were perhaps just a few decimeters (give or take one order of magnitude!) closer than the other, PRESTO! instant frequency-dependent interference, again at a level you could most certainly discern. The summation of 2 equal coherent signals would give +6 dB, while the cancellation of those frequencies out of phase could give -10, -100, or -EVERYTHING! dB, if the amplitudes were similar enough. Ironically, those people seated to the sides probably had better sound than you did, since they "heard" one speaker much more than the other. There are several solutions to this: 1) use a hall with a center aisle, thus no seats have this problem until you get further back from the stage (the problem zone widens as you go back, but those folks won't feel like they got ripped off by paying for a choice seat which did't sound so good!, 2) run a stereo mix - this works for every sound source on the stage except those performers right at center stage, who will probably still be panned equally into each channel, but it can accentuate the feeling of being-off-to-the-side for those people with side seats (stereo mix probably was not done at your concert, I would guess), or 3) use a better speaker placement, i.e., only ONE (if you are mixing in mono), placed above the performer, with a full spread angle (180 degrees or whatever is necessary) - this also has the benefit of increasing the throw of the coherent sound from the speaker into the reverberant field of the room. This last solution is rarely done partly becuz it isn't practical with portable (for hire) sound systems, and partly because engineers have a fixation with two speakers (ya' know, high-end stereo stuff, and all), and sometimes becuz multi-speakers are needed for audience coverage. This last solution also has the benefit of allowing some of the patrons to experience the proximity effect, making them think they are hearing the performer directly rather than the PA system. Unfortunately a lot of (even pro) sound reinforcement engineers don't understand this either. As you can see, the problems of SR'nfrcmnt are usually the result of the actual sound fields, and typically dominate the smaller electronics or signal processing concerns that SR'prdctn involves. Worrying about relative phase response between frequencies in an amplifier, or whatever is misguided when you can have +/- 5 dB in amplitude response from the sound field, lots of phase shifts in microphones, as much as 100%(!) harmonic distortion in (admittedly the cheapest) PA speakers, poor frequency/dispersion characteristics and nonconstant efficiencies (varying impedance) with volumn or frequency, etc., that comes from optimizing a speaker for high efficiency/high power handling ability. Couple this to the fact that anyone can buy PA stuff any then consider that a rigorous quantitive analysis of any concert setting is not practical (so that experience often counts for more than theory) and you can see why there is variability in concert PA system performance. I'm not trying to criticize either sound engineers or your own thoughts on recording, rather just trying to put things into perspective. Notice that I haven't even touched on the question of PERCEPTION. Most of what I have said I can substantiate with experience. I am a semi-pro musician, semi-pro sound reinforcement engineer, and a (professional) signal processor (underwater acoustics) and scientist (seismology). I HAVE probably spent more time thinking these problems than most people in the SR'nfrcmnt business. Yeah, I know I misspelled voluum, and a few other words.... Jim Muller 8 Bates Rd East Watertown, MA 02172 Yes, interference between the direct and speaker sound is real possibility though it is more likely that the requisit similarity of amplitudes for any frequency was obtained between the two speakers.