Unplugged Sound
Disappearing Act
Anton Niedersteiner - Acoustic Sound Reinforcement & Audio Engineering
"That's a nice sounding system" one patron remarked during a pause in the performance. What a pity I said to myself, not because the system didn't sound "nice", but because it did.
What do I mean by that statement? Simply put, refining a sound reinforcement system sufficiently to the point where it sounds good is going only half way. You can select the best components operated with skill and musicality and achieve just that - a "nice" sounding system. What the listeners are hearing is the system and always will be, unless the engineer takes the next step, which is to make the system disappear.
The fact that the term "unplugged sound" exists and is definable is sufficient proof of the need to take that next step. If the instrument sounds different after it is plugged in then whatever it is plugged into is altering the sound in the process of making it louder. The entire concept has taken on a kind of reverse logic. Often critics will evaluate the "unplugged" sound of an instrument, comparing that to the way it sounded plugged in, as if plugging a piece of wire into it somehow changes it's acoustic parameters. The instrument "sounds" just as it always does. It is the system of amplification they hear and are comparing that to the the original (unplugged) sound. (In the purest sense, even an electric guitar played through an amplifier and loudspeaker is an acoustic event. "Plugging" that into a sound system will alter the event just as it does with an acoustic instrument.) So how do you make the system disappear?
When it comes to sound I am a purist at heart. The goal is to hear the instruments as they really sound, just louder so you can hear them in the back row. In trying to do this many systems fall short. Failing to differentiate between a Martin and a Gibson for example, each sounding like an acoustic guitar but missing the subtleties and nuances inherent with either, thereby diminishing the listening experience and nullifying the musician's investment. Before we can go on to the next step of making your system disappear, it must already be at the point where it has enough fidelity to make those subtle differences known. The disappearing act will not happen unless your entire system tracks true, from mics to loudspeakers. That is why I design and build some of my own components.
The next step is rooted in a gem of a discovery (the precedence effect) I learned about while studying psychoacostics (the way people hear). It is uncanny in it's simplicity and even more profound in the way it works. Localization is more dependant on time than on amplitude. When two (nearly) identical sounds reach the listener with a time interval, the earlier signal gets the cue. In other words, the sound appears to come from the source which is heard first.
Popular sound systems place the main loudspeakers ahead of the musicians on stage in order to help control feedback. In this configuration, the sound from the main loudspeakers reaches the listener first. Any sound from the stage will be heard a fraction later and assimilated to the loudspeakers. (We over-rule our auditory cues with visual cues. Our eyes persuade us the sound comes from the performers though our ears tell us otherwise). Assuming everything else is being done properly, merely delaying the main signal so that the PA sound arrives at least 2ms after the stage sound will shift the perceived sound source from the loudspeakers to the stage. It is that simple! Of course you can do the same by physically moving the speakers back, away from the listeners, but feedback becomes much more difficult to manage.
The breakdown point of this psychoacoustic phenomena is where the delayed source is about 12dB or more louder than the original. Then the earlier sound is masked by the louder sound. Given that the stage sound is sometimes 90dB SPL, this shouldn't be a problem. Keeping delays to generally less than 30ms will avoid echoes.
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