Ron Pellegrino's Quest for Audio Excellence Audio Report 3
Wednesday, July 24, 1996
Intel Internet Media Symposium
Hyatt Regency Hotel at the San Francisco Airport
A multipurpose ballroom in a major hotel.
There were two-thousand Intel invitees on the receiving end of an Intel pitch for integrating and fine tuning currently available technology to ride and thrive on the leading edge of the internet explosion.
Good messages from Intel to the industry engine -- developers and system integrators:
Nothing new here. Just a pep talk and an update. A good reason to gather.
However, once more, the care of the audio left a lot to be desired. When are architects going to learn the fundamentals of spatial acoustics? If you build parallel reflecting surfaces, you've created the optimal conditions for an architecturally induced distortion called flutter echo, a rapidly repeating echo of any percussive envelope -- sonic impulses reflect off one surface, head for the opposite parallel surface, reflect off it back to the original surface, and repeat the process until the energy of the original impulse dissipates. Speech consonants provide perfect examples of percussive envelopes and Intel's design for day called for a parade of presenters producing a rain of consonants. The Hyatt ballroom has parallel reflecting walls and, of course, it has more than enough flutter echo if you made the mistake of sitting in the center section of the ballroom. During the presentations I occasionally walked the space to test for flutter echo at different locations with different presenters. The ballroom is rectangle with the length being front-to-back. Midway along the length of the space the flutter echo was so bad that it made it difficult to follow and understand the presenters.
Erica Efinger of North Hollywood's STAGING TECHNIQUES was the audio engineer at the board. With occasional "ringing" carrying the threat of howling feedback and very dangerous levels during musical interludes (people were wincing, covering their ears and bending over in pain), it was an average workday for her and the audio crew. During the early 90s university audio technology programs popped up like dandelions after spring rains. With all those degreed graduates we should expect public audio production to improve, not deteriorate. People who are working the audio venues don't seem to understand the fundamentals. If they do, they certainly aren't applying them.
The ringing problem in audio engineering sound reinforcement venues is basic and common. A sound indoors radiates from its source in straight lines until it encounters walls or some other obstacles. The acoustic properties of a space are determined by how the walls and obstacles reflect and absorb the sound. Just like any acoustic instrument, an acoustic space, depending upon its geometry and the materials involved, has standing waves. A space's resonant frequencies are the frequencies of its standing waves. Every space "rings" or "booms" naturally; its resonant frequencies create a natural sound signature for that particular space. The color of the space is based on the frequencies and amplitudes of those frequencies that ring, the resonant frequencies.
Most folks who enjoy singing in the shower get a charge out of the shower enclosure's capacity to enrich their singing. The enclosure's resonant frequencies serve to emphasize spectral components of the singer's sound. The same principle holds true for any sound source in any acoustic space. As a presenter delivers his message, his audio stream includes fundamental frequencies and spectral components that may or may not match up with the resonant frequencies of the acoustic space he's working. When they do match up, they set the space to ringing -- the resonant frequencies of the presenter that match those of the space reinforce each other and build up in amplitude level. Those frequencies with the build-up in amplitude stand out above the amplitude level of the rest of the spectrum and create the conditions for ringing, an irritating distortion that carries the engineer-as-terrorist threat of howling feedback.
How do you solve the "ringing" problem?
Hire someone with good ears who can move in the space and help you tune it on the fly. Talk about good ears, I heard Stan Getz, the great jazz tenor saxophonist, dealing with the ringing problem by shouting out equalization settings to a clueless audio engineer at an outdoor jazz festival.
Or get a dual trace oscilloscope. Connect the podium microphone to one of the oscilloscope's inputs; connect a traveling microphone to the other input. Sweep a sine tone from 100 Hz to 10 kHz and pick it up with the podium microphone. Take the traveling microphone out into the space and match the amplitude levels of the two oscilloscope traces. Move around in the space and sweep the sine tone generator. Note the frequencies that bulge in amplitude and attenuate the bands on your equalizer that contain those frequencies so as to flatten out the total acoustic/reinforced audio environment. Do this exercise often enough and you'll end up with an educated ear.
How do you solve the problem of dangerous levels in the musical interludes?
Entrance (background) and exit music was used for this event. The amplitude levels of the exit music were positively offensive and dangerous to the hearing health of people close to the rear speaker stacks. I watched as scores of people seemed to be literally blown away by the sound levels -- on cue when the exit music came on, people grimaced, covered their ears, leaned away from the speaker stacks, and stumbled over each other trying to distance themselves from the audio attack. I hope this doesn't become a formula for exit music.
Audio engineers should test levels by playing their material with the highest levels and putting themselves in the various positions of the audience -- front, back, center, side, directly in front of speakers, etc. Join the musical world; tune on the fly. Don't rely 100% on the pre-event sound-check. Send trusted ears out into the space during the event and make adjustments according their feedback. At the Intel event all the technical people were located centered against the back wall of the space, the worst place to make audio level decisions. The rear speakers were stacked on the floor in the corners with many seats parked directly in front of the speakers -- not the least bit considerate. (Audience tip: avoid sitting in front of speaker stacks!)
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