Sound Discipline: Speakers at School Dances

June 29, 2011 by Michael Buonaccorso

YOUTH EVENTS CAN PUSH YOUR SPEAKERS PAST THEIR LIMITS; HERE’S HOW TO GET YOUR SOUND UNDER CONTROL

Before we take our sound systems to school this fall, lets have a little education ABOUT sound systems. Teen events today can require a lot from our rigs, and there’s more to it than simply the attendees wanting you to liquefy their insides with nearly lethal doses of low frequency audio. The music industry itself has been waging the “Loudness Wars” and if we’re not careful, our sound systems can end up as “collateral damage.”

Before the significance of this becomes apparent, I would be remiss if we didn’t get a little nerdy and talk about the science of making sound. Specifically, how we make sound with transducers (speakers, in lay terms). Transducers are devices that convert one form of energy to another. In the case of a loudspeaker, it is converting electrical energy into acoustic energy. It does so by running an electric current through a coil of thin wire (the voice coil in the speaker), which then becomes an electromagnet. The voice coil operates in close proximity to a permanent magnet on the speaker. At one point, I’m sure we all witnessed how magnets of like and opposite polarity react to each other. Like polarities repel and opposite polarities attract. The magnetic reaction between the voice coil and the permanent magnet in a loudspeaker moves the cone in and out, which creates vibrations in the air. Voila! We have acoustic energy (sound).

SPEAKERS UNDER PRESSURE

So, now that you survived a paragraph of pure nerdspeak, you’re probably dying to know that your pain was worth it. It was. By understanding how a loudspeaker operates you’ll be far better equipped to understand why they fail, and thus how to prevent it. Often people believe that putting too much power to a loudspeaker is what causes failure, and while there is some truth to that, there is also a lot of misunderstanding. Loudspeakers can withstand tremendously large amounts of short term (peak) power. Music has numerous peaks and valleys, the latter of which allow the loudspeaker to recover and remain cool. Just about the worst thing you can do for a loudspeaker is to play a continuous tone at high levels for an extended period of time.

Worse yet is a phenomenon known as “speaker compression” or “power compression.” This occurs when the voice coil in a loudspeaker heats up. Some heating is normal during use, but it stands to reason that heavier use causes higher temperatures. As the voice coil heats up, its DC resistance increases. The increased resistance reduces the effective output of the loudspeaker by as much as 6dB. That’s the equivalent of reducing your amplifiers power by 75%! So, of course, as our loudspeakers produce less sound, our natural reaction is to “turn it up”—which only worsens the situation.

There’s another kind of compression to worry about as well. In this case it is the intentional reduction of recorded music’s dynamic range for artistic and other reasons. Judiciously applied, compression can give a track a pleasantly smooth and powerful sound, but when we compress music, we run out of dynamic range and the loudspeaker no longer has the opportunity to recover. Unfortunately, most modern dance tracks are heavily compressed during the recording process. Graphic A shows stereo waveforms from top to bottom for a country song, a hip-hop song, and a dance track. Notice how much more blue we see in the hip-hop and especially the dance track? The loudspeaker has much less chance to recover while playing these tracks at high levels. And the bad news isn’t over yet: This is where the so-called “Loudness Wars” come into play. In Graphic B (courtesy of Song Yanbo) you will see comparison waveforms from Michael Jackson’s “Black or White” from 1991, 1995 and 2007. Again you’ll notice the green sound waveform of the latest version filling up the dynamic range until it’s almost all “peak” during most of the song. Essentially, the Loudness Wars are a result of the record labels competing to make their songs “stand out” by increasing the amplitude of the track until more and more of it reaches the peak level for the recording. This has caused a reduction in the dynamic range, sometimes substantial, which not only degrades the quality of the audio, but makes our loudspeakers much more likely to heat up.

DRIVING THE RIGHT VEHICLE

The solution? Have enough rig for the gig. Don’t ask your loudspeakers to do what they were not designed to do. I strongly recommend having subwoofers for any teen event. They will produce the deep, low bass that’s desired and take the pressure off of your full range boxes.

How much rig do you need? I wish there was an “Easy” button for this, but unfortunately there is not. Loudspeakers vary in efficiency, output, frequency response and pattern of coverage. But here are some rough rules of thumb though, to simplify the equation. For teen dances of 200 to 400 kids in an average-sized gymnasium I would recommend a minimum of a pair of 18” subs and a pair of 12” or 15” 2-way tops, with a peak output of no less than 129 dB (1M). For 400 to 1000, you’re probably going to want at least four 18” subs (or two dual 18s), preferably in a center-coupled arrangement. Placing the subs next to each other will produce a gain of approximately 3dB of output. More than two tops may be required here. If you use full range boxes placed closely together, be sure they are splayed at the horn angles to give adequate coverage over the dancefloor area, and to avoid interference between the boxes. Be sure to get the horns of your top boxes slightly above head level to allow the smaller wavelengths (higher frequencies) to cover the dancefloor. (Loudspeaker placement is another topic, all of its own, and my colleague Stu Chisholm covered the basics of it on page 26 of MB’s March 2011 issue).

It is important to note that all speakers are not equal, no more than all cars are. Taking a family sedan to the racetrack will only lead to its early death, and it won’t perform as well as a car designed to race anyway.

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Michael Buonaccorso Michael Buonaccorso (51 Posts)

Co-founder of Mobile Beat Magazine in 1991, and current producer of Mobile Beat Las Vegas, Mike has worn just about every hat possible in the evolution of the magazine, from writing articles to making sales calls. Formerly a musician and DJ himself, he still enjoys writing for the magazine and released “A Different Spin”, a DJ history book, in 2011.


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