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The Price of 3 DB

A 3 DB LEVEL INCREASE is a modest one. Subjectively, one might describe it as being a ?tweak? to the loudness of a system. Most infrared remote controls for home entertainment systems are about 1.5 dB per step, and most people who go to the trouble to pick one up and point it will change the level by at least two steps. Quit reading and go try it now....

Destined for failure

A loudspeaker will eventually succumb to that “next 3 dB.” It doesn't sound like much, and it came so easy at lower levels. But eventually heat takes its toll and failure occurs. When a loudspeaker is power tested, once its response changes by a few dB due to heat, it's very close to its thermal limits. You can't expect another 3 dB. Level is very deceptive. You many only want “a little more,” but at some point a slight increase produces distortion or component failure due to the logarithmic increase in power.

We hear about “the straw that broke the camel's back.” But in this case, we're doubling the number of straws each time we add to the camel's load. If he's showing signs of fatigue, it isn't advisable to keep piling it on. It's better to add a second camel.

The moral of the story? Don't load your camel logarithmically! Yet that's exactly what happens each time you turn up a loudspeaker by 3 dB.

Power vs. voltage

The truth is that amplifiers seldom run out of power. They're rated using sine waves — a signal that has a much higher RMS voltage (and therefore generates much more power) than typical audio program material. Instead, they run out of voltage swing. When normal program material lights the clip indicator on an amplifier, it's likely at one-tenth or less of its rated sine wave power output. Further level increases produce more severe clipping, and the amplifier power output continues to rise, escalating toward the sine wave power production reflected by its power rating. An inexperienced system operator will often turn the sound level up so much that the amplifier clips. They want it louder yet, so they continue upward with the volume control. Because the amplifier is out of headroom, each 3 dB increase clips the signal more severely. The power output of this amplifier is going up, even though it's heavily distorted.

It takes an amplifier rated at about 100 watts to produce 10 “clean” watts into a loudspeaker. Likewise, it takes an amplifier rated at about 1,000 watts to produce 100 “clean” watts into a loudspeaker. Peak limiters can allow an amplifier to play louder by reducing the program peaks and allowing the amplifier to generate a higher RMS voltage across the loudspeaker. While the fidelity may suffer, the sound definitely gets louder, and the voice coils definitely get hotter.

A system's ability to go 3 dB louder diminishes with increased level. Eventually the next 3 dB will max out a system component, causing amplifier clipping, thermal failure of the loudspeaker, or both.

The bottom line? At some point, a sound system simply can't go “a little louder.” A perfectly reasonable request for a slight loudness increase at low playback levels can be an absurd request at high playback levels. If you ignore the warning indicators, something will fail. A sound system that's operating at “half power” is nearly all the way up.

Pat Brown is president of Synergetic Audio Concepts (Syn-Aud-Con) Inc. and Electro-Acoustic Testing Company (ETC) Inc. Syn-Aud-Con conducts training seminars in audio and acoustics worldwide for those who operate, install, and design sound reinforcement systems. ETC Inc. performs precision loudspeaker testing for the audio industry. He can be reached at pbrown@synaudcon.com.



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