2

For normal home audio systems, the speakers usually have sensitivities around 85~90 dB. So according to the sound pressure chart, it's pretty loud already at 1 Watt of power. Then why do audio amplifier often have several hundreds watts of output power? I understand you need some extra range for the dynamic range of your music, but that still seems very excessive to me.

Let's say you play your music with average sound pressure somewhere between TV level (60dB) and hearing damage (85dB), say 75 dB, then you'll have a good 20 dB dynamic range with just about 10 watt power. Then why do I need amps with 200W power? Or did I completely misunderstand the whole thing?

4

Indeed it's easy to generate very loud noises with a speaker system whose power is only a fraction of what's actually used in PA or HiFi systems. What's not so easy is to get any sound you want to that level, without severely distorting it.

Perhaps most relevantly for music: bass frequencies need way more power to be perceived as loud as a 1 kHz signal. And short transients can have enourmous peak power without sounding particularly loud. A bass drum has short transients in the bass range, and indeed the bass drum hits will by far dominate the power histogram in a typical live situation.

For studio records, mastering has as one of its goals to minimise the peak-power need (what they do is, maximise the loudness for a given sample peak cap); most important tool is various compressors, which roughly speaking shift some of the transients' energy into the gaps (at the cost of reduced dynamic range). But to really get the most efficiency, you'd need to do what speech application have done pretty much forever: reduce the signal for the most audible frequency range, i.e. cut both bass and treble. Then put in some nonlinearities (compressors/limiters or distortion, in doubt in the low-quality speakers) to also reduce the peaks, and voilà: you can build a fire siren with a 5-watt supply, or speak to an entire stadion with only a couple hundred of watts. It'll sound horrible though, definitely not useful for music.

2

Some of that chart isn't very good in my opinion. 85dB isn't that loud. A typical concert will be at 106dB to 110dB or sometimes louder and the power requirement doubles every 6dB or so because it is an exponential scale. Additionally, sound falls off very rapidly as well.

You might be able to produce an 85dB sound with one watt of power at the cone of the speaker itself, but think about a pair of headphones. When you turn them up so they are uncomfortably loud and then you take them off your head, what happens? You can barely hear them at all at arms length even though they were deafening on your head.

Sound falls off quickly and is hard to get loud, so amplifiers need a fair bit of power to overcome these limitations of sound, plus each speaker needs power.

For more details, I suggest reading up on the inverse square law.

  • Yeah that's true. The chart is rated at 1 meter distance. – LWZ Sep 26 '14 at 22:51
  • Note that inverse-square is avoided with line arrays. (And inverse-square is not exponential... that's what happens at obstacles, but not with mere spread of sound.) – leftaroundabout Sep 27 '14 at 0:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.