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Reviewer Keith Howard sent me an interesting note a few days ago that really was an eye opener for me when it comes to understanding how loud music actually gets. I am going to reprint it here. I would like to preface this note by pointing out that a lot depends on the type of music you listen to. For example, classical music is generally very dynamic with average levels quite low but filled with big dynamic peaks. Rock music, on the other hand, is generally the opposite: lots of loud average levels with very few peaks and valleys in the sound. This observation is important when it comes to figuring out what kind of amplifier you need and we'll start to cover some of these needs in the coming days. Meanwhile, here's Keith's post. "This is a subject close to my heart and has been for some years - so I eagerly look forward to reading what you have to say about it. You may be aware that I wrote an article about one aspect of this for Stereophile some years ago (http://www.stereophile.com/reference/707heavy), following a three-part feature on various aspects of the subject for Hi-Fi News. In the course of my experiments, listening at only a little higher than I would consider 'realistic', I clipped Musical Fidelity's most powerful amplifier of the time, rated I recall at 700W into 8 ohms continuous, when driving a pair of B&W 805 stand-mounts in a room of about 64m^3. I also find it 'amusing' that if you record a tambourine with a calibrated microphone, you find that it produces short-term peaks of around 113dB SPL at 1m. The amusing part comes when you calculate the amplifier output required to achieve this with a tweeter sensitivity of, say, 90dB. Not many of us listen to solo tambourine, I know, but it seems not unreasonable to me to expect a hi-fi system to be able to reproduce single instruments without clipping, let alone larger musical forces. The argument that hard clipping of such short-term peaks is inaudible is hogwash - I can hear it when the tambourine recording is processed to clip at 3dB below peak level, and anyone would easily hear it at -6dB clipping. And this without any amplifier latch-up or other misbehaviour when recovering from clipping. This was done using a piece of freeware I've made available on my web site (http://www.audiosignal.co.uk/freeware.html, scroll down to Clipping Simulation and clippingsimulator.zip). But you know what? I suspect that hardly anyone has tried the experiment for themselves and then come to terms with what it means. A 5W SET does not cut the mustard even with 95dB sensitivity loudspeakers!" Thanks Keith. In yesterday's post I mentioned that to reach 100dB with a 90dB efficient loudspeaker, you would need 500 watts. What I left out is some valuable information: distance from the speaker. To get that 500 watts I moved the listener away from the 1 meter mark without telling you. So let me straighten this out. Here's a handy list you can use to help you figure out needed watts for playing music, not sine waves. These figures are all based around a 90dB efficient speaker, meaning 1 watt of sine wave power gives you 90dB at a distance of 1 meter as our reference.
  • 50 watts are needed to get 90dB of music. 90dB average, 107dB peaks
  • 200watts are needed to get 96dB of music. 96dB average, 113dB peaks (our tambourine now works).
  • 500watts are needed to get 100dB of music. 100dB average, 117dB peaks.
But, most of us don't sit 1 meter away from our loudspeakers, now do we? I don't, I am about 3 meter away from my loudspeakers.
For every doubling of the distance we need 6dB more power to reach the same level - so we need 10 times more power for a 3 meter distance. So now let's look at out chart:
The next set of figures are all based around the same 90dB efficient speaker playing music, but this time measured at a 3 meter distance, meaning 10 watt of sine wave power is needed to give you 90dB as a reference.
  • 500 watts are needed to get 90dB of music. 90dB average, 107dB peaks
  • 2000 watts are needed to get 96dB of music. 96dB average, 113dB peaks (our tambourine now works).
  • 5000 watts are needed to get 100dB of music. 100dB average, 117dB peaks.
If your speakers are 87dB efficient, double the above numbers. If your speakers are 93dB efficient, halve the above numbers, 96dB halve them again and so on. How many tweeters can handle all that power? Not many, still the numbers don't lie. A ridiculous amount of power by anyone's standard - except that is unless we were famed audio designer Dick Burwen whose audio system runs on 20,000 watts of power. http://www.burwenaudio.com/Sound_System.html But then you knew this series of posts was going into la la land. We'll return to something reasonable tomorrow.
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Paul McGowan

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