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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: Re: Thanks for the link - disturbing lack of accuracy there posted by Borough33 on March 25, 2007 at 17:25:33:
Very cleverly done. Mention names and provide quotes for specific statements which are correct, then string those statements together in an argument which is not correct.The 6 dB dropoff per doubling of distance is correct, in a free field such as out in the open. In a bounded space like a room, you get that 6 dB dropoff in the near field but not in the far field. The transition between the 2 is sometimes defined as the point where the dropoff changes from 6 dB to 3 dB for each doubling of distance. Depending on speaker and room, that can even occur before you reach a 2 metre distance.
They don't really cover speakers of different impedances. They use a simple dBW table which is correct for a pure 8 ohm impedance which few speakers present. For an 8 ohm impedance, 1 watt does equal 2.83 volts but it does not for a 4 ohm speaker as Duke has pointed out.
Having said that, most ss amps give power ratings for 4 and 8 ohms speakers, with a higher rating being shown for 4 ohm speakers, often 3 dBw higher. Provided you use the 4 ohm amp rating, everything is taken into account. Tube amps usually have output transormers and different binding posts for use with 4 and 8 ohm speakers. Their power ratings tend to be the same regardless of speaker impedance, because of the differences in the way they couple to the speaker as a result of the use of output transformers.
They say to measure the SPL at the listening position and deduct 10 dB for the attenuation over distance. At another point they say that 10 dB relates to a listening distance of 10-11 ft, somewhat under 4 metres. The attenuation would be 12 dB at 4 metres in a free field, but nowhere near as much in a room where 10-11 ft is in the far field and sound is reducing at 3dB or less per doubling of distance. Making this deduction too large means the end result is going to be that you need a larger amp.
The 3 dB add back for 2 speakers is correct. In a HT system with more speakers, you need to add back more for the additional speakers.
The 105 to 110 dB target is excessive in a small room. You need a large space like a concert hall for sounds of that level to work well. You're going to overload a small room well before you reach that volume. Your peak level depends on 2 things, the dynamic range of the music and the noise floor of your room. Concert halls have much higher noise floors than the average home listening room at night. I don't have measurements but it could be as much as 10 or 15 dB depending on where you live and whether you have air conditioning or not. If I try to measure background noise in my home with a Radio Shack meter, it's less than 50 dB and if I add the 46 dB dynamic range they quote from one of JA's measurements (I have no doubt about the accuracy of that measurement) to that 50 dB figure, I end up with a target peak of 96 dB in my house. There's even a safety margin in that since my noise floor is less than 50 dB. If you get a reading for the noise floor, ie the noise floor is above 50 dB using a Rat Shack meter, you can add 2 or 3 dB to the noise floor in order to ensure that the softest passages are clear of it. Even doing that, I doubt you're going to come up with a target of around 105 to 110 dB as the goal, and increasing the level of the goal translates to needing a bigger amp.
I listen at around 2 metres in a near field setup. My speakers are nominally 6 ohms and 86 dB sensitive. I use an amp that delivers 90 w into 8 ohms and a bit more into lower impedances so let's call that 20 dbW. Adding 20 dB W to my 86 dB sensitivity gives me 106 dB from which I deduct 6 dB for the doubling of distance to 2 metres and then add back 3 dB for the second speaker. That gives me a peak SPL of 103 dB. It's probably a bit higher due to the impedance of the speakers coupled with the fact that the falloff at even 2 metres isn't going to be 6 dB. I play most music with peak levels at the listening position rarely exceeding 85 dB. In a room the size of mind, 90 dB sounds quite loud and you wouldn't want to go much louder, definitely not to the 100 dB mark. Such levels would be unlistenable in my room. The sound just hardens and you lose any sense of gradation at high levels when you push them too high.
See how easy it is to mislead. They use JA's measurement of dynamic range which is accurate, but play games with it. In a smaller room the softest passages in a live performance of the work being played when JA did his measurements would be played more softly than 63 dB, considerably more softly and especially when the background noise level is lower. The peaks would be a similar amount lower than 109 dB, perhaps a bit more than a similar amount lower. Peaks are scaled to the room's capacity to deliver them and musicians don't push the room too much. They're chasing a sense of scale and dynamic contrast, and you don't need to play as loud to produce that scale and contrast if the room is smaller and the softest passages are lower in level. There's probably a fair bit more absorption in a concert hall than in your room at home too, so there's a little bit more level you have at home that gets lost in the hall. That saves you a bit more amplifier power too.
Of course, their sums are TOTALLY ACCURATE provide you:
1- use 8 ohm speakers; and
2- listen in a room the size of a concert hall that's full of people.Do that on your own and you probably don't have to worry about the attenuation with listening distance because people absorb a fair bit and you've lost that and got a bit more room support as a result.
And all they have to say to justify what they are presenting is that they're presenting the info in its simplest form, and that any errors provide you with a little bit of safety margin so that you don't have to worry about your amp ever being too small to do what you want. It doesn't take much to end up with a realistic peak level goal that's 3 dB less than what they suggest, and I'd say the average person would be more than happy with a peak level goal around 6-9 dB less than they suggest. For every 3 dB you lower that peak level goal, you halve the amplifier power required. 3 dB doesn't sound like much but when that's at the peak output level and we're talking amps of over 100 w power, it's very easy for that 3 dB to turn into 100 or 200 watts or more. That's a lot of money for peak SPLs you're never going to need to reach in a home listening room.
There are some advantages to having a bigger amp than required, but peak output level is rarely going to be the advantage you're chasing. A much more important difference is simply the fact that bigger amps have better power supplies and speakers respond to that in things like the speed of their attack and the crispness of it. Do you need as much extra power as their calculations recommend in order to gain that benefit. I don't think so.
David Aiken
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Follow Ups
- How to play games with figures, names, and quotes. - David Aiken 01:39:49 03/26/07 (0)
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