It's all pretty academic since variations from unit to unit along with your box tuning and frequency of interest will have a significant effect on the outcome of such a comparison. This is from a recent discussion:Originally Posted by toddalin
http://www.audioheritage.org/vbullet...1&postcount=10
Widget
Forgive me for dragging this up again after such a lapse. I read through the responses, and more than a few times came back.. remaining confused. But (if it's not too late.. ) perhaps I have found a way through, or at least a more specific way to frame the question.
If this statement, if I recall correctly, works for inefficient and efficient drivers, then it would seem to make a case for relative sound pressure being independent of efficiency.
And so in a effort to strip away (if possible) the other issues that were brought into discussion ('which will play the loudest, 'which might sound more dynamic due to better transient, or less compression for example), what if; Our same two systems, sine waves for each at one watt input, then again at 2 watts.
(To pull some made-up driver numbers) does one go from 88 to 91 dB, another from 100 to 103?
Or maybe it would be just as fun to talk about the new monitors I'm going to be trying out.
Either way, wishing a merry Christmass everyone.
Wayne Smith
Yes.
However you will more likely listen to both at about the same SPL so in system A you would go from 1 watt to 2 watts and in system B you would go from 63mW to 125mW. It is very likely that system B will sound more dynamic since it will heat up significantly less at 125mW than system A will with 2 watts.
...and Merry Christmas to you to!
Widget
But (assuming that all to impossible 'all other things being egual
Or unless the more efficient one spends a smaller percentage as heat...
Wayne
This is where the physics of reality gets more complicated than the simplified ideal as used in a school lab to explain different scenarios.But (assuming that all to impossible 'all other things being egual
This would explain part of it, but basically most lower sensitivity drivers heat up and loose dynamics. There are some that vent the heat well and can absorb great amounts of power and remain dynamic... these are rare.
Widget
It is interesting this dB stuff and is perhaps worth a little more explanation.
Once upon a time someone sat down and said how do you measure noise.
They figured out that that the ear is non linear and perceives sound in a logarithmic manner.
1 dB is considered as the smallest change you can hear on a solid state tone
3 dB is considered the smallest change you can hear on music
10 dB is considered as twice as loud.
Now it is a mathematical anomaly that for a speaker to sound twice as loud it needs 10 x the power.
So to make that 100 watt stereo sound twice as loud you need1 Kilowatt.
But this is only part of the story.
Music consists of a whole lot of peaks but the average power can be very low with a whole lot of peaks that need lots of power.
So if we assume a bass transient is driving the amplifier output +ve at the same time as a cymbal crash (HF) is also going +ve our peak requirement can become very high
Lets say we are using 1/10 of watt and need a 40 dB peak that’s 1Kw. (Actually its not but we need the voltage and instant current capacity that only a 1 Kw amp has)
Now the worst thing you can do is let a solid state amp clip, horrible things happen.
The real world is not quite like this as this signal has been “processed” before it’s recorded, one of the things that’s happened is it’s been compressed and all the peaks chopped of so they don’t overload every thing.
We just have to hope the recording engineer used a nice sounding compressor.
But the idea of a big amp to cope with peaks is a valid one that makes a difference.
Now it gets a bit more interesting, speakers are not very efficient things and most of the energy you pump into them gets converted into heat.
When was the last time you removed a working light bulb? Hot wasn’t it.
Same thing happens in a speaker voice coil.
Now when you heat up a voice coil lots of nasty things happen this referred to as power compression.
Now a 3 dB of power compression means that when you double your amp power nothing happens the speaker won’t get louder it just gets hotter.
Does power compression matter? Yes I believe it does.
Recently I was playing with some 2220’s, quite a nice speaker; I was using an H version which has a big ferrite magnet. When you turned it up it just went flat and lifeless.
It so happened that I changed to an A version which has a smaller Alnico magnet.
It went flat and lifeless sooner, less heat sinking more power compression.
I replaced them with a pair of 2227’s which is what 2220,s evolved into.
The result you turn it up and it just gets louder, why? Because these are JBL’s super vented gap transducers capable of dissipating heaps of heat.
Here are the power compression figures for 2227”s
60 watts 0.5 dB
300watts 1.7 dB
600 watts 3.2 dB (rated Power)
Exceptionally good numbers.
And they are 100 dB at watt at 1 meter much the same as a 2220
So what happens when I drive an inefficient speaker, say 80 dB at 1 watt/1meter
That’s 20 dB more grunt I need, instead of 1 watt I need a 100 watts and that’s a lot of heat.
And that means power compression and is a good an explanation as any as to why an inefficient speaker sounds dull flat and lifeless because when you turn it up it stops getting louder and just gets hotter, and it happens a lot faster than you think.
And you need another 20dB out of that amp.
By nature a low efficiency speaker will have a large magnetic gap and the voice coil just can’t radiate away the heat.
This is one of the reasons that high efficiency systems sound better, you can turn them up more before lots of nasty things start happening.
http://www.jblpro.com/pub/technote/tn_v1n18.pd
Quote from http://sound.westhost.com/lr-passive.htm#3.3
“3.3 Temperature
In case you were wondering, the voice coil temperature used in the examples below (150°C) is not as outrageous as it may seem. Since loudspeakers have an efficiency of typically 1% or less, this means that 99% of all the power going to the speaker must be dissipated as heat. Although there is some air movement through the voice coil gap, it cannot keep the temperature down low enough to ensure that the effects described will not disturb the behaviour of the crossover network. An efficiency of 1% indicates just over 92dB/m/W, which is quite a respectable figure in the world of loudspeakers!
Copper has a thermal coefficient of resistance such that its resistance increases by 0.395% per °C (variously listed as anything from 0.39% to 0.43% on different websites). We can safely assume that the impedance is based on "room temperature", which will generally be in the order of 20°C. When power (in the form of music or test signals) is applied to a speaker, the voice coil temperature must rise. Given a typical 6.6 ohm (DC) voice coil for an 8 ohm nominal speaker, at 150°C, the resistive component alone will rise to 10 ohms - and naturally the impedance must be somewhat greater than this figure.”
And this does lots of bad things to a passive crossover...
Johnny Haugen Sørgård
Let's not immediately presume this is of any major consequence at typical home listening levels.
The voice coil temperature is at equilibrium with the average power and the thermal dissipation capabilities of the motor.
When a transient comes through, the temperature rise is not instantaneous, as the voice coil, unlike the above cited light bulb filament, has thermal mass, apart from the dissipation path. Additional power compression does not come into play significantly on transients.
How hot does my woofer's voice coil actually get when I'm listening at 90 dB average from 4 M away, which is pretty damn loud?
Well, hotter, of course, when playing an inefficient driver, but enough so to compress the dynamics of the program.
Let's do! I believe it is one of the reasons that many of us like horns even though they are typically not as neutral as conventional speakers.
The subtleties the ear can discern are really amazing. I believe the lack of even very small amounts of dynamic compression adds to our sense of transient response and a sense the music is alive.
Widget
The instructions that come with the W15GTI address power compression as a result of temperature in reference to voice coil impedience as well as volume level for use in sound level competition.
It simply has to do something with transients, when taking one quite hefty Yamaha amp (260W + 260W) for my 2235's and then adding a new dimension to the listening experience, when bridging two of them, so that each bass driver is controlled by 800W, while still listening at normal SPL's.
BR
Roland
Well, yes, you still have to be able to deliver them, as required by the program....
Yup, equilibrium occurs at a higher SPL with VGC and other thermal dissipation measures. There's more "buffer" to transients, as well, but my point is that's not of consequence at normal home listening levels. It matters to SR guys, too, of course, who are working closer to the operating limits of the drivers.
I believe we can agree that has more to do with diaphragm mass than dynamic power compression, however.
Not sure we can... check this out.
http://www.adireaudio.com/Files/Tech...ooferSpeed.pdf
Widget
Do you by program, mean the music that's on the record or CD?
I've got several CD's, where the dynamic range varies from just above 40dBC to over 100dBC, with all the transients I could ever dream of present, so, I don't think the speakers will fall short, in that respect.
BR
Roland
Roland,
It's just that Zilch has been listening to pink noise too long.
Widget
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