Yah, but now I have a headache..
jn
ps..99 dB spl 1W/1M is pretty much the highest I've dealt with in dynamic drivers. Horns go better, I think the K's were 103.
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Aha, what appears to be a mistake!!!
The velocity of sound is pretty much independent of pressure. However, I forgot to mention that the air, when pressurized, increases it's temperature (pv=nrt). My pondering involves the effect the acoustic power has on the gas as it traverses it. 4 watts going through a tube of roughly a square inch..
Perhaps somebody can do the math?? Heat capacity of Nitrogen is 1.040 kj/kgK.
Once I get past 10, my math skills slow down..have to unlace my shoes to get to 20..
jn
When spot local temp. varies as in places inside the throat close to the compression driver at mid to high freq.(PA?), things may be not so clear, even if γ remains constant.
Theory of Nonlinear Acoustics in Fluids - B.O. Enflo, C.M. Hedberg - ?????? Google
When air is stressed, it becomes non linear.
At freq. above ~25kHz this becomes very evident and it has been exploited.
http://www.akoustic-arts.fr/wp-content/uploads/article_1.pdf
http://ew3.ee.uec.ac.jp/e/wespac9.pdf
There are some products making use of the theory and the developed technology
LRAD Corporation - OVERVIEW
Audio Spotlight - Add sound and preserve the quiet.
Soundlazer
In industry, Air-coupled Ultrasound inspection of materials (f: 50kHz-1 MHz) is utilized for ~10 years now.
George
This touches on a "fetish",
, of mine: eons ago I realised there is an excellent subjective marker of a system working well enough, way superior to the normal test instrument procedures. And this is that one is never aware of distortion coming from a speaker driver, no matter how close the ear is to the driver. Obviously there still is distortion emerging, if one applied a measuring mic, etc, it would still register; but, because the ear/brain automatically turns down the gain when subjected to loud sound and psychoacoustic masking comes into the picture, the distortion subjectively never registers as being present, in your mind the sound is clean.This is what people are experiencing when they say the system sound is at a "magic" level, that they can't imagine it getting better. In essence it is optimisation, lifting every aspect of the system to a very high state of tune that gets you there; it's also very fragile, typically; the slightest maladjustment or deficiency can be sufficient to knock the SQ off that pedestal ...
Frank
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The guys here seem to consider mu zero constant even in the megatesla range. I'll ask one of the carpool guys about the 7 TEV collisions. (chances are, I will not understand the answer.
jn
If you will not, chances are that what happens is worse than (or at least, different from) what you could think of.
Relay here the un-understandable please
George
Speaker Distortion
2 points
Horn Distortion
Olson, in his comprehensive treatment quotes studies going back to 1933. Basically, the smaller the throat and greater the compression ratio, the higher the distortion. Hence my advice to use the least compression ratio to achieve your desired efficiency. Less drastic flares help too.
Loudspeaker Distortions: can we hear them?
The Fryer et al papers I mention investigates the audibility of various distortions. I extended that work in nearly 2 decades of Blind Listening Tests.
One of the most interesting findings is that it is possible to have a speaker with poor measured harmonic distortion at modest levels sound 'less distorted' than one which measured a lot better.
The speakers involved would all be considered good speakers today and the panel were all experienced listeners with proven better than average aural discrimination.
2 points
Horn Distortion
Olson, in his comprehensive treatment quotes studies going back to 1933. Basically, the smaller the throat and greater the compression ratio, the higher the distortion. Hence my advice to use the least compression ratio to achieve your desired efficiency. Less drastic flares help too.
Loudspeaker Distortions: can we hear them?
The Fryer et al papers I mention investigates the audibility of various distortions. I extended that work in nearly 2 decades of Blind Listening Tests.
One of the most interesting findings is that it is possible to have a speaker with poor measured harmonic distortion at modest levels sound 'less distorted' than one which measured a lot better.
The speakers involved would all be considered good speakers today and the panel were all experienced listeners with proven better than average aural discrimination.
Richard, there is indeed a clash between science and the mind in this area, in part because science always wants to separate out one part, concentrate on one aspect, consider just the black box. Trouble is, in audio the black box extends beyond the element that supposedly is solely being "tested", all the ancilliary equipment comes into play, the level of electrical interference from the various sources, etc. And the ear/brain, of course, the most important component of all.
In another post I mentioned in some tertiary coursework being lectured on the need for extreme care in the experimental setup, for the final measurements to have any meaning. In audio the same sort of rigour needs to be applied, and sadly, this seems to be a very rare occurence ...
Frank
Frank, you are perfectly right about this. The measures you need to take to ensure a Blind Listening Test (especially with speakers) is both not biased as well as comfortable to the panel are immense. We pontificate on the details in at least 3 of our papers.
Doing these tests costs a great deal of time and money. Each member of the panel is tested separately and may take up to half a day for a single test. Ideally, the test needs to be repeated at least once. It tied up one of our most important tools, the Listening Room and its gear and several highly trained (expensive) people for long periods.
But IMHO, as listening is the ultimate purpose of all our efforts, we need to extend at least as much rigour in testing listeners & listening.
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Let me interject some experience here.
Horn throat distortion is REAL and measurable. It is ALSO predictable, and there are equations to predict it, (more or less).
When I first mentioned it, some time ago, I stated the 40 years ago, I measured a JBL horn driver with a 1'' throat, driven at 1KHz with 2W input, and I got 3% distortion. This was almost entirely, HORN THROAT DISTORTION!
Now how loud was that? REALLY LOUD! However, the horn was being driven from 1KHz and above with 2W-20W! So, 3% (second harmonic) distortion was rather high, but it got even worse. Now, if the GD had 50 of the horns in parallel and only put a 200W amp (total) on them, then it might have been OK, but the GD used only 4 identical horns, AND we could hear the problem.
Now, at home, there would be no problem. It is almost impossible to sit in front of a horn like this with even 2W input, without losing your hearing, ultimately. BUT with an auditorium with 500+ people, perhaps it is a bit too audible.
Esperado, mentioned working with 10KW and horns! What did he do to get it to sound OK? That was the original question.
OUR solution was to REPLACE the horns with a large bank of quality direct radiators, in the Wall of Sound. It worked.
Horn throat distortion is REAL and measurable. It is ALSO predictable, and there are equations to predict it, (more or less).
When I first mentioned it, some time ago, I stated the 40 years ago, I measured a JBL horn driver with a 1'' throat, driven at 1KHz with 2W input, and I got 3% distortion. This was almost entirely, HORN THROAT DISTORTION!
Now how loud was that? REALLY LOUD! However, the horn was being driven from 1KHz and above with 2W-20W! So, 3% (second harmonic) distortion was rather high, but it got even worse. Now, if the GD had 50 of the horns in parallel and only put a 200W amp (total) on them, then it might have been OK, but the GD used only 4 identical horns, AND we could hear the problem.
Now, at home, there would be no problem. It is almost impossible to sit in front of a horn like this with even 2W input, without losing your hearing, ultimately. BUT with an auditorium with 500+ people, perhaps it is a bit too audible.
Esperado, mentioned working with 10KW and horns! What did he do to get it to sound OK? That was the original question.
OUR solution was to REPLACE the horns with a large bank of quality direct radiators, in the Wall of Sound. It worked.
Of possible interest to some, after doing a quick search, is this: Red Shift: Doppler distortion in loudspeakers | Stereophile.com. It mentions some work done by Fryer ...
Frank
Frank
Hi
Fwiw, sound does not “heat” air with the exception of what ever energy is absorbed. At the same time, air behaves following gas law and so if you compress it momentarily, it’s pressure rises but so does it’s temperature as you have the same heat energy confined into a smaller space, an important detail.
So far as sound, as Dick Heyser said "it's what happens when you push on air", it is a modulation of pressure above and below ambient and so, for that brief instant a cluster of air molecules are in the pressure side of the sound wave, their temperature is slightly higher.
At the same time, when those molecules are in he rarefied side of the sound wave, they are cooler than ambient.
In the 80’s and 90’s I worked developing very high intensity acoustic transducers for acoustic levitation.
The idea then was for acoustically containing a sample within a high temperature furnace and then melting and solidifying materials at very high temperatures without a container or physical contact.
We flew several high temp container less processing experiments on STS-7, STS-51a and several sounding rocket flights.
Anyway, in the link, scroll down to the video.
Scientists levitate liquid in order to develop better pharmaceuticals
Acoustic levitation becomes possible when the intensity is about 150dB SPL for a Styrofoam ball and for a liquid like he is demonstrating, about 165dB SPL.
The transducers here (a type I developed / patented back then) produces a narrow beam of high intensity sound and here around 22KHz.
If you set one of those sources up horizontally and took a microphone happy at >165 dB, one sees the wave shape is a sine wave but as you move say 6 inches away it is changing, a foot away and it is a saw tooth shape, not a sine wave.
What is happening is the pressure side of the wave is traveling slightly faster than the rarified side and so is no longer 180 degrees away. The longer the distance it travels, the more it catches up to the negative wave front ahead of it For what we were doing back then, this was not a good thing because that wave shape with a sharp transition had much more hf energy and so one had acoustic pumping, a moving column of air moving away from the source (which could both cool and destabilize the sample).
For a high frequency compression driver, the internal sound pressure is high enough to reach this level of nonlinearity but……. The thumb rule throat distortion formulas are based on the how fast the horn is expanding and so the SPL falling (via the low frequency corner) compared to the high frequency corner (the shortest wavelength involved) and then the SPL (rate the wave front precession happens) it has to produce.
The assumption is that a horn with a lower frequency corner will have a slower rate of expansion and so the region where the pressure up high is more wavelengths long.
Funny thing too about those levitation sources, there is a larger configuration using 6 orthogonal sources that can produce about 175dB at the intersection. That IS enough intensity to light a cigarette with the acoustic friction of the air rushing back and forth through the tobacco at 22KHz.
What a cool / semi dangerous coffee table conversation starter, great for gift giving eh?
Best,
Tom Danley
Fwiw, sound does not “heat” air with the exception of what ever energy is absorbed. At the same time, air behaves following gas law and so if you compress it momentarily, it’s pressure rises but so does it’s temperature as you have the same heat energy confined into a smaller space, an important detail.
So far as sound, as Dick Heyser said "it's what happens when you push on air", it is a modulation of pressure above and below ambient and so, for that brief instant a cluster of air molecules are in the pressure side of the sound wave, their temperature is slightly higher.
At the same time, when those molecules are in he rarefied side of the sound wave, they are cooler than ambient.
In the 80’s and 90’s I worked developing very high intensity acoustic transducers for acoustic levitation.
The idea then was for acoustically containing a sample within a high temperature furnace and then melting and solidifying materials at very high temperatures without a container or physical contact.
We flew several high temp container less processing experiments on STS-7, STS-51a and several sounding rocket flights.
Anyway, in the link, scroll down to the video.
Scientists levitate liquid in order to develop better pharmaceuticals
Acoustic levitation becomes possible when the intensity is about 150dB SPL for a Styrofoam ball and for a liquid like he is demonstrating, about 165dB SPL.
The transducers here (a type I developed / patented back then) produces a narrow beam of high intensity sound and here around 22KHz.
If you set one of those sources up horizontally and took a microphone happy at >165 dB, one sees the wave shape is a sine wave but as you move say 6 inches away it is changing, a foot away and it is a saw tooth shape, not a sine wave.
What is happening is the pressure side of the wave is traveling slightly faster than the rarified side and so is no longer 180 degrees away. The longer the distance it travels, the more it catches up to the negative wave front ahead of it For what we were doing back then, this was not a good thing because that wave shape with a sharp transition had much more hf energy and so one had acoustic pumping, a moving column of air moving away from the source (which could both cool and destabilize the sample).
For a high frequency compression driver, the internal sound pressure is high enough to reach this level of nonlinearity but……. The thumb rule throat distortion formulas are based on the how fast the horn is expanding and so the SPL falling (via the low frequency corner) compared to the high frequency corner (the shortest wavelength involved) and then the SPL (rate the wave front precession happens) it has to produce.
The assumption is that a horn with a lower frequency corner will have a slower rate of expansion and so the region where the pressure up high is more wavelengths long.
Funny thing too about those levitation sources, there is a larger configuration using 6 orthogonal sources that can produce about 175dB at the intersection. That IS enough intensity to light a cigarette with the acoustic friction of the air rushing back and forth through the tobacco at 22KHz.
What a cool / semi dangerous coffee table conversation starter, great for gift giving eh?
Best,
Tom Danley
Peter's stuff is all in AES. AES E-Library If you have a clean copy of the HFN article, I'd appreciate a scan. There's more detail in internal Engineering Memos but I no longer have access to these and they may have gone to the great library in the sky.
I worked with him when he was at Rank HiFi (Wharfedale/LEAK).
Thanks very much for that fascinating contribution Tom!
I wonder if there would be any benefit, in the levitation app, to predistortion of the electrical signal, to compensate at a specific distance and have, locally, more-sinusoidal pressures?
Where's the Bass?
That's why I have 4-15 high effec. bass drivers and adding 2-18" subs with high power amps on all 6. To attempt some reasonable simulance of realism... realism with low distortion and no compression on my end of things. Two-way with 6.5" "bass" driver?? I don't think so. Nice for background music. -RNM
That's why I have 4-15 high effec. bass drivers and adding 2-18" subs with high power amps on all 6. To attempt some reasonable simulance of realism... realism with low distortion and no compression on my end of things. Two-way with 6.5" "bass" driver?? I don't think so. Nice for background music. -RNM
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