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Old 21st September 2007, 12:44 PM   #1
gedlee is offline gedlee  United States
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Default Dynamics in Loudspeakers

There is much talk about a certain loudspeaker design having better "dynamics" than another, and other concepts along these lines.

I have been recently doing some tests using dynamic signals on loudspeakers rather than the mostly static tests that are performed. Some very interesting results have occured.

The point of this thread is to ask if anyone knows of or has done dynamic tests of a loudspeakers performance. How was the test performed? What were the results? Conclusions?

Please, analytical tests only. I'm not really interested in subjective interpretations of a listening test.

To give you an idea, here is the test that I performed.

I sent out a white nose signal at a very low level. I use this to represent the steady state linear response, lets call it the reference.

Then I blasted the loudspeaker with a very high level noise signal, just about at its thermal limit (the 3 dB compression point) for about 20 seconds. Then the signal is dropped back down to the initial low level. I calculated the frequency responses of the system at the very high level burst and then down into the low level as it cooled. I plot the differences from the reference as a range of curves for different times - 4 sec, 8 sec, 12 sec, etc.

In some designs the frequency response differences from the reference would remain below about 2 dB everywhere and typically at about 1 dB through the large signal burst and would recover almost immediately.

In other designs the frequency response difference would be as high as 4-6 dB with an almost 2 dB variation across the bandwidth. The recovery time was slow.

Does this seem like a viable test? Any comments? Its fairly easy to perform, although you can destroy the speaker if you are not careful and it also gets real loud!!
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Old 21st September 2007, 03:29 PM   #2
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That is a curious test and an interesting result.

Two things that come to my mind are
1) measure something like "Slew Rate" ie how fast can the output go from zero to something "big" If there is something akin to compression (which I think of as the opposite of "dynamic"), then there would be a depressed slew rate. The problem, however, is that you may end up simply with a measure that is contaminated by the device's bandwidth (less bandwidth then a seemingly lower slew rate).
2) The other possibility is to measure some akin to a frequency response using white noise or another "steady state signal" and then perform the following two measures. First measure the frequency response using an MLS sequence or a Golay code (that is a series of impulses). Do this at a "reasonable level" and then do it again but this time increase the level by 20 or 40 dB. Is the response comparable? The obvious disadvantage would be that at 40 dB higher you may simply be running the device into clipping or distortion and any difference may simply represent that. IOW, a change in response could be either caused by the introduction of distortion or by the inability to reproduce "dynamics". Difficult to distinguish. Another difference is that with the extra 40 dB, you simply have a cleaner measure since you are now out of the noise floor (environmental etc).

A very different possibility is look at some of the statistics associated with band passed "white noise". One of those is the "crest factor" (there are others that correspond to the "peakedness" of the waveform). Here you would look at whether the peakedness (perhaps crest factor) on the output measures the same when it is manipulated (again at an level of an extra 40 dB or so).

The caveat on all this is that when you start putting big pulses through your device you could easily ruin the components if you are not careful.

There is a technique used in psychoacoustics, that employs signals that have their peakedness manipulated using "Schroeder Phase". Perhaps that is worth pursuing (varous references in JASA) as a signal of choice

Interesting to speculate,
-Tom
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Old 21st September 2007, 03:50 PM   #3
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Quote:
1) measure something like "Slew Rate" ie how fast can the output go from zero to something "big" If there is something akin to compression (which I think of as the opposite of "dynamic"), then there would be a depressed slew rate. The problem, however, is that you may end up simply with a measure that is contaminated by the device's bandwidth (less bandwidth then a seemingly lower slew rate).
I assume Earl is talking about power compression. This should not be affected by an amps slew rate since both are worlds apart in terms of time.

I did indeed take care of that (though I didn't measure anything) by using a HiFi driver with P.A. driver sized voice coil (3/4" for 180 Watts). But I think this is a bigger issue in the PA busines, isn't it ?

Regards

Charles
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Old 21st September 2007, 04:25 PM   #4
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Hello Earl

You might want to take a look at the Vented Gap Cooling Tech sheet in the JBL Tech library on the Pro site. They test a group of 15" tranducers with a 300watt input signal and ploted the power compression with time. Not what you are doing but you might find it interesting if you have not seen it already.

They have second one called Distortion and Power Compression which looks like it was tied to the release of SFG Motor. Here's the AES paper cited in it.


2. M. R. Gander, “Dynamic Linearity and Power
Compression in Moving-coil Loudspeakers:’ presented
at the 76th Convention of the Audio Engineering Society,
8-11 October 1984, New York; preprint number 2128 (E-11).

Rob
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Old 21st September 2007, 05:15 PM   #5
gedlee is offline gedlee  United States
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I am talking about power compression in loudspeakers and not slew rate (in a linear system slew rate is bandwidth).

I will have to look at the JBL stuff, although I do recal it.

Clearly the higher the power level the bigger this issue is. For low level signals like background music, its not much of an issue. But I suspect that these factors take place at any realistic listening level where one has the playback level in the 90's.

I was testing pretty big stuff at pretty high levels, but I suspect that a 1" tweeter would be affected by what I measured at normal listening levels. These little devices can't take much heat without seriuos changes in their performance.

I think that my point here is that this seems to be an ignored area as only the high output pro guys do anything with it. But I suspect that its an issue even in a home setting.
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Old 21st September 2007, 05:32 PM   #6
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"I was testing pretty big stuff at pretty high levels, but I suspect that a 1" tweeter would be affected by what I measured at normal listening levels. These little devices can't take much heat without seriuos changes in their performance."

There was an article in Stereophille where they used the temperature rise in the voice coil as a method to prove that power compression was not an issue with music played at "average" levels. You might want to look at what they did. The idea of measuring the temperature rise and tracking the temperature as it decreases tied into the measured response changes might be interesting. Just food for thought.

Rob
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Old 21st September 2007, 05:59 PM   #7
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" .....

I assume Earl is talking about power compression. This should not be affected by an amps slew rate since both are worlds apart in terms of time.

...."
You have completely misunderstood my comment. I probably was not being clear. I was certainly not referring to measuring an amplifier's slew rate. Consider the analogy of what a mechanical system is doing: going from zero to some degree of excursion. It would be difficult to measure this at a mechanical evel (perhaps laser interferometry?). But one could measure the response to a step input of various step sizes. This of course would simply be measuring the faithfulness of scaling in terms of a linear system.

The other suggestions were simply descriptions of measuring frequency response with test signals of various degrees of "peakedness" re: to a baseline (steady-state).

-Tom
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Old 21st September 2007, 06:11 PM   #8
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My second thought is that you are correct, I am not referring to power compression, I was talking about compression of instantaneous amplitude (IOW clipping).

I still think the output to step response would "change" if it were now riding on a carrier frequency that was either at a "big level" or a "small level". If the carrier (steady state) were narrow band, its effects could be separated from the response of the step function (or the alternative, "peaked" signals).
-Tom
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Old 21st September 2007, 06:55 PM   #9
gedlee is offline gedlee  United States
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Quote:
Originally posted by WithTarragon
My second thought is that you are correct, I am not referring to power compression, I was talking about compression of instantaneous amplitude (IOW clipping).

I still think the output to step response would "change" if it were now riding on a carrier frequency that was either at a "big level" or a "small level". If the carrier (steady state) were narrow band, its effects could be separated from the response of the step function (or the alternative, "peaked" signals).
-Tom

This is correct, but this is a system nonlinearity and not a thermal compression issue which has a much longer time constant. Nonlinearities act instaneously, but thermal issues have very large lag (compared to the signal changes).
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Old 21st September 2007, 08:31 PM   #10
Pano is offline Pano  United States
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Interesting thoughts - a good subject.

What struck me right away is the time period. 20 seconds at high level? That's not what I would consider dynamics. Tho it may be some measure of ultimate dynamic power handling, it's not what most people refer to as "dynamics."

Isn't the subjective impression of dynamics a much shorter loud burst? Say 1 second or less? The exception might be going into a fortissimo section for a few bars- and being able to maintain it. But my impression of a dynamic system is one that can jump up to a much higher level with ease. Maintaining that level just adds to the sensation, but does not initiate the sensation.

How quickly do the ears start to compress?

I know that Dr. G. was asking for no subjective listening tests, but it seems to me that 20 seconds on-off is not what most people experience as "dynamic." Shorter peaks are.

FWIW, I can use the same driver, same amp, but a different box or baffle and get a different feeling of dynamics. Why? Is it simply different efficiency, or something else?
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