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Distortion in loudspeakers

oivavoi

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#1
I'm interested in this. Loudspeakers probably add more distortion to the reproduced sound than everything else in the chain, but it's rarely talked about. I have a gut feeling that the infamous subjectivist claim that some systems "measure well but sound bad" may partly be about speakers that are equalized in order to have a flat frequency response, but which nevertheless have much distortion (it might even be that equalizing the drivers beyond their comfort zone creates distortion). So:

What do we know about it? What are the causes?
Why do most loudspeakers manufacturers avoid mentioning the issue when talking about their products?
Why is this almost never measured or quantified in reviews, even among us objectivist folks?

And: HOW does one measure this in a good way?

Would love to get some input on this...:)
 

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#2

amirm

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And their description: http://www.soundstagenetwork.com/measurements/test_loudspeakers.htm

  • THD+N variation with frequency at 90dB - Measured at 2 meters (equivalent to 96dB at 1 meter) from 50Hz to 10kHz. The top curve of the chart shows the frequency response of the loudspeaker at the determined SPL level (i.e., 90dB) while the bottom curve shows the distortion component of the signal (values below 40dB should be ignored because they are too close to the noise floor of the test equipment to be of use).

    Both curves are reported in dB which can be read off the vertical axis. In order to convert to a percentage one must read the top line (frequency response) and then determine the dB difference between that line and the bottom line (THD+N line). Translation from dB to % is as follows:

    Equal (or 0dB difference) = 100 %
    -10dB = 31.6%
    -20dB = 10.0%
    -30dB = 3.16%
    -40dB = 1.0%
    -50dB = <0.5%

    Please note: an SPL level of 90dB measured anechoically is very loud and considered far beyond normal listening levels, particularly for small loudspeakers. To give more information for real-world listening levels, if it appears that the speaker is being strained beyond its output abilities at this level we will provide a second measurement at at lower SPL (the SPL level will be printed with the chart).

    Purpose: Measures THD+N output at discrete frequency intervals for above-normal listening levels. Please note that 90dB output at a 2-meter distance is equivalent to an SPL level of 96dB at a 1-meter distance.

    What it tells you: Audibility of distortion varies as to type of distortion and also the frequency at which it is occurring. Distortion measurements for loudspeakers are usually many times that of electronics (i.e., amplifiers, receivers, etc.). Furthermore, certain types of distortions are more audible than others and the audibility of that also depends on the frequency. Our distortion measurements give a general indication of how much distortion is occurring for a given output level at above normal listening levels. Distortion levels will be less (sometimes much less if the speaker is being stressed beyond capabilities at 90dB) at lower SPLs.
THD unfortunately is not psychoacoustically very relevant so hard to draw audibility inferences there.
 
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oivavoi

oivavoi

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Thread Starter #5
Thanks Amir! Wasn't aware of the soundstage measurements. Excellent source of information
 

DonH56

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#6
What do we know about it? What are the causes?
Quite a bit, actually; even I know a little and I am no loudspeaker designer. Some causes off the cuff (by no means complete and random order, as I thought of them):
  • Nonlinearity in the EM field as the voice coil moves back and forth across the magnet, worsens as volume increases -- power compression happens, natch
  • Flexure in the suspension again causing higher distortion (and ultimately compression) as volume increases
  • Flexure in the cone or membranes (sometimes called "breakup modes" when they get really bad) related to mechanical resonances at certain frequencies
  • Thermal distortion due to wire and magnet heating
  • Interaction (or crosstalk) among drivers, e.g. woofer modulating the tweeter and vice-versa (isolation boxes can help but hard to get high isolation in a small cabinet)
  • Crossover issues ranging from poor driver integration (causing frequency and time domain irregularities) to actual distortion as components go nonlinear (e.g. inductors saturate, capacitor hysteresis and voltage coefficients/nonlinearity, etc.)
  • Related is the fact that obviously if a driver receives a signal above or below its frequency range it will not be happy and will let you know (i.e. measure poorly); high-order passive crossovers are difficult to design and implement thus drivers need substantial overlap in response to reduce this distortion
  • Time alignment among drivers (very difficult with conventional designs) -- this does not necessarily add harmonic distortion but corrupts the time response of the speaker
  • Cabinet vibrations
  • Might as well throw in port noise and signal alignment related to vented (ported) speaker designs
  • Cabinet interaction -- how the driver/system responds and how that modifies the response; again can produce frequency errors that are not necessarily nonlinear distortions
  • Etc. etc. etc.
That's not even getting into dispersion effects and interaction with the room and all that jazz.

Why do most loudspeakers manufacturers avoid mentioning the issue when talking about their products?
Because they suck? :) Really, people are used to seeing 0.01 % THD on the electronics and would go bonkers if they knew their speakers were at 1 % to 10 % or more (often much more for loud signals). Marketing... It is also harder to measure because you have to have a well-characterized measurement mic and good anechoic or well-known space in which to test them.

Why is this almost never measured or quantified in reviews, even among us objectivist folks?
See above.

My 0.000001 cents (microcent) - Don
 
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RayDunzl

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#7
Distortion measurements use a Sine Wave source (typically, if not always) as the base tone for measurement. Sine wave is described by the sin trigonometric function over time, deviations from which are easily (if you are a mathematician, which I failed to become) detected and calculated.

Here is a Sine wave calculated and displayed in Audacity, with a fundamental tone of 128Hz

upload_2017-1-27_14-51-58.png


Spectrum

upload_2017-1-27_14-55-7.png



Integer multiples of the fundamental frequency are "harmonic" distortions.

If you blow across the top of a Beer Bottle (or in this case, a Kahlua bottle) the tone generated is almost a pure sine wave. Here is a recording of that:

upload_2017-1-27_14-42-2.png


And Spectrum:

upload_2017-1-27_14-45-57.png


You can see the 128Hz fundamental, and 256Hz 2nd harmonic, and 384Hz 3rd harmonic, then it gets messy as it is recording the whoosh of my breath blowing across the bottle.

The second harmonic is about -40dB, or 1% distortion.

Great! We can easily measure distortion. Using pure tones. Which are relatively uncommon in music.

Voices are not uncommon in music, and I have one of those handy.

Here is mine, trying to make a 128Hz tone:

upload_2017-1-27_15-2-51.png


Ugh... Not very sine-like, though it can be decomposed into a series of sines... Hey, I hit the pitch (fundamental frequency) though. That's good on me.

Spectrum:

upload_2017-1-27_15-4-34.png


The second harmonic is more intense than the fundamental, so, measurement-wise, that's at least 100% distortion. And you can see the series of higher harmonics, musltiples of 128hz trailing off for a while, then it gets too messy to make more sense of.

So...

Music is distortion. Musical instruments get their "tone" from varying levels of harmonic distortion.

How to measure whether what we play is distorted is not so easily determined as the basic THD measurement.

IMD uses two sine tones.

THD + N (noise) adds calculation for sound that is not in the harmonic series.

So, make of that what you want, it's something I know.

I'll show something else in a minute.
 

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RayDunzl

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#8
Cones and Domes...

A dynamic speaker driver moves by electromagnetic force, apply a sine wave and you expect a sine to come out. it almost does.

Simplest first "problem" that comes to mind, would be the cone working against (stretching) the surround and spider. What woould that do to the sine wave? Maybe flatten the peaks a little as the cone encounters more resistance due to stretching the suspension.

That might look like this:

upload_2017-1-27_15-22-39.png




How did I make that wave?

By taking the sine wave at 128Hz and adding to it a smaller sine wave at 384 Hz, the third harmonic.

upload_2017-1-27_15-24-9.png


Spectrum of my fake constrained by the suspension speaker output:

upload_2017-1-27_15-25-41.png


Ok, that's about all I think I know about distortion.

Carry on.
 

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DonH56

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#9
A flute provides about the purest tone as far as musical instruments are concerned. The rest of us produce a complex signal, not always purely harmonic, that generates that "rich" sound you hear out front. An off-topic point that may be of interest is that, when playing in a group, we listen for subharmonic "beat" tones to tell us when we are in tune. Those beat tones are part of what separates a good section from a great section and helps produce the wall of sound you hear.
 

RayDunzl

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#10
Beat Tones - the combination of tones add and subtract as they go (almost) in and out of phase.

128 + 129 Hz = 1 cancellation per second

upload_2017-1-27_15-44-47.png
 

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RayDunzl

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#12
Brass instruments have big peaks...

Trumpet.

 
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oivavoi

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Thread Starter #13
Thanks for all the comments! Really excellent! I will need to chew on all this information for a little while before I grasp it, I think...
 

RayDunzl

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When I simplistically compared my electrostatic (ML reQuest) and cone (JBL LSR 308) for distortion. The low frequency hash is not coming from the speakers... Room noise and USB mic noise and such. The results vary with frequency tested, too.

JBL cone



ML electrostat

 
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oivavoi

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Thread Starter #15
Music is distortion. Musical instruments get their "tone" from varying levels of harmonic distortion.

How to measure whether what we play is distorted is not so easily determined as the basic THD measurement.

IMD uses two sine tones.

THD + N (noise) adds calculation for sound that is not in the harmonic series.

So, make of that what you want, it something I know.
I'm still not sure exactly what to make of that... ;)
But I think I get that simple THD measurement using sine waves will not be perfect, because music doesn't behave like sine waves. Which means that the THD measurements that Soundstage did may not reveal the whole picture, for example?

Here's an idea for how to do it in a different way: Shouldn't it be possible to just play a piece a music through a loudspeaker in a anechoic chamber, and record it using a microphone with known properties? And then compare the original digital file with the recorded file using audiodiffmaker or something similar? The resulting difference would be some kind of distortion, as far as I can understand. I'm not sure how it could be quantified, though. The difficult thing to know would be if the measured differences were due to the microphone or the speaker. But what one could do in this way, was to compare several speakers, and see which one had the least distortion. So it could be a relative indicator of distortion, at least. And one could repeat it with different kinds of musical tracks. Or am I missing something here?
 
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oivavoi

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Thread Starter #16
Quite a bit, actually; even I know a little and I am no loudspeaker designer. Some causes off the cuff (by no means complete and random order, as I thought of them):
  • Nonlinearity in the EM field as the voice coil moves back and forth across the magnet, worsens as volume increases -- power compression happens, natch
  • Flexure in the suspension again causing higher distortion (and ultimately compression) as volume increases
  • Flexure in the cone or membranes (sometimes called "breakup modes" when they get really bad) related to mechanical resonances at certain frequencies
  • Thermal distortion due to wire and magnet heating
  • Interaction (or crosstalk) among drivers, e.g. woofer modulating the tweeter and vice-versa (isolation boxes can help but hard to get high isolation in a small cabinet)
  • Crossover issues ranging from poor driver integration (causing frequency and time domain irregularities) to actual distortion as components go nonlinear (e.g. inductors saturate, capacitor hysteresis and voltage coefficients/nonlinearity, etc.)
  • Related is the fact that obviously if a driver receives a signal above or below its frequency range it will not be happy and will let you know (i.e. measure poorly); high-order passive crossovers are difficult to design and implement thus drivers need substantial overlap in response to reduce this distortion
  • Time alignment among drivers (very difficult with conventional designs) -- this does not necessarily add harmonic distortion but corrupts the time response of the speaker
  • Cabinet vibrations
  • Might as well throw in port noise and signal alignment related to vented (ported) speaker designs
  • Cabinet interaction -- how the driver/system responds and how that modifies the response; again can produce frequency errors that are not necessarily nonlinear distortions
  • Etc. etc. etc.
That's not even getting into dispersion effects and interaction with the room and all that jazz.



Because they suck? :) Really, people are used to seeing 0.01 % THD on the electronics and would go bonkers if they knew their speakers were at 1 % to 10 % or more (often much more for loud signals). Marketing... It is also harder to measure because you have to have a well-characterized measurement mic and good anechoic or well-known space in which to test them.



See above.

My 0.000001 cents (microcent) - Don
Very very enlightening. Thanks.

Then my follow-up question goes like this: Are there any known commercial speaker designs that we can expect to have much lower distortion than others, based on these considerations?
 

watchnerd

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#17
Why do most loudspeakers manufacturers avoid mentioning the issue when talking about their products?
It's not universal, but distortion specs for monitor speakers are a bit more common in the pro audio world. Eve Audio, for example, publishes distortion graphs for all their models.
 

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Blumlein 88

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Very very enlightening. Thanks.

Then my follow-up question goes like this: Are there any known commercial speaker designs that we can expect to have much lower distortion than others, based on these considerations?
Electrostats typically have lower distortion from somewhere around 200 hz and up.
 
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oivavoi

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Thread Starter #20
It's not universal, but distortion specs for monitor speakers are a bit more common in the pro audio world. Eve Audio, for example, publishes distortion graphs for all their models.
Checked out their website now. These seem like seriously good figures. Here's the distortion on the small two-way SC205, with a 5-woofer:


And here's the larger four-way (or technically I would call it a 3 or 3.5-way) SC407:


Seems pretty impressive to me.
 

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