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Loudspeaker THD

Postby SafeandSound Mastering » Thu Sep 26, 2013 5:11 pm

Just got back from a holiday and head a tad fuzzy right now so forgive the slightly clumsy post. An old conundrum popped into my mind on the beach (as it does for the audio obsessive) regarding loudspeaker drivers high distortion levels vs let's say.... a mixer or DA converters. Maybe we can discuss the reasons why 3-4 pct THD is fine for speakers but much less acceptable for many other professional audio devices.

I will throw a few possibility carrots in here....

The frequency spectrum where distortion occurs is different and are thus less objectionable in the loudspeaker.

The harmonic profile of the distortion is different.

At higher SPL levels the distortion products are masked. (possibly related to the Fletcher Munson curves ? But probably not)

They are usually swamped by acoustic room issues.

I would be nice if someone who designs speaker drivers would chime in or who works for a loudspeaker company. (or an avid speaker tweaker) I am sure Hugh will ply us with good info at some point.
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Re: Loudspeaker THD

Postby Hugh Robjohns » Thu Sep 26, 2013 5:19 pm

SafeandSound Mastering wrote:Maybe we can discuss the reasons why 3-4 pct THD is fine for speakers but much less acceptable for many other professional audio devices.


It's not fine! A lot of people have spent considerable parts of their careers trying to improve the situation, and things have steadily improved over the last 100 years or more -- loudspeaker THD used to be so much more than that (it still is in some cases, like the less advanced PA speakers).

There are, in fact, monitor speakers now that manage much less than 5% distortion (in large parts of the spectrum if not yet entirely universally).

But the simple fact is that it is incredibly difficult to control distortion in mechanical loudspeaker drive units. Microphones are also victims of the same problems, although thankfully generally at much lower levels because the amount of mechanical movement involved is orders of magnitude smaller.

Controlling distortion in electronics is trivially simple by comparison, but only because electronic design is so advanced in comparison.

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Re: Loudspeaker THD

Postby Folderol » Thu Sep 26, 2013 7:02 pm

Mmmm. This would make a rather fine SOS article
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Re: Loudspeaker THD

Postby alexis » Thu Sep 26, 2013 7:25 pm

Does the "weakest link" argument apply here? ... meaning if there is 3-4% distortion in your best $5,000 near fields, there is no point in spending thousands of dollars extra on a pre-amp, A/D-D/A converter, etc. for the purpose of reduction of distortion below that of the speakers?

I'm sure that's not the case and I will be schooled ... but enquiring minds would like to know, nonetheless!

Thanks!
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Re: Loudspeaker THD

Postby dmills » Thu Sep 26, 2013 7:56 pm

Sort of, but there are multiple types of distortion, and it is not always additive.

Consider two devices one having a transfer function
f(x) = x + 0.1x^2 (so, a sniff of second harmonic),
and the other having a transfer function of
g(x) = x + 0.1x^3 (a little third),
now if unit f occurs earlier in the chain then unit g we have a composite transfer function
h(x) = g(f(x)) = x+0.1x^2 + 0.1((x+0.1x^2)^3),
which I make it means that our second and third harmonic sources when combined give rise to harmonic content up to order eight.

These are very simple sources, but it serves to demonstrate that distortion early in the chain can make later problems more objectionable.

Of course the linearity problems can become very much more complex as both of these units given in the example will also produce non harmonic tones when fed with multiple simultaneous tones, which will then increase the complexity of the signals fed to the next stage making it produce yet more IMD products..... You cannot see these mixing[1] products when testing with only a single tone source.

None the less, as has often been stated around here the (and is just as often ignored), the electronics for making recordings to the limits of the room are pretty much a solved problem, even mid range gear does that essentially perfectly in the environments we typically have access to, the transducers however are a whole can of worms that most manufacturers are reluctant to open in public.

Regards, Dan.

[1] In Engineering terms, mixing means multiplying rather then the use made in most audio where mixing means adding.
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Re: Loudspeaker THD

Postby SafeandSound Mastering » Fri Sep 27, 2013 7:02 am

"Fine" as in we all use them and never complain, even very expensive well designed speakers have the same issue.(in factors of thousands). As I understand Dynaudio Acoustics have made significant improvements in this area through various manufacturing techniques. (Magnet inside former, very large coil vs cone size, one piece former/cone construction)

I find it very interesting that we can hear the miniscule improvements over some electronic component before the speakers through that layer of transducer distortion many magnitudes higher.("ahh a veil has been lifted", "listen to the clarity and separation", "I can hear all the detail now" etc.)

I would imagine that such distortions could remain true of dynamic mics as well essentially using the principle in reverse.

I am glad my post may have potential to inspire an article.

Got to dive into my backlog and will catch up on this shortly.
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Re: Loudspeaker THD

Postby James Perrett » Fri Sep 27, 2013 8:57 am

SafeandSound Mastering wrote:"

I am glad my post may have potential to inspire an article.


I don't know if you've seen Hugh's review of the new Neumann KH310's in this months SOS but it is one of the few reviews I've seen where a distortion figure is quoted for a monitor.
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Re: Loudspeaker THD

Postby Hugh Robjohns » Fri Sep 27, 2013 9:22 am

Yes, very few speaker manufacturers quote distortion figures, although I think most do measure them during R&D. I think the reason most don't publish them is because they will inherently look pretty bad when compared to the equivalent figures for electronics etc. and, given that most potential buyers are technically illiterate and won't understand the relevance or context of such figures, it is generally considered prudent to simply not mention them.

In Germany, though, technical knowledge and understanding is still respected -- they don't yet have the stupidity culture that we have here where 'nerd' and 'geek' are seen as bad, and ignorance is favoured as the first step to celebrity fame -- and so their manufacturers (and technical magazines) are happy to publish far more technical information.

Neumann (K&H as was) publish masses of very interesting and usually impressive technical information and plots, in part presumably because they are very proud of what they have achieved in their designs. Other German manufacturers generally follow suit, and a few other European companies do too... but few if any in the UK or USA, despite achieving similar levels of technical competence in many cases.

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Re: Loudspeaker THD

Postby Folderol » Fri Sep 27, 2013 5:18 pm

Just a quick question. What's the most prevalent form of distortion speakers produce?
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Re: Loudspeaker THD

Postby Hugh Robjohns » Fri Sep 27, 2013 5:54 pm

Probably second harmonic in most cases, with fairly strong third but generally with much weaker higher harmonics.

I found a review by Martin Colloms of PMCs IB2 speaker, which (like the IB1) has unusually low LF distortion, and he gave the following figures:

Distortion was generally low, particularly in the mid and treble, but that’s not to say the bass was not good too. For 30 Hz, 96dB spl, second and third harmonics were about 3%, (which is) below audibility. At 120 Hz, 86dB, second was 0.1%, third still fine at 0.15%. At a higher 96dB spl some rise was to be expected, now second recorded 0.6%, but third still a fine 0.15% (credit that fine motor). (For the record, fourth and fifth harmonics were better than 0.06%.) By 200Hz some increase was noted, but nothing to cause concern, for example at 96dB second harmonic was at 1%, third a very good 0.13%.

Moving on to that midrange unit (the IB2 has a much better but much more expensive midrange driver than the IB1), for 700Hz and 86 dB spl I measured 0.033% second and 0.003% third, both superb results, and distortion was still better than 0.1% total at 96dB spl, which is pretty loud for sine wave drive. The tweeter also was excellent, with distortion almost as low as that remarkable midrange driver.


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Re: Loudspeaker THD

Postby John Willett » Sat Sep 28, 2013 1:10 pm

Hugh Robjohns wrote:Yes, very few speaker manufacturers quote distortion figures, although I think most do measure them during R&D. I think the reason most don't publish them is because they will inherently look pretty bad when compared to the equivalent figures for electronics etc. and, given that most potential buyers are technically illiterate and won't understand the relevance or context of such figures, it is generally considered prudent to simply not mention them.

In Germany, though, technical knowledge and understanding is still respected -- they don't yet have the stupidity culture that we have here where 'nerd' and 'geek' are seen as bad, and ignorance is favoured as the first step to celebrity fame -- and so their manufacturers (and technical magazines) are happy to publish far more technical information.


Neumann publish graphs of their new monitors - the set for the KH310A are HERE and the KH120A ones are HERE

Geithain also publish all these graphs on-line. they are on both Geithain's site in Germany and on the Sound-Link distributor site in the UK.

This is the plot for the baby RL906 which Hugh reviewed last year:-

Image


This is the distortion plot for the KH120A:-

Image


This is the distortion plot for the KH310A:-

Image

Both the Neumann plots are the 90dB plots.

I have looked for graphs for some of the top UK manufacturers, but can't find them, unfortunately.
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Re: Loudspeaker THD

Postby dmills » Sat Sep 28, 2013 2:25 pm

Now, who publishes two tone the IMD 2 & 3 intercept for microphones as a function of frequency and measured at a few different levels (So we can see where third order law breaks down)?

Yes, I know how to measure it (not hard, just tedious unless you automate it), but that information is actually at least as useful as simple harmonic levels, and tells you something subtly different.

I know for a fact that these figures are **SHOCKING** for some very expensive sdc microphones up near the top of the band and into the low ultrasonic (Where many classical instruments have energy just waiting to form products in the audio band), because I have done this experiment.

It might just be a background in RF, but there any manufacturer that does not give these kinds of numbers gets considered to be very, very, pony.

Regards, Dan.
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Re: Loudspeaker THD

Postby Hugh Robjohns » Sat Sep 28, 2013 4:10 pm

That's why ribbon mics often sound so much better than capacitors on those kinds of instruments.

As for tech specs, you're quite right. The pro-audio world seems to ignore fairly basic specs and measurements that are considered fundamental in other engineering fields.

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Re: Loudspeaker THD

Postby John Willett » Sat Sep 28, 2013 4:33 pm

dmills wrote:Now, who publishes two tone the IMD 2 & 3 intercept for microphones as a function of frequency and measured at a few different levels (So we can see where third order law breaks down)?

I know that Sennheiser publish (or used to publish) the phase response graphs for the MKH 20/30/40/... series.
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Re: Loudspeaker THD

Postby dmills » Sat Sep 28, 2013 4:50 pm

Personally I will admit to still going for the dynamics often as not, I never really figured out why condensers became the goto mic for instruments with gobs of ultrasonic energy it does not exactly play to their strengths, the 'sizzle' is a bug not a feature if you are trying to make a recording that sounds like the instrument.

And we wonder why the audiophools say they can hear things that we cannot measure!
We can measure it, and do if we are using those parts for anything else....

I am sorely tempted to hire half a dozen or so **Shiny** condenser mics and set up some GPIB based tools to measure the things (We have GPIB controlled VNAs and signal generators at work that can do this easily), then publish the results, but suspect it would result in several hits being taken out on me.

Would SOS be interested in publishing such a thing (It would be a somewhat fact heavy article thin on opinion (The only way to write when you are revealing interesting things about emperors and their tailoring IMHO)), or would it be a better fit for JAES with the makers names filed off (JAES as a matter of policy does not name names with this sort of thing).

John, Phase response is interesting and is really part of any properly done frequency response graph (Bode plot), but it inherently says nothing about device linearity, being measured with a single tone.

Regards, Dan.
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Re: Loudspeaker THD

Postby Hugh Robjohns » Sat Sep 28, 2013 5:20 pm

I'd be very happy to talk more about the possibilities of an article on this topic with you Dan. Let's talk about this towards the end of next week. Drop me a line with some contact details and good times to call on hugh BLIP robjohns AT soundonsound BLOB com

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Re: Loudspeaker THD

Postby alexis » Sat Sep 28, 2013 5:24 pm

Another question from the technically naive - a year or so ago there was a blinded pre-amp shoot-out in SOS, and people were surprised at how little the $$$$$ preamps stood out from the $ ones (and I suspect the difference would have been less except for familiarity of some of the responders with the sound of the $$$$$ preamps, a source of bias that can't easily be compensated for by trial design).

Would the overwhelming THD of speakers, in relationship to the preamps, help explain those results to some degree - smearing the technical differences among the preamps, acting as the "great equalizers" among them? Especially since there's at least some chance not all respondents were comparing the preamp sounds on $$$$$-level speakers?
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Re: Loudspeaker THD

Postby dmills » Sat Sep 28, 2013 5:35 pm

It is possible, but that is exactly the point, when used in the way those preamps were used (clean gain) even the cheap stuff was good enough to NOT be the weak link in the chain.

Regards, Dan.
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Re: Loudspeaker THD

Postby Folderol » Sat Sep 28, 2013 6:15 pm

Fascinating stuff. Thanks guys a really good read so far

I was surprised that 2nd harmonic was coming out so far above 3rd. I wonder what the cause is. Anything to do with being easier to push air than to suck it?
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Re: Loudspeaker THD

Postby dmills » Sat Sep 28, 2013 7:05 pm

Second usually comes out higher then third unless the situation is inherently nearly perfectly symmetrical.

Part of of it is the thermodynamics of gasses (As you say air is easier to push then pull, this is mostly an issue in high ratio horn speakers as used in some large PA applications where the pressure fluctuations in the throat can be a significant part of an atmosphere with correspondingly nasty modulation of the speed of sound).
Part of it is thermodynamic non reversibility, and part of it is things like mechanical imperfections in the suspension (non ideal spring) and imperfections in the homogeneity of the field and coil.

The last are easier to correct then the gas laws and second law issues!

Regards, Dan.
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Re: Loudspeaker THD

Postby Random Guitarist » Sun Sep 29, 2013 8:25 pm

Sounds like this could give rise to a very intersting article (or maybe series)
Personally, I'd be fascinated to read more detail in this area.
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Re: Loudspeaker THD

Postby alexis » Sun Sep 29, 2013 9:34 pm

I would be interested in learning how we can hear the tiny improvements in THD achieved by spending thousands of extra dollars on one converter/interface over another, when the signal in each case is then passed through a transducer with THD orders of magnitude greater.
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Re: Loudspeaker THD

Postby dmills » Sun Sep 29, 2013 11:36 pm

Are you sure it is THD that you are hearing, and not say Confirmation Bias?

Anyone who has a rack plate from 'funk logic' can confirm the power of this effect, and how many of us can honestly say we have never tweaked an eq, decided it improved things then noticed five minutes later that it was bypassed.

Level matching is also critical for perceptual comparisons and as I say distortion is not necessarily additive.

It might just be my 40 year old ears, but with things like basically competent converters I have enough difficulty with deciding if they sound different, never mind deciding which one is better, at least once I have the levels matched and the thing blinded.
I do not claim golden ears however.

Regards, Dan.
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Re: Loudspeaker THD

Postby Richie Royale » Mon Sep 30, 2013 9:22 am

dmills wrote:how many of us can honestly say we have never tweaked an eq, decided it improved things then noticed five minutes later that it was bypassed.


My favourite mistake.
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Re: Loudspeaker THD

Postby SafeandSound Mastering » Mon Sep 30, 2013 11:18 am


It might just be my 40 year old ears, but with things like basically competent converters I have enough difficulty with deciding if they sound different, never mind deciding which one is better, at least once I have the levels matched and the thing blinded.
I do not claim golden ears however.


I have this unenviable task coming up in the not too distant future (DA and AD). Part of me is thinking to simply get the ones that offers the right facilities and measures best on audio precision and job done. But......
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