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Author Topic: Sine Sweeps, Extraneous Noises, and Sound Quality  (Read 822 times)

Frank Koenig

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Sine Sweeps, Extraneous Noises, and Sound Quality
« on: February 06, 2017, 01:46:23 pm »

When I started playing with speakers as a teenager (in the early '70s, oh dear) the only test signal source I had was a sine wave generator. (I had square waves too, but they were of little value other than to demonstrate the utter inability of speakers of the day to reproduce them.) So it was very common to listen to sine sweeps, which revealed all sorts of odd, generally unharmonically-related, buzzes, rattles, and so forth. (Nothing smokes out an iffy HF driver like a sine sweep.) Now I have ARTA, LIMP, Smaart, my own analysis tools, and a nice measurement mic, and don't think about listening to sine sweeps very often.

The last few days I've been developing a new passive crossover for some 18 Sound 8CX400F 8 in. coax based utility speakers which I've had for a few years. I took many acoustic and electrical measurements, ran lots of simulations, consulted the Parts Express catalog for available element values, but never once listened to a sine sweep. Then, this morning, as a final sanity check before ordering parts, I hooked up the old analog sine wave generator and "swept it out". I was horrified to hear a note lower than the sine frequency (what some might call a "sub-harmonic", but let's not go there) in the crossover region. My first thought was oh crap, I've got a blown HF diaphragm. I swapped drivers and got the same sound, and furthermore, disconnecting and shorting the tweeter did not make it go away. It was coming from the woofers -- identically from both of them.

The bad news for me is, in the case of these little speakers, there's probably not much to be done. I don't want to move the crossover frequency any lower because of power and distortion concerns in the tweeter.

The takehome is, remember to do sine sweeps. It's quick, easy, and you learn things.

The question is, how often and to what extent is the difference in sound quality between speakers due to extraneous noises of this sort?  We focus so much on what we routinely measure, but, other than distortion measurements, these measurements are blind to extraneous noises.

Best,

--Frank
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John Roberts {JR}

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Re: Sine Sweeps, Extraneous Noises, and Sound Quality
« Reply #1 on: February 06, 2017, 02:19:57 pm »

It seems a little odd for simple distortion to a single frequency sine wave to generate a lower frequency artifact (not uncommon from IM distortion between multiple notes).

Perhaps an excited acoustic resonance?    Are the woofers clean without the crossover?

JR
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Frank Koenig

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Re: Sine Sweeps, Extraneous Noises, and Sound Quality
« Reply #2 on: February 06, 2017, 03:04:34 pm »

Perhaps an excited acoustic resonance?

Yes, it is an acoustic resonance! Not only that, after a little poking around I figured out where it's coming from, which appears to be the acoustically transparent dust cap through which the tweeter's sound comes out. If I place a finger lightly on the dust cap the nasty sound damps out completely. It's a little surprising that the good folks at 18 Sound didn't discover and fix this. Maybe it only reveals itself after the speaker has aged. These are about 8 years old.

Best,

--Frank

« Last Edit: February 06, 2017, 03:06:38 pm by Frank Koenig »
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Ivan Beaver

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Re: Sine Sweeps, Extraneous Noises, and Sound Quality
« Reply #3 on: February 06, 2017, 08:08:04 pm »

Sine waves can revel A LOT about speakers, that normal "noise based" systems don't.

I remember years ago (back when I was in the install business) there was a popular speaker who claimed a really deep freq response.

We got some into the shop to play with.

We took a stepped sine generator and stepped down.  Not at a loud level-just good enough to easily hear

Everything was fine until we somewhere around 40Hz.

The next step down the freq went UP!

We redid the test with the same result.

Obviously the generator was defective.  So we tried it with a known speaker that went low.

The generator was fine.

But the distortion of the "low freq speaker" was so high at that freq (well over an octave HIGHER than its claimed low freq number) you could not hear the fundamental.

The sine waves pointed the "usable freq response" VERY quickly.

Sine waves are fun to play with regarding low freq room modes and where they build up and cancel.

Noise does not point them out as easily.  Yes you can measure them, but you have to move the mic around and look at the screen.  With sine waves you can walk around.  But you can only do 1 freq at a time.
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Ivan Beaver
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John Roberts {JR}

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Re: Sine Sweeps, Extraneous Noises, and Sound Quality
« Reply #4 on: February 07, 2017, 10:30:19 am »



The next step down the freq went UP!

===
But the distortion of the "low freq speaker" was so high at that freq (well over an octave HIGHER than its claimed low freq number) you could not hear the fundamental.
===
Sine waves are fun to play with regarding low freq room modes and where they build up and cancel.

Noise does not point them out as easily.  Yes you can measure them, but you have to move the mic around and look at the screen.  With sine waves you can walk around.  But you can only do 1 freq at a time.
Yes harmonic distortion expresses as multiples of the fundamental (2x, 3x, etc).

Back when I sold the TS-1 many people used the sine wave output to quick test loudspeakers for rubs or buzzing. It only put out mW of power, but enough sound to suss out obvious problems.

JR
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Ivan Beaver

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Re: Sine Sweeps, Extraneous Noises, and Sound Quality
« Reply #5 on: February 07, 2017, 11:47:39 am »

Yes harmonic distortion expresses as multiples of the fundamental (2x, 3x, etc).

Back when I sold the TS-1 many people used the sine wave output to quick test loudspeakers for rubs or buzzing. It only put out mW of power, but enough sound to suss out obvious problems.

JR
Sweeping a sine wave (by hand or setting a automatic sweep) can revel all sorts of construction issues (rubs buzzes).

It is FAR better than a stepped response.  The steps can jump over the problems.

A hand sweep allows you to easily "zero in" on the particular problem freq and location.
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John Roberts {JR}

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Re: Sine Sweeps, Extraneous Noises, and Sound Quality
« Reply #6 on: February 07, 2017, 02:02:19 pm »

Sweeping a sine wave (by hand or setting a automatic sweep) can revel all sorts of construction issues (rubs buzzes).

It is FAR better than a stepped response.  The steps can jump over the problems.

A hand sweep allows you to easily "zero in" on the particular problem freq and location.

The TS-1 would sweep from below 20Hz to 30kHz in one sweep of the knob... nice for listening, but it would make customers crazy looking at the built in frequency counter and trying to dial in 1,000 Hz exactly. (later versions added a fine freq knob).

With digital technology I could have my cake and eat it too, with dynamic control response so it could slow down allowing fine adjustment with same one knob, but.... No point to design hardware products that can be accomplished with a cheap smart phone app.

JR
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Ivan Beaver

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Re: Sine Sweeps, Extraneous Noises, and Sound Quality
« Reply #7 on: February 07, 2017, 05:30:51 pm »

No point to design hardware products that can be accomplished with a cheap smart phone app.

JR
Agreed-it is amazing how much times have changed.

We used to have a sign from the GM of IBM

I have learned a lot from my years in the business-most of which doesn't apply any more :(
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Ivan Beaver
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