Different sweep volume levels result in different responses - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 17 Old 03-19-15, 04:19 PM Thread Starter
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Different sweep volume levels result in different responses

I took 3 sweeps at different volume levels (I calibrated each using SPL meter), no other changes.

purple = 85dB
green = 75dB
red = 65dB

The 200hz and up region is what I was expecting to see across the entire frequency spectrum. Look at how bloated the bass gets on the quieter sweeps. The 65dB response even surpasses the 85dB sweep at 40hz!

Does anyone know why could this be? Which is the accurate response? Is REW calibrated to 75dB only (surely this cannot be)?
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post #2 of 17 Old 03-19-15, 04:41 PM
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Re: Different sweep volume levels result in different responses

Possibly there is Audyssey Dynamic EQ or some other loudness compensation feature active in an AVR?
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post #3 of 17 Old 03-19-15, 04:57 PM Thread Starter
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I don't think that's the case, mac mini output going directly into a sansui 70s vintage integrated. There is a 'loudness' switch on it which is permanently enabled.
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post #4 of 17 Old 03-19-15, 10:13 PM
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Re: Different sweep volume levels result in different responses

You don't think the enabled loudness switch is responsible? It is correct that most are more like tone controls that tilt the response the same at all volume levels. I believe a few did change the response depending on volume setting.

If that is not it, then maybe there is a setting in the mac?

We know it is not a natural sound phenomenon and I don't know of any setting or feature in REW that would cause this.
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post #5 of 17 Old 03-19-15, 10:30 PM
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Re: Different sweep volume levels result in different responses

You could put a loopback cable on your soundcard - on the channel that you are measuring on. The same as you did if you performed a soundcard calibration. That way you can take measurements the wire at different volume settings. If you get the same shifted response at several SPL levels then that rules out the receiver.

I would then first suspect a settings in the mac, but that is just because I can't think of a way to get this effect with REW. I'm not always the most imaginative person around however.

At least this will narrow to the possibilities.

Last edited by jtalden; 03-19-15 at 10:44 PM. Reason: Confused logic originally.
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post #6 of 17 Old 03-20-15, 02:12 AM Thread Starter
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Thanks, I'll try this tonight, first disabling loudness on the amp, then creating a loop back. Just to confirm, I could do a loop back from Mac output to mic input (to rule out Mac), and integrated amp headphone output to mic input to rule out the amp? I cannot create the loop with the actual signal going to the speakers (speaker cable) correct?
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post #7 of 17 Old 03-20-15, 05:36 AM
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Re: Different sweep volume levels result in different responses

It can not be any clearer ( a least for the red curve) that this behaviour is generated by the Loudness Circuit.

Small Signal --> Loudness amplifies/lifts Bass and High Treble to compensate the hearing curve.

Loud signals --> Loundess is off because no compensation needed anymore
http://en.wikipedia.org/wiki/Loudness

Don't know what the difference between green and purple coudl be, but first switch off that Loudness.
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post #8 of 17 Old 03-20-15, 05:54 AM Thread Starter
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Re: Different sweep volume levels result in different responses

Guys, this is great, looks like this is the root cause. I will confirm this evening by repeating measurements with loudness off.
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post #9 of 17 Old 03-20-15, 09:45 AM Thread Starter
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Re: Different sweep volume levels result in different responses

Looking at the manual online (Sansui AU 505) it says the following about the loudness switch:

+10dB at 50Hz, +8dB at 10kHz, volume control at -30dB. Looks familiar....
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post #10 of 17 Old 03-20-15, 03:44 PM Thread Starter
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Re: Different sweep volume levels result in different responses

looks like it is something else, measurements with loudness switch set to off:

same as before, 65/75/85dB - new measurements do not rise as much in the treble but the bass is pretty much unaffected
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