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Discussion Starter #1
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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Discussion Starter #3
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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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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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.
 

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Discussion Starter #6
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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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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Discussion Starter #8
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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Discussion Starter #9
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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Discussion Starter #10
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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Have you looked at the RTA with and without the white noise signal? That 30 Hz bass looks like it could be an external source and not part of the signal since it doesn't vary.
Also it would be interesting to see the waterfall graphs.

Or, as others have said, it could be something in the mac.
 

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It is not unusual to see response vary with level. Speakers are not necessarily linear nor is room response.
 

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65dB is a really low level for an in room measurement. 75dB isn't much better and you can get HVAC, refrigerator, outside elements etc creeping in. I'd do a sweep with no signal to the speakers and see what kind of noise floor you're working with. Low bass response can be quite surprising even with no signal present depending on your situation. IMO measurements at 85/95/105 would be more interesting and get the noise floor out of the way.
 

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Discussion Starter #14
Just took another set of measurements.

65dB loudness off
75dB loudness off
85dB loudness off
amp off - room noise floor
macmini output to audio device in - line out frequency response.

I think this proves that the amp is being sent a linear response and that there are no external 30-50Hz frequencies in the room (unexpected, I live in a detached house in a quiet area, no washing machines running etc).

I am starting to conclude that the shift in response is due to my 1971 Sansui AU-505 integrated amp which it seems cannot deliver the current in the sub-80Hz range - and in turn not able to properly render the 85Hz sweep.

The amp is also past it's prime, sometimes the right channel will drop out or start distorting which a restart of the amp will fix.

The elevated bass (in 65dB and 75dB sweeps) is likely due to room modes and the upwards treble may be the voicing of the speakers, or the amp applying EQ even though the treble is set to 0dB (I doubt the linearity of frequency response of this amp). I guess I need to get a proper amp and repeat the measurements. Does this sound logical?
 

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The 65 dB and 75 dB sweeps look like they are offset by the right amount (you can check that by using the graph controls to subtract 10 dB from the 75 dB sweep). The 85 dB sweep also looks right above 1 kHz but pretty odd below. It is worth making sure that you are not encountering clipping in the chain anywhere, so not overloading the amp input or the mic or SPL meter input (didn't see a mention of what you are using to capture the measurements). You could check the output side by using a 10 dB lower sweep level in REW and turning up your amp to compensate.
 
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