Different sweep volume levels result in different responses - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 17 Old 03-20-15, 04:21 PM
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Re: Different sweep volume levels result in different responses

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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post #12 of 17 Old 03-20-15, 05:20 PM
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Re: Different sweep volume levels result in different responses

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




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post #13 of 17 Old 03-22-15, 09:22 AM
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Re: Different sweep volume levels result in different responses

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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post #14 of 17 Old 03-22-15, 10:11 AM Thread Starter
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Re: Different sweep volume levels result in different responses

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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post #15 of 17 Old 03-22-15, 02:27 PM
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Re: Different sweep volume levels result in different responses

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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post #16 of 17 Old 03-22-15, 04:18 PM Thread Starter
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Re: Different sweep volume levels result in different responses

Thanks, the level matched graphs below. I will remeasure tomorrow with your suggestion, something could be clipping somewhere indeed.
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post #17 of 17 Old 03-22-15, 04:37 PM Thread Starter
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Re: Different sweep volume levels result in different responses

oh yes, i'm using a behringer ecm8000 with a m-audio fast track pro
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