measurement of Acoustic time reference for 2.1 - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 8 Old 10-15-16, 06:13 AM Thread Starter
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rudy
 
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measurement of Acoustic time reference for 2.1

Hi,

I have a 2.1 system with a miniDSP .
I have the latest REW version.
crossover is 80 Hz .
By the groupdelay of the sub, it isn't possible to determine the actual distance for the sub.
So I want messure the timing delay (phase) with REW for main speakers and sub, and put these settings into the minidsp.
This works fine for the main speakers but not for the sub because for the messure REW use a high reference tone ,so when i messure the sub REW don't accepte the measurement because of the high reference tone is not audible on the sub.
the subfilter is disabled as well as in the minidsp no crossover filter is set, so full signal is possible .

How can I measure the sub so that I can enter the delays in the minidsp?

is there another way to work out this ?
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post #2 of 8 Old 10-15-16, 03:21 PM
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Wayne Myers
 
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Re: measurement of Acoustic time reference for 2.1

Measuring sub distance / timing is not an easy matter, as you are discovering. With a 60 Hz crossover, the wavelength of a 60 Hz tone is 18.8 feet, so a few inches either way is not going to be audible or measurable.

A sub's impulse response is also very difficult to measure for the same reason.

If your sub allows bypassing the built-in crossover and running it temporarily with, say, a 200 Hz upper bandwidth limit, you can begin to get more meaningful measurement/delay numbers, but that is a lot of work for little gain in terms of accuracy.

Your best bet is to measure physically with a laser distance measurer (my preference) or tape measure, and set delays accordingly. Then verify by measuring the smoothness of frequency response at the crossover frequency. If it is smooth within 1 or 2 dB, you are doing great.


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post #3 of 8 Old 10-16-16, 03:30 AM Thread Starter
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rudy
 
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Re: measurement of Acoustic time reference for 2.1

Hi,
thanks for the quick respons !
for the distances to measure, I know it from the front of the speakers to the listening place, but I have a downfiring sub BK Monolith,
is the measurement then from the front of the Cabinet or the center point of the woofer down below?

I have also read that the groupdelay of most subs would be between 8 and 12 ms .
But in my setup the crossover is bypassed ( crossover and delay for the speakers and sub is regulated in the minidsp)
So must i calculate de delay of 8-12ms to?
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post #4 of 8 Old 10-16-16, 09:44 AM
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Re: measurement of Acoustic time reference for 2.1

One way to evaluate the situation is:
> Set the time delay in the MiniDSP to account for the difference in distance; mains to LP vs SW to LP.
> Leave the 80 Hz XO and any EQ you applied active.
> Take 5 measurements: (USB mic? - acoustic timing suggested, XLR mic? - loopback timing suggested) [USB mic? - be sure to use the same main speaker for the acoustic timing reference channel.]
  1. SW
  2. L
  3. SW+L
  4. R
  5. SW+R

If there is good SPL fill/support in the XO range then all is good - done.

If not then:
> Possibly a delay adjustment is needed.
> Possibly the timing is correct, but there is a significant room mode in the XO range.

This can be done without the timing reference, but:
> If the SPL fill is good; done.
> If the fill is poor; the delay timing can be changed and the measurements retaken to see if there is improvement. This trial and error method takes some time and many measurements, but often works okay.

When timing reference used; optional timing can be evaluated within REW by shifting the SW timing manually to see the impact on results. No additional measurements are needed.

If you use the above method for evaluation and timing adjustment and want me to take a look at the data, just post the .mdat.

Below is example of good SPL fill in the XO range:
[Blue was initial timing, Black is adjusted timing]
measurement of Acoustic time reference for 2.1-post-1b-new-spl.png
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post #5 of 8 Old 10-16-16, 11:56 AM Thread Starter
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rudy
 
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Re: measurement of Acoustic time reference for 2.1

Hi,
I have an UMIK mic USB.
For the acoustic timing reference , REW use a high reference tone ,
so first i messure left speaker as reference = OK,
than the right speaker result distance =OK
but when i do the messure for the sub REW don't accepte the measurement because of the high reference tone is not audible on the sub.(crossover on sub is bypassed )
Is this correct?
How do you do the acoustic time reference for the sub?

So if not possible i do the messure like you descript in the graphic.
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post #6 of 8 Old 10-16-16, 02:16 PM
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Re: measurement of Acoustic time reference for 2.1

I don't have this setup but maybe...

Assuming the L is the common reference channel:
> Set the right channel for the measurement channel. [Have the Right main disconnected for this measurement process].

Thus the left should get and play the reference sweep and the measurement sweep will be in the right channel where the R main is disconnected, leaving only the SW to play.
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post #7 of 8 Old 10-16-16, 04:12 PM
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Re: measurement of Acoustic time reference for 2.1

Please use a full range sweep for all 5 measurements (12Hz-20kHz for example). If any post measurement analysis is done in REW it is very helpful to see the full range phase response.

Do not run a shorter sweep for the SW. It should be the same range as all the others.
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post #8 of 8 Old 10-19-16, 12:40 PM
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John
 
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Re: measurement of Acoustic time reference for 2.1

Remember that the channel used for the timing reference should not change whichever channel you are measuring, otherwise it cannot act as a reference.
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