inconsistent delay readings in ref. REW

[Sacenti]

Hi John! I need your help again, please ...
I can't make reference measurements of my sub's time through the ref acoustic process ... the measurements are very strange ...
When I measure the channels' delay, the reference is consistent, but when I measure the individual subs, the times are far from reality ... I have already done several microphone level calibrations in the REW but the problem persists.
In the SW1a curve I only got the right measure just once and it was consistent, then I changed to make the SW4 curve, the delay was strange ... I changed the sub and beep levels to see if it was a low signal but nothing changed .. .
Attached is the file of some measures to compare.
Then I went back to taking measurements in the same sub SW1, see the SWb curve and it was incoherent again as in SW4.
I'm using an analog LinearX mic connected to an external usb sound blaster card, I use a mic converted to usb by the external card, it seems to work fine with the HW part ... all the other measures work well ... sub delays in the overlay impulse graph
What confuses me is the reading in the notes that is not true with times and distances inconsistent with my 6m long room ...
REW does not inform the correct delays in the note fields. See the impulse graph and compare please ... in the impulse graph this is the problem.
When I use loop back everything is fine ... the sub are as expected ...
Thank you for your usual attention ...

 

[EarlK]

If you're getting good ( and expected ) distance results using the cabled Loop Back method ( the more accurate of the two ) then stick with that method.

 

[Sacenti]

Thank you for the advice EarlK! I need both methods for my work, when I configure AV receiver where everything is integrated amp + preamp the acoustic reference method is more suitable and when I have separate pre amp the loop back method is better and more secure ... when I can turn off the 7.1 amp / pre amp level output
I don't know why REW does not record the correct delay value in the notes field and this only happens for sub output regardless if I'm using bass or LFE management on channel 4. It is very strange that delay information outside the reality..50m / 160ms ... and I'm limited in the multisub alignment by the acoustic ref method

 

[EarlK]

Hi,

You are using a full range speaker as your Time =0 acoustic marker , correct ??

Now the problem here is >> I'm strictly a 2.0 music sort of guy here at home ( though I do run subs in my Pro Sound work >> which is of course all on hold during Covid time > so I have some interest in aligning sub woofers ).

John has mentioned ( many times in the past ) that full range frequency sweeps ( or almost ) must be used ( even with subs ) so that all the waveforms are playing by the same set of rules// deck of cards ( more or less ).
- If I remember correctly.

He has also mentioned that within the Gear Sprocket "Control" window . found top-right when one is in the "SPL & Phase" window ) the user should ask REW to "Estimate IR Delay" and use this delay number instead of the one that initially shows up in notes window.

I realize this ( that they aren't the same ) is all a bit confusing ( counter-intuitive even ) but there is good reason for the differences between the two delay numbers .
- The "Notes" delay time includes all room effects as found within the measurement and therefore is limited in its' estimation to using the highest peak in the waveform ( even though that peak might actually be a room effect >> node ).
- The "Estimate IR Delay" removes the room effects as best as it can and then uses the beginning of the waveform change ( not the peak ) as the time marker ( these are two different areas of the same waveform > so will give 2 different timings ).

Clear as mud right ??? Right ?

I'll look around for some relevant info to your observations ( I have seen this before quite a few times where the delay numbers for subs were non-sensical when using the acoustic timer ) .

 

[Sacenti]

Thanks for your explanation, yes in the Sprocket "Control" window I can read the measure of the delay I am looking for when the information is not valid in the notes field, I am also collaborating in informing this type of inconsistency to John if it is a failure of REW or I'm not doing it right.
Thanks for your valuable help EarlK!

 

[John Mulcahy]

You don't need to make full range sweeps when measuring the subs, just make sure you measure to the point their response drops into the noise floor. For the settings you have looks like 200 Hz would be enough for that. The main problem is the signal to noise ratio of your sub measurements is very low, only about 24 dB. That means a little extra noise can easily confuse the search for the start of the impulse response. Using a lower end frequency for the measurement sweep will help, as will increasing the sweep level - I suggest another 10 dB or so.

 

[Sacenti]

Even on Christmas Day you promptly attended to me! You are very kind John! Your information was my Christmas present. Merry Christmas to you and your family!
I redid the measurements and calibrations by increasing the signal level and reducing the frequency sweep to 200Hz and everything went to the right place! I am very happy with the result now!
I am interested in the REW PRO license, but I wanted to study more about miniDSP multi-microphone measurements ... I don't have enough information to do my up grade ... miniDSP reports very little, or I didn't find it on the website of them ... would you have anything to tell me about measuring with multi microphones, please? The literature is more focused on TRINNOV and JBL I wanted something specific from the REW + miniDSP solution and its microphones. Thanks again!

 

[John Mulcahy]

If you routinely measure in multiple positions then multi-channel capture could be a time saver by capturing multiple mic inputs with one sweep. There are also situations where the sound field varies so much that a single position doesn't give useful information, such as inside vehicle cabins. In those cases a multi-mic solution like the UMIK-X can give a better picture of the environment. There are alternatives though, such as the Moving Mic Method.

 

[Sacenti]

Thank you very much and have a good day John

 

[dandi36]

You don't need to make full range sweeps when measuring the subs, just make sure you measure to the point their response drops into the noise floor. For the settings you have looks like 200 Hz would be enough for that. The main problem is the signal to noise ratio of your sub measurements is very low, only about 24 dB. That means a little extra noise can easily confuse the search for the start of the impulse response. Using a lower end frequency for the measurement sweep will help, as will increasing the sweep level - I suggest another 10 dB or so.

Hi John and happy Thanksgiving!!

I have been looking for this subject for a long time - if you have any more refrences for this subject will be happy yp know.

a few questions just to get this clearer to me:

1.100% clear and make sense - your suggestion to increase the sweep level.

2.how do you conclde that about 200hz is the correct approch for this specific setting and why lowering the end freqancy for the measurment will help ?

3.do we have an option in REW to have info about the start and end point of the frequncy that a measurment was taken ? such as the info that we have in the "Notes" .


Thank you very much John for your kind support!!

 

[John Mulcahy]

200 Hz was right for that specific measurement because the response was in the noise floor after that. Reducing the end frequency means the energy of the sweep is put into a smaller frequency span, so the frequencies in that span get more energy (compared to a sweep with a higher end frequency) and so the response to them is further above the noise floor. Click the Info button for information about the measurement, including its frequency range.

 

[dandi36]

200 Hz was right for that specific measurement because the response was in the noise floor after that. Reducing the end frequency means the energy of the sweep is put into a smaller frequency span, so the frequencies in that span get more energy (compared to a sweep with a higher end frequency) and so the response to them is further above the noise floor. Click the Info button for information about the measurement, including its frequency range.

John,

Thank you very much for your kind support!!

much appricated.