Which measurement is correct ?

Hello !

I was trying many hours to get consistent measurements with REW, i guess this was due to Windows issues. There are error messages appearing all the time, different ones. I am using REW with UMIK-2.

I was fighting a lot with "no input data", drivers were deleted automatically, the measurement won´t start, the mic was not recognized, clipping even at -25dBfs, you name it. I am sure these are Windows issues, i was setting up REW correctly ( and Windows, too, but yeah.. )

However, i got some measurements of my subwoofers, outside, i always got the identical measurement spot. It is an 8Ohm Sub, i was applying 2,83V measured with a true RMS Voltmeter and i was going to make 1w/1m measurements.

The measurements are different.

1-3 is the subwoofer measured today
4-6 is the same subwoofer measured yesterday

Both times at the exact same spot with exact same reflections, outside.


Which measurement is the correct one ? I would think that 4-6 are correct - but why are the other ones distorted ? What kind of phase is that ?


What went wrong with 1-3 ??


Thank you !


Kind regards

You would need to attach an mdat file rather than some images to get meaningful comments on the measurements. No input data and input selection reverting to default sounds like the mic is disconnecting, so USB cable length or connector issues.

Hi,

thank you, of course i should add the mdat files. i have done so, now. The USB cable is the original one, the USB port is working. The cable and the mic are pretty new.

It would be great if you could have a look at those two, which one do you think is the correct measurement ? The low end cutoff is very different f.ex.


Thank you !

Here are your two MDAT's, which I have re-labelled according to your filenames since there is no other information provided.



These are outdoor subwoofer measurements:

- taken on different dates (14 Jun and 22 Jun)
- the sampling rates are different with different impulse lengths (red = 48kHz, 131072 samples, green = 24kHz, 65536 samples)
- different sweep frequencies (red = 20-12kHz, green 20-2kHz)
- neither measurement has a timing reference
- both measurements are taken at different levels (red = -5dBFS, green = -25dBFS)

Just looking at the FR's you can see that the general shape of both measurements is similar. Even if you use the same measuring position, variations in microphone height can cause the difference observed.



The green impulse response is a complete mess, I would think the red measurement looks more realistic.

Of the two measurements you posted, I would say that the MDAT labelled "1w1m2" taken on 14 Jun looks more realistic. However, I still wouldn't trust it, given all the errors that you encountered during your measurement. Maybe you could try swapping the cable, or trying another USB port? How long is your USB cable?

Here are a couple more tips for more reliable measurements:

- Choose a longer sweep, 512k or 1M. You get a better signal-noise ratio. This is especially important if you are taking outdoors measurements.
- Always take multiple sweeps, not just one. Examine each sweep for signal-noise and reject all the bad ones. Overlay the measurements and look for consistency.
- Always choose 48kHz sampling rate as a minimum. Don't use 24kHz.
- Please confirm you are using WASAPI Exclusive mode (devices should be labelled EXCL in REW's drop-down menu).
- Use a timing reference if you can. It helps a lot.

Hi Keith,


thank you so much for the information. I have just made another test, trying to find out where the error(s) come from. The cable from the UMIK-2 is the original 2m long cable. I have tested the mic with help of windows and it works. It works with the REW signal generator, too. I am at the moment measuring my laptop speakers.

Still, even with low level Laptop speakers, i get very high distortion ( 24% )

Why is that ?


The Mic is new, the cable is new.


What can i try to lower distortion ?

Hello Folks,


SOLVED

i found the error.


I switched the 2m original USB cable to a 3m USB cable and the error is solved. I have rechecked it, i dont get any distortion anymore and the phase is beautfiully consistent by now.

What i have learned is, cables can cause days of trouble, if you experience very strange issues, distortion issues and "no input data" error messages, change the bloody cable.

Thank you !

I was going to say, I wouldn't trust any measurements (distortion or otherwise) until REW stops giving you error messages. If you are still concerned about distortion, take another set of measurements. Your mic needs to be 1m from the speaker, and the output of the speaker adjusted to 76dB at mic distance. Proper volume is important when doing distortion measurements. Look up online measurements of distortion from spinorama.org (if available) and see how your speaker compares. You ideally want <1% across the entire spectrum, but 5-10% is acceptable for low bass.