Repeatable accuracy of levels for drivers measurements

[Zoltan]

Hi Guys,

Before everything else I have to announce that I am a beginner in REW but also in the concept of acoustic measurements, I have the following equipment :

• Behringer UMC204HD
• Dayton AudioEMM-6
• T.Amp PM40C amplifier
• And all the cables in between including the loopback

I have made some search but without finding very clear solutions to my problem.

To make sure that every-time I use REW to make measurements of a driver that I am at precisely the same level, how would you proceed ?
But before you tell me I have to say that leaving the potentiometers (Audio Interface and Amplifier) at the same position is not possible

To me the best would be a solution to make sure that my high level output at the amplifier are at 2.83V, but connecting a multi-meter give me nothing or inaccurate when signal is present, I did not find anything on google that I trust or that seem to work.

This way I would have the correct sensitivity scaling between the drivers I measure

Also I don't have an external dB meter besides a phone app, only if that is absolutely necessary I could buy one.

Can you walk me trough your workflow?

Thanks

 

[Grayson Dere]

Hi Zoltan,

This is an interesting procedure you have on hand. I am trying to fully understand...you mention the potentiometers can't
be set at the same position during testing. Does this mean your gain/output is always varying? Please correct my misunderstanding.

Thanks! : )

 

[Zoltan]

Hi,

Thanks for responding

No fortunately It is not that bad as you imagine, so allow me to detail a bit more

It's between each test session that I'd like to get back to the same levels, but also have the habit to lower the gain level to 0 on the amplifier and the main before on the audio interface before unplugging or manipulating the driver I test (call me parano)

But independently of that I have no Idea what voltage I am sending to the driver, the measure at the amplifier are not giving me anything reliable to work with

To make a measurement I have to set the level on :

Input1 on the UMC204 (The microphone)
Input2 on the UMC204 (The loopback)
Main output on the UMC204 (signal to amp)
Gain control on the amplifier
(And not forget to set the Windows volume to 100%)

Its getting difficult If I use anything for something else between sessions

EDIT: Ho, and I forget to add none of the UMC204 have any precise "click" stop that I can use.

 

[John Mulcahy]

To me the best would be a solution to make sure that my high level output at the amplifier are at 2.83V, but connecting a multi-meter give me nothing or inaccurate when signal is present

You should probably deal with that first, make sure your multimeter is on the AC scale and the test tone frequency you are using isn't too high, multimeters tend to work best at frequencies around the mains frequency.

If you change any gain setting on the input path you will need to repeat the REW SPL calibration. I suggest you leave that mic gain pot alone once you set it. Rather than changing gains you could turn off the amplifier.

 

[Zoltan]

Hi John,

Thanks for the help

I have tested the voltage with a signal at 1khz as it is some sort of standard I have read, I have now tried with a sine-wave of 50Hz (as the grid in europe) and it seem stable but something still isn't right or I don't understand the concept.

Am I wrong to believe it's only in mV that I can get a reading, setting my multimeter to 1.5V show only 0.00 to 0.01V

My multi-meter show the following value when the gain on the amplifier is at minimum : https://i.imgur.com/YzFaHUx.jpg
And this is at maximum : https://i.imgur.com/AiLvaZA.jpg

When I adjust the gain on the UMC Main output it change momentarily and then revert back to the value I set to the amplifier, even if loudness level change.



I can manage to leave the input level set during the session, I had read but section about SPL Calibration but even with it some parts are still obscure to me, when I set it to Z-weighted and do the calibration it activate C and Z together and then revert back to C automatically but in the mic preference it's still set to Z, so I suppose it don't matter what it's set in the SPL meter?

Ho and and in 5.20 beta 9 (I will upgrade to 10 to test) when the SPL Meter is on and I open the measurement I get a popup that indicate the soundcard is not responding and the SPL meter then turn off, I can still do the measurement anyway.


It's quiet difficult to get a driver measurement close to the dB that it is advertised, I know the data are often not reliable but I am far (about -10dB)

 

[John Mulcahy]

Your meter is set to measure the DC voltage. According to the manual pressing the FUNC key switches between DC and AC. The manual shows the range in AC as 50 to 400 Hz, so keep the test signal in that span.

 

[Zoltan]

, I'm sorry John, I usually RTFM but not this time, Func!
Work Ok now