Very strange measurement problem

[mick007]

Hi all.

I hope someone can answer this and tell me what is going on.

I am testing some scanspeak 10f/8424g00 midrange drivers, I have 5 drivers, 4 installed in enclosures and one on the bench. My first tests were all done 2 days ago using the one on the bench to get some free air measurements. Sweep range is 20 to 10000 The SPL graph matched the drivers almost perfectly on the bench and in the enclosure which to my suprise put a smile on my face as I knew I have done everything right. It was 2am so I packed up and come inside. Nothing was changed in the test environment, the speaker on the bench and the microphone have not moved, no change to the amplifier. I simply unplugged the laptop and come inside for the night.

Tonight when I went out I ran the same test sweep on the driver on the bench, to my suprise there is now a rise in amplitude from 60db to over 110db. I have another laptop with an older version of REW installed and it does the same thing. You can here the audible rise in the sweep to the point where I have to block my ears. This was not there the night before. I removed and reinstalled REW, no change. SO this appears to be pointing to the laptop but the fact it is doing it on both laptops has me perplexed.

My nice flat frequency response +/-2db from 300 to 4000 is now a pile of crap rising from 60db to over 110db.

What in gods name is going on?

 

[John Mulcahy]

Could there be any monitoring active? Difficult for the sweep to get louder on its own, but if the mic input is being fed back to the output that might do it. If on Windows check that "Listen to this device" has not become selected.

 

[mick007]

Could there be any monitoring active? Difficult for the sweep to get louder on its own, but if the mic input is being fed back to the output that might do it. If on Windows check that "Listen to this device" has not become selected.

Thanks John,

Im not sure where to look for that, in windows or REW?

Mick

 

[John Mulcahy]

That's in the Windows audio properties for the recording device. Can find it from Settings - Sound - Sound control panel (on right hand side) - Recording - select the input device then Properties - Listen tab.

 

[mick007]

Just checked the settings its OK. I have attached the sweep file. I didn't save any from the night before as I wasn't expecting this to happen. You can see what is happening.

 

[John Mulcahy]

You haven't selected the output device or output, so I can't see what interface you are using on the output side, but the measurement looks normal as far as settings and configuration goes. If you have some DSP in the path that might be the culprit, also need to check Windows is not applying any effects.

 

[mick007]

What I was getting 2 nights ago was the same as what is in the spec sheet. Got me beat what is going on here.

 

[mick007]

It wont allow me to select any output device only default. I was thinking the same thing re DSP but nothing is selected.

 

[mick007]

I took one picture from the night before everything changed and you can see how flat this is just laying on the bench. Whatever this is has more than likely happened to someone else. Same driver, same position, same amplifier, same microphone. All I did was unplug the laptop and come back 24 hours later.

 

[EarlK]

LOUDSPEAKER MEASUREMENT

Understanding the basics of making audio measurements can help one get control of ones unfounded expectation biases.

 

[mick007]

LOUDSPEAKER MEASUREMENT

Understanding the basics of making audio measurements can help one get control of ones unfounded expectation biases.

What tab would I find that under???

 

[mick007]

These are the outputs from my old laptop with 5.19 and the new laptop with 5.20. Remembering that the new one worked fine for one night. This is driving me bat crazy.

 

[John Mulcahy]

That looks to me like the kind of response shaping a Windows audio effect would apply. Make sure all "enhancements" are off on the Advanced tab of the Windows audio properties for the output device.

 

[mick007]

Everything is off. To make matters worse when I plug the UMIK in to the old laptop the output reverts to the same old output signal.

How in the hell does plugging the UMIK in to either laptop change the output signal?

 

[John Mulcahy]

If by "same old output signal" you mean the non-flat sweep then for a mic to affect the output some monitoring would need to be active, feeding some of the mic input back to the output. You could test that by setting the UMIK input volume setting (in REW or in the Windows audio properties Levels tab) to its minimum level (zero on the control) and see if the output shape alters.

You didn't mention what output device you are using, only that it is somehow not available for selection. What is it?

 

[mick007]

All this is now from the old laptop which used to work.

 

[mick007]

Downloaded the umik cal file again and I now get this. That hole at 10k is visible on the scope too but it's getting closer to what I measured on the first night. Nowhere near the flatness though.

 

[John Mulcahy]

Best check again that "Listen to this device" is not selected in the UMIK Windows audio properties, that is pretty much the only way to feed the UMIK input back to the output.

 

[mick007]

Im using Windows 10 and that option is not available under the Windows properties for the UMIK. Thanks for your help John but I'm not getting anywhere with this. Its gone from being an almost perfect reflection of the manufacturers specs to what it is now.

 

[John Mulcahy]

It is definitely an option under Windows 10, it looks like this:

 

[mick007]

I compared all settings with the working laptop and everything is identical. Thanks for your help John i will go back to the old laptop.