# Measurements change



## Phillips (Aug 12, 2011)

Hi 

When i take several measurements in the same position /settings nothing changes at at all for each measurement the bass seems to change, please can someone explain?

Maybe the mic is picking up noises that i can't hear?

Thanks in advance


----------



## fusseli (May 1, 2007)

How much are they changing? Slight movements in mic position can cause variations in measurement. Background noise could also be the culprit. Can you post your measurements?


----------



## Phillips (Aug 12, 2011)

> How much are they changing? Slight movements in mic position can cause variations in measurement.


Thanks for your response. 

No change at all all measurements were taken straight after each other.



> Background noise could also be the culprit.


Wondered that

Seems to be from about 55hz down.



> Can you post your measurements?


----------



## fusseli (May 1, 2007)

What kind of mic are you using? I'm wondering if that's the issue, since I see both the HF and LF show variance in measurements. Hmmm.


----------



## Wayne A. Pflughaupt (Apr 13, 2006)

How much time elapsed between the measurements? Temperature and humidity have an effect on transducers (both speakers and mic elements), so you can expect somewhat different readings from say, a rainy vs. sun shining day, or even a cool morning vs. hot afternoon. However, the difference between the red trace and the other two is significant enough to suspect that something else is at play.

Regards, 
Wayne


----------



## Phillips (Aug 12, 2011)

fusseli said:


> What kind of mic are you using? I'm wondering if that's the issue, since I see both the HF and LF show variance in measurements. Hmmm.


Thank you

The mic is a calibrated: 

*Behringer ECM8000

Microphone Sensitivity: -41.8 dB (8 mV/Pa) 

Noise Floor: 31 dBA*


----------



## Phillips (Aug 12, 2011)

> How much time elapsed between the measurements?


Thanks for your reply

About 30 seconds.



> Temperature and humidity have an effect on transducers (both speakers and mic elements), so you can expect somewhat different readings from say, a rainy vs. sun shining day, or even a cool morning vs. hot afternoon.


I can't remember the type of day, actually i was playing around.



> However, the difference between the red trace and the other two is significant enough to suspect that something else is at play.


Maybe soundcard setup?

Stability of the mic or soundcard?

Is there any way i could test?

Could it be background noise that i can't hear?


----------



## Steve1533 (May 12, 2013)

I definitely pick up low frequency background noise from time to time when using REW, and sometimes I don't always here it by ear during the scan.

This problem is easy to observe and evaluate using REW: just repeat your usual REW measurement, but with the amp gain lowered, or with the amp power lddudeff, so that the REW sweep is not audible. REW will still record during the (unheard) sweep, and the measurement obtained is a sample of the background noise (both real and electronic) that your measurement would have seen. Repeat the background scans over time to see if varying noise is your problem. The noise should be well below your normally recorded sine-wave-sweep signal.

I used this method both to detect a bad mic (which I exchanged) and also to observe varying ambient noise coming in from my neighbors (I live in an apartment).

Hope this helps, Steve. lddude:


----------



## Phillips (Aug 12, 2011)

> This problem is easy to observe and evaluate using REW: just repeat your usual REW measurement, but with the amp gain lowered, or with the amp power lddudeff, so that the REW sweep is not audible. REW will still record during the (unheard) sweep, and the measurement obtained is a sample of the background noise (both real and electronic) that your measurement would have seen. Repeat the background scans over time to see if varying noise is your problem. The noise should be well below your normally recorded sine-wave-sweep signal.


Thank you for your reply.

I will try this. 

Maybe at different times of the day and night.



> I used this method both to detect a bad mic


How did you determine this?



> Hope this helps, Steve.


Yes thank you


----------



## Steve1533 (May 12, 2013)

Phillips,

You are right to suspect that detecting mic and other electronics problems are a little tricky -- basically the REW scans present some sort of signature that is pretty clearly not caused by ambient noise, and then you go on from there with further diagnostics tests of various kinds.

In my case where the mic was bad, I was getting routine REW scans where at low frequencies the scan varied wildly and was up at say ~95 dB, while the rest of the scan was nominally flat at ~75 dB from a few hundreds Hz up to 20 kHz, as expected. The 95 dB low frequency noise signal hardly reduced at all when the "background" REW scan was performed with no output, so it was obvious there was a gross electronics problem (and there certainly wasn't 95 dB of sound!). The mic noise fell as 1/frequency (another sign of electronics-xrelated issues) in the background scans, which was why the regular REW scans tended to look OK as the frequency of the scan increased.

To isolate the problem specifically to the mic, I removed the mic (with phantom power off), and then I put phantom power back on with all of the rest of the set up also unchanged from the usual REW measurements. I then repeated the REW background measurement with no mic. This step measures the electronics noise of the whole set up sans the mic. In my case, the noise in the scan without the mic immediately fell by tens of dB at the lower frequencies, making it clear the problem was very likely the mic. 

Before returning the mic, I did double check for other problems like say, bad cables (swapped cables, checked cable resistance and isolation, etc) but could not identify anything. I had gotten the mic at Cross Spectrum Labs, as suggested by many here on the site, and Herb (the owner of Cross Spectrum) was very helpful and prompt with the return/exchange.:bigsmile: Happily enough, the REW measurements worked normally as soon as I was able to get the replacement mic.:T:T

If your background scans give very high low-frequency noise that always tends to approach the intended scan levels and also decreases as ~1/f, your case could perhaps be mic problem somewhat similar to my own, though less severe, and I would see if removing the mic greatly reduces the noise...

Cheers, Steve


----------



## Steve1533 (May 12, 2013)

P.S. Phillips, it occurs to me that maybe you don't say "Cheers" in New Zealand (as in Britain). If not, what's a friendly way to sign off in your part of the world? --Steve:dontknow:


----------



## weverb (Aug 15, 2008)

Steve1533 said:


> This problem is easy to observe and evaluate using REW: just repeat your usual REW measurement, but with the amp gain lowered, or with the amp power lddudeff, so that the REW sweep is not audible. REW will still record during the (unheard) sweep, and the measurement obtained is a sample of the background noise (both real and electronic) that your measurement would have seen. Repeat the background scans over time to see if varying noise is your problem. The noise should be well below your normally recorded sine-wave-sweep signal.


Wouldn't the RTA feature with no sound do the same thing?

http://www.hometheatershack.com/forums/rew-forum/9872-spectrum-rta-feature.html#post86006


----------



## Steve1533 (May 12, 2013)

weverb,

Probably you are right that RTA could be used as well, but the background sweep approach gives a documented record over time that can be save with each measurement if needed.

Note the newbie tag for me -- I have not use the RTA feature much, so I can't speak to the advantages or disadvantages with any confidence. I will check out your link and see what I can learn.

Thanks, Steve


----------



## weverb (Aug 15, 2008)

Steve1533 said:


> Probably you are right that RTA could be used as well, but the background sweep approach gives a documented record over time that can be save with each measurement if needed.


Valid point. I just save a jpg of the graph which is time stamped. The nice thing about the RTA is you can move the mic or make adjustments and see "real time" changes.


----------



## Phillips (Aug 12, 2011)

Steve1533 said:


> Phillips,
> 
> You are right to suspect that detecting mic and other electronics problems are a little tricky -- basically the REW scans present some sort of signature that is pretty clearly not caused by ambient noise, and then you go on from there with further diagnostics tests of various kinds.
> 
> ...


Thanks for that

Which mic had the problem?


----------



## Phillips (Aug 12, 2011)

Steve1533 said:


> P.S. Phillips, it occurs to me that maybe you don't say "Cheers" in New Zealand (as in Britain). If not, what's a friendly way to sign off in your part of the world? --Steve:dontknow:


Yep we do say cheers, funny enough i tend to say Cheers in conversation but in posts i don't, no real reason.

Laid back over here, anything goes.

Interesting with the Americas cup, anything can happen.


----------



## Phillips (Aug 12, 2011)

Here are the RTA + Sweep measurements.



Red is the peak


----------



## Steve1533 (May 12, 2013)

Phillips,

I use a Dayton EMM-6 calibrated by Cross Spectrum. If I remember correctly, Herb/Cross Spectrum used to sell calibrated Behringer mics, but recently, so many of the ones that he bought from the distributor were becoming so faulty that he had to stop offering them (doesn't mean your particular mic is bad).

In your Post #17, the scans are the so-called background (no output) REW sweeps made with the mic, and with settings just like in your original post for the real sweeps?

Steve


----------



## Phillips (Aug 12, 2011)

> Phillips,
> 
> I use a Dayton EMM-6 calibrated by Cross Spectrum. If I remember correctly, Herb/Cross Spectrum used to sell calibrated Behringer mics, but recently, so many of the ones that he bought from the distributor were becoming so faulty that he had to stop offering them (doesn't mean your particular mic is bad).


Thanks

Any idea what type of problem?



> In your Post #17, the scans are the so-called background (no output) REW sweeps made with the mic, and with settings just like in your original post for the real sweeps?
> 
> Steve


Yep the original sweeps were made as normal test tone etc and post 17 were no output just back ground noise.

Thanks again

I have attached more background measurements different day.

Interesting with the 2 that fall of early.

But below 60 hz is prominent.


----------



## Steve1533 (May 12, 2013)

Phillips,

I went and looked at my current background REW scans to see how they compare to yours. I would post them but I don't have time -- instead I'll just summarize the highlights:

(1) My background scan with REW behave very much like those you have measured. Mine jumps up a little at low frequence just like yours except in my case the jump is a few dB more and occurs closer to say 40 Hz or less.
(2) I have some 5-15 dB spikes at 60 and 120 Hz that go up and down with time. This is electricity noise and its first harmonic. You don't have this problem it seems.
(3) I can reproduce the drop-outs to < 0 dB in the background-scan signal that you see. I do it by making some brief sharp noise(s) during the early portion of the background scan. The drop-out cannot be made to happen every single time, but every few times that I make some rattle or thunk near the start of the scan, the REW signal falls below zero just like you saw in your last post (you could try this too). Maybe noise occuring at just the wrong time causes REW to initialize the scan incorrectly or some such.

My conclusion is that your set up is very likely operating normally, and the problem is that you are picking up various background noises, some audible to you and some very likely not. Before scanning, have you done stuff like turning off things like refrigerators, AC fans, computer fans, florescent lights that hum, etc. I do all this but often still see varying low frequency noise because of my apartment neighbors... they don't have an OFF switch! (REMEMBER TO TURN YOUR FRIDGE BACK ON!!!)

So I think you may be good to go for the most part.:T Be careful about the background -- most especially in the REW waterfall plots, decay plots, and spectrograms -- because the noise will easily show up at long decay times (say >200 millisec). It will vary a lot and look at times like something that maybe needs bass traps, but maybe it doesn't actually.lddude: I think that any significant low frequency modal problems in your room will still be easy to spot using a careful and thoughtful measurement routine, even with the varying low frequency noise in the background.

Regarding the Behringer mic problems and your question about them, I don't recall exactly the problem(s), but Herb/Cross Spectrum maintain a thread here (a sticky I think) and you could either look through the thread for clues, or just post there, if you want to pursue the question further.

--Steve


----------



## Phillips (Aug 12, 2011)

Thanks for your reply.

How does your measurements (speaker measuring sweeps) look in this range and what extra level / volume above the background noise do you measure?

Thanks again


----------



## Steve1533 (May 12, 2013)

In do real speaker sweeps from 20 Hz to 22 kHz. In the 20-100 Hz range the measured signal in a SPL vs frequency plot is about 80 dB, and the measured signal decreases slowly to about 70 dB at 22 kHz. My background signal in the 20-40 Hz range is often as high as 50 dB, maybe higher on rare occasion if my scan coincides with footfalls on my ceiling from neighbors above.

Under typical circumstances my real sweeps show much less variation (usually <1dB) in the low frequency range compared to what you see in post #3 on this thread. In the 20-25 Hz range where acoustic noise is worst for me, I might on every few scans see a variation in the SPL vs frequency scan of say 2 dB. Above ~25 Hz I see good repeatability almost always (so long as I have not changed anything).

The story changes for time dependent waterfall plots where one is always much more sensitive to the background noise because the real signal decreases with time. In waterfalls for the 20-40 Hz range I always see some variation after say 150-250 msec because the signal has decayed by 20-30 dB, and it approaches the level of the varying background. I also often see the 60 and 120 Hz electrical noise show up at about this same time as well, again as the real signal decays in the region too.

If you still see obvious variations in your real scans (do you?), and if background scans repeatedly made in the same time frame never show noise signals up near say > 60-70 dB in SPL vs frequency plots, then I would begin to suspect some other problem than your mic, cable, preamp, and soundcard input chain. You can Google about how to add together SPL signals (real signal + background signal, in our case), and I think you will find that the background has to get within about 10 dB of the real signal in order to cause even a small ~1 dB error due to background (you might want to verify this for yourself).

Could there instead be a problem on the sound-source side with the sweep output and originating in the soundcard, output cable, preamp/amp, and speaker chain? I am grasping at straws here (ugh:dontknow, but perhaps you need to consider other possible problems if the background scans don't ever read high.

In my own case, my speakers (no subwoofer) were not dealing well with scans that started at 10 Hz instead of 20 Hz (sometimes my amp would trip out on overload), so I stopped pushing the ultra low end so hard. This was also back when my mic was bad and I also had little experience -- so, things were confusing in any case at the time. I don't know how my system would behave in the 10-20 Hz range now ... and I'm not going to explore because these frequencies are not important to my main interest in music reproduction (and not "boom-boom" movies and their sound effects). 

I was wondering, do you set your output levels and verify real signal levels each REW session using a SPL meter? Basically this would insure that you real signal (70-80 dB) is always and truly 30-40 db above your worst background (~40-45 dB). Sounds like you may already do this... 

Beginning to run out of thoughts for you,onder: but I would keep periodically checking background behavior whenever your real signal does not reproduce well in the short term (and no changes have been made to the room, the measuring set up, and the sound equipment settings -- or the positioning of anything). 

Cheers, Steve


----------



## Phillips (Aug 12, 2011)

> Could there instead be a problem on the sound-source side with the sweep output and originating in the soundcard, output cable, preamp/amp, and speaker chain? I am grasping at straws here (ugh:dontknow, but perhaps you need to consider other possible problems if the background scans don't ever read high.


Thanks again
Yeah i wondered that too.



> I was wondering, do you set your output levels and verify real signal levels each REW session using a SPL meter? Basically this would insure that you real signal (70-80 dB) is always and truly 30-40 db above your worst background (~40-45 dB). Sounds like you may already do this...


I normally measure about 75db.



> Beginning to run out of thoughts for you,onder: but I would keep periodically checking background behavior whenever your real signal does not reproduce well in the short term (and no changes have been made to the room, the measuring set up, and the sound equipment settings -- or the positioning of anything).
> 
> Cheers, Steve


Thanks again

I am going post a picture of my sound card settings.


----------



## AudiocRaver (Jun 6, 2012)

Catching up on this interesting thread, beg pardon if I missed something obvious.

It is very true that LF acoustical noise can be hard to hear, but what your plots are showing is high amounts of LF noise, a lot of it at frequencies that you should be able to hear. Have you tried

a different mic?
a different sound card input?
a different sound card?
a different mic cable? (not likely it's the cable, but easy to try)
Try a few measurements with mic hooked up but phantom power turned off. If your sound card's mic preamp is noisy you should see some of that noise.
Try a few measurements with no mic or cable hooked up but with phantom power turned on - if the phantom power supply is noise, you might see it.
If you have some kind of phantom powered mic preamp or mixer around, run your ECM8000 through it and from that into the line input of your sound card - this also could show if your sound card's mic input is noisy.
These are just some ideas. It looks like it could be electronic noise to me.

Looking back through the thread, I see that Steve1533 has already made some of these suggestions, plus other excellent ones - including _don't forget to turn the refrigerator back on!_ (yeah, I've done it, more than once). The RTA view is a great way to diagnose in real time. Watch the changing LF levels and listen carefully while watching. With the visual cue from the RTA, you will find that you should be able to hear at least some of that noise.

It is true that LF acoustical noise can be difficult to hear and hard to track down. But here is what is bugging me about your original plots in post #3. At 40 Hz, the measurement is 72 dB +/- 2dB dB, and it looks very regular, not random, like a steady vibration at that frequency. An acoustical noise source, in order to change your measurement by 2 dB, would have to be at around 65 or 66 dB or so. And at 40 Hz, you should be able to hear that.

*Try this first:* With your mic connected to make measurements, bury your mic in pillows, completely surrounded isolated as much as possible, use soft pillows, blankets, whatever, so it is completely smothered and isolated as much as possible. Now listen to your sound card's output with some headphones that have good LF response - most do these days:bigsmile: - not through speakers! - start out with the headphones just off your ears in case something loud happens, and carefully turn up the mic gain and the headphone output gain, all the way if you have to, listening for any kind of LF noise. There is a kind of electronic noise called popcorn noise that occurs sometimes in transistors that are going bad, and it sounds like - you guessed it - popcorn, although all at low frequencies so it will sound like giant muffled popcorn pops, maybe just once every few seconds, maybe more often, pretty random. LIsten for that or any other kind of LF noise that could be indication of an electronic noise problem. You can be watching the RTA screen at the same time for cues. Acoustical noise at low frequencies could still get through the pillows, but should be reduced. If you get something with this test, then the other tests above could help you track it down to mic or sound card.

Hope that helps. Good luck.


----------



## Phillips (Aug 12, 2011)

> Catching up on this interesting thread, beg pardon if I missed something obvious.


Thank you very much for your reply



> It is very true that LF acoustical noise can be hard to hear, but what your plots are showing is high amounts of LF noise, a lot of it at frequencies that you should be able to hear. Have you tried
> 
> a different mic?
> a different sound card input?
> ...


Try it in this order (some i haven't got e.g soundcard, mic cable)?



> Looking back through the thread, I see that Steve1533 has already made some of these suggestions, plus other excellent ones - including _don't forget to turn the refrigerator back on!_ (yeah, I've done it, more than once). The RTA view is a great way to diagnose in real time. Watch the changing LF levels and listen carefully while watching. With the visual cue from the RTA, you will find that you should be able to hear at least some of that noise.


Maybe when it is not turned on at the wall but when the refrigerator compressor starts?



> It is true that LF acoustical noise can be difficult to hear and hard to track down. But here is what is bugging me about your original plots in post #3. At 40 Hz, the measurement is 72 dB +/- 2dB dB, and it looks very regular, not random, like a steady vibration at that frequency. An acoustical noise source, in order to change your measurement by 2 dB, would have to be at around 65 or 66 dB or so. And at 40 Hz, you should be able to hear that.


Possibly room / cabinet vibration or something like that?



> *Try this first:* With your mic connected to make measurements, bury your mic in pillows, completely surrounded isolated as much as possible, use soft pillows, blankets, whatever, so it is completely smothered and isolated as much as possible. Now listen to your sound card's output with some headphones that have good LF response - most do these days:bigsmile: - not through speakers! - start out with the headphones just off your ears in case something loud happens, and carefully turn up the mic gain and the headphone output gain, all the way if you have to, listening for any kind of LF noise. There is a kind of electronic noise called popcorn noise that occurs sometimes in transistors that are going bad, and it sounds like - you guessed it - popcorn, although all at low frequencies so it will sound like giant muffled popcorn pops, maybe just once every few seconds, maybe more often, pretty random. LIsten for that or any other kind of LF noise that could be indication of an electronic noise problem. You can be watching the RTA screen at the same time for cues. Acoustical noise at low frequencies could still get through the pillows, but should be reduced. If you get something with this test, then the other tests above could help you track it down to mic or sound card.


Unfortunately i don't have any headphones or anyone i could borrow some?



> Hope that helps. Good luck.


Yes thank you i will try as much as i can.

Thanks again


----------



## weverb (Aug 15, 2008)

Silly question. Is there an a/c vent or ceiling fan very close to your measurement spot? I noticed some very lf changes in my RTA graphs when the a/c was on. I have a vent very close above my listening spot. :dontknow:


----------



## AudiocRaver (Jun 6, 2012)

weverb said:


> Silly question. Is there an a/c vent or ceiling fan very close to your measurement spot? I noticed some very lf changes in my RTA graphs when the a/c was on. I have a vent very close above my listening spot. :dontknow:


A very GOOD question, that could cause a lot of LF noise, too.


----------



## AudiocRaver (Jun 6, 2012)

Phillips, I threw a lot at you in the earlier post. You might start with the "mic buried in pillows" test with the mic in the next room, door closed (so no feedback), listening through speakers while watching the RTA display, for any LF electronic noise. Then at least that question can be eliminated.


----------



## Phillips (Aug 12, 2011)

weverb said:


> Silly question. Is there an a/c vent or ceiling fan very close to your measurement spot? I noticed some very lf changes in my RTA graphs when the a/c was on. I have a vent very close above my listening spot. :dontknow:


Thanks for your reply

No but i do have a Dehumidifier which was turned off at the wall.

Thanks again


----------



## Gaugster (Nov 6, 2013)

Have a concern with my 1st set of bass measurements. Saw this thread and the description may match the quirk that I noticed?

Under a certain condition, it seems that the output signal slowly ramps up to the predefined level. See below images for the progression. Output set to start at 5Hz @ -20dB. However I was getting various readings at these subsonic starting points. I am using REW V5.0 2142 with a Dayton UMM-6 with Cross Spectrum Calibration file.

It only happens when I do a "Check Level" step followed by the first "Start Measuring". This was my initial procedure first time through the software. At the time I did not notice that the output was not immediately at the set point level of -20dB. Repeat measurements did not have this issue from what I remember. Poked around with REW without the connections and was able to confirm this quirk.

On the other hand - This "soft start" quirk could be a "feature" if the initial level is set too high by mistake. Could have saved a lot of subs from a XMAX death?

























Below has other variations too but I was gathering SPL data for multiple seating positions so not unexpected. At first I was thinking that perhaps the floor shake was getting coupled into the mic/stand at a certain mic position.


----------



## Phillips (Aug 12, 2011)

Down in those low frequencies that looks ok, could be a few things Mine were happening higher.

Matter of interest you have the length set at 1M is there a reason for this?


----------



## Gaugster (Nov 6, 2013)

Phillips said:


> Down in those low frequencies that looks ok, could be a few things Mine were happening higher.
> 
> Matter of interest you have the length set at 1M is there a reason for this?


My issue is definatly related to the beginning frequency which could be set anywhere.

No real reason for the 1M data length setting. During setup I read that a higher setting could load down the computer. However I am using a fairly new laptop and don't noctice any delay or lag. Also was watching CPU/memory resources and no change from idle really. Higher settings are also suppose to reduce the noise floor or something. However that should not matter in this case.


----------



## AudiocRaver (Jun 6, 2012)

Gaugster said:


> My issue is definatly related to the beginning frequency which could be set anywhere.
> 
> No real reason for the 1M data length setting. During setup I read that a higher setting could load down the computer. However I am using a fairly new laptop and don't noctice any delay or lag. Also was watching CPU/memory resources and no change from idle really. Higher settings are also suppose to reduce the noise floor or something. However that should not matter in this case.


Everything said above is correct, AFAIK. I stick with standard length sweeps so there is a shorter opportunity window for an interrupting background noise, but that's just me & my environment.:hsd:

Haven't played with the beginning frequency thing. Noticed, slightly annoyed, thereafter ignored as VLF noise - where it matters, I take a few sweeps & use one that starts in the middle of the pack. Interesting.


----------



## Gaugster (Nov 6, 2013)

AudiocRaver said:


> Haven't played with the begining frequency thing. Noticed, slightly annoyed, thereafter ignored as VLF noise - where it matters, I take a few sweeps & use one that starts in the middle of the pack. Interesting.


I had two experiments going. First was to measure a single sub output at a central listening position while I turned the box/terminus to different positions. Observing if the output changes in any way. It was then that I noticed some inconsistency in the subsonic frequency. Result was that two out four positions worked better. So it would seem my sub is not exactly a point source. No surprise.

Secondly I moved the mic to six different listening positions and recorded the SPL. Next I plan to keep the sub/mic position the same and add some stuffing to the enclosure and observe SPL to see if there is any impact.


----------



## AudiocRaver (Jun 6, 2012)

The varying-beginning-of-the-measurement thing could be annoying doing subwoofer measurements.

I will experiment with it later, maybe PM John for insight.


----------

