# Measuring Noise Criteria with REW



## aackthpt (Jan 24, 2011)

I'd like to measure noise criterion, and I'd prefer to use REW to do so if it's possible.

There have been two threads on The Shack about measuring background noise, This one in Jan 2008 and this one in Oct 2008.

It was indicated the best tool was the RTA display, except that I know how to overlay the requirement curves if I use the SPL graphs and I don't think this can be done using the RTA. One of the earlier threads suggested to just run sweeps without the output channel connected which I _can_ get onto the SPL graphs, so is this worse than the RTA result for any reason?

Also, Nyal Mellor recently told me special equipment was needed to do this measurement. In particular the mic (which I had already guessed anyway). Unfortunately I did not find very clear direction on how to figure out the self-noise of my mic. I currently use a Dayton EMM-6 which is pretty typical 'round here. Ratings on it are:
Sensitivity at 1 kHz into 1K ohm: 10mV/Pa (-40 dBV, re. 0 dB = 1V/Pa) 
Max SPL for 1% THD @ 1 kHz: 127 dB
S/N ratio: 70 dB A-weighted
My question, then, is what would the noise level be? If I use the 127-70=57 dB SPL which is not really good enough to do a decent job. However, the REW instructions have you set a 75dB SPL noise signal to be ~-18 dBFS, so full scale is 93 dB and 70dB down would be 23 dB SPL, which suggests that you could reliably measure down to perhaps NC25, and presumably one could set the gain lower and therefore measure lower. However I am not sure I am thinking about this clearly; can anyone lend any insight? I have a difficult time believing this mic can be remotely close to a ~$450 Earthworks M23 which is clearly spec'd at 22dB SPL equivalent A-weighted.

In his "stereo listening rooms" paper, Nyal recommends RC20 as a target for a dedicated room. This would appear to require a system capable of measuring down to ~15dB SPL at 4000 Hz just to measure RC25.

If a quieter mic is required, what are the choices?


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## JohnM (Apr 11, 2006)

The noise floor of the preamp is often more of a limitation than the mic, but can't help you with mic self-noise data, sorry. For noise measurements it would be worth looking at ARTA, which provides NC figures.


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## aackthpt (Jan 24, 2011)

"ARTA Software is classified as "shareware". To use it you have to pay for the user license key. If you have no license key, you can freely use this software only in a demo mode. The demo mode of programs is fully functional except loading and saving of files."

Interestingly, for the simple purpose of measuring noise criteria, one could presumably even avoid buying a license. Thanks for the tip, John, whether or not I buy a license I plan to try it.


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## jtalden (Mar 12, 2009)

aackthpt said:


> ...
> 
> 
> > It was indicated the best tool was the RTA display, except that I know how to overlay the requirement curves if I use the SPL graphs and I don't think this can be done using the RTA.


I may misunderstand your comment here, but once "save" is selected for an RTA measurement it shows up in the list of measurements just as sweeps do. So if a target FR for noise floor is imported they can be overlaid normally.



> One of the earlier threads suggested to just run sweeps without the output channel connected which I _can_ get onto the SPL graphs, so is this worse than the RTA result for any reason?


The noise floor that is measured when no signal is present is a combination of the measurement system noise floor and the room noise floor. I will call this Total Effective Noise Floor (TENF).

The TENF changes with signal type. Sweep measurements lower the TENF by about 10 dB compared to RTA measurements. If you are using a sweep for room measurements then a sweep TENF is appropriate. If you are using the RTA for room measurements then an RTA TENF is appropriate. 
If you are looking for an absolute measurement of the TENF then use the RTA.

As an example, here is the TENF in my room under my best conditions (Sweep Vs RTA):








There is about a 10 dB difference. While not technically accurate, I think of the sweep as being able to look down into the noise floor at the signal and thus it gives an additional 10 dB of SN ratio.

Ramblings:
Let's look just at the RTA curve (since it is the appropriate one for absolute noise values)
After measuring at my LP, I placed the mic on the floor and covered it with lots of pillows and blankets. The isolation is probably reasonably effective for HF, but not at LF.

The curve was changed only slightly:








It actually looks a little worse in the 20-30 Hz and 80-150 Hz ranges otherwise it tracks pretty closely.

The portion that is measuring system vs Room noise is not possible to identify for sure. If I were guessing however, I would say this measuring system noise is being represented above about 400 Hz. It measures just under 30 dB. There is a relatively smooth increase in level below 400 Hz reaching up to about 65 dB at 15 Hz. Since the general trend of the curve is relatively smooth and relatively unchanged by the covering, I would guess the measurement noise floor probably follows somewhere near the red line I placed on the chart below:








I attribute the area above the red line to be actual room noise exceeding my measurement noise.

The above measurement was taken with as much of my noise sources (HVAC, Water Heater, etc) turned off. With the HVAC on, it was much worse as shown below: 








None of this helps at all with any of your main questions, but I thought you might like to see someone else’s measurements.


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## aackthpt (Jan 24, 2011)

jtalden said:


> I may misunderstand your comment here, but once "save" is selected for an RTA measurement it shows up in the list of measurements just as sweeps do. So if a target FR for noise floor is imported they can be overlaid normally.


Excellent, this is definitely the sort of feedback I was looking for. I have never saved an RTA response before so I was not aware that it does this.



> The TENF changes with signal type. Sweep measurements lower the TENF by about 10 dB compared to RTA measurements. If you are using a sweep for room measurements then a sweep TENF is appropriate. If you are using the RTA for room measurements then an RTA TENF is appropriate.
> If you are looking for an absolute measurement of the TENF then use the RTA.


I do use a sweep for room measurements. But (a) I am not really looking for TENF, I am looking for the room noise floor alone (which you probably realized) (b) I do want "absolute measurement" because this is not to compare to my sweeps, it is to compare to the standard curves for noise criterion and noise rating. I've actually already translated the curves into the REW-readable format and expected to publish them to the community once I had a decent method for the whole measurement and a read on how to estimate the system self-noise and knew how to ensure it doesn't interfere with the measurements.

It is awesome to see all your measurements for sure, thanks for putting them up! Judging by the number of labeled measurements you have there, you have definitely done some serious analysis of your room noise, so props for that! :T Your room has a big difference with the HVAC on! Have you taken any measures to reduce the noise from it? I plan to build some mufflers for my outlets (one will sit on the floor, one built into a freestanding riser) but I can't do the real deal since I'm not working on a dedicated room (in this house anyway).

I thought about that mic blanketing procedure, though I thought of building a box with ~24 inches of fluffy fiberglass in every direction around the mic; I figured that might give me a good idea of the system noise at various input chain gain settings, at least above a few hundred hertz. Also, having read your post, I am thinking maybe if I take a measurement with the mic disconnected, or the gain knob turned all the way down, or maybe just phantom power off perhaps that will give an idea of the noise contribution from the preamp without the mic. Again, that mic self-noise that I don't know how to account for!

I wasn't sure from your discussion if you are saying you expect the measurement noise floor to be spectrally flat or curved? I'd kind of think it might be spectrally flat, so (going down in frequency) where it begins to rise is where I'd think the measurement begins to have some significance. I don't really get the idea of expecting the area above the red line to be the room noise.

Wow, OK this explains why I was confused about RC curves this morning. There are THREE scales for noise?? Gah. :unbelievable: :gulp::crying:

Oh well. I guess I'll have to carve out some time to read the ASHRAE paper on this.


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## aackthpt (Jan 24, 2011)

I note that you get a noise floor reading if you get the "premium" calibration package from Cross-Spectrum Labs. Maybe someone who got that could chime in so we could get a feel for what a typical value might be?

Hmm, a CSL mic was on my list, this will probably add haste if typical values are reasonable.


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## Anechoic (Jan 16, 2009)

aackthpt said:


> I note that you get a noise floor reading if you get the "premium" calibration package from Cross-Spectrum Labs. Maybe someone who got that could chime in so we could get a feel for what a typical value might be?
> 
> Hmm, a CSL mic was on my list, this will probably add haste if typical values are reasonable.


A typical noise floor measurement for the Dayton Mic is in the 30 to 34 dBA range. Behringers can vary widely (anywhere from 30 to 45 dBA) but a value of around 36-38 dBA is probably typical.

For contrast, my Type 1 and Type 2 mics (ACO Pacific 7052, PCB 377B02, Rion NA-52, BSWA MP201) have noise floors in the 15-17 dBA range.

Most mics in the ECM8000 class (RTA-420, Nady CM-100, etc) have similar noise floors, you can't get much lower without spending a lot more money.


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## SAC (Dec 3, 2009)

...Not to mention preamps.

An example comparing:

PreSonus Audiobox: noise floors of -73 dBFS* and a frequency response of 20Hz-20kHz +0-6dB with all inputs and outputs exhibiting pin 1 issues.

Duran DAudio: noise floor of -92 dBFS* and a frequency response of 20Hz -20kHz +-0.5dB with all inputs and outputs free of pin1 issues.

Plus the Audiobox exhibited increasing phase variation from 1kHz to 20 Hz while the Duran D-Audio is ruler flat over the entire bandpass.

* Noise floor measurements: 36 dB preamp gain; input terminated with 150 ohm resistor.


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## jtalden (Mar 12, 2009)

aackthpt said:


> I wasn't sure from your discussion if you are saying you expect the measurement noise floor to be spectrally flat or curved? I'd kind of think it might be spectrally flat, so (going down in frequency) where it begins to rise is where I'd think the measurement begins to have some significance. I don't really get the idea of expecting the area above the red line to be the room noise.


Just to be clear, I don't how to clearly separate room noise from the total noise. You have probably done much more reading on that subject than I have. I was only speculating that my measurement system noise may be at the red line that I drew in. I did this only from looking at the nature of these and other measurements I have taken. It could easily be wrong. If it is there then the portion above red line is due to the room noise floor rising above it.

I have no evidence as to whether the noise floor of my mic alone is, or is not, relatively flat. The total measurement system has other components that infuence the result as well as the mic and I think my mic preamp may be adding significant LF noise rise. I've measured the noise floor of the mic preamp before via loopback, but do not have this data at hand now. It showed a significant LF rise in this manner, but I would have to look up that data to so see how it compares with my red line. It's pretty clear the red line could not be any higher than I indicated here, but it could be lower. I would be a little surprised if it is significantly lower however.

I am in a 8000 cu. ft. basement multipurpose room and the HVAC is in an adjoning room. I have not taken any significant steps to mitigate the noise the HVAC creates. That's why it is so bad. The noise can even be a little worse when other appliances kick on also. Maybe some day I will get a proper room, but probably not.


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## aackthpt (Jan 24, 2011)

Thank you Anechoic and SAC, that is exactly the type of data I hoped to find - extremely useful. It sounds the hobbyist mics are not going to be suitable for measuring any sort of purpose-built room or even quieter space.

SAC - do you make the preamp measurements yourself? What do you mean by "pin 1 issues"?

Anechoic - can you give us some idea how the mic self-noise is measured? I have some guesses but I don't think I'm on the right track. Is the noise spectrum measured or just the level, and would flat be a reasonable expectation?



jtalden said:


> [snip] I think my mic preamp may be adding significant LF noise rise. I've measured the noise floor of the mic preamp before via loopback, but do not have this data at hand now. It showed a significant LF rise in this manner, but I would have to look up that data to so see how it compares with my red line.


Thanks... the idea that the preamp may be adding LF noise is definitely interesting.

I need to quiet my HVAC noise but also my projector noise - when the room is warmer that is way intrusive. But this is mostly motivated by a desire to be able to measure all of the criteria set by Mellor and Hedbeck in that paper. And anyway it makes sense to be able to verify success if one builds a "soundproofed" room. It's looking like the answers are (a) spend a nice wad of cash for better gear (b) rent the gear when you need it [if anyone near me does that] (c) hire a pro (d) phone a friend [if any of them have the right level of gear].


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## SAC (Dec 3, 2009)

Pin1 problem.
If you need more info PM me. I can provide schematics for a simple 'sniffer' and process guide.

Pat Brown made the measurements.

I have previously tried to explain the common need for higher quality mics and preamps once one moves beyond simply setting up subs or EQ, but after repeatedly having one of the principles here specifically dismiss the concern (based on more than 25 years actual experience in acoustics and physics data acquisition) and then being warned that my disagreement is not welcome, if you would like to know more, please PM me and I would be glad to discuss it offline.


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## aackthpt (Jan 24, 2011)

SAC said:


> Pin1 problem.
> 
> Pat Brown made the measurements.
> 
> I have previously tried to explain the common need for higher quality mics and preamps once one moves beyond simply setting up subs or EQ[snip]


Thanks for the info, I will have to read more on the pin 1 problem. My current preamp has a plastic housing, so I don't even know if it has a chassis ground as it were.

I've seen the discussions. I'll just stay focused on accomplishing specific tasks; if they require different equipment then I'll discuss what they require (unless it is completely unrelated to REW in which case I will take it elsewhere). I appreciate the offer, and I probably _am_ interested in what tasks for which one would want improved equipment. I believe I already have your contact info in an old PM; I will probably look for you on Skype sometime.


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## SAC (Dec 3, 2009)

The cheapest and best product I know of for precision NC measurements (and MUCH more) is ARTA, available for 79 Euros.

And you are afforded the opportunity, which greatly expands measurement possibilities, to use the two channel capabilities of the pre-amp as well as two mics...


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## aackthpt (Jan 24, 2011)

What does two channel measurements get you the ability to do? Obviously I could make two measurements at once which would actually be very valuable in its halving the time to measure multiple seating positions in my home theatre. Beyond that I'd think maybe some Ando approach, but the extent I know of that is this article by Szymanski so I'm not really sure what this gets me.

I did notice that besides measuring the room noise NR, NC, PNC, RC, NCB it will also measure speech measures MTF, STI, RASTI, %AL in which I am also interested.

No use buying ARTA for speaker measuring stuff (LIMP) which can be done with REW, and the harmonic distortion stuff that the STEPS part does I believe can be done with HOLMImpulse.


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## SAC (Dec 3, 2009)

If you only perused the gross description above, you are missing the actual power and flexibility of ARTA. I would seriously suggest perusing the user manual. Especially as you miss the power that the ability for extensive customization of tests can afford.

Its capabilities go far beyond those afforded by other platforms until you get in the realm of those costing well north of $1000.
Aside from configurable stimulus not limited to the generation of an IR via periodic noise excitation, swept-sine excitation, Maximum Length Sequence (MLS), and triggered signal recording, you are afforded the ability to compare actual input to output, as well as various cross correlative measurements.

One of the most common applications of FFT spectrum analyzers is to measure the transfer function of a mechanical or electrical system. A transfer function is the ratio of the output spectrum to the input spectrum. 

Single-channel analyzers cannot measure transfer functions. Single-channel analyzers with integrated sources can measure transfer functions but only by assuming that the input spectrum of the system is equal to the spectrum of the integrated source. 

In general, to measure a general transfer function, a two-channel analyzer is required. One channel measures the spectrum of the input, the other measures the spectrum of the output, and the analyzer performs a complex division to extract the magnitude and phase of the transfer function. Because the input spectrum is actually measured and divided out, you’re not limited to using a predetermined signal as the input to the system under test—any signal will do.

Also, imagine comparing an input to output of an acoustical system, be it treatments, sound transmission, or any of a myriad possible scenarios where the actual effectiveness of a system is desired comparing the actual stimulus with the resulting signal as modified by some variable… This actual transfer function is fundamental and critical to so many applications! And while single channel FFT assumes a comparison with the ideal, dual channel analysis assesses the actual behavior.

This capability affords an incredible flexibility and utility in SO many applications.

Additionally ARTA has incorporated the ability to perform _“lateral energy fraction (LF) and inter-aural cross-correlation coefficient (IACC) capabilities that correlate well with the subjective quality "spatial impression" in a concert hall. Spatial impression relates to the perception of the sound source width -effect called “spaciousness”, and a state of diffusion of the reverberant sound field - called “listener envelopment “.

Early lateral energy is being measured with two close spaced microphones: omni-directional and bidirectional (with figure of eight directional pattern). Inter-aural cross correlation coefficients are measured with either a dummy head, or a real head, and with two small microphones placed at the entrance to the ear canals…

Although the LF and IACC parameters relate to the same subjective quality, they are not highly correlated in practice. The fact is that LF and IACC emphasize different frequency regions being of importance. LF is primarily measured in the four lowest octaves, 125 Hz, 250 Hz, 500 Hz and 1000 Hz while IACC should rather be measured in the octave bands above 500Hz. IACC values would always be high in the lower octaves, because the distance between the ears (< 30 cm) is small compared to 1/4 of the wave length (. 70 cm at 125 Hz).

Measurement of spatial parameters
To measure spatial parameters a sound system with two microphone input channels is required. For measurement of IACC two small identical microphones must be used at the entrance of the ear channel of a dummy or a real head. For measurement of lateral energy fraction, an omni-directional microphone should be connected to left channel and bidirectional microphone to right channel. Microphones should be calibrated, at least average difference in sensitivity have to be known.”_

The fact is, once you get a taste of dual channel measurements and the power and flexibility they offer, I doubt anyone will voluntarily go back…

Also, as far as LIMP, you might want to read the actual manual as well, as it utilizes the actual source reference in a 2 channel configuration,. It does not assume an ideal uncorrelated source, which affords all sorts of additional accuracy as well as the identification of any non-ideal behavior that would otherwise be missed by assumptions made in the configuration of the experiment/test.

REW does a great job as far as a single channel system can. But a true dual channel system simply adds an entirely new dimension to both testing capabilities as well as the identification of real behavior that is necessarily otherwise ignored.


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## aackthpt (Jan 24, 2011)

Fantastic feedback, thank you SAC. I'll read those manuals (actually, and read the post more completely) then return to make more-informed comments. 

One cool thing, since I am interested in spatial parameter measurements, is that I have also fancied getting a mobile setup to make binaural recordings for which I'd need the same mic set assuming I'm going to provide the dummy head as my own. :bigsmile:


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