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Speaker Measurement Thread

Jeffery,

I'm very impressed with HOLMImpulse. However, it's probably not the best way to start learning this stuff. For one thing, it's fairly sophisticated in its approach and is challenging even to very experienced users. Also, being new, the program is under-documented and not bug-free. To get going, I recommend something simple, such as the free version of TrueRTA.

http://www.trueaudio.com/rta_abt1.htm

This will allow you to see intuitive results on your screen in real time. You can play around with mic placement, correlate what you see with what you hear, try different signals, etc. Speaker measurement is a very complex subject, and few "experts" fully agree on the ideal methodology. So, it might be optimistic to expect a clear-cut approach to emerge on AK... But, questions are welcomed!

-k




Jeffery said:
I got the MicMate Pro today, hooked it up and was able to see the "Record Meter" moving in HOLMImpulse. So I ran a test thru my computer speakers and got a graph, but I really have no idea of what all the various settings are supposed to be set to.

A "HOLMImpulse for Total Idiots" thread would sure be nice, I just hate to clog up this forum with literally 1000 questions. The thread on DIY seems to be more for people who are already adept at running measurement software, and the user manual skips over the mundane things that I guess people taking measurements take for granted.

Wear earplugs. Even the blast from my computer speakers caused my tinnitus to flare up
Click to expand...
 
This is a fairly well documented project with Speaker Workshop:
http://www.speakerworkshop.com/SW/Project/Main.htm

I'd imagine that you're more interested in acoustic measurements.

One issue here is that to do a good job with speaker design you really want a setup with line level probes and sometimes a jig in order to measure T&S parameters, crossover transfer functions, calibrate the setup, etc. The Wallin jig was designed for use with Speaker Workshop but it is rather unintuitive to use. Something is needed to protect the sound card inputs from overload when probing power amp output signals:
http://mysite.verizon.net/tammie_eric/audio/audua/audua.html
 
Thanks Ken and Pete for your links. I will get them linked from the first page.
Thanks for all of your hard work here Gentleman.:yes:
 
Praxis I/O limitation

I took a quick look at Praxis in demo mode where it can be used as an RTA. I've found a limitation with the program as far as acoustic measurements go in that it cannot use one audio device for output and another for input. Most USB mic preamps are input only - once it is selected for input no other device can be selected for output with Praxis.
I otherwise liked the way that Praxis worked, good averaging, format and other controls. I did see a reduction in the LF noise level when averaging was used.
I did encounter what I think is a bug in that the save restore config did not handle the word width properly - always restored to 24 even when 16 was saved.

The solution is to only use devices that are both input and output such as the E-MU 0404. Or to use a line level I/O device such as the M-Audio Transit with a mic pre in front of it (ART Audio Dual Pre should work) with the analog pre-out going to the M-Audio Transit line in. This has the advantage that the line in can be used for other measurements such as T&S - be sure to use probes with overload protection.

I'm going to try ARTA next to see how it handles I/O and if it gets around this issue.
 
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Correction: ARTA is free in demo mode only where overlays cannot be saved but measurement settings
and measurement data can be saved. Overlays allow previous measurements to be compared and this
is not too bad of a limitation. There is pricing at $79 and $149 Euros for the full version.
ARTA has what looks to be a decent manual for those who need some guidance with these tools:
http://www.fesb.hr/~mateljan/arta/download.htm

The program seems to work well and is written by:
Ivo Mateljan
Electroacoustic Laboratory, Faculty of Electrical Engineering, 21000 Split, Croatia.
Who also publishes in the area of acoustics.

The features are on a more professional level than most of these other tools; just a few from the web page:
The ARTA has functions of following measurement systems:

Impulse response measurement system with signal generators: periodic white noise, periodic pink noise, MLS, linear and logarithmic swept-sine.
Dual channel Fourier analyzer with signal generators: white noise, pink noise, periodic white noise and periodic pink noise.
Single channel Fourier analyzer with signal generators: periodic white noise and periodic pink noise.
Spectrum, octave band and THD analyzer with signal generators: sine, two sine, multitone, white noise, pink noise, periodic white noise and periodic pink noise.
Triggered storage scope with gated spectrum analysis and Short-time Fourier transform.
Two-channel voltage level meter and third octave analyzer


With calibrated microphone, ARTA can be used as virtual IEC class 1 SPL meter with real time modes:

Integrating SPL meter with 24 hours data logging,
Octave SPL meter with noise rating (NR, NC, PNC, RC, NCB),
Third octave SPL meter with report of specific loudness, loudness in sone and loudness level in phone.


The ARTA is also a powerfull analyzer of:

Gated frequency response,
Smoothed frequency response (in 1/n-octave bands),
Step response,
Impulse response envelope (ETC – curve),
Cumulative spectrum and burst decay waterfall curves and sonograms,
Energy decay in reverberant environments,
Room acoustical parameters,
Speech intelligibility measures: MTF, STI, RASTI, %AL
Loudspeaker directivity pattern
​

The help system provides extensive background on many of these measurements and might help those
who are just getting started - just noticed that it is the same as the user manual previously linked.

It is interesting that the main window provides frequency response, and spectrum analysis
views, but the RTA has its own window that cannot be resized. The RTA also only provides one
and 1/3 octave resolutions. It does offer a user defined integration time.

This software is in the running for a free, (slightly limited as stated above as far as saving data for overlays),
windowed and compliant measurement system. I'll probably also supplement it with the free demo
version of Praxis which has a fully functional RTA with up to 1/48 th octave resolution and averaging.
I'll be curious to see if ARTA will work as a replacement for LAUD which requires special hardware
and an ISA slot. I won't mind paying the small fee if it does.

Ken, I'd be curious to hear what you think of it.
 
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ken kantor said:
Pete,

To my way of thinking, a room doesn't have a single, stable response. The room response varies substantially with integration time. The easiest way to see this is to measure a room with pink noise, then measure it with a chirp, then with a slowly swept sine. Same room, but very different response curves will result. Using something like REW to investigate room response means accepting an integration time and window that someone else has arbitrarily decided is correct. It's like those old, rack-mount "auto eq" boxes that flattened out the 1/3-octave pink noise bands... universally unlistenable results.

Plus, if you can't window things, you can't easily isolate which room features are causing which response aberrations.

-k
Click to expand...

Ah, now I see where you're coming from well that is a highly advanced view that might be important if you were doing a research paper, or had a lot of control over the speaker design and/or room. My intent was to work with finished speakers mainly working with placement, minimal EQ, and perhaps minor room treatment. I think many of us are able to look at the unwindowed response and at least get the big picture of response irregularities caused by room modes, reflections etc. I don't think the general audio world is ready for complex cepstrum transforms and quefrency analysis:
http://www.libinst.com/cepst.htm

I'm just trying to get the equivalent of a traditional RTA system so that I can see the effects of minor room treatment and EQ - I am aware and have already debated the limitations with simple EQ. If the system also supports a windowed measurement mode that is fine also and I might use it for this analysis.
 
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Hi Pete.

Would you mind putting your thoughts in the post above for ARTA as you acquire them and then I will link your post?


OOPS. I posted to late. The second one above.!!
Thanks Junior.:yes:
 
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ken kantor said:
Pete, I know and respect your background. Just trying to make things clear for all readers here. The first sentence of your first citation says it all:

"Signal averaging is a signal processing technique applied in the time domain..."

The operative words here are, "time domain." Noise mitigation via signal averaging cannot be done in the frequency domain.

-k
Click to expand...

Yes, however such statements are based on certain assumptions about the nature of the system, signals, and noise. One might say that 60 Hz interference does not meet the requirement as a "noise" source and it may or may not be correlated with the sample interval. That article suggests a way to maintain the stimulus as synchronous with the sample interval but force a 60 Hz "noise" component to be asynchronous and therefore to be mitigated through averaging. There are many exceptions or ways to force exceptions to the basic assumptions for signal averaging. But I agree that this is in the time domain.
I'll just mention for those not familiar with MLS methods, that MLS measurements are by nature synchronous in that the pseudo random MLS sequence is a unique "key" that is "matched" in the analysis phase when the analysis transforms are performed. I'd expect these RTA systems to use pseudo random pink noise so that every stimulus is the same, and since the random sequence is known it is possible to make all the measurement trials synchronous for averaging. Not sure how each of these different programs handles averaging would have to look into it more.

My observations on the noise with these mics below 100Hz, is that it is what I think is popcorn noise. This is on top of the usual broadband noise, and 1/f noise. I'm wondering if you see the same thing when you look at the noise floor of an ECM8000 in say an RTA view without averaging? I see 10 to 20 dB short term peaks in the noise around and below 50 Hz and these peaks are narrow band limited usually to one band. I see the noise floor drop about 20 dB if I turn off the power to the Mic and then look at the mic pre-amp noise - which is to be expected as I understand it with these mics.

I see the same thing, with battery phantom power, MXL Mic Mate power, and a Rolls phantom power source - and therefore do not think it is from the phantom power source.
 
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ART USB Dual Pre - Project Series

This is a low cost, about $79, stereo USB mic pre with XLR inputs and phantom power.
It is a 16bit 44.1/48 kHz only device and can be powered by the USB interface, a 9V
battery, or a wall wart (not included).

A quick glance at the literature suggests that it is an input only device, but it does have an output
device that drives the monitor and headphone outputs. These outputs can be panned between
the live analog mic signals coming through the box, or the digital audio feed from the PC.

Taking a look inside the DAC is a TI PCM2902 which is not recommended for new designs:
http://focus.ti.com/docs/prod/folders/print/pcm2902.html

There is a TI 8 pin TPA6111A2 headphone amp:
http://focus-webapps.ti.com/general...nid=HYZ4EHZBT0DTTQC1JAWBF2Q?partNumber=6111A2

ST micro SO14 924I which is probably a TS924I OP amp:
http://www.st.com/stonline/stappl/p...ClickOfOPNLink&code=28589&producttype=product

ST Micro 339 quad comparator for the clip and signal indicators.

Fairchild 74HC240 Octal inverting buffer probably for the voltage octoupler to multiply
the 5V to over 30V for the phantom power. There is a long array of diodes (14) and
caps nearby for the multiplier.

ST Micro TS974 Very low noise OP Amp:
http://www.st.com/stonline/stappl/p...ClickOfOPNLink&code=28635&producttype=product

There are many JunFu 105deg C electrolytic caps around the board.

The open circuit phantom voltage is 41.6 V, and 31.5V loaded with an ECM8000. The
ECM8000 is rated to work with phantom voltage from 15 to 48V. I was hoping to improve
the phantom power supply, but it is not worth the risk given the surface mount construction.

It works with the generic USB audio drivers and worked in Praxis for both input and output
just by selecting the USB Audio device.
 
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Pete B said:
This is a low cost, about $79, stereo USB mic pre with XLR inputs and phantom power. It is a 16bit 44.1/48 kHz only device and can be powered by the USB interface, a 9V battery, or a wall wart (not included).

A quick glance at the literature suggests that it is an input only device, but it does have an output device that drives the monitor and headphone outputs. These outputs can be panned between the live analog mic signals coming through the box, or the digital audio feed from the PC. It works with the generic USB audio drivers and worked in Praxis for both input and output
just by selecting the USB Audio device.
Click to expand...
Thanks, Pete. Working fine here with HOLMImpulse, too.

No drivers to download or anything, plug-'n'-play so far.

$79 ART USB pre + $48 PE Dayton EMM-6 + $20 15-foot XLR mic cable + $50 DR Pro tripod mic stand w/boom = $197.

http://www.amazon.com/DR-Pro-Tripod-Boom-Stand/dp/B0018TG8MQ

Print and take to Guitar Center to get this price on the stand; their "Guaranteed Lowest" price is $10 more:

http://www.guitarcenter.com/DR-Pro-Tripod-Mic-Boom-Stand-101810125-i1170322.gc

OR, watch for them to go on sale for $35.... :tongue:
 
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Pete B said:
These educational papers are a bit difficult to find at the liberty Audio web site:
http://www.libinst.com/technical.htm

This paper covers MLS methods which are common to most of the impulse type measurement systems:
http://www.libinst.com/mlsmeas.htm
Click to expand...


Just FYI, I posted data on the MXL and ECM-8000 the other day:

http://www.audiokarma.org/forums/showthread.php?t=279240


Also, some trivia: the use of Maximum Length Sequences in audio was first introduced by Jeff Barish (ne' Borish) in 1985.

http://www.aes.org/e-lib/browse.cfm?elib=4459


We also did a signal processing project together some time later. He's an amazingly smart guy.

http://www.google.com/patents?id=mp...rce=gbs_overview_r&cad=0#v=onepage&q=&f=false

-k
 
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I need to study your post more carefully, so I am sure I understand it, then I need to re-test the mics with more attention to the noise. I can compare them with known good units to get a handle on things. Should be easy to do by substitution, when operating below a few 100 Hz. Maybe this weekend, I can find time to try various analyzers, too, to see how quiet they are. But, I suspect as you do, that this noise sources are in the mic.






Pete B said:
Yes, however such statements are based on certain assumptions about the nature of the system, signals, and noise. One might say that 60 Hz interference does not meet the requirement as a "noise" source and it may or may not be correlated with the sample interval. That article suggests a way to maintain the stimulus as synchronous with the sample interval but force a 60 Hz "noise" component to be asynchronous and therefore to be mitigated through averaging. There are many exceptions or ways to force exceptions to the basic assumptions for signal averaging. But I agree that this is in the time domain.
I'll just mention for those not familiar with MLS methods, that MLS measurements are by nature synchronous in that the pseudo random MLS sequence is a unique "key" that is "matched" in the analysis phase when the analysis transforms are performed. I'd expect these RTA systems to use pseudo random pink noise so that every stimulus is the same, and since the random sequence is known it is possible to make all the measurement trials synchronous for averaging. Not sure how each of these different programs handles averaging would have to look into it more.

My observations on the noise with these mics below 100Hz, is that it is what I think is popcorn noise. This is on top of the usual broadband noise, and 1/f noise. I'm wondering if you see the same thing when you look at the noise floor of an ECM8000 in say an RTA view without averaging? I see 10 to 20 dB short term peaks in the noise around and below 50 Hz and these peaks are narrow band limited usually to one band. I see the noise floor drop about 20 dB if I turn off the power to the Mic and then look at the mic pre-amp noise - which is to be expected as I understand it with these mics.

I see the same thing, with battery phantom power, MXL Mic Mate power, and a Rolls phantom power source - and therefore do not think it is from the phantom power source.
Click to expand...
 
I need to study your post more carefully, so I am sure I understand it, then I need to re-test the mics with more attention to the noise. I can compare them with known good units to get a handle on things. Should be easy to do by substitution, when operating below a few 100 Hz. Maybe this weekend, I can find time to try various analyzers, too, to see how quiet they are. But, I suspect as you do, that these noise sources are in the mic.

I'll ask a friend if he can pour some concrete into large mic road case I have. I think, with judicious of foam, fiberglass and concrete, we could make a mic chamber that is <5bBA, at least above a few Hz. I want to try!!!

I can also make a fake mike, with the capsule replaced by a controllable uV source. That could be fun.

-k

PS- can you tell I have a lot of really work to do this weekend......:(.





Pete B said:
Yes, however such statements are based on certain assumptions about the nature of the system, signals, and noise. One might say that 60 Hz interference does not meet the requirement as a "noise" source and it may or may not be correlated with the sample interval. That article suggests a way to maintain the stimulus as synchronous with the sample interval but force a 60 Hz "noise" component to be asynchronous and therefore to be mitigated through averaging. There are many exceptions or ways to force exceptions to the basic assumptions for signal averaging. But I agree that this is in the time domain.
I'll just mention for those not familiar with MLS methods, that MLS measurements are by nature synchronous in that the pseudo random MLS sequence is a unique "key" that is "matched" in the analysis phase when the analysis transforms are performed. I'd expect these RTA systems to use pseudo random pink noise so that every stimulus is the same, and since the random sequence is known it is possible to make all the measurement trials synchronous for averaging. Not sure how each of these different programs handles averaging would have to look into it more.

My observations on the noise with these mics below 100Hz, is that it is what I think is popcorn noise. This is on top of the usual broadband noise, and 1/f noise. I'm wondering if you see the same thing when you look at the noise floor of an ECM8000 in say an RTA view without averaging? I see 10 to 20 dB short term peaks in the noise around and below 50 Hz and these peaks are narrow band limited usually to one band. I see the noise floor drop about 20 dB if I turn off the power to the Mic and then look at the mic pre-amp noise - which is to be expected as I understand it with these mics.

I see the same thing, with battery phantom power, MXL Mic Mate power, and a Rolls phantom power source - and therefore do not think it is from the phantom power source.
Click to expand...
 
Concrete in a road case! I was thinking about a chamber but, hmmm that's a bit extreme!
I did try stuffing it under pillows in the sofa here.
I'll have to think about what I might use for a chamber here ...

Well, no rush on this. Electret mics are known to be noisy as I'm sure you know.
For the group:
The best mics are 5-7 dBA, most high end electrets are around 20, and these
cheap units are about 30 - 35 as I understand it.
 
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I actually prefer independent threads - measurement for room optimization, another measurement for design, and finally software for box design. These thread otherwise get long and difficult to navigate.
 
OK I will start some other threads in the next few days. I will link to them from my first post here so they will be under the sticky. Pete if you or anyone else starts one let me know and I will link to it as well.

Sound good?
 
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