DIY measurement mics

[thadman]

I'm interested in assembling a measurement setup to serve dual purpose as a loudspeaker design tool and as a tool to optimally setup loudspeakers/subwoofers in rooms.

I have a Macbook Pro and plan on combining the Apogee Duet external firewire soundcard with Fuzzmeasure Pro acoustic software.

The last component of the measurement setup to be determined is the mic, of which there are several options.

Behringer ECM8000. Apparently Panasonic stopped producing the capsules for these microphones and they no longer include the previous output transformer. I haven't heard a lot of positive information over on gearslutz about these mics.

Earthworks M23/M30. Expensive ($500 for the M23), but nearly perfect omni mics.

Calibrated DIY? Would it be possible to DIY a mic that was equivalent to the M30 or even M50 for reasonable cost? Upon completion of the mic it would be calibrated of course. I know capsules are available, but are the bodies? Assuming bodies can be sourced, this begs the question ... which capsule to choose?

Has anybody completed a DIY mic kit? Was it worth it?

Thanks,
Thadman

[JonP]

See a recent thread here, and search for others in the past...

The ECM8000 probably was a flatter product in the past, with the Panasonic capsules. I don't think they ever had an output transformer.

They or any mic can be calibrated... or more accurately, measured and a response file made that gets put into the software you use to cancel out the nonlinearities.

Kim Girardin (sp?) has been doing this for years, will cal your sent mic, or he used to have pre-measured Panasonic capsules for sale. About $40-50. Search him up.

There's another guy as well, see the link that was recently brought to life.

As for making your own whole mic, there was a site or two out there on packaging a mic capsule. Typical thing was to find the size of copper tubing that snugly fits the little Panasonic type capsule, mount it in the end, and provide a wider body at the other end to fit in a standard mic stand clamp. The thin tube/wand makes for a low reflection mic.

With the ability to get a mic calibrated, the need for a ultra flat (and expensive) mic isn't really there...

[ch83575]

With the ability to get a mic calibrated, the need for a ultra flat (and expensive) mic isn't really there...

This is true with the notable exception of doing distortion measurements. I have a DIY mic, and I am coming to realize that as I go up in SPL a larger and larger portion of the distortion I measure is from the mic, not the driver. This is where the high quality commercial mics are desirable. I do not know how the ECM8000 holds up to my mic/pre in terms of high SPL.

My mic is just a panasonic capsule in a wand as described above. I use it with a preamp designed by Eric Wallin. It gets the job done. If I were building another I would use the Linkwitz capsule mod and the Elliot Sound mic driver. and any high quality mic pre (there are a few on the ESP site that look good). It is a lot of work to do all that. That combo I would guess would get you well out of the ECM8000 range and into sight of the Earthworks M30, but that would just be a guess from what I have gathered on the internet.

-Chad

[Bukem]

Hi Thadman,

I'm using a Macbook with Fuzzmeasure, going into a TC Electronic Konnekt 24D firewire sound card. I am using an Audix TR-40 mic which has been calibrated in Germany by IBF Akustik.

It measured +/- 1.2 db from 10hz-20Khz or +/- 0.9db between 12.5 and 18Khz. It's pretty flat.

Rgds,

Bukem

[thadman]

With the ability to get a mic calibrated, the need for a ultra flat (and expensive) mic isn't really there...

Is there really no point in going with a premium mic since calibration is readily available? What about beyond normal operation, such as high SPLs? How loud of a sound can a Behringer ecm8000 measure while still being objectively valid (frequency response and distortion)? Is compression a real issue with microphones?

FYI I can't imagine testing anything over 120dB @ 2m, although measuring at short distances may appreciably increase the sound pressure level experienced by the microphone.

[JonMarsh]

when doing testing at 1 watt input power levels, it's trivial to hit levels well over 100 dB in nearfield testing.

The input dynamic range of true instrumentation microphones is in a different class from electret capsules.

Pick your poison and what you're willing to pay, based on what you think you need to do. I've had a B&K 4144 for almost 30 years, which stays in Denver now at ThomasW's; I use an ACO 7012 at home and for travel.

[JonP]

Is there really no point in going with a premium mic since calibration is readily available? What about beyond normal operation, such as high SPLs? How loud of a sound can a Behringer ecm8000 measure while still being objectively valid (frequency response and distortion)? Is compression a real issue with microphones?

FYI I can't imagine testing anything over 120dB @ 2m, although measuring at short distances may appreciably increase the sound pressure level experienced by the microphone.

Chad's point is well taken... Jon's as well. I should have qualified my statement as only applying to the flat frequency response requirement...

In another recent thread, the distortion thing comes up, and some comparisons were made between a "known" high SPL capable mic and a ECM8000. There was a larger and increasing distortion at high levels on the ECM8K. To be expected... we are, after all, talking about a $2 capsule vs some $hundreds of test mic.

Doing the "Linkwitz mod" on the capsule should gain you some SPL handling, which should reflect in lower distortion at a given high SPL. Notice the use of the word "should". I'm not sure if you can do it on all recent capsules, it was easy on the now defunct WM-60a.

It would be nice if someone with access to both would do some before and after mod testing to see if the distortion threshold goes up and by how much... for all us cheapskates...

[ch83575]

It would be nice if someone with access to both would do some before and after mod testing to see if the distortion threshold goes up and by how much... for all us cheapskates...

Its on my list... unfortunately it is a very long list, and this one isn't at the top. I was personally never very good at doing the Linkwitz mod. I toasted 5-6 capsules before giving up. Cutting the trace is easy, but soldering the 3rd tab to the case is very difficult as they are not in contact, you actually have to bridge .5-.75mm of insulation with your solder. The physical mod increases the range of the capsule for sure, but I am just as interested in the benefit of the ESP mic driver linked to in my previous post. It is balanced, so I don't need to keep the preamp within a few feet of the mic anymore. I imagine that having something like that directly attached to the capsule (only a few inches of wire) will have an effect on distortion and dynamic range as well. I really think it would make a great project, and I already have the bare capsule setup to compare it to when the time comes.

-Chad

[BretH]

What is the best source for someone wanting a small number of Panasonic capsules - 5 or so?

[Curt C]

The WM-61A was touted as a replacement for the WM-60A's, but I've never used them. The price is right, tho...

C

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=WM-61A102A-ND

[Dennis H]

Cutting the trace is easy, but soldering the 3rd tab to the case is very difficult as they are not in contact, you actually have to bridge .5-.75mm of insulation with your solder.

Somebody recommended using conductive epoxy. I wonder how much R between the contacts would be acceptable? I googled conductive epoxy and you could buy stuff for a reasonable price that had more resistivity than the really good stuff which was expensive and I didn't see anyone selling it in small quantities.

[Amphiprion]

Here's the link for the Panasonic if you don't want to buy them in a pack of 1000

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P9925-ND

Praxis has the schematic for their AudPod posted fyi. I sketched out the circuit for the mic the other day, it's very simple. 4V phantom power for the electret condenser from an LDO reg off the USB power and an OPA2340 with rails of +/- 3.1V fed from 431 shunt regulators. Negative rail is generated by a low noise inverting switching regulator.

CORRECTION!!! I looked at the data sheet for the OPA2340 and it WILL NOT WORK at the rail voltages specified in the AudPod schematic. It will only handle +/-2.5V operating rails (single supply operation spec'd at 5V, max 5.5V). I opened my AudPod and it is using the LMC662 op amp at +/- 3.1V. Bill must have made a change and not noted it on the schematic. The 2340 will not work unless you reconfigure the 431 shunt regulator resistor values to output +/-2.5V.

[JonP]

There is a Yahoo group called Micbuilders: http://tech.groups.yahoo.com/group/micbuilders/ that I've semi followed for a few years.. The DIY crowd of microphones. Lots of cool stuff, and several sites with much info for "us" in the basic measurment mic catagory...

I seem to recall that using aspirin as a soldering flux (!) you can have good luck at soldering aluminum mic bodies. Haven't tried that one myself... A few dozen ohms won't be a big deal, so if it's between that and one or two ohms with the $$ stuff, I wouldn't worry about it. A few hundred might be though... so I'd buy some cheap epoxy and measure a few test joints first...

Something I just stumbled across again, that I had downloaded a while ago from the Arta site, might be interesting to this discussion... how to make a DIY mic calibration chamber. Only good for below 500hz or so, but still, could be a handy thing, especially since it's so easy to make...
http://www.fesb.hr/~mateljan/arta/Ap...r-Rev03Eng.pdf Rather cool little project... would also be just the thing to investigate mic distortion testing, another current question that has many inquiring minds wondering the answers...

And Mark... you might want to look at a wider range opamp, check out the National LM4562/LME49720. Killer, I mean really killer... audio specs, FET high impedance input, low V and I offsets, strong output to drive low impedance lines... should be a good mic preamp chip, and it's good to +-18V.

Another thing on the Panasonic capsules... Kim Girardin used to (as of a couple years back) offer pre calibrated capsules for $40 or so, as well as his calibrate your mic service for about the same price. Have no idea if this is still the case, dig back into the thread with his email to find out...