Measurement Microphone Suggestions

aether_nz · 25th January 2012

Hello, I'm a student looking to build an impedance tube setup and need some expert knowledge on the microphones. For those unfamiliar with impedance tube they basically setup a standing wave along the length of the tube and the pressure is measured at various locations which in turn allows you to calculate the acoustic impedance at the sample end. Here is a pic of a typical setup:

[img]******//www.me.mtu.edu/courses/meem4704/images/impedance_tube.jpg[/img]

I am currently using very cheap microphones (<$10) as a quick mock up and they are proving quite unreliable. My budget would preferably be in the few hundred dollar range, the less the better

For this application:
-the smaller the width the better (ie 1/2" or 1/4")
-unidirectional, the only pressure I'm interested in is from the plane waves at the surface of the tube.
-I am using measurement equipment that has a maximum input voltage of +-7 V @ 50 ohms, so I'm not sure which power system to use. Currently with the cheap electret mics, I am just using a simple op amp pre-amplifier. Can I get away with not having a high voltage phantom power setup?
-Flat frequency response is not so important as I will be measuring relative changes at specific frequencies. My biggest need is for them to be highly repeatable so to have very small changes with temperature and a linear response with amplitude.

Could you suggest any microphones that you have used or know of that could perform this task well? Thanks for your time!

Les · 27th January 2012

Hi

For impedance tube measurements I would suggest a purely pressure transducer. A unidirectional one measures a combination of pressure (p)
and pressure gradient (dp/dt) in a complex manner and is a complicating factor.

Linkwitz modded (internal fet converted to source follower) panasonic type
electret capsules would be ok if their relative sensitivity was calculated with a common sound source. They are not typically environmentally stable.

Better might be the new Knowles MEMs ultrasonic capable chip mic...it's tiny and has good response from about 100 to 75KHz. It's cheap too.

It would be good with either one to do a pistonphone calibration for absolute
pressure sensitivity if you can find or borrow one.

You can also do absolute calibration by reciprocity but you need reversible tranducers. Neither of the ones I mention are, which is why a pistonphone would be good to have.

If you are doing transfer function method I think just relative sensitivity is enough. In that case you can calibrate with known load (like infinite impedance) and not worry about the stuff I mentioned above.

aether_nz · 30th January 2012

Thanks for your reply. Upon further research I was mistaken by the function of unidirectional microphones. Omnidirectional sounds like what I'm after.

I was also looking at these types of mics RTA-M :: dbx® Professional Products . Would they be more stable that the cheap electret capsules or would they use similar quality mics?

The Knowles MEMs ultrasonic capable chip mics look interesting. I am after the microphone with the most repeatable/stable properties which could come from the precision of a SMD chip, although it might be tough to install flush on the inside of the impedance tube.

The pistonphone suggestion also sounds good for calibration, I'll look into getting one.

Cheers.

25th January 2012	#1
aether_nz Gear interested Joined: Jan 2012 Posts: 2 Thread Starter	Measurement Microphone Suggestions Hello, I'm a student looking to build an impedance tube setup and need some expert knowledge on the microphones. For those unfamiliar with impedance tube they basically setup a standing wave along the length of the tube and the pressure is measured at various locations which in turn allows you to calculate the acoustic impedance at the sample end. Here is a pic of a typical setup: [img]******//www.me.mtu.edu/courses/meem4704/images/impedance_tube.jpg[/img] I am currently using very cheap microphones (<$10) as a quick mock up and they are proving quite unreliable. My budget would preferably be in the few hundred dollar range, the less the better For this application: -the smaller the width the better (ie 1/2" or 1/4") -unidirectional, the only pressure I'm interested in is from the plane waves at the surface of the tube. -I am using measurement equipment that has a maximum input voltage of +-7 V @ 50 ohms, so I'm not sure which power system to use. Currently with the cheap electret mics, I am just using a simple op amp pre-amplifier. Can I get away with not having a high voltage phantom power setup? -Flat frequency response is not so important as I will be measuring relative changes at specific frequencies. My biggest need is for them to be highly repeatable so to have very small changes with temperature and a linear response with amplitude. Could you suggest any microphones that you have used or know of that could perform this task well? Thanks for your time!

27th January 2012	#2
Les Gear addict Joined: Feb 2008 Location: Tiger, Ga Posts: 475	Hi For impedance tube measurements I would suggest a purely pressure transducer. A unidirectional one measures a combination of pressure (p) and pressure gradient (dp/dt) in a complex manner and is a complicating factor. Linkwitz modded (internal fet converted to source follower) panasonic type electret capsules would be ok if their relative sensitivity was calculated with a common sound source. They are not typically environmentally stable. Better might be the new Knowles MEMs ultrasonic capable chip mic...it's tiny and has good response from about 100 to 75KHz. It's cheap too. It would be good with either one to do a pistonphone calibration for absolute pressure sensitivity if you can find or borrow one. You can also do absolute calibration by reciprocity but you need reversible tranducers. Neither of the ones I mention are, which is why a pistonphone would be good to have. If you are doing transfer function method I think just relative sensitivity is enough. In that case you can calibrate with known load (like infinite impedance) and not worry about the stuff I mentioned above. __________________ Les L M Watts Technology

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30th January 2012	#3
aether_nz Gear interested Joined: Jan 2012 Posts: 2 Thread Starter	Thanks for your reply. Upon further research I was mistaken by the function of unidirectional microphones. Omnidirectional sounds like what I'm after. I was also looking at these types of mics RTA-M :: dbx® Professional Products . Would they be more stable that the cheap electret capsules or would they use similar quality mics? The Knowles MEMs ultrasonic capable chip mics look interesting. I am after the microphone with the most repeatable/stable properties which could come from the precision of a SMD chip, although it might be tough to install flush on the inside of the impedance tube. The pistonphone suggestion also sounds good for calibration, I'll look into getting one. Cheers.