Rational Acoustics



merlijnv
August 21st, 2012, 11:22 AM
During a 3 hour drive, this thought came into mind. Do microphones exhibit phase shift? When we make a transfer function of a loudspeaker, we measure at least the combination of the speaker, it’s acoustical environment and the measurement microphone. Because of this, one would most likely prefer a microphone with an equal amplitude or “flat” frequency response in order to neutralize it’s contribution. Which leaves (with the exception of an anechoic chamber) the former two. This made me wonder however, if microphones are also “flat” in the temporal sense. In other words, do microphones exhibit phase shift? The way I see it, the workings of a microphone are more or less the opposite of a loudspeaker. The latter exhibits phase shift quite often if not all the time. If memory serves, the diaphragm of a microphone has physical properties like tension, stiffness and mass. Properties which can be modeled in the electrical domain by means of inductors, capacitors and resistors. Components which are known to exhibit phase shift. Not to mention the actual electronics used in the microphone itself. For all I know, these temporal artifacts, if they exist, fall out of the frequency range of interest, say i.e. 20 Hz to 20 kHz. As far as I can remember, I’ve never witnessed any or little differences in phase, when comparing measurement microphones (using the same speaker at the same distance/time off-set). Which could mean that the microphones themselves don’t alter time. This however is merely an observation not an explanation. In the field of system tuning this shouldn’t pose a problem. As long as we use one or more identical microphones, we’ll be most likely looking for differences with regards to phase instead of absolute values. Any contributions made by the microphone itself in the temporal domain should be equal in all measurements. Your thoughts please?

Regards,

Merlijn

Arthur Skudra
August 21st, 2012, 03:56 PM
Interesting question! I'm sure every microphone out there exhibits some phase shift, some less than others. Attached you will find a frequency response and phase curve for a Bruel & Kjaer measurement microphone, which clearly shows 90 degrees worth of phase shift in the high frequencies! Does it really matter? Probably not! But it certainly presents a case where using the same set of microphones in a multi mic measurement setup might be helpful. I don't think with the present version of Smaart phase is taken into account with the mic correction curves.

556

luigichelli
August 25th, 2012, 10:10 AM
AFAIK only dpa shows some phase response curves... sad but true :(

phasetransitions
September 25th, 2012, 09:33 AM
Merlijn,

As your measurement mic is a bandpass device, just like a loudspeaker, it exhibits phase shifts at the edges of the bandpass as a consequence of being a (primarily) minimum phase device. It is conceivable that the electronics could add even more phase rotation, but the damped mass spring of behavior of the diaphragm as part of a causal system dictates it will exhibit some phase shift, though it may well be far outside the band of human hearing.

Harry Brill Jr.
January 28th, 2013, 11:31 PM
Yes of course it does. It doesn't matter because we are more interested in relative phase of this speaker vs that speaker. As long as the mic doesn't change during the comparison it doesn't matter too much. You can see a difference in phase response between measurement mics when you do a mic compare if the mics have a different response. If they are the same then you will not SEE the phase trace slope. Even with a mic compare it is not possible to see the absolute phase response of a particular measurement mic. I'm sure there is some very expensive lab gear for measuring this.
Any time you have a non-flat magnitude response, you are going to have a non-flat phase response to go with it (see Phil's post).
If you are only using 1 measurement mic then you can pretty much disregard this issue altogether. If you have multiple mics, it might be good to do several mic compare measurements and see if they have the same response. If they do then you can also disregard. If they don't then you just need to be a little careful how you use them together. Not too careful, just a little. Most of the time when we make phase measurements for the purpose of comparing 2 systems, we use a single mic in the position where the 2 systems are equal level so it does not matter if your other mics are not responding exactly the same.