Keith_W
Major Contributor
One thing that has bugged me for a long time is why we sometimes point microphones on axis, and why we point it at the ceiling. For convenience I will call this 0deg and 90deg orientation respectively. I thought that it had something to do with whether we wish to capture reflections and measure surround speakers (90deg) or the on axis sound (0deg). For all my measurements and corrections, I have always pointed the microphone at the speaker. But - diligent audiophiles must always question if what we are doing is wrong.
There is certainly a measurable difference at 90deg. Jochen Schulz compared three microphones at different orientations and found most variation at high frequencies:
Note that the nonlinear frequency response below 2kHz is not the microphone, but the speaker he was measuring. His conclusion - always point the microphone at the speakers.
This lead me to do further reading about pressure field, diffuse field, and free field microphones. Bruel & Kjaer have a nice document explaining the difference here. In brief, free field microphones are for measurement in a sound field where sound waves propagate without any reflections. They are suitable for anechoic chambers, or measurement of speakers outdoors. Diffuse field (sometimes known as random incidence) microphones are used in sound fields with many reflective surfaces, producing reverberant conditions - as in listening rooms beyond the critical distance. Pressure field microphones are used where sound pressure and phase are the same in any position within the field, such as in the case in small enclosed spaces (like artificial ears) and nearfield measurement of drivers. The problem is, we want to do all 3 types of measurement!
The difference between the three types is in the capsule design:
You will notice that the pressure field microphone has a hole in the centre, and the other two don't. Why is this hole needed? As Sound on Sound explains, if a mic is placed on axis to a speaker, sound will strike the diaphragm at a 90deg incidence. At long wavelengths, this is not a problem. But at short wavelengths, a pressure build-up in the cavity will occur, causing an error of up to +10dB at 20kHz. The purpose of the hole is to relieve the pressure.
Bruel & Kjaer published this graph showing the different response of their Type 4971 pressure-field microphone (with the hole in the top) when used in the pressure field (blue curve, the response is flat), random incidence aka diffuse field (red curve, about a 2.5dB rise in frequency response at high frequencies), and free field (grey and yellow curves) at 0deg and 90deg respectively. The graph can be found here.
This made me realize that my "omnidirectional" Earthworks M30 mic isn't as omnidirectional as I thought. Microphone orientation matters, depending on what type of microphone you own! Nowhere in their product page do they indicate what type of microphone it is. Their calibration file does not indicate whether it is for 0deg or 90deg orientation (I am aware that the iSemCon 7150 comes with two calibration files).
So I compared the capsule grid of my Behringer ECM8000 (left) and Earthworks M30 (right). The Earthworks has a mesh covering the capsule, with no side hole slits. I am guessing that both are pressure-field microphones.
At this point the only thing left to do is to take my own measurement and see if I can replicate the theory. I used my Earthworks M30, swept the left speaker only from MLP at 0 deg (red) and 90 deg (green). There is a dip in the treble because of a mistake I made with the mixer (only noticed it when I was processing the curves ... doh!!). Only the orientation was changed between the two measurements, the position was otherwise not moved.
The first thing we observe is that the gain in the vertical position is about 2dB compared to 0deg.
When the two curves are normalised, we see that the mic loses 2dB of treble in the vertical orientation, similar to what Jochen Schulz observed. We also see a marked difference in bass which I was not expecting. I do not have a good explanation for this - the wavelengths are so long that in theory, neither microphone orientation nor any inadvertent small shift in microphone position should produce this boost. Maybe bass is reflecting from the ceiling and striking the diaphragm, but that SOS article says that bass freqs should pass through the microphone as if it's not there, so the angle of incidence should not matter.
This made me realize that even omnidirectional microphones have individual polar response and the manufacturers should publish their own polar measurements, the same way we want speaker manufacturers to publish spinoramas. I have to say I am a little bit miffed that even a microphone as expensive as my Earthworks M30 does not say what kind of mic it is on their website, nor do they supply calibration files for horizontal and vertical orientation. There is no market imperative for them to provide more information, because awareness of the issue is so poor. I did a search for "diffuse field microphone" and came up with no results - so it looks as if this has not been discussed on ASR before.
So now I have some questions!!
1. Is the Earthworks M30 a diffuse-field, pressure-field, or free-field microphone? Neither their website nor their manual says what it is. I have emailed them.
2. Anybody have a good explanation for the different bass behaviour at 0deg and 90deg?
3. Does this mean that if you own a pressure field microphone (hole in the top) you should be pointing your mic at 0deg to the speaker when doing nearfield measurements, and 90deg at the MLP where you are taking diffuse field measurements?
I assume that the Klippel @amirm is using has a pressure field microphone. Would you be kind enough to post a picture of the capsule?
There is certainly a measurable difference at 90deg. Jochen Schulz compared three microphones at different orientations and found most variation at high frequencies:
Note that the nonlinear frequency response below 2kHz is not the microphone, but the speaker he was measuring. His conclusion - always point the microphone at the speakers.
This lead me to do further reading about pressure field, diffuse field, and free field microphones. Bruel & Kjaer have a nice document explaining the difference here. In brief, free field microphones are for measurement in a sound field where sound waves propagate without any reflections. They are suitable for anechoic chambers, or measurement of speakers outdoors. Diffuse field (sometimes known as random incidence) microphones are used in sound fields with many reflective surfaces, producing reverberant conditions - as in listening rooms beyond the critical distance. Pressure field microphones are used where sound pressure and phase are the same in any position within the field, such as in the case in small enclosed spaces (like artificial ears) and nearfield measurement of drivers. The problem is, we want to do all 3 types of measurement!
The difference between the three types is in the capsule design:
You will notice that the pressure field microphone has a hole in the centre, and the other two don't. Why is this hole needed? As Sound on Sound explains, if a mic is placed on axis to a speaker, sound will strike the diaphragm at a 90deg incidence. At long wavelengths, this is not a problem. But at short wavelengths, a pressure build-up in the cavity will occur, causing an error of up to +10dB at 20kHz. The purpose of the hole is to relieve the pressure.
Bruel & Kjaer published this graph showing the different response of their Type 4971 pressure-field microphone (with the hole in the top) when used in the pressure field (blue curve, the response is flat), random incidence aka diffuse field (red curve, about a 2.5dB rise in frequency response at high frequencies), and free field (grey and yellow curves) at 0deg and 90deg respectively. The graph can be found here.
This made me realize that my "omnidirectional" Earthworks M30 mic isn't as omnidirectional as I thought. Microphone orientation matters, depending on what type of microphone you own! Nowhere in their product page do they indicate what type of microphone it is. Their calibration file does not indicate whether it is for 0deg or 90deg orientation (I am aware that the iSemCon 7150 comes with two calibration files).
So I compared the capsule grid of my Behringer ECM8000 (left) and Earthworks M30 (right). The Earthworks has a mesh covering the capsule, with no side hole slits. I am guessing that both are pressure-field microphones.
At this point the only thing left to do is to take my own measurement and see if I can replicate the theory. I used my Earthworks M30, swept the left speaker only from MLP at 0 deg (red) and 90 deg (green). There is a dip in the treble because of a mistake I made with the mixer (only noticed it when I was processing the curves ... doh!!). Only the orientation was changed between the two measurements, the position was otherwise not moved.
The first thing we observe is that the gain in the vertical position is about 2dB compared to 0deg.
When the two curves are normalised, we see that the mic loses 2dB of treble in the vertical orientation, similar to what Jochen Schulz observed. We also see a marked difference in bass which I was not expecting. I do not have a good explanation for this - the wavelengths are so long that in theory, neither microphone orientation nor any inadvertent small shift in microphone position should produce this boost. Maybe bass is reflecting from the ceiling and striking the diaphragm, but that SOS article says that bass freqs should pass through the microphone as if it's not there, so the angle of incidence should not matter.
This made me realize that even omnidirectional microphones have individual polar response and the manufacturers should publish their own polar measurements, the same way we want speaker manufacturers to publish spinoramas. I have to say I am a little bit miffed that even a microphone as expensive as my Earthworks M30 does not say what kind of mic it is on their website, nor do they supply calibration files for horizontal and vertical orientation. There is no market imperative for them to provide more information, because awareness of the issue is so poor. I did a search for "diffuse field microphone" and came up with no results - so it looks as if this has not been discussed on ASR before.
So now I have some questions!!
1. Is the Earthworks M30 a diffuse-field, pressure-field, or free-field microphone? Neither their website nor their manual says what it is. I have emailed them.
2. Anybody have a good explanation for the different bass behaviour at 0deg and 90deg?
3. Does this mean that if you own a pressure field microphone (hole in the top) you should be pointing your mic at 0deg to the speaker when doing nearfield measurements, and 90deg at the MLP where you are taking diffuse field measurements?
I assume that the Klippel @amirm is using has a pressure field microphone. Would you be kind enough to post a picture of the capsule?