Question about ground plane measurement

barhrecords

barhrecords

Axe-Master
I am trying to position a microphone in the ground plane.

The microphone is not a cylinder. The capsule end is smaller in diameter than the XLR connector end.

Should I tilt the microphone so that the capsule itself touches the ground surface?

Also, has anyone got a good solution for isolating (shockmount) the mic from the ground surface too when using this technique?

Thanks,
Richard
 
In that case it is an omnidirectional microphone, and placing it at a boundary makes it a pzm. Get the capsule as close to the boundary as possible, without touching it (could use foam or more creative solutions for shockmount). The surface should be hard and reflective. If the surface is somewhat uneven, do something like this:
image005.jpg
 
I think he is referring to Jay Mitchell's method of capturing IRs:

He has spoke about it many times. Here is an early quote I found:

Quote:
It's important to note tough that the lower bass region will be smeared depending on your room and microphone positioning.
A ground plane measurement technique helps minimize this problem: place the microphone on the floor (needs to be a hard surface) aimed at the speaker. The floor reflection will then sum coherently with the direct sound at all frequencies of interest. If you put the mic at ~2 meters from the cab, with both the mic and the cab 10 feet or more from the nearest wall sufrace, you can get a clean 20-ms farfield IR that contains direct sound only.


 
javajunkie said:
A ground plane measurement technique helps minimize this problem: place the microphone on the floor (needs to be a hard surface) aimed at the speaker. The floor reflection will then sum coherently with the direct sound at all frequencies of interest.
Click to expand...

Yep, he is describing boundary micing. Since Richard has an omnidirectional mic, he'll be making a PZM - pressure zone microphone. The picture gives a good description of how he can position his mic.
 
Trazan said:
Yep, he is describing boundary micing. Since Richard has an omnidirectional mic, he'll be making a PZM - pressure zone microphone. The picture gives a good description of how he can position his mic.
Click to expand...

Sort of,
However the mic jay used was A josephson C550h (iirc) which is also omnidirectional.
He , from the description above, pointed it at the speaker rather than the ground.
 
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javajunkie said:
Sort of,
However the mic jay used was A josephson C550h (iirc) which is also omnidirectional.
He , from the description above, pointed it at the speaker rather than the ground.
Click to expand...

He may try pointing it downwards or more towards the source. It's an omni, it doesn't make all that much of a difference and it's easier to get the diaphragm close to the ground pointing it down. The most important part in this is the benefits of the boundary method.

Now, if the microphone was a cardioide he would have to point it at the source.
 
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If it is omnidirectional then the exact orientation isn't important.

The capsule needs to be less than, say, 5mm from the surface. If I'm doing my math correctly, the critical frequency is f = c/4D where c is speed of sound and D is distance from ground plane. For a critical frequency of, say, 20 kHz, then D would be 4.25 mm.
 
Trazan said:
He may try pointing it downwards or more towards the source. It's an omni, it doesn't make all that much of a difference and it's easier to get the diaphragm close to the ground pointing it down. The most important part in this is the benefits of the boundary method.

Now, if the microphone was a cardioide he would have to point it at the source.
Click to expand...
That makes sense.
 
FractalAudio said:
If it is omnidirectional then the exact orientation isn't important.

The capsule needs to be less than, say, 5mm from the surface. If I'm doing my math correctly, the critical frequency is f = c/4D where c is speed of sound and D is distance from ground plane. For a critical frequency of, say, 20 kHz, then D would be 4.25 mm.
Click to expand...

Thanks so much. I was looking for the math.

Richard
 
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barhrecords said:
Totally missed that... thanks.

I didn't associate PZM with acoustic measurement, but I can see it is based on similar principles.

Richard
Click to expand...

Yeah, it's the exact same principle when using an omni and the ground plane. 1/13th wavelenght is a safe bet (-1dB)...so if you're measuring a sub you're fine placing the mic within some 10cm from the ground 8)
 
There are other factors in play if one intends to capture a *proper* FF IR BTW. One would be the *room* being used (walls, ceiling). Transducer size is the variable in determining how far the walls/ceiling must be, how far the mic must be to capture the influence of the cab, etc.

Java's links in post 11 are good IMO.
More: Old Fractal forum, Jay's replies are gold IMO.
Another old forum/Jay
Insight from Angelo Farina
IMO, between Jay, AlbertA, and Angelo Farina, all the knowledge concerning IRs with regard to speakers/cabs is there for the taking. Implementing the knowledge properly and effectively is the key though.

IMO, and since it was how I was shown, utilizing a strict practice while learning is key. Keep records of everything. Measure, take pictures, etc. Every time you capture an IR, also capture a direct signal of you playing guitar through a patch, as well as the mic'd signal. This will allow you to compare the mic'd signal with the direct one plus the resultant IR afterward. IMO, the Axe is an invaluable tool for this. I utilized my computer for the IR capture process though, as it afforded me greater control and flexability. I use Octave for my convolution processes, Axe-O-Matic to convert to Axe .syx, and AlbertA's utilities for various things.

In the end, if done properly, an IR can nearly identically replicate the mic + speaker + mic pre portion of a recorded signal. If the cab is to be part of the equation, this can also be accounted for, but requires proper attention as well. The room is a separate consideration IMO. There are certain (various amounts of subtlety) nonlinear, time variant aspects that will be missing/static, but these can often be accounted for decently enough as part of the compromise IMO.
 
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