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PZM frequency response depending on boundary size - frequency analysis questions
Old 8th September 2012
  #1
Here for the gear
 
🎧 5 years
PZM frequency response depending on boundary size - frequency analysis questions

G'day everyone, this is the first thread i'm creating and just trying to find out if some of you guys can help me with some frequency analysis questions...

here's the rundown,

im doing a report on the frequency response of pressure zone microphones
heres the title...

"An experimental study on how the frequency response of a pressure zone microphone is affected by the distance from the sound source and the surface area that the microphone is mounted on."

i have conducted testing using full frequency sweeps with the microphone mounted on 3 perspex surfaces (85mm2, 255mm2 and 425mm2) and at 3 distances from the monitor (1m, 2m and 4m)

below are the 3 sizes at each distance. with the 85mm panel in red, 255mm in green and 425mm in blue. distance, 1m first, then 2m then 4m.

i'm having a hard time analysing this info, if anyone can give me some insight it would be very much appreciated.

thanks
Attached Images
Old 8th September 2012
  #2
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Lotus 7's Avatar
Clearly, you have a lot going on in your data sets.

Things you should be thinking about and included in your interpretation of the data:
Were the measurements taken in an anechoic chamber or outdoors?
Do you have a reference plot taken with a calibrated reference microphone of the frequency response of the source speaker?

Presumably, your measurements were made with the sound wavefront at 90 degrees to the boundary layer panels on axis. Measurements at other incident angles, possibly 45 and 0 degrees would further characterize the effects of the panel size.

That data would be of help for you to interpret your measurements.

From the info you have provided, there are some conclusions that may be drawn:

1. The only effects of the differing panel surface areas you have used is in the midrange frequencies between 300 Hz and 2 or 3kHz.

2. The larger area panels clearly increase the mic [midrange] sensitivity, with the most change being between the 85 mm and 255 mm sq. panels. However, this data represents only one incident angle and may vary with that angle.

Because of the relatively small size of the boundary layer panels compared to the wavelengths of the sound, their effects are limited to the mid range frequencies. Much larger panels may extend the effects to lower frequencies.

3.The high-frequency roll-off above 3kHz is likely due to the mic being placed beyond the diffuse field distance in the 4m curve set. The notch at 9kHz in the 1m set is probably due to local phase interference effects (possibly reflections from the mic panel to the front of the speaker and back to the mic), or alignment issues with the HF driver in the speaker at the close distance.

4. There are low-frequency anomalies probably due to either room resonance effects or ground reflections if the measurements were done out-of-doors. Those effects obviously have nothing to do with the boundary layer panel size.

5. The effect of changes in the boundary panel size appears to decrease when the mic is beyond the diffuse field distance. This may be an incident angle effect.

See notes on curves below:
Attached Thumbnails
PZM frequency response depending on boundary size - frequency analysis questions-response-curves.jpg  
Old 9th September 2012
  #3
Here for the gear
 
🎧 5 years
Lotus 7: thank you so much for your insights, i'm still trying to wrap my head around all of this but your info and graphic really help.

the test was done in a studio using a krk rokit 6 g2 on a stand. the middle of the monitor was 1100mm high, and the audio technica u843r pzm was set at the same height on the panels parallel to the monitor.

with the 4m measurement, the panel was almost flat against the back wall, which has foam panel covering the entire wall, so i thought that the highs would just be getting lost in that.

i really appreciate your reply mate and the time you took with the pic, thanks
Old 9th September 2012 | Show parent
  #4
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Lotus 7's Avatar
Quote:
Originally Posted by Laidlaw ➡️
the test was done in a studio using a krk rokit 6 g2 on a stand. the middle of the monitor was 1100mm high, and the audio technica u843r pzm was set at the same height on the panels parallel to the monitor.

with the 4m measurement, the panel was almost flat against the back wall, which has foam panel covering the entire wall, so i thought that the highs would just be getting lost in that....
I suspected that it was an indoor measurement in a non-anechoic space. Most of the various notches and peaks that change with the speaker to mic distance are a result of phase interference and room nodes because of wall, floor and ceiling reflections. Covering the back wall with foam may affect the reflections at higher frequencies (depending on the foam type, shape and thickness), but will do little for the room resonance nodes.

The KRK Rokit 6 G2 is a decent speaker, but is not noted for its particularly flat frequency response on axis or for a uniform off-axis response (most small near-field monitors like the KRKs are designed to be pretty directional and have a narrow "sweet spot"). Even though your measurements were all on-axis, because of the nearly uniform half-hemisphere response of the PZM mics you're really measuring the whole, integrated room response.

The additional HF roll-off is certainly a diffuse field effect. (look up "diffuse field distance"). As you move the microphone farther from the speaker the direct sound level drops according to the inverse square law, BUT the reflected sound from all surrounding surfaces remains essentially the same. Up close, the direct sound is louder, at a specific distance they are equal, and at greater distances the room reflections are the main component of the sound (those reflections do not follow the inverse square law). The room absorption at short wavelengths is higher so the amount of energy at high frequencies is less in the diffuse field. Note: The diffuse field distance in a given space is dependent on the microphone spatial characteristics. Its much different for a omni mic compared to a narrow cardioid.

On the other hand, your measurements do reflect (no pun intended) how the mic will perform in a room of similar size and shape.

Good Luck.
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