diy shotgun microphone interference tube.

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flashlamp

Well-known member
Joined
Apr 14, 2021
Messages
89
diy shotgun microphone interference tube.

im trying to mod one of these I have 2 , to make a short directional mic i have a vblock , would you recomend a dremel is good enough to cut the vents? was going to use some alluminum or cooper pipe.

the capsules unscrew., just like akg cms.

heres the specs

Tech Specs

Microphone Type: Condenser Mono/Stereo: Stereo Pair Polar Pattern: Cardioid Diaphragm Size: 0.74" (19mm), 6-micron Mylar Frequency Response: 40Hz-18kHz Max SPL: 135 dB SPL Sensitivity: 8mV/Pa Self Noise: 22dB.
 
If you have dimensions for cutting the slots would you post them or at least a page discussing this.
Mikek
 
thats what I was wondering is there a golden standard for the slots, i was thinking something along the lines of the 416 or the rode ntg i did read somewhere you get better bass response the longer the length of pipe.
 
ok ,
here are the measurements of the vents
Note i used a cloth tape measure.
vent width = 6.2mm
0.2 mm height of vents
0.2 mm spacing between vents.
 
flashlamp said:
diy shotgun microphone interference tube.

im trying to mod one of these I have 2 , to make a short directional mic i have a vblock , would you recomend a dremel is good enough to cut the vents? was going to use some alluminum or cooper pipe.

the capsules unscrew., just like akg cms.

heres the specs

Tech Specs

Microphone Type: Condenser Mono/Stereo: Stereo Pair Polar Pattern: Cardioid Diaphragm Size: 0.74" (19mm), 6-micron Mylar Frequency Response: 40Hz-18kHz Max SPL: 135 dB SPL Sensitivity: 8mV/Pa Self Noise: 22dB.
Click to expand...
I just don't get it.
Am I correct in thinking you want to modify a Presonus PM2 to turn it into a shotgun?
What is a "vblock"?
Where do you want to cut vents?
flashlamp said:
ok ,
here are the measurements of the vents
Note i used a cloth tape measure.
vent width = 6.2mm
0.2 mm height of vents
0.2 mm spacing between vents.
Click to expand...
How can you measure 0.2mm with a tape measure?
If you want to make 0.2mm cuts, a Dremel is way too big. You need a laser torch or a hydraulic shear.
 
millmeter not micrometer it was a cloth tape measure , the kind they use for clothing. had inches and centimeters i just used the millmeters inbetween each centimeter tickmark. as inches were a bit off, im attaching the tubing with tape , or maybe a hose clamp. so the cuts are in some aluminum pipe. i used from mop handle. it was a slightly larger outer diameter. it was .2 millmeter
 
flashlamp said:
millmeter not micrometer it was a cloth tape measure , the kind they use for clothing. had inches and centimeters i just used the millmeters inbetween each centimeter tickmark. as inches were a bit off, im attaching the tubing with tape , or maybe a hose clamp. so the cuts are in some aluminum pipe. i used from mop handle. it was a slightly larger outer diameter. it was .2 millmeter
Click to expand...
I think you got your units wrong. There is no way you can evaluate 0.2mm with a tape measure. 0.2mm is 8 thou.
0.2 cm 2 mm) looks more likely.
 
flashlamp said:
millmeter not micrometer it was a cloth tape measure , the kind they use for clothing. had inches and centimeters i just used the millmeters inbetween each centimeter tickmark. as inches were a bit off, im attaching the tubing with tape , or maybe a hose clamp. so the cuts are in some aluminum pipe. i used from mop handle. it was a slightly larger outer diameter. it was .2 millmeter
Click to expand...
Run to harbor freight and get a set of dial calipers. I wouldn't be without mine. They are cheap enough that you can use them to scribe lines and when they don't work get a new set. I prefer the dial indicator to the electronic, because batteries!
Also what mic did you measure and how are the slots, 1 on 1 side, 2 on 2 sides, 3 on 3 sides? Are they all the way around evenly spaced?
Thanks, Mikek
 
yes i have a dial calpiers i there are 2 sets of 3 vents on the top on both sides of the mic body.
 
I think if I was going to attempt to make my own interference tube for a shotgun mic, I might try and beg steal or borrow one of the new Rode NTG5
mics ( RØDE NTG5 Professional Shotgun Microphone | RØDE Microphones ) and attempt to measure and then copy that hole pattern?
I would think that drilling a selection of circular holes is likely to be a lot simpler than trying to copy a slot pattern with any accuracy?...
 
flashlamp said:
yes i have a dial calpiers i there are 2 sets of 3 vents on the top on both sides of the mic body.
Click to expand...
You're talking about the PM2, aren't you? There are 4 sets of 3, if I see the picture correctly.
The vents on cardioid mics have a different function than the ones on an interference tube. They're here to provide a path to the back of the capsule. Their geometry is almost irrelevant, only their combined area matters.
You can't use these vents' geometry as the basis for an interference tube.
 
rogs said:
I would think that drilling a selection of circular holes is likely to be a lot simpler than trying to copy a slot pattern with any accuracy?...
Click to expand...
Geometry accuracy is not a determinant parameter in the performance of an interference tube.
It just turns out that industrial production techniques lend themselves naturally to a nice regular geometry, but it is not a necessity.
Some may argue that progressive spacing, even logarithmic spacing may provide better performance.
I am convinced a DIY interference tube with the typical 0.5mm (20 thou) accuracy is capable of delivering the same performance as an industrial one.
I would think the most difficult part is finding the proper liner.
My claims must be taken knowing that I don't DIY mics, but I have a long expertise in line-array systems and end-fire arrays, which are based on the same physics.
 
wait I do have a ck8 akg capsule, I could measure that instead and size up the dimensions i think that capsule also uses holes
 
ok it's a pattern of 4 holes on each side then 3 holes then 4 holes again and that pattern repeats up the tube.
 
flashlamp said:
wait I do have a ck8 akg capsule, I could measure that instead and size up the dimensions i think that capsule also uses holes
Click to expand...
On the CK8, the vents are replaced with a piece of perforated sheet metal. The slots at the back are the usual openings that allow the cardioid pattern to happen at low frequencies, because the interference tube works only at mid and high frequencies.
 

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