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DIY Interference Tubes

A standard shotgun microphone is essentially just a microphone capsule and an interference tube.  The capsules are simple enough, it's these interference tubes that are (apparently) complicated.

An interference tube is a tube with slots cut into it to cancel out any sound coming from off axis.  Any sound coming from on axis (straight down the shotgun tube) will hit the microphone without any interference ie the loudest sound.  Any sound coming from off-axis will encounter the slots and bounce around in the tube until they are cancelled out with each other or attenuated down to nothing (inverse-square law).  The pattern of slots on the tube can vary, depending on the amount of rejection wanted, and the effects on different frequencies.  Also, generally speaking, the longer the interference tube, the better the off-axis rejection (more directionality).

So I decided, why not make my own interference tube and save a bunch of money?  I did a whole lot of research on the web, and was surprised to that no one had tried this.  At least, no one had documented trying this, maybe it was so complicated that they failed and didn't bother posting their results online.

Well I hope to change all that.

I've got a fairly simple plan, I'm using a Electret capsule preamp project I found online, from Elliot Sound Products http://sound.westhost.com , it's phantom powered and is designed for measurement microphones.  I've already ordered the parts and I'm waiting for the PCB to be shipped from Australia.

It's going to be a system style microphone, similar to the Schoeps Colette system.  The preamp will be a modular piece, with each microphone/tube module separate.  I'm thinking of using a BNC connector to connect the two, as it's a locking connecter, that won't spin around.  That way when I experiment will different interference tube designs, all I'm replacing is the tube, capsule and connecter, instead of the entire microphone.  In order to reduce costs I'm going to use 3/4" EMT conduit for the tubing.  I'll drill holes and connect them with a dremel to create the slots.

The first tube design I will be testing is: OBM-A-00, (OrangeBug Microphones - Style A - 00) a simple unaltered tube.  I'll be doing this as a control, and also to test my pickup pattern rig.

The method of measuring the pickup pattern is pretty simple (probably too simple).  I'll place the microphone on my tripod, with a sound source 1 meter away (I"ll use a SPL meter to ensure consistent sound levels).  I'll take a measurement on-axis, as well as 45 degree increments rotating the mic on the tripod.  I'll record the measurement by recording the mics output onto my H4n, and analyzing them later on the computer.  There's one main issue with this, I'm not in an anechoic chamber, so sound reflections will be an issue.  I'll try to do every measurement at the exact same place, and I'm going to take noise floor measurement at each test.  I'm also going to test the electret capsules on their own, to compare with their data-sheets.

I'll post again once the PCB gets here, as well as after each test, regardless of the success.

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