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[J.J. Blair]

I don't know if any of you remember the Ohm Walsh speakers.  They were quite revolutionary, and sounded amazing.  The idea was that with a single cone, you could vibrate across the entire frequency range, with phase coherency and in omnidirectional manner.  Part of this was due to using different materials in the cone for the various frequency areas.  But what's of more interest to me as the omnidirectional aspect of the speaker. 

So, bearing in mind that a speaker is a reverse transducer, has anybody ever made a moving coil mic that employed a conical diaphragm, to create what in theory could be an almost omni directional dynamic mic?  Any thoughts on this approach, or is everybody here simply capacitor mic nerds?

[klaus]

... to create what in theory could be an almost omni directional dynamic mic?  Any thoughts on this approach, or is everybody here simply capacitor mic nerds?

Everybody (I speak with conviction here) appreciates each and every of your posts. Straight talking, and experienced. Cannot ask for more. That alone is a standing invitation for you to keep laying it on.

On this subject though, and tell me if I am off: a new dynamic mic design would need to offer something that is

a. not available in existing mics, condenser, dynamic or otherwise, and

b. desirable and practical for everyday use in the recording environment of today


These things look like unwieldy monsters, covering up the gold record on the wall, and I am not sure what the use of an omni dynamic, with its relatively low sensitivity common to all dynamics, would be? Usually, omnis, regardless of operating principle, need distance and a good room to blossom.

Distance recording (orchestra, big band, choir, etc.) is a relatively rare bird theses days, and so is good rooms. The added disadvantage of an omni dynamic then, is the low sensitivity of the passive design, which, paired with relatively low SPLs in quiet passages of distant-recorded material, brings up noise issues.
Am I wrong?

[Kai]

A Walsh transducer used as microphone would not have an omndirectional, but somewhat cylindrical directivity in the upper frequency range.
In the lower range where it does not behave like a transmission line it would be omnidirectional.
The transition frequency from one characteristic to the other would depend on the size of the transducer.
Additionally the directivity would be asymmetrical from top to bottom.
The transmission line principle of the Walsh transducer does only work perpendicular to its axis.
Off axis it would behave quite similar to a shotgun mic, with a lot of side lobes and irregularities caused by interferences of transmission line with air sound.

If this all would be desirable, other problems, especially with noise, could be overcome by making the whole thing active. Using active electronics in dynamic mics have the advantage that it's possible to use very high impedance coils that give lots of output.
 

[mbrebes]

Another problem with thinking of it as an omnidirectional mic is that the transmission occurs with the voice coil moving on one axis with the stationary magnet.  Any movement on any other axis would either be minimal or damaging to the structure.

[soapfoot]

speaking to Klaus's point-- omni dynamics do occasionally prove useful to me on things like guitar cab.

On a live tracking date, something like an MD21 or EV635a right up on a guitar amp speaker grille can be quite nice due to the lack of proximity effect, and if baffled off, the spill isn't much greater than a cardioid most times, especially considering it can be placed much closer to the grille without proximity effect.

[Kai]

Another problem with thinking of it as an omnidirectional mic is that the transmission occurs with the voice coil moving on one axis with the stationary magnet.  Any movement on any other axis would either be minimal or damaging to the structure.

Am omnidirectional microphone is picking up sound (air) pressure.
Pressure does not have a direction, opposed to air movement like wind.
In no way would air pressure caused by natural events be strong enough to damage the voice coil  of any dynamic mic.
Only things like explosions are able to do that.

The idea behind the Walsh transducer is, that sound travels faster in hard materials then in air. If you make a sound wave travel down a cone it radiates a cylindrical sound wave.
This does only work for frequencies where the wavelength is shorter then the size of the cone.
For frequencies below the Walsh does behave like any normal dynamic speaker.

[J.J. Blair]

Thanks for your input.  It was just an idea that had occurred to me, and I was hoping somebody here had some insight on it.  Thanks, Kai!  Very informative. 

[J.J. Blair]

BTW, Klaus, yes these are unwieldy.  I was wondering if doing a much smaller version would be possible, or have the same effect.  Also, I was just curious about the basic premise of a conical diaphragm, as opposed to the standard dynamic diaphragm shape, and how that might work with directionality. 

[leswatts]

Hmm.
I think in a microphone form factor (much smaller) it could be done. At mid frequency (before the dipole transition) it acts as a mechano-acoustic bending wave transformer of high ratio. one would be hard pressed to get anything out of it at low frequency.

Mics like the 1935 western electric 630A eight ball would have much better omnidirectionality.
It was designed to cutoff at 10 kHz though. I think with modern materials it would be possible to do a 20kHz one.

FWIW we make a pressure ribbon microphone. It's only omnidirectional at low/mid frequencies due to diffraction. With the very low mass it gets response to 24kHz. One could do that with a low enough inertance dynamic diaphragm as well.

Great advantage of dynamic mics is the HUGE dynamic range and simplicity. 150 dB+ DR is common.

Les