Why is it that ribbon microphones go no longer than ~15 kHz?

[zebra50]

The ML19/ML219 also have back-chamber, but somewhat different structure. If you want I can snap some pics.

Hi Mark,

That would be good - I haven't seen inside those mics so would be good.

I've got hold of a gaussmeter so will be posting magnetic field readings along with photos in future. I think it would be a good idea to build some kind of database of readings, so when a 'weak' microphone like that RCA varacoustic comes up, at least we have a benchmark.

Why would you want a mic working from DC?...

I was wondering... what do we mean by ~DC? As I understand it, microphones transducers measure through the change in some physical property with respect to time. If nothing is changing then it's not really sound, and not really a microphone. At least not in the audio sense. I guess there are some applications like geological processes where something might be moving extremely slowly, which might be measured. But then I don't think I'd choose a ribbon mic.

One needs to define what one wants to measure, and go from there.

 

[Speedskater]

@speedskater: Thanks for replying. Just to make sure that I get the terminology right: background noise is that the noise from the surroundings or is it "intrinsic" noise, i.e. the noise that the microphone in itself generates (through electronics etc.)? And if it is intrinsic noise is it then because of low signal levels (small membrane means less signal per area) relative to the noise of the microphone itself?

Just would like to clarify.
Greetings,
Jesper

Yes, it is "intrinsic" noise.

(small membrane means less signal per area)

Make that less area to generate the signal voltage.

 

[SSLtech]

Guess that explains why I don't mind my nailgun anymore 

Happiness is a warm nailgun.

 

[Eric H]

Guess that explains why I don't mind my nailgun anymore 

Happiness is a warm nailgun.

If you were a carpenter.

 

[xmvlk]

why ribbon microphones typically go no longer than ~15 kHz upwards in frequency. Is it because of the size of the membrane + magnet structure or is there another reason?

Size of structure plays role, but slightly more complicated.

HF roll-off of ribbon microphones is rather sharp. Sharpness sugests, that it is sound field deformation effect.
(Ribbon velocity microphone is mass-controlled and mass of the ribbon only lowers sensitivity in MF)
Another effect is HF enhancement in the condenser microphone. Deformation of sound field in front
of microphone causes velocity zero and then pressure rise at HF (theoretically 6 dB).
Frequency (and polar diaphragm) of this rising can be changed by body attached to mic (Neumann M50 have sphere,
Schoeps have disc, DPAs have set of different things...)


In the velocity ribbon microphone situation is dual, because ribbon microphone picks-up velocity.
deformation of the soundfield (caused by pole pieces) causes HF loss.
Some ribbon mics have also differences in frequency when this occurs (RCA have narrow pole pieces, EMI have holes in the pole pieces,
older RESLOs? have wide pole pieces, but modification in the field is not so simple as in the  capacitor mic.

Pole pieces thickness plays opposite role and serves like concentrator.

The effects together causes that frequency modulus characteristic of ribbon mic is somewhat evenly wave-shaped at the HF.

.... added  ....
Effect of sound field deformation is used also in another velocity converter
(not transducer) - hot wire microphone:
http://www.microflown.com/products/standard-probes/pu-regular.html
Here are two wires nearly together (made by some micromechanic technology). These two wires are lower resistors in the Christie (...Wheatstone) bridge. DC (may be also high frequency AC - less problems with 1/f noise) current goes through the bridge and heats the wires. Air flowing between the wires causes heath exchange and then imbalance the bridge (platinum wires have thermally dependent resistance) and output signal
is present at the diagonal.
Of course, mass of the wires makes HF range poor and then cylindrical concentrators (which are not pole pieces) are used to equalise it in some frequency band. It looks nearly like ribbon mic, only each concentrator have different collour. It is for rear/front discrimination (and also because the transducer (the wires) have base plate and thus is not perfectly symmetrical.).

 

[zebra50]

Hi Michal!

I was just looking recently for your ribbon mic buffer...

http://www.groupdiy.com/index.php?topic=2086.msg25725#msg25725

The link doesn't work. Do you still have the circuit?

Thanks!

Stewart

 

[xmvlk]

The link doesn't work. Do you still have the circuit?

I do not remember, its long ago.
But I had used current signal transfer. It may be somewhat
similar to this.

(Note, that CB stage may be used twice, one in the mic and other (with Zener
instead of LED) in front of console to obtain low impedance on the cable)

 

[zebra50]

Thanks for that! It does look similar to the one I remembered. I've been experimenting with buffers and wanted to try yours too.

Stewart

 

[pstamler]

About using electromagnets in ribbon mics...some of the very first RCA ribbon designs used electromagnets. They were big and bulky, and no doubt had bad acoustical effects.

See for example Harry Olson's US Pat. # 1,885,001, filed 1931, granted 1932.

Peace,
Paul

 

[xmvlk]

Harry Olson's US Pat. # 1,885,001, filed 1931, granted 1932.

Olson patent is about effect of pole pieces on mic hi freq response. Ribbon microphones
were used before Olson, by Schottky and Gerlach -  but hi freq equalisation was done rather by front waveguide than by pole piece minimalisation therefore these microphones have not had figure 8 chatracteristics.

But Olson needed true fig 8, he needed ribbons for music theatre amplification, he needed
fig 8 for feedback elimination. It was discussed here with schematics of preamps (some theatre in New-York, RCA, cca 1932 ?????)