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

Hello 

I'm currently studying microphone techniques and have been wondering 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?

Hope one of you knows 

Greetings,

Jesper

There are two main reasons for that:
The inductive nature of the source and the inevitable transformer conjure to attenuate HF, but there is no reason why this could not be overcome, although at some cost.
The size of the ribbon, which makes its vertical directivity increasingly narrower, and the time-delay created between the front face and the rear face of the ribbon, leading to almost perfect phase cancellation.
Reducing the size of the ribbon AND the magnetic structure would allow some HF extension, at the cost of an almost redhibitory loss of sensitivity.
Anyway, ribbon mics seem to be favored today for their lack of HF response (which some call smoothness), so there is not much incentive in trying.

I think this topic should go to the Drawing Board...

I suspect that eddy currents in the ribbon might contribute to HF roll-off too.

I ain't hearing nothin over 3K, DON'T WASTE MY BANDWIDTH SON!

;D I had a couple of thoughts when I saw that.

What do manufacturers mean when they quote a frequency range? You need both a frequency and amplitude to describe the behaviour. So the same mic might be 10KHz (-2 dB) and -15KHz (-4dB) and so on.

So part of it is marketing. If you look at old data sheets, everything used to go up to 10 KHz. Then it was 15 KHz, and now 20 KHz.

Also, the difference between 15KHz and 20KHz is only about 5 semitones....
http://www.drewdaniels.com/FREQ.pdf

Finally...

"All the highest notes neither sharp nor flat,
The ear can't hear as high as that.
Still, I ought to please any passing bat,
With my high fidelity.
(Flanders & Swann)

Did a little testing (thanks to this thread) and my hearing hits a brick wall at around 14k;
13k is great, and nothing at 15k. It's nothing like myopia, where squinting helps, either
I'm 57, and there are reasons (rock, and a life in the building trades).
Guess that explains why I don't mind my nailgun anymore 

Hi everyone,

My apology for not answering earlier. A little awkward  :-[ but I started this thread and subsequently could not find it in prodigy's search system so assumed it was "gone" for some reason.

Regarding what humans can hear/feel I have attached a link to a website - the journal of neurophysiology, search for: "Inaudible High-Frequency Sounds Affect Brain Activity: Hypersonic Effect" - that I find quite interesting.

See www.jn.physiology.org .The authors have - in what to me appears to be a scientific way - investigated the objective (measurements on the brain) and subjective effects of in- or excluding frequencies above 22 kHz, if I remember correctly. Please note that I attach this not with the intention of "opening" a discussion, more like an input of information, if you are interested.

There's also this article: http://www.cco.caltech.edu/~boyk/spectra/spectra.htm

@ abbey road d enfer:

The inductive nature of the source

Click to expand...

Hmmm... but wouldn't the inductance be very low? It's a few centimeters of straight wire ... The transformer, yes, but would the wire/ribbon itself cut off at ~15 kHz?

And then I have been considering how it would work if a ribbon was used as a sort of "ventilated pressure" microphone? I.e. like the Neumann TLM 50 but with a variable "length and size" opening in a sphere similar to the TLM50's?

Regarding the sensitivity of the microphone: Do you know if it is possible to transfer the magnetic flux from e.g. a thick neodymium magnet through a medium of some kind to the place where the ribbon is? The idea is to have a much smaller structure around the ribbon, thus hopefully enabling a much higher cut-off frequency.

@zebra50: Yups, know about marketing issues (worked ~7 years in a quite serious audio retailer). But hope to get/think around/beyond them  :

To me the ribbon has some potentially interesting qualities: no tensioning, low resonance frequency, very low impedance, thus in itself less susceptible to stray electromagnetic fields, very low weight. I also find some drawbacks, like the bandwidth, and the exceptionally low output!, but hope they may be overcome ...

Thanks for reading & maybe replying 

Jesper

gentlevoice1 said:

The inductive nature of the source

Click to expand...

Hmmm... but wouldn't the inductance be very low? It's a few centimeters of straight wire ...

Click to expand...

Very low... compared to what? You have to put this inductance in perspective with the very low impedance it sees. The load impedance reflects at the primary as a fraction of ohms. A typical ribbon mic xfmr is rated at 0.1-0.2 ohms at the primary.

The transformer, yes, but would the wire/ribbon itself cut off at ~15 kHz?

Click to expand...

Electronically no. In itself, its frequency response is illimited. That's in conjunction with a load that it forms a low-pass filter.

Regarding the sensitivity of the microphone: Do you know if it is possible to transfer the magnetic flux from e.g. a thick neodymium magnet through a medium of some kind to the place where the ribbon is? The idea is to have a much smaller structure around the ribbon, thus hopefully enabling a much higher cut-off frequency.

Click to expand...

The availability of Nd magnets has made possible somme improvements related to the smaller size of the pole pieces; however there is a very basic limit. Magnetic materials can "transmit" a limited magnetic flux. It's almost impossible to have the induction above 2 Tesla. Current ribbon mics operate with about 1 T. So there is not much improvement in this direction.

To me the ribbon has some potentially interesting qualities: ...very low impedance, thus in itself less susceptible to stray electromagnetic fields,

Click to expand...

Again, you have to put that in perspective with the actual overall gain needed (or the output level of the ribbon, which is also very low). The sensitivity of a ribbon mic to electromagnetic fields, one you have factored in the overall gain, is higher than that of a dynamic mic. A ribbon picks hum 360° in the horizontal plane. A dynamic picks up only in its axis direction.

@ abbey road d enfer: Very good information, it puts things into perspective - thank you. Can I ask you if you have any thoughts about the TLM 50 analogy?

Regards,

Jesper

... or maybe I can ask you where you find that improvements can be made to a ribbon microphone?

Jesper

abbey road d enfer said:

gentlevoice1 said:

The inductive nature of the source

Click to expand...

Hmmm... but wouldn't the inductance be very low? It's a few centimeters of straight wire ...

Click to expand...

Very low... compared to what?  

Click to expand...

I measured the inductance of the ribbon and from memory it was a few uH.

abbey road d enfer said:

Electronically no. In itself, its frequency response is illimited. That's in conjunction with a load that it forms a low-pass filter.

Click to expand...

Because of its mass, the ribbon can go at least to some good 50-70KHz, however, it is meaningless to talk about electronic response of the ribbon itself, since the main limiting factor is the phenomenon of its acoustical response on both bottom and top ends.

abbey road d enfer said:

The availability of Nd magnets has made possible somme improvements related to the smaller size of the pole pieces; however there is a very basic limit. Magnetic materials can "transmit" a limited magnetic flux. It's almost impossible to have the induction above 2 Tesla. Current ribbon mics operate with about 1 T. So there is not much improvement in this direction.

Click to expand...

A few years ago I made extensive experiments and calculations of the theoretical limits of the magnetic field strength in the gap for magnets of different sizes. It really depends on the gap width, but indeed there is no way of making 2T in the gap. Even 1T is quite a stretch--you will need quite big magnets (by no means of practical size) to get that much in ~1/4" gap. The Apex 205/Nady RSM2 types of ribbons give only 0.35T.

Best, M

And then I have been considering how it would work if a ribbon was used as a sort of "ventilated pressure" microphone?

Click to expand...

A few classic microphones use acoustic chambers behind the ribbon to alter pattern or response. Have a look at the Oktava ML11 and ML17 photos on this page.

http://www.xaudia.com/xaudia/Galleries/Pages/Oktava_Lomo.html

Both those microphones have substantial cardioid character, and brighter than many ribbon mics. Also the RCA varacoustic has a chamber and mechanical switch to move a tube over the rear of the ribbon - there should be some pictures at coutant.org.

And lots of good info here.
http://www.groupdiy.com/index.php?topic=1108.0

Stewart

Is it possible to use electro magnets instead of normal magnets?

Maybe if we could build a ribbon mic with powerful electro magnets, powered from a external power supply, the magnets could be physically small enough, bla bla bla... or am I missing something?

J

Matthew Jacobs said:

Is it possible to use electro magnets instead of normal magnets?

Click to expand...

I'm sure this has come up before - I asked the very same questionb, but I couldn't find the link - maybe it was a very long time ago, in the other place. Anyway, if my memory is good, some of the very early microphones did use electromagnets, but were big and bulky, and took considerable power - those were the drawbacks.

Hopefully someone has a better memory than I have, and can dig up the thread.

Matthew Jacobs said:

Is it possible to use electro magnets instead of normal magnets?

Maybe if we could build a ribbon mic with powerful electro magnets, powered from a external power supply, the magnets could be physically small enough, bla bla bla... or am I missing something?

Click to expand...

No electromagnet could give the same field strength as neodymium for a given size (not one that would work at more than a few degrees above absolute zero, anyway).

Hi Stewart & Matthew,

In my recollection I also considered electromagnets but one day happened to ask an audio designer here in Denmark about the feasibility of this and learned that it would require quite a lot of current to generate the necessary magnetic field. I guess it may also require physically huge electromagnets so I decided to let it be, although I find the concept intriguing 

@Stewart: I've taken a look at the ML11 and ML17's - it looks interesting the small chamber at the back. Thanks for directing me towards these models & pictures. And coutant.org - impressive how many microphones are displayed here ...

And then I have this question popping up: Has anyone ever designed a microphone with a bandwidth going from ~DC - ~50 kHz that sounds magnificent?  I realize that technically this may not be that easy but just out of curiosity - and also for design inspiration in case it exists. Earthworks QTC50 goes from 3 Hz to 50 kHz but any microphone working from ~DC?

Regards,

Jesper

All microphone design is a trade-off between frequency response, background noise and pattern.
A very small diaphragm can have good high frequency response but lots of background noise.

@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

gentlevoice1 said:

And then I have this question popping up: Has anyone ever designed a microphone with a bandwidth going from ~DC - ~50 kHz that sounds magnificent?  I realize that technically this may not be that easy but just out of curiosity - and also for design inspiration in case it exists. Earthworks QTC50 goes from 3 Hz to 50 kHz but any microphone working from ~DC?

Click to expand...

Why would you want a mic working from DC?... but yes, it is possible. The B&K 4138 capsule works 6Hz-140KHz. Seal the pressure equalizing tunnel and you will have a mic from DC to 140KHz. Because of noise, hardly useful for recording, though.

On the other hand, Gefell MK201, or B&K 4133 (3Hz-40KHz) are wonderful for recording and with sealed tunnel can go from DC.

Best, M

zebra50 said:

And then I have been considering how it would work if a ribbon was used as a sort of "ventilated pressure" microphone?

Click to expand...

A few classic microphones use acoustic chambers behind the ribbon to alter pattern or response. Have a look at the Oktava ML11 and ML17 photos on this page.

http://www.xaudia.com/xaudia/Galleries/Pages/Oktava_Lomo.html

Click to expand...

Hi Stewart,

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

Best, M