difference between condensor and ribbon technology

[C.Lambrechts]

I'm interested in learning a bit more about the differences in technology between a ribbon and a condensor microphone.

Here's what I think to know allready :

One of the main differences between them is that a ribbon measures the 'speed of air' ... dunno how to put it better then that .... and a condensor measures the 'presure of the air'. Hope that makes sence but in simple human language that's what I understood from it.


Another difference would be that they have very different 'resonance frequencies'. A ribbons resonance frequency would be way low .... (lower then 500 Hz ?) where as a condensors resonance frequency would be way up high .... above 15kHz ?? .... the higher resonance frequencies would explain the often grainy / harsh sounding condensor where as the low resonance frequency of a ribbon would explain it's often blurry sounding bass frequencies ??


I hope this makes a little bit of sence. Someone care to discuss / explain these diferences , how they work and what they mean ?

I'm afraid I wont be able to add much scientifical information to this topic but maybe someone else can do that.

If at all possible ... please try to keep explanations understandable for people like myself ....

[Paul Frindle]

Both ribbon and condenser mic respond to air pressure, the differences lie only in the method of converting this to an electrical signal - and of course a whole host of other factors that affect sound quality and angular responses.

The ribbon mic is a variant of the dynamic mic. The ribbon is basically the 'coil' of the dynamic mic, it is suspended in a lateral linear magnetic field and generates voltage when it moves within the field. Because the ribbon essentially makes only one turn of a coil, the impedance and output voltage is very low. This means that a very high ratio transformer is needed to match it to 200 or 600 ohms impedance.

The capacitor mic uses conservation of charge principle to function. Basically the diapragm makes a capacitance with the stator. A voltage is applied across it and a charge is set up. When the diaphragm moves with air pressure the capacitance changes in sympathy. Because the charge must stay constant, the terminal voltage must change - rising for reduction of capacitance and falling for an increase etc.. The output is therefore a voltage of very hign impedance, so a high impedance buffer amp (usually and FET circuit) is needed to produce the output current to match 200 or 600 ohms. There are many variants of capacitor mics with very different qualities.

But the main thing that sets the ribbon mic apart from most other mics is it's particular pick-up pattern. Since the diaphragm is essentially linear with a large length to width ratio, it exhibits very large differences in frequency response depending of the angle the sound arrives at the mic. For instance, if the mic is used with the ribbon vertically orientated (most usual) sounds entering the mic from above or below the centre axis will be very steeply rolled off, whereas sounds arriving off axis horizontally will retain a good degree of HF response even at quite large angles. This means that full spectrum response is only available from a very narrow angle corresponding to the vertical right angle centre of the mic and left to right, all other angles of arrival will be heavily rolled-off. The reason therefore that the mic sounds 'bassy' is that a large proportion of the sound spill is being added to the total output with a heavy loss of HF response.

The reason the ribbon mic initially gained a large following is that it's rolled-off response to extraneous spill sounds reduced MF and HF feedback in PA equipment, that at the time usually had significant LF roll-off anyway. This meant that higher PA gains could be used in live situations. Also the mic's good response to horizontal angular displacement was very useful in live performance situations, as it resulted in more consistent results in the presence of the performer's natural movements, which are of course more often left to right rather than up and down.

I hope this rather simple and general explanation is useful :-)

[C.Lambrechts]

Originally posted by Paul Frindle
I hope this rather simple and general explanation is useful :-)

It is ... thank you very much.

[McQ]

Originally posted by C.Lambrechts
It is ... thank you very much.

Then there's the issue of "fidelity" and the fact that moving coil mics act as really competent differentiators while their capacitive brethren have the potential (har, har) of maintaining amplitude/phase integrity...

This can be understood when you consider that displacement current at the motional extremes goes to zero (i.e., output current from the mic is zero at the loudest part of the waveform) for the moving-coil element while the condenser-element tracks maximum excursion/maximum output.

Just a thought (if it could be called that),
McQ

[smoothmoniker]

There is a unique response to transients with ribbon mics, as well (transient = rapid state change from quiet to loud, etc.). They respond very quickly, unlike a dynamic, but they don't stop immediately when the sound source does. There is a little bit of a resonance to the ribbon that "rings out" after the sound source.

It's not audible as an echo, or a tail. It's audible as a resonance, especially on complex acoustic signals (strings, piano, brass).

I think this has to do with mounting of the ribbon within the capsule - it's not a highly resistant mounting, so it moves more freely.

-sm

[Paul Frindle]

Originally posted by McQ
Then there's the issue of "fidelity" and the fact that moving coil mics act as really competent differentiators while their capacitive brethren have the potential (har, har) of maintaining amplitude/phase integrity...

This can be understood when you consider that displacement current at the motional extremes goes to zero (i.e., output current from the mic is zero at the loudest part of the waveform) for the moving-coil element while the condenser-element tracks maximum excursion/maximum output.

Just a thought (if it could be called that),
McQ

This is an interesting thought and is based on the observation that any dynamic (magnetic/coil) arrangement produces it's maximum output at the largest rate of change of displacement. For a sine wave this occurs as it passes through zero, which is 90degs out of phase with the signal peak - therefore all the signals from the mic are 90deg out of phase with the sound etc..
If this is what you are refering to, the answer is of course that because ALL freqs are subjected to this treatment, the relative phase between the signals is in fact unchanged and nothing therefore is affected. The provisor is of course - that you have no reference to the original signal to compare it with.

The post on resonance and transients is also interesting. The ribbon mic has a fast transient response because the ribbon has a low effective mass, but thats not the whole story. In calculating the transient response we must also consider the collected energy to the diaphragm, in that a larger diaphragm collects more acoustic energy to counteract its mass. Therefore it is not a given that a large diaphragm mic will necessarily always have a slower on-axis transient response. However a larger diaphragm WILL have a narrower HF pick up pattern due to roll-off caused by phasing of off-axis sound arrival at its surface. So the larger the area, the narrower its HF pick-up pattern angle becomes generally.
Therefore the main point about the ribbon diaphragm is that its largely linear diaphragm presents a small dimension in the lateral plane, so it has very wide HF pick-up angle in the lateral plane centred on the Mic. Generally this would not be the case with a circular diaphagm of significant size.

Following on from this it can be appreciated that in mic design generally, there is a trade off between diaphragm size, sensitivity and HF pick angle, in that a larger area diaphragm will produce more output energy (and have a lower noise), but will result in increasingly narrow HF pick-up patterns. It is therefore important to get the maximum conversion efficiency from the transducer to maintain low noise and good pick-up patterns - by using smaller area diaphragms if possible. The capacitor Mic is usually better in this regard because it can form a fairly efficient transducer without the added mass of the coil required in the dynamic mic. The dynamic mic is compromised because it needs a larger diaphragm to overcome the mass of the coil and retain good overall on-axis HF response. The ribbon mic is somewhat better off than a conventional dynamic because it doesn't have extra coil mass to contend with. But its lower air coupling area normally means that it produces less overall output.

But back to the ribbon and resonances etc.. All diaphragms must have at least a tendancy for a fundamental resonance formed by its mass and the compliance of its mountings. Obviously a heavier mass and a looser mounting stiffness will result in a lower resonance freq - just like a loudspeaker. But because the overall moving mass of a mic is quite low it has a fairly small kinetic moment, therefore the resonance formed by the diaphragm is significantly damped by air mass loading. (BTW this is one of the reasons why it is easier to get a good response from a mic than a speaker). The ribbon mic suffers a bit in this respect since the the total air coupling is quite low in comparison with the mass of the diaphragm material - and the tension required to maintain its physical integrity in the magnetic field is often quite high in comparison to its effective area. This means that ribbon mics generally have larger fundamental resonances and are more prone to differences in resonance freq and responses that can result from changes in the tension of the ribbon, either due to manufacturing tolerance, temperature changes, ageing - or just blowing hard into it once too often :-)

This seems like a long post , I hope some of this stuff makes some sense.

[McQ]

Originally posted by Paul Frindle
...(snip happens)...
The provisor is of course - that you have no reference to the original signal to compare it with.

Which would occur in multi-mic'ing situations with mixed dynamics and condensors...

The post on resonance and transients is also interesting. The ribbon mic has a fast transient response because the ribbon has a low effective mass, but thats not the whole story...The dynamic mic is compromised because it needs a larger diaphragm to overcome the mass of the coil and retain good overall on-axis HF response. The ribbon mic is somewhat better off than a conventional dynamic because it doesn't have extra coil mass to contend with. But its lower air coupling area normally means that it produces less overall output...

...and, presumably, is more "sensitive" (I'm not sure that's the correct term...maybe, "responsive") to motional EMF and circuit loading?

Thanks for your insights (cheers!),
McQ

[Paul Frindle]

Originally posted by McQ
Which would occur in multi-mic'ing situations with mixed dynamics and condensors...

...and, presumably, is more "sensitive" (I'm not sure that's the correct term...maybe, "responsive") to motional EMF and circuit loading?

Thanks for your insights (cheers!),
McQ [/b]

Yes hmm.. I thought of that too after I wrote the post, but I haven't yet got my head around what a phase difference of 90degs at all freqs would add up to if added together?! It's an interesting concept to be sure. It's certainly not a delay, neither is it a classical complex phase addition problem. I'm not really sure what it is - I need to dwell on this one. Are we discovering an effect that is not widely understood or documented? That would be really exciting :-)