Post by markfouxman on Apr 26, 2015 16:13:51 GMT -6
Long ago as a response to somebody on another forum I wrote this little paper on ribbon microphones physics behind frequency response. It's long got buried in the multipage thread, so thought it would be useful to post it here:
I will try to outline the main points in the simplest form I can manage, so less technically inclined folks could also grasp the idea.
In order to understand how the ribbon microphone works, how its flat response is formed, and how this response is different from capacitor mic, it is important to know three concepts: 1) Resistance Controlled system, 2) Mass Controlled system, and 3) Forces on the diaphragm.
1) The best example of the resistance Controlled system is a condenser microphone capsule, where the diaphragm is tuned in the middle of the band (usually somewhere between 900Hz to 1500Hz, depending on a mic). That exhibits a huge (sometimes up to 60db) peak. Naturally, for the flat response we need to damp this peak, which is done by means of air cushion trapped between the diaphragm and back-plate. The amount of viscosity of this cushion is regulated by the certain size holes (or sometimes grooves) in the back-plate.
2) On the contrary, the ribbon microphones are tuned into the lowest frequency of the band, so actually their natural response FALLS with 6db8 rate.
So, where is the flat response in ribbons coming from?
In the "native" fig8 pattern the sound wave strikes the front of the diaphragm and creates acoustic pressure p1. Since the back of the ribbon is exposed, the same sound wave flows around the ribbon and pole pieces/magnet structure some distance (called acoustic path d) and creates some acoustical pressure p2 at the back of the ribbon. This results in pressure difference p1-p2, which in fact, is a driving force to move the ribbon.
For example, why there is a null at the 90 degrees polar response? Because the sound wave reaches both, front and back at the same time, so there is no pressure difference.
The interesting feature of this driving force is that it doubles with every octave, so the acoustical response of the ribbon actually RAISES with 6db8 rate. Now, remember that the Mass Controlled system naturally has a falling response? When we combine those two that gives us an overall flat response.
Now let's see what's going on on the extremes of the bandwidth.
1) Low end:
Obviously, the tuning frequency of the ribbon would determine the lowest response. However, in the real system there always will be a slit between the ribbon itself and magnet/pole, so because of the viscosity of the air in that slit below some certain point the system turns into the STIFFNESS Controlled one (the one, which defines true omni operation) and the response below that point rapidly falls--that's why it is not practical to tune the ribbon much lower that point.
2) Top end:
As we talked earlier, there is a distance d, which represents the path between front and back and obviously this distance can be translated into the wavelength.
As we talked, the driving force p1-p2 increases with each octave, but only to the point where the d represents 1/2 wavelength, because when the d reaches the full wavelength our driving force p1-p2 becomes ZERO. That is, in the condition when the wavelength of the signal becomes equal to the distance of the acoustic path d obviously the p2 will become equal to p1 (because it is 360 degree shift) and the ribbon won't be moving.
That's why if we know the acoustical path d and ribbon dimensions, it is very easy to calculate the top frequency response.
It is important to notice, the top response will also be somewhat affected by diffractions caused by magnet system cavity, but there are special graphs, which help to correct the calculations to the very high degree of accuracy.
Hopefully it helps and puts more light on how the ribbon microphones work.
Best, Mark Fouxman
I will try to outline the main points in the simplest form I can manage, so less technically inclined folks could also grasp the idea.
In order to understand how the ribbon microphone works, how its flat response is formed, and how this response is different from capacitor mic, it is important to know three concepts: 1) Resistance Controlled system, 2) Mass Controlled system, and 3) Forces on the diaphragm.
1) The best example of the resistance Controlled system is a condenser microphone capsule, where the diaphragm is tuned in the middle of the band (usually somewhere between 900Hz to 1500Hz, depending on a mic). That exhibits a huge (sometimes up to 60db) peak. Naturally, for the flat response we need to damp this peak, which is done by means of air cushion trapped between the diaphragm and back-plate. The amount of viscosity of this cushion is regulated by the certain size holes (or sometimes grooves) in the back-plate.
2) On the contrary, the ribbon microphones are tuned into the lowest frequency of the band, so actually their natural response FALLS with 6db8 rate.
So, where is the flat response in ribbons coming from?
In the "native" fig8 pattern the sound wave strikes the front of the diaphragm and creates acoustic pressure p1. Since the back of the ribbon is exposed, the same sound wave flows around the ribbon and pole pieces/magnet structure some distance (called acoustic path d) and creates some acoustical pressure p2 at the back of the ribbon. This results in pressure difference p1-p2, which in fact, is a driving force to move the ribbon.
For example, why there is a null at the 90 degrees polar response? Because the sound wave reaches both, front and back at the same time, so there is no pressure difference.
The interesting feature of this driving force is that it doubles with every octave, so the acoustical response of the ribbon actually RAISES with 6db8 rate. Now, remember that the Mass Controlled system naturally has a falling response? When we combine those two that gives us an overall flat response.
Now let's see what's going on on the extremes of the bandwidth.
1) Low end:
Obviously, the tuning frequency of the ribbon would determine the lowest response. However, in the real system there always will be a slit between the ribbon itself and magnet/pole, so because of the viscosity of the air in that slit below some certain point the system turns into the STIFFNESS Controlled one (the one, which defines true omni operation) and the response below that point rapidly falls--that's why it is not practical to tune the ribbon much lower that point.
2) Top end:
As we talked earlier, there is a distance d, which represents the path between front and back and obviously this distance can be translated into the wavelength.
As we talked, the driving force p1-p2 increases with each octave, but only to the point where the d represents 1/2 wavelength, because when the d reaches the full wavelength our driving force p1-p2 becomes ZERO. That is, in the condition when the wavelength of the signal becomes equal to the distance of the acoustic path d obviously the p2 will become equal to p1 (because it is 360 degree shift) and the ribbon won't be moving.
That's why if we know the acoustical path d and ribbon dimensions, it is very easy to calculate the top frequency response.
It is important to notice, the top response will also be somewhat affected by diffractions caused by magnet system cavity, but there are special graphs, which help to correct the calculations to the very high degree of accuracy.
Hopefully it helps and puts more light on how the ribbon microphones work.
Best, Mark Fouxman
