Measuring Capsule Membrane Resonance

[Scrappersa]

With glued membranes, it's easy to measure the resonant frequency of a capsule membrane because you can take it off the backplate, check it on top of a speaker with a sweep with some salt on top of it or with a measurement mic.

Any ideas how to check the resonance of a membrane on a backplate that is screwed down and cannot be removed? This is a problem that has stumped me for a while.

@ricardo mentioned in some older posts that at Calrec they would apply polarization voltage to the capsule and check the capacitance. I'm not sure how that would work though since the capacitance in each backplate is variable depending on shim height and inconsistencies in the machining/lapping.

I'm wondering if anybody knows of a way to check the tuning of a membrane while it's on the backplate. Or if you have any other techniques for figuring out if a membrane is within an acceptable range once it's screwed down. Even if there is a device like a tonometer or a tension gauge that can let you know if the tension is set within a spec.

 

[soliloqueen]

i saw someone run a very quiet frequency sweep thru the capsule as an electrostatic speaker once and then they measured...something else about the capsule? while they did that? not sure what. but that sounds like it could get destructive.

 

[gyraf]

capacitance change vs. applied polarization voltage will give you stiffness-of-membrane, which translates to resonance frequency in a particular type known membrane (i.e. all other parameters being the same)

Once mounted in a mic, there's no way of deducting resonance "directly" anymore - as this is exactly what is (meant to be) counter-acted upon by damping and delay networks

/Jakob E.

 

[Scrappersa]

Hi Jakob,

That makes perfect sense. Of course.

So we're just looking at the difference in the capacitance of a capsule with and without polarization voltage applied. That capacitance value (the difference) would then be what we're aiming for. Is that correct?

Also, let's say the capacitance of one capsule without voltage is 65pf and a different capsule is 85pf but the resonant frequency in both capsules are the same. When we apply voltage to them, should the capacitance change in both capsules theoretically be the same, assuming all other parameters are also the same?

 

[gyraf]

..my gut-feel tells me to look for a similar percentage-change, but I fear there's quite some math in there if it needs to be really precise.. does it?

/Jakob E.

 

[Scrappersa]

Hi Jakob,

In this design, the front membrane has larger range of 775hz to 875hz. The back needs to be much more precise, weirdly and unfortunately. I can play around with it a bit and see what I find. In the final design, the capacitance should be around 80pf so I'll aim to make it more consistent.

 

[soliloqueen]

i feel like if you can get the distance off the backplate exactly the same, you could match them adequately just by making sure they have the same capacitance under voltage. if you've controlled for literally everything else, the only difference is the tightness of the membrane

 

[GeorgeToledo]

https://downloads.bbc.co.uk/rd/pubs/reports/1956-12.pdf

 

[Murdock]

Sorry for reviving this older thread but I would really like to know if there is a “simple” method of measuring the resonant frequency of a assembled capsule.
There is this really interesting picture of what seems to be the Neumann tuning jig. It looks like a KK53 capsule.
What’s interesting is the cable going in (or out of) to the jig.
What could it be they measured? I don’t know if the unit in the background belongs to the jig but there it seems one can switch between three settings. Maybe different frequencies? Or Polarisation voltages?
Or they just measured the capacitance as the distance between backplate and membrane was already set.

 

[soliloqueen]

There is, but I can't remember it. It's purely electrical. Is it impedance over frequency? Capacitance over frequency? I earnestly can't remember. There is some way to do it. I just haven't been able to find the paper on it in a long time.

 

[gyraf]

Repeat: Once mounted in a mic, there's no way of deducting resonance "directly" anymore - as this is exactly what is (meant to be) counter-acted upon by damping and delay networks, as I understand the mechanics..

/Jakob E.

 

[ccaudle]

Once mounted in a mic, there's no way of deducting resonance "directly" anymore

Couldn't you measure impedance over frequency, like measuring the resonance and Q of a speaker in an enclosure? If there is a resonance it implies energy storage, which would have to show up as a change in motional impedance, which should be reflected in the electrical measurements. Crystal resonators are measured that way, some of the same equipment might be usable for measuring capsules, although the Q of a capsule would be orders of magnitude lower. Should be able to use audio measurement tools to measure since the resonance is in the low audio range and not MHz.

 

[Murdock]

But with pressure transducers it’s a different story, right?
Also, does the damping via air cushion just reduces the resonance peak of the membrane or shifts it alltogether?

 

[kingkorg]

Absolute resonant frequency of the whole system in the end is not in audible range anyways, so no real need to be paranoid about it. Especially with k67, k47 and others with center screw right in the middle.

 

[Murdock]

Yes, I know that. But I’m more interested in how the diaphragm tuning of an Omni pressure mic “resonates” with the air chamber of the whole system.
Or in other words, if I tune the membrane to, let’s say 7Khz, does the air chambers move the resonance point or just dampens it?

 

[Murdock]

Tune what above 20khz? The membrane? Or is the membrane itself tuned lower but together with the backplate and air cushion the resonance frequency goes higher?

 

[MagnetoSound]

I can’t imagine the tension required to get a membrane to 40kHz in free air …!

 

[molke]

B&K capsules are not dampened at all

The AIP Handbook seems to describe the principle of a (1 inch) omni B&K capsule as “[…] an airflow, which introduces damping primarily at two places: in the gap, and in the openings (holes ans slots) in the backplate.”?

Or is the membrane itself tuned lower but together with the backplate and air cushion the resonance frequency goes higher?

Although not the most up-to-date reference, the BBC Engineering Training Manual (p. 89) states the following: “With the foil materials available at present, the natural frequency of a practical diaphragm cannot exceed 8,000 c/s by the action of tension alone. However the small volume of air between the diaphragm and back plate raises the natural frequency of the system.”

This Shure paper lists the resonance frequency in Table 1 as 3.5 kHz, which is clearly not the resonance in the simulated and measured response in Section 3.

In my own experiments with an omni capsule (18 mm membrane diameter), I wasn’t able to tune “the membrane itself” (5 µm aluminized Mylar) to a very high frequency—certainly not above 20 kHz. With smaller diameters and/or metal membranes, this could well be possible.

 

[Tim Campbell]

The AIP Handbook seems to describe the principle of a (1 inch) omni B&K capsule as “[…] an airflow, which introduces damping primarily at two places: in the gap, and in the openings (holes ans slots) in the backplate

In talking with friends at Bruel & Kjaer they had said that the membrane tension dampened more than the backplate but I am sure they were discussing high polarization voltages where the backplate is further removed. Their backplates are free floating, very thin and have fairly large holes.

 

[Murdock]

The AIP Handbook seems to describe the principle of a (1 inch) omni B&K capsule as “[…] an airflow, which introduces damping primarily at two places: in the gap, and in the openings (holes ans slots) in the backplate.”?

Although not the most up-to-date reference, the BBC Engineering Training Manual (p. 89) states the following: “With the foil materials available at present, the natural frequency of a practical diaphragm cannot exceed 8,000 c/s by the action of tension alone. However the small volume of air between the diaphragm and back plate raises the natural frequency of the system.”

This Shure paper lists the resonance frequency in Table 1 as 3.5 kHz, which is clearly not the resonance in the simulated and measured response in Section 3.

In my own experiments with an omni capsule (18 mm membrane diameter), I wasn’t able to tune “the membrane itself” (5 µm aluminzed Mylar) to a very high frequency—certainly not above 20 kHz. With smaller diameters and/or metal membranes, this could well be possible.

And that‘s exactly what I thought and would like to understand better.
Is there a “simpler” way to determine/measure at what frequency the membrane needs to be tuned to have a certain resonance in a certain system. I know there are probably a lot of Formulars to know and calculations to be done.
Can one maybe calculate it when knowing the volume of the air behind the membrane?