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Mic building science question

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Mic building science question

Postby ManFromGlass » Wed May 27, 2020 12:42 pm

Curious about frequency ranges in mics. I’m generalising here but were really old microphones designed to record up to 15-16kHz due to the limitations of playback technology?
Even today some mics (mostly ribbons?) still top out around there while newer designs go closer to 20kHz and some way beyond. Is it easier to build a mic that covers a smaller frequency range? And yet almost all mics I’ve researched can record really low frequencies.
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Re: Mic building science question

Postby CS70 » Wed May 27, 2020 12:52 pm

ManFromGlass wrote:Curious about frequency ranges in mics. I’m generalising here but were really old microphones designed to record up to 15-16Hz due to the limitations of playback technology?
Even today some mics (mostly ribbons?) still top out around there while newer designs go closer to 20Hz and some way beyond. Is it easier to build a mic that covers a smaller frequency range? And yet almost all mics I’ve researched can record really low frequencies.

I guess high frequencies are very short and carry very little energy, so you need very thin materials to have them move while preserving fidelity (and sensitive components to detect the voltage changes).

Low freqs carry a lot of energy (i.e. move a lot of air) so it's easy for them to move a comparatively thick membrane (you can use a speaker cone as a kick mic, for example).
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Re: Mic building science question

Postby John Willett » Wed May 27, 2020 2:36 pm

ManFromGlass wrote:Curious about frequency ranges in mics. I’m generalising here but were really old microphones designed to record up to 15-16Hz due to the limitations of playback technology?
Even today some mics (mostly ribbons?) still top out around there while newer designs go closer to 20Hz and some way beyond. Is it easier to build a mic that covers a smaller frequency range? And yet almost all mics I’ve researched can record really low frequencies.

It's to do with the technology and polar-pattern.

Low Frequencies - An omni microphone goes much lower that a directional microphone. A cardioid directional mic. starts rolling off quite early and a fig-8 starts even earlier - an omni can go very low. The Sennheiser MKH 110-1 went down to 0.1 Hz :o

High Frequencies - even the ancient Neumann U47 goes up to 20kHz. Ribbons tend not to go so high as a ribbon, even a light one, has a lot more mass than a capacitor mic. Dynamic mics, also tend to have a more limited HF due to the mass of the diaphragm and attached coil.
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Re: Mic building science question

Postby Hugh Robjohns » Wed May 27, 2020 2:57 pm

ManFromGlass wrote:...were really old microphones designed to record up to 15-16kHz due to the limitations of playback technology?

I've amended your original post to avoid confusion over the frequency ranges you are considering.

But to answer your question, Yes, at least in part. If the amplifiers, recorders, transmission systems and loudspeakers couldn't reproduce higher frequencies -- and they couldn't before the late 1950s at the earliest -- how would you know if your shiny new microphone design went all the way to 20kHz? :lol:

Even today some mics (mostly ribbons?) still top out around there while newer designs go closer to 20kHz and some way beyond.

It'll have a lot to do with the pesky physics again. Diaphragm mass is a big limiting factor because there is relatively little sound energy at high frequencies to move the diaphragm. Diaphragm size (its length/width/diameter) is also significant, especially for off-axis sounds, as it relates to the sound wavelength at high frequencies. If both the compression and rarefaction of a soundwave act on the diaphragm at the same time as the sound wave moves across its surface, the diaphragm isn't going to move very far! And that's going to happen when the sound wavelength is smaller than the diaphragm's dimensions.

There are specialist mic manufacturers (Earthworks and Sanken spring to mind) that claim some of their microphones can capture frequencies up to 50 and 100kHz, respectively, but they all use very small capsules, largely to overcome both of the limitations mentioned above. And very small capsules are inherently noisy becuase they sample the sound wave over such a small portion, which makes life difficult when there isn't much signal energy in the first place!

But the frequency responses of most large diaphragm capacitor mics, ribbons and moving-coil mics are rolling off by 12-15kHz, if not lower. Small diaphragm capacitor mics will be rolling off by around 16-20kHz -- all as rough ball-park figures.

JW's claim that "even the ancient Neumann U47 goes up to 20kHz" is rather optimistic! Neumann themselves claimed the upper bandwidth limit for that mic was 15kHz, which was in accordance with the normal engineering measurement of where the response had fallen by 3dB. Here's the published response chart for the U47:

FQ_U47fet.png


But John is right insofar as whatever the published highest (or lowest) limit of the frequency response, the mic still captures some signals above (or below) that limit, just with progressively reducing sensitivity. So yes, a U47 will capture some signals at 20kHz, but they will be sensed at least 6-8dB lower than they really are, and higher frequencies much more so.

Is it easier to build a mic that covers a smaller frequency range?

Yes, much! And it's the same for any transducer -- including loudspeakers, for example. A smaller bandwidth allows much greater sensitivity and linearity, and lower distortion.

And yet almost all mics I’ve researched can record really low frequencies.

Not really... the low end response varies just as much as the high-end depending on the technology and -- as John rightly says -- the operating principle (pressure or pressure-gradient operation).

Most omnidirectional mics are flat to below 20Hz, and if you combine pressure operation with RF-bias technology (rather than tradirional DC-bias) you can make a mic that's flat down to just a 1 or 2 Hertz!

Directional mics of all types inherently have a much less well-extended low end because of the physics again. Fundamentally, the native frequency response of pressure-gradient mics slopes upwards at 6dB/Octave -- so they inherently have a very poor bass response which has to be corrected in some way, and there are practical limits as to how far that correction can go without introducing other significant problems (such as being insanely sensitive to mechanical vibrations)!

As a result, most directional mics are already well into their LF roll-off by 40Hz, and some start higher still. The more directional the polar pattern (fig-8, supercardioid, hypercardioid...) the greater the proportion of pressure-gradient operation and the higher the roll-off tends to start. Conversely, the smaller the proportion of pressure-gradient operation and the greater the pressure operation element, (e.g hypo-cardioid), the more bass extension there will be.
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