Compromises in audio (I): Microphones

3 Nov

Since I started my engineering degree, I started to acknowledge (assuming it was way harder) that everything in life is a compromise between a great solution with a high cost and a “just OK” solution that’s much more convenient. Or as his Satanic Majesties The Rolling Stones very wisely said: “You can’t always get what you want/ but if you try, sometimes/ you get what you need“. Pop lyrics are sometimes more than great. Try to express it better and simpler, Yeats!

As I was saying before digressing, there is no perfect solution that covers all of the issues that we might face with no drawbacks at any point. The audio and acoustic world is one of the best exponents of compromises being made to achieve the best possible solution, which is far from perfect but is the best we can get with the technology or knowledge that we have at the present moment.

If we start by the first link in the chain from acoustic source to storage medium, we have the transducer, this is, microphones. And microphones are one of the biggest examples of a compromise solution if we have a look at them. Let’s take the difference between condenser and dynamic mics to start with. Simply put:

  • Condenser mics: Good (fast) time response able to capture accurately transients due to the diaphragm’s low mass, which means great high frequency response… they directivity might be switchable from omnidirectional to cardoid or figure of 8. The drawback? They’re mechanically very delicate and they need a constant voltage between the diaphragm and its backplate (48 V Phantom power or prepolarized electret).

A condenser microphone (Photo by Bill Selak, under CC BY-ND 2.0)

 

  • Dynamic mics: They are cheap to produce, they mechanically robust, resistant to humidity, they can handle a high SPL and they have good middle frequency response. Their directivity is moer commonly cardioid, hypercardioid or supercardioid. The drawbacks? Their time response is very slow due to the diaphragm’s high mass, and their low and high frequency response is not great due to the damping of the mid-band, which makes them not very accurate for orchestral sounds.

A dynamic microhpone (image by arbyreed under CC BY-NC-SA 2.0)

  • Ribbon mics: They were the first microphones to be made since the 30s and 40s. They are usually quite fragile due to the nature of a suspended ribbon rugged coil placed between two magnets. Their sensitivity is quite low, and their high frequency response is not very good either, as it starts to roll off at around 6 kHz, but still they are sought after for their full-bodied, vintage warm sound. This is due to their pronounced proximity effect, as the diaphragm finds its resonance frequency between 50 to 100 Hz. Their directivity is figure of 8 or bidirectional.
Coles microphone

Coles 4038, a classic ribbon mic

Depending on what you need, you’ll make your microphone choice. Let’s go a bit further with this. If we talk only about the capsule size of the microphone, we’ll get more compromise solutions.

  1. Self noise: Smaller diaphragms produce more noise than big ones because of a tricky phenomena called Brownian Motion. This happens because air molecules are constantly hitting the diaphragm thus creating a noise that is inherent to the physic nature of it. In a smaller diaphragm, we have that it acts as a very stiff surface; then, air molecules hit it “harder”, exchaning more energy and producing more noise than on a large diaphragm microphone.
  2. Frequency response: Low frequency response in condenser microphones is controlled only by the small air vents that it has in the backplate of the capsule which prevents it from moving according to changes in barometric air pressure (which is something we don’t want to happen!). Taking that out of the equation, we have high frequency response, which is determined mainly by the diaphragm weight, so a large one will be heavier and then it will have a harder time vibrating at a very fast pace (or frequency) when compared to a small diaphragm microphone. Small diaphragm: very extended HF response; big diaphragm: good HF response.
  3. Sensitivity: As we just said,a smaller diaphragm will act as a hard surface, so sound pressure waves hitting it will produce a smaller electric output than in a bigger diaphragm because it will be easier to move and then translate it more easily into electric signals.
  4. Sound Pressure Level admitted: Then, by the same reason as before, a smaller and harder to move diaphragm will be able to handle a higher SPL than a bigger diaphragm. Just because the diaphragm will move less, it will be more unlikely to reach the backplate, which is something that causes horrible distortion and capsule damage. Small diaphragm: outstanding high SPL handling; bigger diaphragm: less SPL handling, we have to be more careful about the source loudness.
  5. Dynamic range: We call Dynamic range to the difference between the microphone’s self noise floor measured in dB(A) and the maximum SPL that it’s able to handle before producing too much Harmonic distortion. As we have just seen, the noise floor is bigger for small diaphragm microphones, but SPL handling is bigger also than in large diaphragm mics, so we reach an equilibrium between both. I’ll take DPA microphones table for some of their models so that you can compare:

Dynamic range comparison between small and large diaphragm microphones (DPA Microphones)

Not to mention the effect of directivity on the microphone: as the microphone capsule grows bigger, it starts acting as an acoustic “shade area” for wavelengths smaller than it. The wave hitting the diaphragm will be reflected at the surface, creating a sound pressure build-up between incoming and outgoing sound, becoming more directive as frequency rises.

Another issue that we adressed before is that of the directivity of a microphone. This means how well a microphone responds to a sound source according to its position. I think that almost all readers are familiar with terms like “Omnidirectional, bidirectional and cardioid” which describe the pickup polar pattern of mics. These describe from where is the microphone more sensitive to the arrival of sound waves. This figure from Excelsior Audio might help:

Microphone polar patterns

Microphone polar patterns

This means that an omnidirectional microphone will be likely to capture sound pressure waves equally well if the source is behind it or at a side of it. For bidirectional microphones, it is only likely to pick up the sound if the source is either in front of them or behind them, but it will theorically produce no output if the source it placed at a side of it. For cardioid microphones, they will work if the source is only in front of them or slightly at the sides, but it will produce no output if the source is behind it.

That’s what the theory says. But there are more complex facts in directivity:

  • Directivity is not constant with frequency: Microphones tend to be almost omnidirectional at low frequencies and very directional at high frequencies.
  • Directional microphones have an effect at low frequencies called “Proximity effect” that makes their response be too hyped when the source is very near the microphone (less than 15 cm. mostly).

Depending on the application, we will need a directive microphone (cardioid, super or hypercardioid) for live sound reinforcement (avoid feedback from monitors) or to prevent leakage in a studio because we need to separate the sound sources playing at the same time.

So, as you might have seen, we are always making decisions on what suits our particular application, as there is no ideal solution. We get what we need, most of the times.

Next time, I’ll be talking about the following piece in the recording equipment, which is the microphone preamplifier and the electronics underneath. As microphones are analog in nature, they need at least something capable of handling voltage (not bits of 0s and 1s) so we can’t get away from the Analog stage before going to digital. Very soon, in your screens!

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