A microphone with FOUR diaphragms! Really?

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Record-Producer.com.

Friday September 22, 2017
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Firstly, I don't expect every visitor to this site to be an audio expert, so it's worth saying that the diaphragm of the microphone is the part that contacts the air and detects sound. Normally it is round and flat. And normally there is just one diaphragm in a typical microphone. Exceptions are ribbon mics where the diaphragm is likely to be rectangular and corrugated. And mics that have one diaphragm for low frequencies and another for high. But these exceptions are very much in the minority.

So, one diaphragm that is round and flat. Not good enough says the Audio Technica AT5047. Here are larger versions of the images for your detailed consideration...

Audio-Technica AT5047 external

Audio-Technica AT5047 internal

Why?

Well that has to be the obvious question. One answer might be that the microphone marketplace is extremely crowded and Audio-Technica just wanted something different, and an interesting feature to promote. This might be true of many pro audio products and is worth considering when you are reading manufacturers' marketing materials.

But there is an interesting point to this development. The virtue of a large diaphragm in a microphone is that it contacts a lot of air molecules. Air molecules, as you might remember from school science, are subject to Brownian motion where the molecules vibrate at random. Microphones interpret Brownian motion as noise. But the great thing is that a larger diaphragm will average out the noise better than a smaller diaphragm will. This is of considerable practical importance and you will hear the difference if you record, such as in classical music, with microphones at a distance from the sound source. Classical guitar is a good example because it is a quiet instrument, yet sounds better when the mics are positioned to capture the acoustics of the recording venue (assuming it is a venue suitable for classical guitar).

So a large-diaphragm microphone can be expected to be quiet. But at what cost?

The cost is that the diaphragm is now large enough to have resonances in the audio band. In part this accounts for the sound texture of the large-diaphragm mic and often it is something you would want, to add character to the signal you are recording. But it would be nice to have an ultra-quiet mic that was effectively resonance-free.

So this is the thinking behind the AT5047. Multiple diaphragms mean that each diaphragm can be smaller. Making them rectangular seems obvious because rectangles tessellate where circles do not. Multiple circular diaphragms would waste space.

Also, although it isn't the case that there will be no resonance, a rectangular diaphragm will have one resonant frequency for the long dimension and a different resonant frequency for the short. Two smaller resonances are, to the ear, better than one strong one.

I have to wonder though whether the rectangle is the best shape possible. Maths enthusiasts will probably know of irregular shapes that tessellate, and not necessarily have straight edges. Shapes such as these could distribute resonances more widely. Something for microphone manufacturers to consider perhaps?

Directivity

Another feature, or issue if you like, of large-diaphragm microphones is that they tend to be more directional than small-diaphragm mics. This is due to cancellation effects when sound strikes from an angle. It's something that is worth looking up if you want more detail. I found a polar response chart among Audio-Technica's published information...

Audio-Technica AT5047 polar response

It is a little difficult to make out the details, but if you peer closely enough you will see that the response tightens considerably at higher frequencies. This could make the mic less suitable for use in a stereo pair, or as drum overheads. But never say never - interesting results can often be found by going against conventional wisdom. However the conventional wisdom says that this mic is most likely to perform at its best pointing head-on at an instrument or vocal.

In summary, this is an interesting microphone and new developments are always exciting. By the way, did I mention the price? Er, no I didn't. List price is $3499 USD!

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