Genius!3: Shioto Okita and the rectangular diaphragm

The brains behind Audio-Technica's pioneering products - part three
By Jon Chapple,

The head of Audio-Technica’s microphone development division pioneered a design that combines four tatami-style diaphragms into one mega transducer…

Tell us about your invention.

I figured out how to combine four rectangular diaphragms into one ultra-large diaphragm in order to achieve improved performance. This is the design concept used in the AT5040 and AT5047 microphones.

How did you come up with the design?

There are certain factors limiting the performance of microphones, which work by combining sound waves, mechanical vibrations and the electrical signal in the diaphragm.

If we are considering, for example, a frequency response of 20Hz to 20kHz – the hearing range for humans – with a sound velocity of 340m/s – the speed of sound – wavelength can vary from 17m to 17mm. Additionally, from 0dB SPL to 140 dB SPL, the sound pressure varies from 0.0002Pa to 200 Pa. The diaphragm, which is the most important part of the microphone, needs ideally to ‘accept’ these characteristics as they are – but in reality, the diaphragm has its own weight and a specific stiffness, which can limit the overall performance of the microphone.

Regarding the electronic signal, condenser microphones need phantom power, which also impacts the maximum output level – while electrical noise and distortion can also affect the performance.

To achieve the highest performance, a microphone needs to balance each of these aspects. This is something I, as a microphone developer, need to consider, and I am continuously thinking about new ideas every day.

One day, after many years of trial and error, the AT5040 was suddenly born. The idea was to put the rectangular-shaped diaphragms together like a Japanese tatami mat [a traditional rectangular floor covering] in order to construct one large transducer to improve the performance.

Different timings and different ideas overlapped and the prototype for the AT5040 was finished straight away.

How does it work?

To achieve the greatest purity of sound, the circuit uses a buffer amplifier for the first and third elements of the four-part diaphragm, and through these feeds signals to the summing elements of the second and fourth elements. With this proprietary summing method, I was able to quadruple the microphone’s sensitivity while keeping self-noise low.

The AT5040 provides a high sensitivity of -25dB SPL, and a S/N ratio of 89 dB, while keeping the maximum input sound level at 142dB SPL.

Was anyone else involved in its development?

Yes – this idea did not come from me alone. All the know-how of microphone engineering pioneers, from whom I learnt a lot, and my engineering colleagues, who gave me a lot of inspiration, were also important factors in this idea.

What are your plans for the future?

I want to continuously research the unknown possibilities of acoustic transducers.

Modern analogue microphones are required to be compatible with digital equipment –but this digital processing comes after the transducer process, and I think that the fundamental part of the microphone, which is the diaphragm, will not change. Hence, we can say that if the diaphragm does not change, the basic performance of the aimed microphone will not change in the future either.

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