vibration research at the Kobayasi Institute of Physical Research, knows the whole story of how Professor. Ishii’s idea was made into a product. Dr. Hirao explains.“e earliest model of the acoustical capacity and volume meters was actual-ly created based on an idea put forth by Professor. Ishii to measure the volume of a baby to calculate its body fat per-centage. He was looking for a way to achieve the practical use of a mecha-nism for measuring the volume of a baby’s body using sound, by placing the baby inside a large tube-like container. While the mechanism in this project never came into practical use, related know-how was accumulated. Aer his retirement from the university, the very rst instrument created as a product was an instrument for measuring the volume of reference weights for scales.”e instrument was developed and marketed jointly by the National Institute of Advanced Industrial Science and Technology, Measurement Science Laboratory, Kobayasi Institute of Physical Research, and Rion in 1995. Development of the acoustical capacity meter then began aer a major Japanese engine manufacturer submitted a request for an acoustical capacity meter.“Professor. Ishii wanted to produce an instrument that would measure the volume of a baby in just a few seconds aer placing the bay inside a container resembling a cradle. e underlying principle of such an instrument lies at the foundation of our current instru-ments. e real innovation of these instruments is that they can measure capacity and volume quickly, while keeping the measurement target dry.”e instrument is currently used in a limited range of applications, like measuring engine capacity, inspecting the dimples on golf balls, and measur-ing the volume of reference weights for scales. I asked Dr. Hirao about potential applications in other elds.“ere was an attempt recently to measure surface area using sound. For example, the surface area of gears. You know how gears have jagged edges around their sides? You can’t really use a ruler to measure the surface area of fea-tures like this. Coatings are applied to these complex surfaces during the gear manufacturing process. To know the amount of coating agent needed before-hand, you have to measure the precise surface area. While this attempt was not successful, I think it’s highly likely that an instrument for measuring other tar-gets using sound will appear in the future. As long as the measurement target doesn’t deform when subjected to sound waves, then theoretically we can measure it quickly, precisely, and in a dry condition.”e acoustical capacity and volume meters highlighted in this interview are by no means widely used around the world, but they have undoubtedly come to play an important role in industries that sought these functions. I asked Dr. Hirao one last time about what he saw as the signicance of these instruments.“First, there’s no doubt at all about the groundbreaking nature of Professor. Ishii’s theory. I also admire the knowl-edge and eorts of all those involved in putting it to actual use. Above all, I think it’s wonderful that even if no practical application was realized ini-tially, the know-how was steadily accumulated and passed on to the next generation of product development, and even if the form, conguration, and purpose diered from the original con-cept, it was introduced to the world as products which contribute to society. I’m honored to have been a part of that development.”Principle of measurementAccording to the laws of Boyle and Charles, “The volume V of a given mass of a gas is inversely proportional to its pressure P at a constant temperature.” Based on this principle and using a calibrator, acoustical capacity and volume meters measure capacity or volume by artificially inducing a change in pressure.MicrophonesElectret condenser microphones are used inside the instrument. These microphones detect pressure changes, and the ratio is used to calculate volume or capacity.V2V1⊿P1⊿P2=-VV2V0=（V1：constant)γ⊿V=⊿P1P0P0V1γ⊿V=⊿P2P0V2（V0：constant）（Pressure）×（Volume）=constant(γ: ratio of specific heat of air; γ = 1.4)：Static pressure inside chamber (atmospheric pressure)⊿P1：Minute pressure change inside reference chamber⊿P2：Minute change in pressure of the space combining the inside of the attachment and the measurement target (for capacity meter)Minute change in pressure of the space combining the inside of the attachment and the measurement chamber excluding the measurement target (for volume meter)γ5Yoshihiro HiraoSenior researcher (Ph.D. in engineer-ing), Kobayasi Institute of Physical Research. Yoshihiro Hirao is an engineer and expert in noise and vibration mea-surements of machines, structures, road trafc, and aircraft using microphones for acoustic measurement, sound level meters, and acceleration sensors. Previously, during a temporary transfer to Rion, he contributed to the develop-ment of the acoustical capacity meter and acoustical volume meter.
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