Takeshi Fujimori

Final report on key comparison APMP.AUV.A-K1

A regional key comparison APMP.AUV.A-K1 has been carried out for the pressure sensitivity of laboratory standard microphones. The National Metrology Institute of Japan (NMIJ) piloted this project. Two LS1P microphones were circulated through nine national metrology institutes and calibrated in the frequency range from 63 Hz to 8 kHz. Deviations from the mean value are below ? 0.05 dB for all the frequencies. Results have been linked to CIPM key comparison CCAUV.A-K1 by introducing a correction factor. For every participant, the degree of equivalence with a key comparison reference value is within the corresponding expanded uncertainty. Main text. To reach the main text of this paper, click on Final Report. The final report has been peer-reviewed and approved for publication by the CCAUV, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Instability of the voltage transfer function for an MR103 microphone in a coupler calibration technique

Abstract MR103 microphones are commonly used laboratory standard microphones in Japan. When the pressure sensitivity of MR103 is calibrated by using a coupler calibration technique, higher accuracy is difficult to achieve due to deviations in the measured voltage transfer function. These deviations are peculiar to MR103, but not to B&K4160, which is also a commonly used laboratory standard microphone throughout the world. Such deviations occur even when the measurements are done consecutively under the same measurement conditions, such as polarization voltage, temperature, and static pressure. This study experimentally and theoretically considered one of the possible reasons for this deviation. The results reveal that (a) this deviation can be explained by changes in the microphone’s parameters, such as the distance between a microphone’s membrane and back-plate, and the tension of the membrane, (b) grease used to prevent leakage of gas and sound out of the coupler might be one of the reasons for this deviation, and (c) insertion of polished sapphire spacers between the microphones and the coupler might help diminish this deviation.

A new traceability system in Japan

In 1993, the measurement law was revised and enacted in Japan. The old law had provided only the use of legal metrology units and the approval and verification systems for measuring equipment. The new one has added the provision of a traceability system to match the needs for a high accuracy measurement technique and international mutual recognition. In the Electrotechnical Laboratory (ETL), the national standard of sound‐pressure level has been established and maintained in an audible frequency range. The type approval test and verification of sound level meters have been carried out in the Japan Quality Assurance Organization (JQA). Now, in conformity with the law, ETL has started to construct the new traceability system for the acoustical standards in cooperation with JQA. An outline, is made of the traceability system in Japan and of the newly constructed pressure calibration systems for laboratory standard microphones.

Free‐field calibration of 1/4 inch microphones for ultrasound by reciprocity technique

Recently, equipment that radiates ultrasound radiation at frequencies far beyond the audible range is increasing in our environment. Such electronic equipment has switching regulators or inverter circuits, and many devices are unintended sources of ultrasound radiation. However, the effects of airborne ultrasound on human hearing and the human body have not been well investigated. To estimate the potential damage of airborne ultrasound radiation quantitatively, it is necessary to establish an acoustic standard for airborne ultrasound because the standard is a basis of acoustic measurement. With the intention of establishing a standard on airborne ultrasound, a free‐field calibration system with an anechoic chamber was produced. The principle of free‐field calibration techniques is introduced in this presentation. Type WS3 microphones (B&K 4939) were calibrated in the system to examine the calibration ability to be achieved. Results showed that it can calibrate a microphone from 10 to 100 kHz with dispersi...

Development of a laser‐pistonphone for an infrasonic measurement standard

Acoustical standards for audio frequencies are based on pressure sensitivities of laboratory standard microphones calibrated using a coupler reciprocity technique. There is a growing need to extend the frequency range downward for reliable infrasonic measurement. The reciprocity technique, however, has limitations on low‐frequency calibration (1–20 Hz) because signal‐to‐noise ratio deteriorates and a sound leak occurs from capillary tubes that equalize the static pressure inside and outside of the coupler. These factors rapidly increase the measurement uncertainty as the frequency is lowered. NMIJ has therefore recently developed a laser‐pistonphone prototype, which enables precise calibration of microphones at low frequencies. Compared with the reciprocity technique, the laser‐pistonphone produces a higher sound pressure within a cavity by the sinusoidal motion of a piston and has a significantly improved signal‐to‐noise ratio. Sound pressure is calculated from the piston displacement, which is determine...

Free‐field calibration of ‘‘1/2‐in.’’ microphones

A precise free‐field calibration system of type LS2 microphones by the reciprocity technique is proposed. A source and a receiver microphone are each attached to a cylinder of 1/2 in. diameter, respectively, and the cylinders are set up perpendicularly at the center of the anechoic room. The output voltage of the receiver microphone and the input voltage of the source microphone are measured by an FFT analyzer, and the frequency transfer functions are averaged to improve the signal‐to‐noise ratio. To reduce the influence of the disturbing reflections in the anechoic room, the gating technique is adopted. Values for the position of the acoustic centers of MR112 microphones were measured by two methods—gating and conventional. The uncertainty of the values by the gating method was less than by the conventional method.

An estimation of absorption coefficients measured in a small reverberation chamber

This paper describes a simple and practical technique for measuring absorption coefficients of a material utilizing a small reverberation chamber. The chamber is used as a sound insulation box as well as a chamber for a reverberant sound field. The chamber does not have an ideal diffuse sound field but a sound field just for absorption measurements. A method based on the sound intensity technique using array microphones has been adopted to measure the absorption coefficients. Many loudspeakers are arranged in the chamber in order to improve the directivity distribution of the incident sound power near the surface of the absorbing material and m simulate a random incident sound field. Theoretical and experimental considerations regarding the effect of multiple sound sources in a small reverberation chamber and regarding the dependence of measured reverberant absorption coefficients on the distribution of sound power directivity are presented.

A study on the standard for sound intensity measurement

A study on a traceability system for sound intensity standards required for accurate sound intensity measurements is described in detail. Required characteristics for the system, such as a calibration method for sound intensity meters involving a field calibrator and a working standard, are discussed. A free‐field calibration method using a progressive wave or a standing wave in an anechoic room is proposed. This is valid for the estimation of accuracies of intensity measurements in complicated actual sound fields. Some ideas on the generation of a calibrating sound field and a standard sound intensity probe are given. The standard probe is composed of several pressure microphones mounted on a rigid sphere and is precisely calibrated using a laboratory standard condenser microphone. To obtain enough accuracy in intensity measurements using the system, five sound intensity meters on the market were tested in various sound fields. The results show that the proposed traceability system is valid for sound int...

An automated coupler calibration system for laboratory standard condenser microphones

An automated coupler calibration system for laboratory standard condenser microphones is manufactured for trial use. Active couplers whose cylindrical wall vibrates as a sound source are designed. These couplers are used so as to get pressure response levels of microphones in a short time and to reduce manual transaction. A microcomputer controls the system, processes measured values, examines them, and stores final results in a diskette. In addition, many digital techniques are adopted for noise reduction and for the convenience of calibration. Accuracies were tested for 1‐in. condenser microphones, such as MR103, using a coupler which has the same dimensions as the 20‐cm3 a coupler. Pressure response levels calibrated by the system agree well with those calibrated by the ordinary precise coupler calibration system within 0.2 dB at the frequencies 100–10 000 Hz.