Acoustic Standard

We have established and maintained the national standard of sound pressure level in the frequency range of 20Hz to 20kHz.

We are responsible for calibration of laboratory standard microphones by the Measurement Law in Japan. Model MR103 microphones whose diameter is about one inch, and model MR112 microphones whose diameter is about a half inch are used as the laboratory standard microphones, which are named type I and type II in the Japanese Industrial Standard (JIS C 5515), respectively. These are also conformed to type LS1P and LS2bP in the IEC publication 61094-2, respectively.

An acoustic coupler is used for pressure calibration of the laboratory standard microphones, based on the reciprocity technique. Correcting several effects in the coupler such as wave-motion and heat conducting, accuracy in the calibration is confirmed within 0.04 dB for type I and within 0.10 dB for type II.

For the free-field calibration, precise researches have been conducted in an anechoic chamber. In the case of LS1P microphones, analytical values of free field correction, which are difference values between the pressure sensitivity and the free-field sensitivity, agreed with measured ones. We now have been invesitigating in case of LS2P microphones.

Verification of Sound Level Meters

1 . Inspection of Verification Standards

The manufactures of sound level meters are imposed by the law to be always equipped with verification standards which passed the inspection. They are type I condenser microphones used as laboratory standards. In this inspection, which is carried out in the NMIJ, pressure sensitivity is calibrated by the reciprocity technique.

2. Type Approval Test

In verifying sound level meters, the type approval system is adopted in order to improve its efficiency. The inspection of the structure and the performance as well as the endurance test is carried out in the JQA (Japan Quality Assurance Organization).

3. Inspection of Individual Sound Level Meters

For the individual sound level meters which belong to the approved type, an inspection of instrument error is only required to determine the acceptance. The verification is also carried out by the JQA.

The Quality System

Activities

-Primary calibration of pressure sensitivity level of laboratory standard microphones (LS1P & LS2P) by using the reciprocity technique (IEC 61094-2).
-Primary calibration of free-field sensitivity level of laboratory standard microphones (LS1P & LS2P) by using the reciprocity technique (IEC 61094-3).
-Comparative calibration of working standard microphones in an anechoic chamber.
-Participation in inter-comparisons, such as pressure calibration of type LS1P microphones in CCAUV.A-K1 key-comparison (1999-2000).

Calibration services

-Transfer of primary standards based on the Japan Calibration Service System (jcss) to applicants
-Transfer of primary standards based on a verification service system of sound level meters to applicants

Peer review

October 2002, the peer review was done to evaluate the quality system of the sound pressure level standard against the technical requirements of the ISO/IEC 17025. The reviewers were Dr. Sang Joon Suh (KRISS), Mr. Bruce Meldrum (CRIRO), and Dr. H-J von Martens (PTB).