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Anechoic Chambers
NMIJ has two anechoic chambers (large and small ones) for the various acoustical measurements.
Sound noise and echoes always disturb precise measurements of sounds.
In the anechoic chambers, such disturbances are controlled to become very small.
To eliminate sound echoes, sound absorbing wedges that are made of glass wool are attached to all the walls, the ceiling and the bottom.
The floor is made of steel wire net.
The large anechoic chamber is set in the large concrete building with thick walls so that sounds from the outside can hardly come into the chamber.
Further, the chamber is isolated from the building by rubber spacers in order to remove the vibration that travels from the ground.
The Large Anechoic Chamber
| Name | Inner Space(m) | Cutoff Frequency
of Wedges |
|---|
| Large Chamber | 9.5W X 8.0D X 7.2H
(5.6H Above Floor) | lower than 40Hz |
| Small Chamber | 5.4W X 3.8D X 4.6H
(3.7H Above Floor) | lower than 85Hz |
Free-Field Calibration of Laboratory Standard Microphones
The standard microphone has been calibrated with an acoustic coupler based on the reciprocity theorem.
As the coupler calibration method has low uncertainty,
we have been providing the pressure sensitivity of a standard microphone as the acoustic standard.
On the other hand, we need the free-field sensitivity for deciding sound pressure in the free space.
For the free-field calibration, a high performance anechoic chamber and high precision measurement technologies are needed.
Revising the Equal-Loudness Level Contours
Researchers from Tohoku University and Sendai National College of Technology occasionally use the anechoic chamber for measurement of loudness.
They are working for revision of the equal-loudness level contours.
Equal-Loudness Level Contours
When tones of different frequencies are perceived to be of the same loudness,
the levels of these tones can be combined into the equal-loudness level contour.
Equal-loudness level contours measured by Robinson and Dadson (1956) have been adopted by ISO.
Since it has been claimed that recent data do not agree with those of Robinson and Dadson,
revision of the equal-loudness level contours is required.