90 dB low-frequency sound recorded in classroom during Osprey take off
November 16, 2012 Kenta Masuda of Ryukyu Shimpo
On October 29, Takeshi Tokashiki, an associate professor of the University of the Ryukyus, conducted a low-frequency sound measurement survey in No. 2 Futenma Elementary School when the MV-22 Osprey transport aircraft took off from U.S. Marine Corps Air Station Futenma in Ginowan. According to the survey, a 90.9 dB low-frequency sound was measured at 20 Hz in a closed classroom fitted with soundproof windows. This exceeded the measurement taken on the roof of the school building on a different day.
The Ministry of Defense has installed the best quality soundproofing insulation in the school, but questions are being raised about its effectiveness because the characteristic noise generated by the Osprey aircraft was recorded in the room.
According to the Ministry of the Environment, the low-frequency noise spectrum is in the range of 1-100 Hz, which creates a feeling of pressure, discomfort and shakes furniture. The Ministry of Defense organizes the noise insulation work, which is carried out with gradations of effectiveness from 1 to 4. While the government requires an average noise reduction of more than 35 dB at 125-4000 Hz, in the provisions for Grade 1 there is no provision for mitigation of low-frequency sound.
The team led by the associate professor carried out the low-frequency sound survey at nine steps from 12.5 Hz to 80 Hz on the roof of the school building on October 10 and in the classroom on October 29, when the Osprey took off in helicopter-mode using the same flight path.
In particular, the maximum value of the low-frequency sound was confirmed at 20 Hz. While 85.2 dB were recorded on the roof at 1:31pm, on October 10, 90.9 dB were measured in the closed room at 19:06pm, on October 29.
These figures are greater than the 80 dB at 20 Hz, which is the threshold value for physical impact stated in the environmental impact assessment that the Okinawa Defense Bureau created as part of the Henoko relocation plan for U.S. Marine Corps Futenma Air Station.
In its decision on the Futenma roar lawsuit in 2010, for the first time the appeal court recognized the causal relationship between low-frequency sound and mental and physical damage. On November 7, in the Cabinet Committee of the House of Representatives, Chief Cabinet Secretary Osamu Fujimura commented, “If necessary, the government will continue to conduct scientific investigations on noise.” However, the government does not carry out low-frequency sound measurements.
Residents of the Ojana and Kakazu districts of Ginowan complained that since the Osprey has been deployed chairs tremble when there is a deep bass sound, and that dishes have fallen off shelves.
Tokashiki said, “The building is made of concrete, but the low-frequency sound enters the rooms through the aluminum sash windows. The problem will not be solved unless the government provides adequate sound proofing measures.”
(English translation by T&CT, Mark Ealey)
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