hyperacoustic zone

hyperacoustic zone

[¦hī·pər·ə¦kü·stik ‚zōn]
(geophysics)
The region in the upper atmosphere, between 62 and 100 miles (100 and 160 kilometers), where the distance between the rarefied air molecules roughly equals the wavelength of sound, so that sound is transmitted with less volume than at lower levels.