High-frequency-noise-induced hearing loss: a field study on the role of intensity level and accumulated noise dose
SummaryBased on audiometric tests in the range of 10 to 20 kHz, of 106 ultrasound operators, as well as on measurements of high-frequency noise, the problem of safety limits for high-frequency noise exposure was investigated. Analyzing the relation between noise levels of 1/3 octave bands at center frequencies of 10, 12.5 and 16 kHz and the accumulated noise dose on the one hand, and changes of hearing at 10 to 12, 11 to 13 and 14 to 16 kHz respectively, on the other hand, a harmless level up to 80 dB and a harmless noise dose up to 1 unit for people not older than 40 years have been found. For older people this level and this noise dose can be dangerous.
Does airborne ultrasound lead to activation of the auditory cortex?
As airborne ultrasound can be found in many technical applications and everyday situations, the question as to whether sounds at these frequencies can be heard by human beings or whether they present a risk to their hearing system is of great practical relevance. To objectively study these issues, the monaural hearing threshold in the frequency range from 14 to 24 kHz was determined for 26 test subjects between 19 and 33 years of age using pure tone audiometry. The hearing threshold values increased strongly with increasing frequency up to around 21 kHz, followed by a range with a smaller slope toward 24 kHz. The number of subjects who could respond positively to the threshold measurements decreased dramatically above 21 kHz. Brain activation was then measured by means of magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI) and with acoustic stimuli at the same frequencies, with sound pressure levels (SPLs) above and below the individual threshold. No auditory cortex activation was found for levels below the threshold. Although test subjects reported audible sounds above the threshold, no brain activity was identified in the above-threshold case under current experimental conditions except at the highest sensation level, which was presented at the lowest test frequency.
Tentative Audiometric Threshold‐Level Standards from 8 through 18 kHz
Both ears of 100 young healthy men with no previous military noise exposure, aged 17–23 yr and normal by otoscopy but otherwise unselected, were given Bekesy‐type audiometry through 18 kHz with the Rudmose high‐frequency ear‐canal insertion transducer. Minimum audible pressures (MAP) were obtained in approximate agreement with published studies on children and on adolescent males. There is also correspondence with the British 1954 Standard MAP up to 15 kHz. It is suggested that the time is approaching for standardizing audiometry in the octave 10–20 kHz by using all available data.