H. Håkanson

Comparison of the Hearing Threshold Measured by Manual Pure-Tone and by Self-Recording (Békésy) Audiometry

The hearing threshold of 115 subjects aged 25--63 years and working on a shipyard were determined both by Békésy sweep audiometry and by conventional manual octave pure-tone audiometry at fixed audiometric frequencies. The attenuation rate was 2.5 dB s-1 with pulsed-tone presentation and the sweep time from 0.25 to 10 kHz was 400 s for the Békésy audiometer. Manual pure-tone audiometry was performed in 5-dB steps. The Békésy method gave the lowest values for the hearing thresholds. It has been possible to find a useful linear relation between pure-tone and Békésy hearing thresholds. With the help of a retest experiment, it has been established, that the standard deviations of hearing thresholds obtained under similar conditions in a pure-tone investigation are about twice as large as those obtained in a Békésy investigation.

The Reliability Of Békésy Sweep Audiometry Recording And Effects Of The Earphone Position

The hearing thresholds of subjects with hearing impairment of different severity were tested several times. The reliability of the hearing threshold measurements were analysed and compared to the inter-aural variations, the learning effects, the degree of noise-induced hearing impairment and the application of the earphones. The main factor determining the reliability of the hearing threshold is the application of the conventional earphone. A preliminary test-retest with the earphone adapted to an ear speculum for sound transmission to the external ear canal suggests that the precision of the Békésy sweep hearing threshold measurement can obviously be increased in the frequency range of 2 to 8 kHz.

The effect of static middle ear pressures on the hearing threshold.

The effect of static tympanic pressure gradients on hearing sensitivity was studied by introducing relative underpressure of 5, 10 and 15 cmH2O in the middle ear cavity of six normal ears. A self-recording Békésy audiometer was used to measure the hearing threshold shifts during middle ear pressure equilibrations. The threshold loss was most prominent for 0.5 and 1 kHZ and less for 4 kHZ. A threshold gain was shown for 2 and 6 kHZ. Over all test frequencies the threshold shifts were increased with higher relative underpressure in the middle ear cavity. In order to evaluate the hearing impairment caused by noise, it is therefore important to control the middle ear pressure before hearing is tested. A small change in middle ear pressure can be ignored when using the summed hearing thresholds between 2 and 8 kHZ.

Hearing Deterioration in Shipyard Workers Serial Audiometry over a Four-year Period

The hearing thresholds of a total of 58 workers aged 20 to 65 years were measured annually over a period of 4 years using Békésy sweep-frequency audiometry. Using these data, accurate values of the yearly rate of change of hearing threshold level were calculated. The rate of change of hearing threshold level due to presbyacusis was obtained from several data sets, and the rate of change of hearing threshold level due to noise was calculated. When the average hearing threshold level over the frequency range 2-8 kHz was considered, it was found to be possible to establish a significant rate of threshold shift caused by exposure to noise for yearly changes as small as approximately 2 dB. In the cases investigated about 30% of the ears showed a significant deterioration in hearing ability due to noise. When comparing the permanent threshold shift caused by noise in groups of younger and older workers, it was found that the younger workers exhibited less threshold shift than expected, whereas the older workers showed a greater threshold shift than expected.

Hair cell damage in the inner ear of the guinea pig due to noise in a workshop.

Guinea pigs were exposed to noise in various workshops at a shipyard. The equivalent sound level ranged from 87 to 90 dB(A) and the exposure time was 30 days of 8 hours. The numbers of lost outer hair cells were greater than in control animals, thus establishing a relation between the number of lost hair cells and the total noise dose. A comparison between the results obtained here and results obtained in laboratory experiments shows that although many physical sound parameters may be identical, the effect on the hair cells can be quite different. In particular, the total noise dose is not a parameter which can be directly related to hair cell loss. The most probable explanation for the difference in damage between the laboratory and workshop animals seems to be the rest periods of 16 hours between each of the 8-hour exposure periods.

Comparison between Hearing Threshold Measurements Made with and without the Aid of an Ear Speculum

The hearing threshold levels of a small group of shipyard workers having differing degrees of hearing impairment were measured five times using the following audiometric techniques: fixed-tone Békésy audiometry with MX 41/AR cushions, fixed-tone Békésy audiometry with an ear speculum, and sweep-frequency Békésy audiometry with an ear speculum. From these data the mean standard deviations of the hearing threshold levels for the frequencies 0.5, 1, 2, 3, 4, 5, 6, 7 and 8 kHz were calculated, thus giving a measure of the accuracy of each method. The hearing threshold levels obtained by the three methods are compared, and the merits of data handling, expressing hearing threshold levels in pascals rather than decibels, are also discussed.

An Impulse Sound Generator And Some Comparative Experiments With Different Noise Dosimeters

An impulse sound generator has been constructed which gives the same sound pressure wave pattern during over 10(5) impacts with very small variations. This device has been used in evaluating the response of different types of noise dosimeters for impulse sounds.

Comparison of the hearing threshold measured by pure-tone audiometry and by Békésy sweep audiometry.

The hearing thresholds of 115 subjects, workers in a shipyard, were determined both by Békésy sweep audiometry and by conventional individual pure-tone audiometry at fixed audiometric frequencies. The Békésy method gave the lowest values for the hearing thresholds. It has been possible to find a useful linear relation between pure-tone and Békésy hearing thresholds. With the help of a retest experiment it has been established that the standard deviations of hearing thresholds, obtained under similar conditions in a pure-tone investigation, are about twice as large as those obtained in a Békésy investigation.

Calculation Of Noise Dose From Time Distribution Of Sound Levels

The noise from ten different sites in a machine factory was measured and cumulative time distributions of the sound pressure were constructed. From these distributions it is possible to calculate the energy mean level, the equivalent sound level as well as the average sound pressure level. Calculations based on knowledge of the times during which only a few sound pressure levels are exceeded give values deviating systematically from the exact value. Because of the systematic nature of the deviations corrections can be made for them. The accuracy of this method is relatively high.

Noise Dose Measurements With Stationary And Ear-Borne Microphones

Precision measurement of noise doses with stationary and mobile, ear-borne, microphones were made on ten workers (emery-grinders, welders and platers). The workers selected in each group differed from one another in the amount they moved about during their working day. The differences in the reverberant field at their working places were also taken into account. Very large noise dose differences, up to 20 dB, between stationary and mobile dose measurements were found and it was not possible to find any correction factors for these differences.