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To investigate the predicted threshold shift associated with the use of nonlinear hearing aids fitted to the NAL-NL2 or the DSL m[i/o] prescription for children with the same audiograms. For medium and high input levels, we asked: (1) How does predicted asymptotic threshold shifts (ATS) differ according to the choice of prescription? (2) How does predicted ATS vary with hearing level for gains prescribed by the two prescriptions?A mathematical model consisting of the modified power law combined with equations for predicting temporary threshold shift (Macrae, 1994b) was used to predict ATS.Predicted threshold shift were determined for 57 audiograms at medium and high input levels.For the 57 audiograms, DSL m[i/o] gains for high input levels were associated with increased risk relative to NAL-NL2. The variation of ATS with hearing level suggests that NAL-NL2 gains became unsafe when hearing loss90 dB HL. The gains prescribed by DSL m[i/o] became unsafe when hearing loss80 dB HL at a medium input level, and70 dB HL at a high input level.There is a risk of damage to hearing for children using nonlinear amplification. Vigilant checking for threshold shift is recommended.

Excessive amplification by hearing aids causes temporary threshold shift (TTS) and permanent threshold shift (PTS). This investigation addressed the question whether it might be possible to predict the eventual amount of PTS caused by excessive amplification from the amount of TTS it causes after a day of hearing aid use. Asymptotic TTS (ATS) to be expected as a result of hearing aid use was predicted for 8 children with sensorineural hearing loss and the predicted ATS was compared with observed permanent deterioration of their thresholds attributed to hearing aid use. There was good agreement between the predicted ATS and observed PTS at 500 to 2000 Hz. It follows that, for prediction of PTS caused by hearing aid use, the mean of the sound levels produced in the ear by the hearing aid is the correct equivalent continuous level (ECL) to use and that the Modified Power Law (MPL) is the appropriate method of adjusting the predictions for sensorineural hearing loss, because these have been shown to be appropriate for prediction of TTS caused by hearing aid use. Predictions of the PTS to be expected for the children that were carried out using the MPL and the mean level as the ECL were in good agreement with the observed PTS at 500 to 2000 Hz, whereas predictions of PTS based on an alternative method of correction for sensorineural hearing loss (the Continuation Hypothesis) were significantly less than the observed amounts. The results of the PTS predictions therefore confirmed the conclusions drawn from the results of the ATS predictions.

To evaluate the validity of the National Acoustic Laboratories procedure for determining percentage loss of hearing as a measure of hearing disability.The percentage hearing losses of war veterans who had hearing ranging from normal to profound deafness were determined and compared with their scores on two hearing questionnaires.A self-report hearing questionnaire was completed by 282 war veterans and 154 of those veterans were given the hearing measurement scale in the form of a structured interview.A percentage loss of hearing of 0 agreed well with the questionnaire scores representing the limit of normal hearing, and a percentage loss of hearing of 100 agreed well with the questionnaire scores representing total loss of hearing. Percentage loss of hearing accounted for 83% of the variance in scores on the hearing questionnaire and 81% of the variance in scores on the hearing measurement scale.The National Acoustic Laboratories procedure for determining percentage loss of hearing provides a valid measure of hearing disability.

This study used a well-verified mathematical model to predict asymptotic temporary threshold shift (ATS) caused by hearing aid use. The model determined the amounts of ATS to be expected if real ear insertion gains (REIGs) recommended by the current National Acoustic Laboratories (NAL) procedure are used. It also determined the consequences of use of excess REIG and of high input levels to hearing aids. If recommended REIGs are used and input levels are normal (average A-weighted input levels of about 61 dB SPL), ATS is unlikely to occur for clients who have typical audiograms with three-frequency average pure-tone thresholds (PTAs) less than 60 dB HL. For people with PTAs greater than 60 dB HL, small amounts of ATS can be expected to occur during hearing aid use, but these amounts of ATS are safe, that is, unlikely to be associated with permanent threshold shift (PTS) for individuals with all PTAs except those greater than about 100 dB HL. If REIGs are 15 dB greater than those recommended, the amounts of ATS will be unsafe for people with PTAs greater than about 80 dB HL. It appears unwise for clients who have this degree of hearing loss to use excess REIG. The use of excess REIG in high ambient levels of sound (average A-weighted input levels of about 75 dB SPL) is likely to cause PTS for hearing aid users with PTAs of about 50 dB HL or greater. Clients who prefer to use excess REIG should therefore avoid high ambient sound levels. The amount of amplification required for people with PTAs greater than about 100 dB HL is likely to cause PTS and is therefore inherently unsafe.

There is a definite risk of overamplification by hearing aids. Guidelines should therefore be established that will minimize the risk of damage to hearing involved in hearing aid use. A mathematical model that can be used for this purpose is derived from equations for predicting noise-induced permanent threshold shift given in International Standard ISO 1999 combined with the Modified Power Law. The model implies that any noise exposure that would cause deterioration of the hearing threshold levels of a person with normal hearing would also be harmful to the hearing of a person with sensorineural hearing impairment. It follows that, in order to ensure that no deterioration occurs in the hearing of a hearing aid user, the output levels from the aid must be such that they would not cause any damage to a person with normal hearing. This constraint can be met for hearing aid users with mild-to-moderate sensorineural hearing loss but cannot be met for users with severe-to-profound loss because it would result in the provision of insufficient gain, particularly at the higher frequencies. If the model is valid, then for this group, some appropriately small amount of hearing damage must be accepted as the cost of the advantages gained from the use of a hearing aid. Verification of the model is essential before the model is used in clinical practice to determine the risk of deterioration in hearing due to hearing aid use.

Humes and Jesteadt have proposed that the Modified Power Law (MPL) provides a means of predicting permanent threshold shift (PTS) due to noise exposure in subjects with preexisting sensorineural hearing loss. Data concerning PTS attributed to overamplification by hearing aids in 8 children with severe sensorineural hearing loss were used to evaluate the MPL hypothesis. The excessive amplification was partly due to use by the children of very high volume-control settings instead of mid-range volume-control settings. The PTS tended to be flat across frequency. Its course in time was a miniature version of the time course of PTS that would be induced by a similar noise exposure in a person with normal hearing. It began to occur soon after the start of hearing aid use and its rate of development was slower than that which would occur in a person with normal hearing. The growth of PTS could be predicted from the estimated real ear output levels of the children’s hearing aids by means of the MPL combined with the logarithmic equation proposed by Kraak for predicting the effect of noise exposure on hearing.

An investigation was carried out into effects of three types of impression build-up (patting down of impressions, special earmould-maker build-up and the multistage impression technique) on the dimensions, static pressure seal, degree of acoustic seal and the subjective tightness and comfort of earmoulds. Patting down the impression significantly improved the degree of acoustic seal provided by earmoulds without making them feel tighter or less comfortable. However, special build-up was much more effective than patting down and the multistage impression technique was slightly more effective than special build-up in improving the degree of acoustic seal. The improvement in acoustic seal provided by both multistage and specially built-up earmoulds can usually be obtained without an unacceptable level of discomfort. Patting down the impression did not improve the chance of obtaining a static pressure seal. Special build-up of the impression by the earmould-maker significantly increased the proportion of earmoulds which provided a static pressure seal but an even higher proportion of earmoulds made from multistage impressions provided a seal. Dimension results indicated that an increase in earmould-maker build-up of the minor axis at the beginning of the canal segment of the impression would improve the acoustic seal provided by specially built-up earmoulds and that earmoulds with rounder tips are more likely to provide a static pressure seal than earmoulds with more elliptical tips. The better the impression material fills the ear canal, the rounder the tip of the impression, and the rounder the tip of the earmould made from the impression.

Temporary threshold shift (TTS) caused by hearing aid use was measured by Bekesy audiometry in a group of individuals with severe sensorineural hearing loss. The accuracy with which a mathematical model consisting of the Modified Power Law (MPL) (Humes & Jesteadt, 1991) combined with equations for predicting TTS in normal listeners (Mills, Gilbert, & Adkins, 1979) could predict the TTS was evaluated. When the exponent of the MPL was set to 0.15, the predicted TTS was significantly greater than the observed TTS at two out of six frequencies. When the exponent was increased to 0.20, there were no significant differences between the predictions and the observations. With this value of the exponent, the mathematical model was able to predict the observed TTS as accurately as it could be measured. The MPL was used to derive safety limits for TTS, and gain reduction was recommended as the best method of reducing TTS to the safety limits.

Temporary threshold shift (TTS) over a wide range of frequencies was found after 4 hours of hearing aid use by a 15-year-old student with severe sensorineural hearing loss who was using real-ear insertion gains 10 to 20 dB greater than those recommended by the current National Acoustic Laboratories (NAL) procedure for selecting the gain and frequency response of hearing aids. Measurements were made of her noise exposure during hearing aid use with a noise dosimeter. The real-ear insertion response and input-output function of her hearing aid were measured with a real-ear gain analyzer and were used to calculate in-ear noise levels from the noise levels measured by the dosimeter. The amount of TTS could be predicted from the in-ear noise levels and the student’s hearing levels (HLs) by means of a mathematical model consisting of the Modified Power Law (MPL) of Humes and Jesteadt (1991) combined with equations for predicting TTS in listeners with normal hearing published by Mills, Gilbert, and Adkins (1979). The mean of the instantaneous A-weighted in-ear noise levels proved to be the appropriate equivalent continuous level (ECL) for use in the predictions. The MPL was also used to determine safety limits for TTS due to hearing aid use. The observed TTS exceeded the safety limits at all frequencies up to and including 2000 Hz. It was therefore considered desirable for the girl to use less gain at frequencies from 500 to 1500 Hz.

The concepts of the modified power law (MPL) and the continuation hypothesis (CH) are outlined and then empirical evidence is presented which suggests that the MPL, not the CH, is the appropriate method for predicting noise‐induced permanent threshold shift (NIPTS) in persons with sensorineural hearing impairment. The ISO1999 rule for combining presbyacusis and NIPTS is explained and empirical evidence is presented which indicates that the ISO1999 rule, not the MPL, is the appropriate method for combining NIPTS and presbyacusis. These findings are then used to predict the progression of hearing loss in a person whose thresholds are affected both by aging and by two separate periods of occupational noise exposure. The results are compared with those obtained using the CH instead of the MPL to predict the progression of NIPTS and with those obtained using the MPL instead of the ISO1999 rule to combine presbyacusis and NIPTS.