The National Acoustic Laboratories' procedure for selecting the saturation sound pressure level of hearing aids: theoretical derivation.

This paper presents the derivation of a procedure for prescribing the saturation sound pressure level (SSPL) of hearing aids. The procedure is designed to be used with either measured values of loudness discomfort level (LDL) or with hearing threshold values alone. SSPL needs to be low enough to prevent the hearing aid from causing loudness discomfort to the aid wearer but high enough to prevent the hearing aid from being excessively saturated by speech. The maximum SSPL likely to be acceptable can be predicted by measuring LDL or by estimating LDL from hearing thresholds. The minimum SSPL likely to be acceptable can be predicted by calculating, for any particular hearing loss, the amount of gain likely to be needed and hence the SSPL needed if the speech input signal is continuous discourse at an overall level of 75 dB SPL. The midpoint between the minimum and maximum acceptable SSPL values is defined as the optimal or prescribed SSPL, and the three frequency average (3FA; 500, 1000, and 2000 Hz) value of this can be predicted from the 3FA hearing thresholds. Alternatively, the SSPL prescription at each frequency can be prescribed on the basis of the hearing aid gain at each frequency. For either method, the SSPL prescription needs to be increased for people with a conductive component to their hearing losses. The SSPL prescription, when referred to a 2-cc coupler, needs to be decreased for infants, for deeply inserted hearing aids, for multichannel hearing aids that limit SSPL separately in each band, and possibly for nonlinear hearing aids. The 3FA SSPL prescribed for persons with a sensorineural hearing loss increases linearly from 89 dB SPL for normal hearing to 107 dB SPL for a person with a 60 dB HL 3FA loss, and then linearly again to 139 dB SPL for a person with a 120 dB HL 3FA loss. The procedure predicts that the acceptable range of SSPLs is very wide for people with mild losses but is vanishingly small for people with profound losses.