Fitting Formulas Estimate amplification requirements of individual patients Maximize intelligibility of speech Provide good overall sound quality Keep amplification within a comfortable range Different fitting formulas used for linear and compression hearing aids Source: Philip T. McCandless (2000). Principles and clinical utility of hearing aid formulas. Textbook of hearing aid amplification. Robert E. Sandlin, (Ed.). San Diego, CA: Cengage Learning.

POGO II: Prescription of Gain and Output ◦ Provides gain and MPO recommendations for mild to severe sensorineural hearing loss. ◦ Slight reduction in low-frequency gain to aid in speech intelligibility in noise ◦ One of the simplest formulas to implement Linear Fitting Formulas 2

Berger o Assumes that speech is presented at an average 55 to 70 dB SPL. o Assumes that the most important frequency range related to speech intelligibility is 2000 to 4000 Hz. o Based upon the 1/2 gain rule: Speech requires amplification of slightly less than half of the audiometric threshold loss o An additional one-fifth gain factor is added for those with conductive losses. Linear Fitting Formulas 3

NAL-R: National Acoustic Laboratories-Revised o Similar to POGO II o Attempts to make all bands of speech equally loud o Uses mathematical scaling corrections slightly different from ½ gain rule o Simple to implement See Figure 1 here for a comparison of the different linear fitting formulas.here Linear Fitting Formulas 4

Compression Formulas Fig6 o Calculates the gain on a frequency-by-frequency basis for each of three input levels (40, 65, and 90 dB). o Useful for losses up to 60 dB and losses with little need for amplification of louder inputs

Compression Formulas IHAFF: Independent Hearing Aid Fitting Forum ◦ Based upon the restoration of loudness perception across a wide frequency range ◦ Means that soft sound and loud sounds to normal hearing person should be soft and loud to a hearing- impaired person ◦ Uses VIOLA software to translate the loudness scale ◦ Applicable to a wide range of losses and hearing instruments ◦ Takes a lot of time to implement in clinical settings

Compression Formulas DSL I/O: Desired Sensation Level Input/Output o Designed to maximize speech intelligibility without exceeding UCL o Designates three separate amplification regions: – Input levels below compression threshold – Input levels that will exceed the compression threshold when amplified – The region between these two limits

Compression Formulas NAL-NL1: National Acoustics Laboratories, Non- Linear, version 1 Aims to normalize loudness across normal speech frequencies rather than across all frequencies. Maximizes speech intelligibility while keeping loudness no greater than that perceived by a person with normal hearing. See Figure 5 here for a comparison of the different compression formulas.here