What is Camfit?

Camfit is a program that is used to determine initial fittings for multi-channel compression hearing aids using the audiogram alone. The aid can have any number of channels up to 20. The program determines the gains and compression ratios that should be applied in each channel, or at the standard audiometric frequencies.

To use the program it is necessary to specify the audiogram of the client and the characteristics of the aid; specifically, the edge frequencies of the channels and the compression threshold of each channel. Several common makes of aid are pre-defined, and there is an option to enter and save the specification of any other aid. The program is used separately for each ear, but the recommendations are intended to give good results when the client is fitted bilaterally.

Two different theoretical rationales can be used. One is based on the idea of giving a flat specific loudness pattern over the frequency range important for speech when the input is speech with a level of 65 dB SPL or 85 dB SPL. The procedure also aims to give about the same overall loudness as normal for speech at 65 or 85 dB SPL. This procedure is called "Equalisation" . For a description of this rationale, see:

Moore, B. C. J., Glasberg, B. R., and Stone, M. A. ( 1999 ). "Use of a loudness model for hearing aid fitting. III. A general method for deriving initial fittings for hearing aids with multi-channel compression," Br. J. Audiol. 33 , 241-258.

For a description of the specific loudness pattern, and how it can be calculated for a hearing-impaired person, see:

Moore, B. C. J., and Glasberg, B. R. ( 1997 ). "A model of loudness perception applied to cochlear hearing loss," Auditory Neurosci. 3 , 289-311.

and

Moore, B. C. J., and Glasberg, B. R. ( 1998 ). "Use of a loudness model for hearing aid fitting. I. Linear hearing aids," Br. J. Audiol. 32 , 301-319.

The second procedure determines the gains needed to give "normal" specific loudness patterns for speech-shaped noise at 65 dB SPL and 85 dB SPL. This procedure is called "Restoration" . For a description of this rationale, see:

Moore, B. C. J. ( 2000 ). "Use of a loudness model for hearing aid fitting. IV. Fitting hearing aids with multi-channel compression so as to restore "normal" loudness for speech at different levels," Br. J. Audiol. 34 , 165-177.

The desired rationale - Equalisation or Restoration - must be selected before the results are displayed. The recommended fitting parameters can be displayed on screen in the form of tables or graphs, by selecting the appropriate "tab". The results can also be saved to a file by clicking on "File" and then "Save as". This file is given the extension ".set", and it has a specific format. This format is used by the Camfit program to retrieve the necessary information. The tables can also be saved as a plain text file, without any specific formatting, by clicking on "File" and then "Save text report". The tables and graphs can be printed by clicking on "File" and then "Print".

The program calculates target insertion gains (IG) as a function of frequency for a sinusoidal input with a level of 50, 65 and 80 dB SPL. It also calculates corresponding gains at the eardrum, measured relative to the level at the reference microphone of a probe microphone system. These are known as real ear aided gains (REAGs). We recommend that fitting be based on these REAGs, using a probe microphone system for verification and adjustment where necessary to achieve the target values. These adjustments must be made using sinusoidal stimuli. Some real-ear measurement systems display the real-ear aided response (REAR), i.e. the actual sound level in the ear canal. The REAG at a given frequency is equal to the REAR minus the input level. For example, if the input level is 65 dB SPL and the REAR is 86 dB SPL, then the REAG is 21 dB.

Both fitting methods - Equalisation and Restoration - have been found to give good results, in that only a small amount of "fine tuning" is required to achieve client satisfaction.

Buying Camfit

Camfit is sold in two forms. For individual hearing aid dispensers, it is available as a stand-alone Windows program for 150 (UK) or $300 (US). This fee is for is a single-practice license; the purchaser agrees not to pass the program to any other user. Purchasers should send a cheque for 150 or a check for $300 to:

PROF. BRIAN C.J. MOORE
DEPARTMENT OF PSYCHOLOGY
UNIVERSITY OF CAMBRIDGE,
DOWNING STREET, CAMBRIDGE CB2 3EB
ENGLAND

The cheque or check should be made payable to B.C.J. Moore. Credit cards cannot be accepted.

For manufacturers wishing to incorporate Camfit within their own fitting software, source code or DLLs for Windows programs can be made available. Interested manufacturers should contact:

bcjm@cam.ac.uk

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