Hearing aid verification using fitting systems has become very popular since the introduction of digital hearing aids. Real-ear measurements (REM) is the main way in which a clinician can measure the hearing aids' effects objectively and independently, on the patients' ear, to gain a better idea of the hearing aids' performance and suitability (Aazh and Moore, 2007; Mueller, 2001).
There are two methods of performing REM: Aided Response and Insertion Gain. Let's begin by explaining the difference between them.
The difference between Aided Response and Insertion Gain
Aided response is a method of verifying the hearing aid's output in dB SPL. It involves measuring the total sound achieved at the probe microphone. This includes the signal, the patients' own ear canal effect and the hearing aids amplification (Pumford and Sinclair, 2001; Mueller, 2001).
Because Aided Response is working with the total sound, no REUR/REUG needs to be measured as it is being included in all measurements that are being taken. It is also for this reason that the gain target curves will not change after measurement of the REUG/REUR because regardless of the patients' personal differences, the total effect should be the same outcome. You can still choose to include REUR/REUG to ensure correct tube insertion (Dillon, 2012).
Insertion Gain is a method of verifying the hearing aids output solely, in dB Gain. Whilst still using the measured sound at the probe microphone, the dB Gain view is showing only the amplification applied by the hearing aid (Pumford and Sinclair, 2001). Therefore the effects of the signal and the patients' own ear canal amplification are removed to focus purely on what the hearing aid needs to achieve the target output signal (Mueller, 2001).
Real Ear Insertion Gain equals Real Ear Aided Response minus Real Ear Unaided Response.
As Insertion gain is a calculation, it is a requirement that the REUR/REUG exists ahead of making any measurements with the hearing aid enabled.
Which method is better?
People have been encouraged and educated to perform REM using a certain method. Both methods still yield the same outcome but they may be showing different things. Historically both methods have been suggested for different fitting algorithms (BSA, 2007), with Insertion Gain being most suitable for NAL and Aided Response being most suitable for DSL. However, both algorithms now have compatibility to be performed in each way (BSA, 2018).
Insertion gain only shows the hearing aids' amplification, therefore it is difficult to understand the full audibility provided when working in this way.
Aided Response shows the complete effect achieved in the ear from all components, which helps to understand the overall effect but can make it difficult to understand solely what one of the components (i.e. hearing aid alone) is contributing. It also gives greater visibility regarding the total effect of the sound achieved within the patients' dynamic range and their proximity to one another and the patient's threshold and uncomfortable levels (Bentler et al, 2016).
Remember that the probe-tube microphone placement for the Aided Response method requires a deeper insertion. This is because your objective here is to measure the sound level close to the eardrum, normally within 5 mm (Dillon, 2012; Pumford and Sinclair, 2001; BS ISO 12124, 2001).
So which method should I use?
Overall, you need to understand what it is you want to see. Most clinicians are focused on the hearing aid, but the overall performance in the patients' ear is the main effect of their efforts.
Aided Response is seen to be easier to understand from an audibility perspective, therefore it may simplify your efforts when explaining the hearing aids' effect to your patient. Aided Response is also a much easier method to use when starting to work with REM (BSA, 2018).
Insertion Gain is a very objective way to look at verification of hearing aids as it is singling out just one of the components contributing to the solution achieved in the patients' ear canal. Should you be interested in the hearing aids performance alone then this is the best method to use.
All the fitting systems offered by Interacoustics can perform both methods. Contact your local distributor if you want to hear more about your options to perform real-ear measurement with an Interacoustics system.
Aazh H. and Moore B.C. (2007). The Value of Routine Real Ear Measurement of the Gain of Digital Hearing Aids. J Am Acad Audiol, 18, 653–664
Bentler, R., Mueller, H.G., Ricketts, T.A. (2016). Modern Hearing Aids Verification, Outcome Measures, and Follow-Up. ISBN13: 978-1-59756-482-3
British Society of Audiology. (2007). Guidance on the Use of Real Ear Measurement to Verify the Fitting of Digital Signal Processing Hearing Aids.
British Society of Audiology. (2018). Guidance on the verification of hearing devices using probe microphone measurements.
BS ISO 12124. (2001). Acoustics: Procedures for the Measurement of real-ear acoustical characteristics of hearing aids. Geneva, International Standards Organisation.
Mueller, H. G. (2001). Probe Microphone measurements: 20 years of progress. Trends in amplification, 5(2), 35
Pumford, J. and Sinclair, S. (2001). Real-Ear Measurement: Basic Terminology and Procedures. AudiologyOnline.