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Hearing aid fitting

The skull simulator is a mechanism for checking bone anchored hearing devices in a Hearing Instrument Test (HIT) box.

Traditional HIT measurements are performed for air conduction hearing aids and do not provide the necessary stimulation or coupling mechanism required for a bone anchored hearing devices.

The skull simulator allows you to connect a bone anchored hearing device, e.g. Oticon Medical Ponto devices, to an accelerometer (skull simulator). This provides the correct coupling mechanism and vibratory stimulation that is required to test a bone anchored device within a HIT chamber.

Outcome measures are similar to those in air conduction HIT measurements and include; battery drain, frequency response, harmonic distortion, peak responses and equivalent noise input.


References and caveats
The skull simulator requires the Interacoustics Affinity instrument with HIT and SKS, licenses and hardware.

March 2018

When verifying a hearing aid fitting, a Real Ear Measurement (REM) is used to show how well the hearing aids are matching a prescribed target, and to adjust the performance where necessary. This process is referred to as verification of hearing aids. 

Most commonly in late childhood through to adult populations, one of several types of real ear measures are used for verification; Real Ear Aided Gain, Real Ear Aided Responses or Real Ear Insertion Gain (REAG, REAR or REIG). These techniques require the patient to remain relatively still for some minutes whilst the measurements at several different signal levels (and any adjustments to the hearing aid output) are being carried out. So, your question as to whether one should use REAG or RECD might be more generalisable; when should we verify using procedures that all take place via a probe-mic in the real ear (REAG,REAR,REIG), and when should we adopt a different approach?

A Real Ear to Coupler Difference (RECD) measure allows the clinician to derive the real-ear performance of a hearing aid while performing most of the verification procedures in a coupler, within a Hearing Instrument Test box (HIT). This can be ideal for use with patient populations who cannot or will not maintain stillness for the longer probe-mic procedures mentioned above, or indeed be inclined to pull at cables needed for communicating with the hearing aid and probe-mic instruments on the real-ear. The RECD does involve a single real-ear measurement (whereby a signal such as a pure-tone sweep is presented to the occluded ear of the patient, often via their ear-mould, allowing the level in the real-ear to be measured via the probe tube microphone). However, the remainder of the verification procedure is carried out in the HIT box. The next step is to measure the level of the same signal (pure tone sweep) in the coupler within the HIT box. The difference between the level in the real-ear and the coupler is the RECD. This transform can now be applied to any further measurements made in the coupler (i.e. the performance of the hearing aid that the patient is due to wear), and it allows the audiologist to derive the performance of the hearing aid in real-ear while verifying it in the coupler; the patient isn’t required to sit still and cooperate for any longer than a few moments to obtain the initial real-ear measure. 

Generally this procedure relates to infants and children who may not cooperate for REAG, REAR or REIG routinely used in adults. However, it is also useful in those of any who may have mental or physical disabilities that mean sitting still for several minutes is not practical. We might also consider clinicians attending patients in their home environment or other remote consultations where travelling with REM equipment may be difficult; in this case a previously stored RECD can be used to verify the hearing aid in advance. 

A second application of the RECD (but not REAG, REAR or REIG) is prior to the verification of the hearing aid.

Immediately prior to the verification of the hearing aid (but immediately after the audiogram has been measured) there is a need to convert hearing thresholds (as measured in dB HL) into dB SPL in the real ear. This step, performed by the hearing aid fitting software, is necessary so that the prescription of hearing aid gain for the real ear can be formulated (e.g. using validated formulas from NAL or DSL or other sources). When hearing thresholds are measured using insert phones, the individuals RECD (be that adult or infant patients) can be used to accurately derive the patient’s hearing thresholds in dB SPL and thus enable the hearing aid fitting software to formulate an individualised prescription of hearing aid gain. If the individuals RECD is not measured at this point, the fitting software can still prescribe hearing aid gain using the same formula, but it will not be individualised; instead, an average RECD would be used and so potential inaccuracies would ensue.

April 2018

When a Real Ear Measurement probe tube is placed into the adult ear canal and a REUR test performed, we expect to see a typical result in associated with a typical anatomy of an adult ear. This is reflective of an ear (i.e. pinna, concha and external ear canal) that has an approximate ear canal length of 28mm and is essentially a tube closed at one end (by the tympanic membrane) and open at the other. Those that are younger (with shorter ear canals), or have had modifications to the ear anatomy through surgery or trauma, will not necessarily have the typical result. 

The typical adult ears natural resonance response produces amplification of incident sounds in the frequency range of around 1500 Hz to 7000 Hz, with a peak at approximately 2700 to 3000 Hz. When we review a REUR, this peak typically occurs with approximately 10-20dB of gain. An additional feature of the typical adult REUR is that there is little or no amplification below 1500 Hz.

The below graph displays an average REUG result in a typical adult ear canal. 

REUG for “typical” adult ear taken from Interacoustics Affinity Suite software

An REUR is the same concept as a REUG but is displayed in dB SPL 

April 2018

The “calibrate for open fit” is used when fitting an open fit hearing aid. It prevents sound leaking from the ear and picked up by the reference microphone during aided measurements. It is performed after the Real-ear unaided gain (REUG) in real ear insertion gain measurements and it is performed as the first measurement in real ear aided response measurements. 

It consists of a single step. The hearing aid is placed its correct position on the ear and the hearing aid must be switched off or muted (with the help of hearing aid fitting software). Once in position the calibrate for open fit measurement can be run. 

After the calibration for open fit has been performed, the hearing aid gets switched on (unmuted) and immediately afterwards, the real-ear aided response (REAR) measurements can be performed without the patient moving between these actions.

April 2016

Both are equally valid approaches to hearing aid verification, and historically are associated with the philosophies of prescribing gain according to the well-known NAL (REIG) and DSL (REAR) prescription methods for hearing aid gain.

One of the most commonly cited advantages of the REIG is that it is a relative measure; the difference between REUR and REAR. This means that precision probe-tube placement is somewhat less critical than an absolute measure (REAR), so long as the probe-tube does not move between measurement of the two steps. On the other hand, such an advantage is at the same time a hindrance; if, as part of the REIG process, one needs to measure the REUR and REAR, why not skip the REUR step and concentrate on the REAR? Using REAR would also imply fitting and verifying the hearing aid using the SPLogram (rather than showing hearing aid gain). This arguably makes visualisation of the patients dynamic range easier. This point is important both for clinicians (who must not only match the hearing aid to target, but also try to ensure audibility across the frequency range for different input levels, while bearing in mind the uncomfortable loudness level) and patients (who are often involved in the hearing aid fitting and counselling process via speech mapping and other visual tools that based on the SPLogram).

There is much literature covering this topic and it is something of a long-standing debate amongst experts in fitting hearing aids. For a brief review please see Jorgensen (2016).


References

Jorgensen, L.E. (2016) Verification and validation of hearing aids: Opportunity not an obstacle. Journal of Otology, 11, pages 57-62

October 2018

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