Traditionally, prescriptive fitting formulas have been the place to start when it comes to fitting hearing instruments. Formulas such as DSL and NAL recommend targets for adjusting the amplified signal to compensate for an individual’s hearing loss, and they are the gold standard for optimal speech perception (Seewald et. al., 2005; Keidser et. al. 2011). These formulas have been improved and validated over time as well as modified to account for improvements in amplification capability (Scollie et. al., 2005; Keidser et. al. 2012). The goal of the most popular fitting formulas is to provide optimal speech intelligibility and normal loudness perception.
Sometimes the fitting formula target isn’t the best place to start. Listeners who haven’t experienced amplification before often don’t have a favorable impression of the sound quality when fitted at the full target gain. Their reactions typically include comments such as “too harsh”, “too tinny”, “unnatural”, or “too loud”. Not surprisingly, this experience can affect their acceptance of hearing instruments. Instead of proactively counselling new listeners that “you’ll get used to it”, another approach is to modify the amplification they receive at the first fit by reducing the amount of gain provided – especially in the high frequencies. The result is a more familiar and natural sounding starting point. Over time the response of the hearing instrument can be adapted to full target.
That’s where Unitron’s Automatic Adaption Manager (AAM) comes in. Just as people adapt to hearing loss, the AAM helps them adapt to amplification over a period of time. Our AAM initially provides reduced amplification and then seamlessly adjusts so that the listener reaches full target. Before the introduction of our Discover platform, the AAM calculated a percentage reduction in the delivered frequency response relative to the full target. This percentage reduction was based on the hearing instrument wearer’s age, audiogram, and previous experience with amplification. Because the reduction was relative to full target, an individual with a sloping hearing loss that is worse in the high frequencies would have a larger reduction (in dB) in the high frequencies than in the low frequencies, even though the percentage was the same for all frequencies.
The most familiar and natural sounding amplified response for a listener that has not previously been exposed to amplification will be a frequency response that matches the unamplified sound.
Although an approach that uses a percentage reduction from full target can lead to improved sound quality, it’s not an ideal first fit response. Instead, Unitron’s new AAM approach with Discover has been optimized based on research and experience with hearing impaired listeners’ preferred listening levels (Pumford et. al., 2012) and Unitron’s experience with products over the years. For an inexperienced listener, the most natural sounding frequency response will be one that is transparent – that is, matches the response of their open ear1. This response is closest to the sound that they’ve been used to hearing, even as their hearing loss progressed. While it’s possible to provide this response in a hearing instrument, it’s also necessary to provide some incremental gain in order to provide any noticeable improvement from amplification. This is where Unitron’s eight years of experience with the AAM is invaluable. Because our new AAM is based on listeners’ preferred listening levels, we can calculate the appropriate gain to be added to the transparent gain in order to help new wearers enjoy the full benefits of amplification.
1 The open ear response can also be referred to as: 1) the Real Ear Unaided Response (REUR), 2) the head related transfer function (HRTF) at 0 azimuth, 3) transparent gain, and 4) 0 dB insertion gain.
The frequency response of an unamplified signal can be measured in the listener’s ear canal with a probe tube. A test signal is presented in a sound field, and commercially available real ear test equipment is used to measure the response level in the ear canal. The measured response of the unamplified signal (with no hearing instrument in the ear) is called the Real Ear Unaided Response (REUR).
When a hearing instrument is placed in the listener’s ear and the same test signal is presented, the level of the amplified signal in the ear canal can be measured. The measured response of the amplified signal is called the Real Ear Aided Response (REAR). The amount of gain is the difference between these two measurements (aided minus unaided) and is called the Real Ear Insertion Gain (REIG).
Representative plots of the REAR, REUR and REIG are shown in Figure 1. Note that if the REAR equals the REUR, then the real ear insertion gain is 0 dB and the hearing instrument would be perceived as being acoustically transparent. That is, the delivered amplified signal is equivalent to the unamplified signal. When that same frequency response shape includes additional gain (for example REIG +10 dB) for a loudness boost, then the result is a very natural sounding instrument for a firsttime wearer.
The calculation workflow for the new first fit response for novice listeners is shown in Figure 2. In order to create the new AAM first fit response, we started with a flat 0 dB insertion gain response at all frequencies rather than applying a percentage reduction from full target. With this starting point, the listener’s experience would be as if the device was acoustically transparent. In order to provide benefit, we then adjust the 0 dB insertion gain by an input level dependent gain offset. The gain offset is calculated based on our estimate of the listener’s preferred listening level at each input level. This preferred listening level has a built-in compression characteristic and is calculated using an algorithm that considers the listener’s age, audiogram and fitting formula full target gain. The first fit response is then a combination of the flat 0 dB insertion gain and the additional preferred listening level gain. The result is a compressive frequency dependent fitting best suited for a novice listener with a natural sound experience right out of the box.
Finally, the AAM seamlessly transitions the response of the hearing instrument from this very natural sounding first fit response to the full fitting formula target response at a pace that’s comfortable for the listener. In this way, a new hearing instrument wearer not only has a great first listening experience, but quickly and automatically achieves the full benefit of amplification and optimal speech perception.
Unitron’s new first fit response and AAM for Discover builds on our knowledge of a new user’s first experience with amplification. Unitron has designed a first fit response that will be familiar for the listener, out-of-the box, while providing sufficient audibility for a noticeable improvement. Over time the response of the instrument will automatically adjust to match the full target response to help new users hear at their best.
Keidser G., Dillon H., Flax M., Ching T., and Brewer S., 2011, The NAL-NL2 Prescription Procedure, Audiol Res. 2011 May 10; 1(1): e24.
Keidser G., Dillon H., Carter L., and O’Brien A.,2012, NAL-NL2 Empirical Adjustments, Trends Amplif. 2012 Dec; 16(4): 211–223.
Pumford J., Hayes D., Cornelisse L., 2012, Automatic Adaptation Management: Addressing First-fit Amplification Considerations Hearing Review, 19(03): 34-37.
Scollie S., Seewald R., Cornelisse L., Moodie S., Bagatto M., Laurnagaray D., Beaulac S., and Pumford J., 2005, The Desired Sensation Level Multistate Input/Output Algorithm, Trends In Amplification, 9(4).
Seewald R., Moodie S., Scollie S., and Bagatto M., 2005, The DSL Method for Pediatric Hearing Instrument Fitting: Historical Perspective and Current Issues, Trends In Amplification, 9(4).