Receiver-in-canal hearing aids with instant-fit ear tips have become increasingly popular. And they have many advantages, both for the professional and the end user. But they also come with some challenges. A new study has examined the effect of different ear tips on gain and sound quality.
Instant-fit ear tips are popular among hearing care professionals because they’re so efficient to work with. And for the end user, they’re generally more comfortable.
While this may sound like a win-win situation, instant-fit ear tips have their challenges too. For instance, how do you ensure the appropriate gain and output, and the optimal sound quality for each client?
In a new real-ear study of instant-fit ear tips, researchers found that the different types of ear tips impact the insertion loss and vent effect in different ways, and that there is a lot of variation between individual users. Figure 1: Instant-fit ear tips that were used in the study: Open (a), Tulip (b), Round, 2-vent (c), Round, 1-vent (d), Double domes (e).
The researchers carried out two tests for each of the ear tips: One that focused on measuring the insertion loss and another that focused on measuring the vent effect.
Insertion loss measures how much the direct sound is attenuated because there is an ear tip in the ear canal. If an ear tip is very close-fitting, insertion loss is high, because not much direct sound gets through the ear tip. For open ear tips, by contrast, insertion loss is low, which means that the direct sound enters the ear canal with little attenuation.
The vent effect refers to how much of the amplified sound escapes out of the ear canal – this happens particularly for open ear tips and must be taken into account to avoid too little amplification, particularly in low frequencies.
You can read the details of the study’s methodology here
Both tests confirmed that there are differences between ear tips, and that there is much variation between study participants.
What the results mean for your fitting
Because of the great variation between ear tips and between individuals’ ears, the study found that it is key for hearing care professionals to take the specific ear-tip style into account when fitting.
Some fitting software, like COMPASS™ GPS, automatically helps you do this. That way it’s easier to achieve the right gain and output targets.
At the same time, the results of this study show that it’s crucial to consider the individual vent size for each ear tip too – instead of relying on averages
. If you’re using a Widex hearing aid, you can use the feedback test in COMPASS™ GPS to estimate the effective vent size and have it factored into the gain prescription.
This way, a person with a larger-than-average effective vent doesn’t get too little gain in the low frequencies (which gives a tinny sound quality), and an individual with a smaller-than-average effective vent doesn’t get too much gain (which gives a boomy effect.)
Overall, you get a more precise fit and optimal sound quality.
What the results mean for sound quality – the comb-filter effect
In the study report, the researchers also discuss the comb-filter effect.
The comb-filter effect occurs when the direct sound that comes through any venting and leakage mixes with the sound processed in the hearing aid, which creates an artificial sound quality.
Typically, the hearing aid sound is delayed between 2 and 8 milliseconds, but even that means that some sounds are out of phase at certain frequencies. This results in peaks and troughs in the signal – and in a gain-frequency curve, which looks like the teeth of a comb. Figure 2: The comb-filter effect.
The comb-filter effect is most pronounced with open fits, but, as the study shows, all the different ear tips could be effectively open for the individual ear, including those we think of as closed. This means that the sound quality could be compromised, even for supposedly closed fits.
That’s why the researchers in the study emphasize how important it is to have a precise estimation of the vent size. This will help identify the frequency bands that give the most pronounced comb-filter effects, and where you should turn gain down.
Overall, the study concludes that, when fitting with instant-fit ear tips, it’s important to keep in mind the variation in direct sound reaching the eardrum – both between ear tips and between individual users – to achieve the best possible sound quality for each of your clients.
Do you want to learn more?Read the full study article in Hearing Review