Jack Bennett, International Clinical Trainer at the Interacoustics Academy, introduces Real Ear Measurements and the steps needed to perform them. He also discusses why they are so important in Hearing Aid Fittings and some of the benefits to the patient and the clinician.
If you prefer reading, find the transcript below.
The first thing that we should consider when looking at real ear measurements is why should we perform them? And why should we perform verification?
In very simple terms, the reason for doing verification and real ear measurements is that we want to confirm that the hearing aid is performing in a certain manner to provide benefit. Verification should be considered alongside validation.
So first of all, let's consider what validation is. Validation normally means conducting a functional assessment. This could be an aided hearing test, for example. Validation does require acclimatization however, as we need the patient to get used to wearing the hearing aids to fully function as well as possible.
When doing aided measurements, these are often binaural. So we don't get information about the quality of fitting on either side, just both of them together. These are subjective measurements, in lots of cases as well. Validation also requires significant cooperation from the patient, which can be difficult in some cases.
So verification then normally, what we look at here is a hearing aid assessment or real ear measurement. There's no delay here, we don't need the patient to get used to wearing the hearing aids in order to perform verification. We can look at monaural data. So although we verify both sides, we're actually checking that each one is doing what it should be doing.
These are objective measurements, so we don't rely on any subjective components. And finally, minimal cooperation is needed. And there are different methods you can use, where you need more or less cooperation.
So what is real ear measurement? Essentially, what we're trying to find out is when we play a sound into the ear canal, what sound is being received at the eardrum?
First of all, we do this in what's called an unaided situation, where we place a probe microphone into the ear canal and then play a sound down and receive that sound at the eardrum. The probe microphone listens to that sound and graphically represents it as an unaided response.
We can then place a hearing aid in the ear canal and play that same sound again measuring the response at the eardrum with the probe microphone and now we have an aided response. That aided response can then be adjusted in the hearing aid fitting software to bring it closer to the target.
That target is defined by the prescription formula that we have chosen to use, we could use a proprietary formula, DSL, or NAL. However, the decision-making process around choosing these is outside of the scope of this lecture.
So the alternative to doing real ear measurements is to use a manufacturer first fit. This paper by Valente and colleagues looked at how well first fits will match to target. They compared this against using real ear measurements or in this paper it was referred to as a programmed fitting. This was assessed at 50 dB, 65 dB, and 80 dB input levels.
For the manufacturer first fit, there's significant deviation from target, especially as you go towards the higher frequencies. Not only does the manufacturer first fit lead to a poor match to target, but there's vast dispersion across the different patients that were measured. So it'd be difficult to predict how far away the patient would be from target if you use the manufacturer at first fit.
With a programmed or real ear measurement fitting, the median dispersion was much lower, giving a much closer match to target. But especially in the low and mid frequencies, the individual dispersion was also much lower. This shows that real ear measurements lead to a closer match to target than first fittings.
This paper also looked at the word score outcomes of these different fittings. At the 50 dB input level, so this would be equivalent to quiet speech level, there's a significant improvement in the word score for the patients that are wearing the hearing aids that have been fitted by REM. The significant difference is also seen at the 65 dB input level.
So for quiet and normal speech levels, then there is a much better word score when fitted with real ear measurements. This improvement is reduced at the higher input levels such as 80 dB SPL.
So we know that fittings are closer to match to target and that the outcomes are better. But what does the patient prefer? In this paper, it's also stated that patients preferred REMs 79% of the time, whereas patients preferred the first fitting, or the manufacturer first fit just 21% of the time.
So how do we actually perform real ear measurements?
So the probe and hearing aid are placed into the ear canals. And there's a speaker which can play a stimulus. So the sound is created by that loudspeaker. And that sound is actually measured by a reference microphone. This ensures that the sound is at the correct intensity. The effect of the ear and the hearing aid are measured by the ear canal probe microphone. The software will then display this result graphically, so that the audiologist or person fitting the hearing aid knows how much to adjust the gain response of the hearing aid.
This is a simplified version of the REM process. But there are a few steps that lead to good quality, real ear measurements. So this is the process that you must go through in order to perform a really good quality real ear measurement: