EarTalk Q&A: Real Ear Measurement

Time to Read: 8 minutes

This question and answer is from the February Ear Talk webinar with Value Hearing founder Christo Fourie. You can find the full presentation on YouTube. If you would like to have your question answered live, leave a comment below RSVP to the next webinar which is on the third Wednesday of the month at 11am AEST.

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Question:

Can you take us through Verification Test & Validity Test? Is Real Ear Measurement a necessary step in fitting for better hearing aids?

Answer:

The next question actually flows very nicely into the previous question (setting up a hearing aid) and taking you through the verification process. The question is, ‘are real ear measurements important?’ and the short answer is yes they are.

What real ear measurements allow us to do is individualise the fitting of the hearing aid away from the average which hearing aids are set at. 

When we feed your hearing  result into the computer, the manufacturer gives us a ‘first fit’, so it essentially knows that with this kind of loss (based on the average of thousands of people), we need to give you this much volume in these different areas. But an average is exactly that - it's an average.

With real ear verification we take that average and fine tune it to your specific ear - including things like, how deep the hearing aid sits in your ear, the length of the tube, and a whole bunch of other features. So, it is really important if you want to get the best possible benefit from the hearing aid.

We often see if you take a first fit and verify it on a individual, they could be very far off the average and you might actually not be giving the client all the audibility that you'd like or should be giving them. 

So typically, real ear measurement is done at fitting appointments to make sure the hearing aid is supplying what we're promising it’s going to supply.

Real ear measurement can also be used as a troubleshooting tool so if someone has a specific complaint, we can then use something called live speech mapping. Say, for example a client might have issues of keys being too loud, you can then record in the ear, through the hearing aid, where those peaks of that sounds coming through and you can pull those down so it's more comfortable.

It is an absolutely necessary tool when first fitted but also a great troubleshooting tool and it's something that should be used if the hearing aid changes in a dramatic way such as you've had a new ear mould made or something of that nature.

This graph (at right) is just an indication of how ears can differ. What it's showing us here is if we put the sound in the ear, the calibration equipment basically plays a set sound -  in this case it's 65 decibels. When that sound comes in we can see that at 125 hertz no volumes added there.

Here’s where a little bit of volume added because what happens, is the ear is a tube that's closed off by the eardrum at the other side and that creates a Helmholtz Resonance which means certain ears boost certain sounds at certain points along its length.

The ear, very interestingly, is actually designed to give the highest peak around two to four kilohertz which is essentially where all the clarity parts of speech lays, so our ear is actually tuned to speech based on its shape.

When you measure it, you put in a sound with a little microphone going about three millimetres away from the eardrum and you play a sound without any hearing aid in there, that sound actually gets additional volume added to it at these frequencies. In this case you can see about 18 decibels at just under three kilohertz for this person (person one). Whereas this person (person two)  at three kilohertz, it's only ten decibels and almost nothing at four kilohertz, so both these hearing losses would have been given the same amount of boost by the manufacture software.

In this person's case (person one), the ear shape itself adds volume. So if you do the manufacturer setting, it might be too loud. Whereas in this person's case (person two), the ear doesn't add as much as the manufacturer expects so you might actually be under amplifying those very important speech sounds for this person.

It’s almost like taking a fingerprint of your ear and it's one of the first steps when you do a real ear measurement.

At right is an example of an real ear measurement machine.

We've got this in many of our clinics as well as some other options.

A real ear measurement machine consists of a speaker which sits about 30 centimetres to a metre away from you. It could be at an angle depending on how they set the software.

This neck loop goes around your neck and these probes, tubes, get hung over your ears and this  little fine little rubber tubing gets placed deep inside the ear. And the REM probe assembly sits over the ear.

You've got the tube coming in your ear and that's placed about three millimetres from the eardrum. What we do first of all is measure what the signature or the fingerprint of your ear is. We then put the hearing aid in and run that again while the hearing aids turned off and we can see how much of your ear resonance does the hearing aid destroys just by sitting there because we've got to compensate for that drop.

Then we turn the hearing aid on and measure again with the tube staying in the same place and we can see what sound actually reaches your eardrum relative to your prescriptive target.

A prescriptive target is basically a range of values or volumes that needs to be set for any particular hearing loss at different frequency points so you get the best audibility and there's a whole range of them.

National Acoustic Laboratories in Australia has one of the most popular ones and essentially it says if you fit that way, you are most likely to get the best speech understanding possible. Then the audiologist's role is to adjust to the target, so in this case this green line dotted line at the top is the target based on that person's hearing loss.

We put in a sound and typically it's going to be a jumble of speech, not just a hiss, because the hearing aid is simply going to drop the volume based on its automatic features because  they're trying to reduce noise.

So what they did is they built a signal that consists of 29 different speech signals in different languages and they cut and pasted so that within about 10 to 15 seconds you get all the speech sounds in human speech playing through the signal.

The machine can then measure what's reaching you at the different pitches inside your ear and we measure that for soft sounds, which is the purple, on their medium sounds which is typically normal speech levels and loud sounds which are the blue line there, and that target here in this case would be for loud sound so we don't want allowed sounds to exceed that.

We can measure it for soft medium and loud and then we can adjust the hearing aid accordingly. There's different adjustments with soft sounds to medium sounds to loud sounds.

Typically with loud sounds you don't want it too large to cause damage. Soft sounds you want to boost it a little bit but not too much you get all the noise, but medium sounds which is speech level, you really want optimal match to the target. 

We can use those controls you saw previously to adjust this graph up to your target at those different points so you get the best audibility possible.

There's currently new manufacturer fitting software that connects to these real ear measurement machines and can actually run that test for us and do all those levels at all the different frequency points in about three minutes.

There's absolutely no reason an audiologist shouldn't be using that because it's now a lot more accurate, a lot quicker and gives great results so hopefully that answers that question.

Yes it's absolutely vital. You wouldn't expect it with every visit, but you'd expect it at the fitting and you'd expect it if there's major changes made to the hearing aid or if the audiologist needs to troubleshoot specific issues for you.