What are Technical Hearing Instrument Test (HIT) Box Measurements?

In this video, Jack Bennett, Audiologist and International Clinical Trainer at the Interacoustics Academy, talks about technical measurements of hearing aids, and covers all of the most common measurements. You can read the full transcript below.

Standards for technical HIT box measurements

Jack Bennett: When performing technical HIT box measurements, there are two main standards that are applied: the ANSI 3.22 and the IEC 60118.

Both of these standards set out the series of parameters covering the:

• Test conditions
• Equipment needed
• Stimuli
• Procedures
• Tests
• Tolerances

Although they have slight differences, broadly, they are very similar. For both sets of standards, you can run either a full protocol or a reduced protocol. Reduced protocols are more commonly used in the clinical setting where a full protocol may be used if there is a particular concern about particular features or factors of the hearing aids themselves.

Demonstration of technical HIT box measurement

Dennis Mistry: So now I'd like to give an introduction to technical HIT measurements using the Affinity Compact, and a quick overview of the Affinity Compact test box.

Now, for technical HIT measurements or for any hearing instrument testing measurements, you'll be using certain transducers. So first of all, the reference microphone which is on the gooseneck. And the coupler microphone, which we described before. And I've explained that as to how that's positioned.

For this scenario, we're going to be using a 2cc coupler and a BTE adapter, which is why this is configured as it is. The other sockets here are for battery drain. So if you want to measure the battery test drain of a hearing aid, then you'd need to insert a battery drain pill into this measurement.

And the additional point as well here is the telecoil, the telecoil socket. And if you want to measure the telecoil functionality of this device or your devices, you'd need to insert the TMFS coil here. In this example, I won't be using these two sockets.

Now, ahead of beginning our testing for any clinical or technical hit measurements, we need to configure the test box without device. So the first thing I want to do is take my hearing aid that I've already connected to my hearing aid software and couple that onto the coupler here.

Now, it's important that when you position this in the box, that the hearing aid and the reference mic are close, but not touching and also orientated towards the center of the box, which you can see here at this point. Now that that's in the correct configuration, we're able to close the box and move into our software.

Now it's always good to start with your hearing aid spreadsheet. So download the specification sheet that's relevant to the hearing aid that you're testing. In this example, here, we want to use a 2cc coupler and the protocols for which you need to test that.

Now I wish to use in this example an IEC 60118- 7 2005 protocol with a 2cc coupler. So I'll be looking at this information over here. I now need to make sure that the protocol chosen in the Affinity software reflects that. And that I can specify here so I wish to use a reduced protocol.

Now ahead of running the test or pressing start, I just need to go into the hearing aid software and make sure that that's set to Full on Gain. And I'm just going to program that to that specification so that when I run this measurement, I can then look at its performance in comparison to that spec sheet that I just showed.

So now that's ready, I'll go into the software and press start. Now the process is automated. Because the protocol is against the standard, the test parameters and everything are predefined for you. So it's just a case of hitting start and letting the test run through. The system will also alert to me when I need to swap it to reference test gain. And I'll do that when that happens.

What is the OSPL 90 measurement?

Jack Bennett: The first test in this protocol is the output sound pressure level 90 dB measurement, also known as the OSPL 90. The purpose of this measurement is to find out if the aid output is correct for loud input levels.

We're measuring the hearing aid output in dB SPL as a function of frequency in kilohertz. The hearing aids will need to be set to the full-on gain setting. The test box will be using a stimulus of 90 dB SPL input which is a pure tone swept from 200 hertz up to 8000 Hertz.

When the measurement is complete, you get a readout that looks like this. The max SPL 90 level will be shown. There is a tolerance of plus three decibels versus the manufacturer standard. You'll also be able to see the high frequency average or HFA level. The HFA is made up of 1000 hertz, 1600 hertz, and 2500 hertz, the HFA level should be plus or minus 4 decibels from the manufacturer standard.

What is the full-on gain measurement?

The second measurement in this protocol is the full-on gain measurement. The purpose of this measurement is to find out if the hearing instrument applies appropriate gain to moderate stimuli. Here we'll be measuring hearing gain in dB gain as a function of frequency in kilohertz. The hearing aid again will be set to full on gain.

This time we'll be using a 50 dB SPL input, still a pure tone sweep from 200 Hertz to 8000 hertz. You'll be able to read the max gain and the max gain frequency. This is the point at which the hearing aid applies the most gain and the amount of gain that it applies. You will also be able to read the HFA level. For this measurement, the HFA level should be plus or minus five decibels versus the manufacturer standard.

Demonstration of OSPL 90 and full-on gain measurements

Dennis Mistry: So you can see that the system is drawing the curves that we see here, we now get to the reference test gain. And it's prompting me to change that setting in the in the configuration here and you'll notice that in your hearing aid software, you'll have settings to either IEC reference test gain or ANSI reference test gain. And it's at this point that you need to swap it to an IEC reference test gain because that's what's reflected in the protocol that we're running and program that device to that setting.

Once that's been done, and the system is reflected back when it's programmed to that setting and go back to our software and hit continue to complete the rest of the test battery.

What is the reference test gain measurement?

Jack Bennett: The next test and an essential step before taking any other measurements is the reference test gain. Note that this is not actually a test but a hearing aid adjustment which is needed to complete the rest of the remaining tests in the test box.

You'll be measuring hearing aid gain in dB gain as a function of frequency in kilohertz. The hearing aid will need to be set to reference test gain and this time the stimulus will be a 1600 hertz 60 decibel SPL pure tone. From a reading you will get a reference test gain. This is the HFA level. This can be noted for reference but isn't set against any of the standards.

What is the harmonic distortion test?

The final test in this particular reduced protocol is the harmonic distortion test. Here we'd like to find out if the instrument exhibits harmonic distortion, which is when the instrument produces harmonics in the output signal that are not present in the input signal.

We'll be measuring percentage distortion as a function of frequency. Again, the hearing aid will be set to the reference test gain. Here we will actually do three separate measurements:

• 500 hertz stimulus at 70 decibels
• 800 hertz stimulus at 70 decibels
• 1600 hertz stimulus at 65 decibels

When interpreting this test, you will actually have three separate measurements to interpret. For each of them you will be able to read the total distortion. The total harmonic distortion can be no more than 3% than what is stated in the manufacturer spec sheet.

Demonstration of harmonic distortion test

Dennis Mistry: Okay, that's the test complete. Now that we have this data, we can now go to our spec sheet again and compare what we've seen as the output of the device to the to what they recommend on the spec sheet according to the test settings.