Introduction to Hearing Instrument Testing (HIT)

In this video, Jack Bennett, Audiologist and International Clinical Trainer at the Interacoustics Academy, will introduce the concepts of hearing instrument testing (HIT). You can read the full transcript below.

The purpose of HIT box measurements

Jack Bennett: Hello and welcome to getting started in hearing instrument testing. There are two main purposes for test box measurements in audiology clinics.

The first is clinical measurements. These are a record of frequency response in the hearing aid user settings. They're a quick reference for measuring the performance of the hearing aids, and a helpful guide for other professionals who might be working with those hearing aids.

The other main purpose of test box measurements is to perform technical measurements. These help you gain knowledge if the instrument is working as it should, you can check against the manufacturer's reference sheet also known as the spec sheet. You can detect faults which cannot be heard by the listening check.

Equipment you need to perform HIT box measurements

To perform HIT box measurements, you need a:

• Sound treated test chamber
• Loudspeaker
• Coupler microphone
• 2cc coupler or ear simulator
• Coupler adapter

There are different styles of coupler adapters, which we'll look at later on. And the reference microphone helps to regulate the stimulus level.

So looking in slightly more detail. Here you can see the reference microphone and now the coupler microphone. This is the microphone that's actually picking up the measurements coming from the hearing aids.

Mounted onto that you have a 2cc coupler. You may also have an ear simulator. Mounted on there is an adapter. Here you can see the HA2 adapter, also referred to as a BTE adapter. A different style of BTE adapter is the BTE L. This is designed for power and super-power hearing aids avoiding excessive leakage or standing waves. There is also the HA1 adapter.

By using a small amount of acoustic putty, you can mount a custom hearing aid onto this coupler adapter. For slim tube and receiver in the ear style hearing aids you can also use an Aidapter™. This is a silicone sleeve, which firmly and correctly holds the receiver or slim tube in the correct position in the coupler.

ANSI and IEC standards for hearing aid characteristics

There are two main sets of standards that are applied in the hearing instrument world: the ANSI 3.22 and the IEC 60118.

Both of these standards set out a series of parameters, which cover the:

• Testing conditions
• Equipment needed
• Different stimuli
• Different procedures
• Tests
• Tolerances

Before any hearing aid is sent out from the factory, they are checked and measured against the standards.

When to perform HIT box measurements

So although testing the hearing instrument in a test chamber is an essential step in many stages of the fitting journey, it's most commonly performed when the hearing aid is initially delivered to the clinic as issues may actually develop in transit. This could be due to weather, temperature, or just the nature of transit.

However, once a hearing aid is in the clinic, there will be many other points at which tests box measurements will be made along the hearing aid journey.

So not just before the first fitting but also as a more accurate process than listening checks with static lips. This is especially important if the audiologist feels that their hearing may not be reliable enough to notice any faults or distortion in the hearing aid performance. They are also commonly performed before and after repairs. Also as accurate record keeping and for the reference of other professionals.