Hearing Instrument Testing: A Complete Guide

Intermediate
10 - 30 mins
Reading
14 May 2024

Description

Hearing instrument testing, also known as test box measurements or HIT box measurements, are tests that verify hearing instruments are working as they should.

 

Table of contents

 

 

Types of HIT box measurements

There are two main types of 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, create a useful record of amplification, and are a helpful guide for other professionals who might be working with those hearing aids.

The other main type of test box measurements is to perform technical measurements. These help you gain knowledge if the instrument is working as it should 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:

  • Loudspeaker
  • Coupler adapter
  • Coupler microphone
  • 2cc coupler or ear simulator

You also need a sound-treated test chamber, of which Interacoustics offer the following (Table 1).

 

Product Type
Affinity Compact Hearing aid fitting system
Affinity 2.0 Hearing aid fitting system
TBS10 Hearing aid test box
TBS25 Hearing aid test box

Table 1: Sound-treated test chambers offered by Interacoustics.

 

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/ASA S3.22 and the IEC 60118-0.

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

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

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.

 

Clinician sitting at desk with the Affinity Compact resting on the desk. The clinician is operating the Affinity Compact’s HIT software module.
Figure 1: With a hearing aid fitting system such as the Affinity Compact, you can perform HIT box measurements at the comfort of your desk.

 

Technical HIT box measurements

The most common technical HIT box measurements are:

  • OSPL 90
  • Full-on gain
  • Reference test gain
  • Harmonic distortion test

We’ll describe each of these tests briefly. You can also learn about the different technical HIT box measurements by watching the video below, which includes demonstrations of each.

 

 

What is the OSPL 90 measurement?

The output sound pressure level 90 dB measurement, also known as the OSPL 90, is a measurement that finds out if the hearing aid output is correct for loud input levels. The OSPL 90 measures hearing aid output in dB SPL as a function of frequency in kHz.

 

What is the full-on gain measurement?

The full-on gain measurement is a measurement that finds out if the hearing instrument applies appropriate gain to moderate stimuli. The full-on gain measurement measures hearing aid gain in dB gain as a function of frequency in kHz.

 

What is the reference test gain measurement?

The reference test gain is a hearing aid adjustment which is needed to complete the rest of the remaining tests in the test box. The reference test gain is used to set the output of the hearing aid to produce a specific intensity level in response to a 60 dB input level which is then maintained for the following test box measurements.

 

What is the harmonic distortion test?

The harmonic distortion test finds out if the hearing instrument exhibits harmonic distortion, which is when the instrument produces harmonics in the output signal that are not present in the input signal. The harmonic distortion test measures percentage distortion as a function of frequency.

 

Clinical HIT box measurements

An alternative to running a technical HIT box measurement is running a clinical HIT box measurement. The clinical HIT is becoming an increasingly popular test box measurement which can provide several benefits. For example, the purpose could be to:

  • Compare and analyze the device's performance
  • Capture the settings of the hearing aid for future use
  • Share the hearing aid’s performance for insurance purposes

You can learn more about clinical HIT box measurements by watching the video below, which includes demonstrations.

 

 

Additional tests

We've already looked at most of the tests in the reduced protocol. Now we're going to look at some of the additional tests in the full protocol, specifically:

  • Frequency response
  • Equivalent input noise
  • Input/output
  • Attack/recovery time

We’ll describe each of these tests briefly. You can also learn more about these additional tests by watching the video below, which includes demonstrations of each.

 

 

What is the frequency response?

The frequency response measures if the response of the hearing aid is appropriate across the frequency range to a 60 dB SPL input. It measures hearing aid output in dB SPL as a function of frequency in kilohertz using the reference test gain.

 

What is the equivalent input noise?

The equivalent input noise determines the level of internal microphone noise in the hearing aid. It measures decibels as a function of frequency in reference test gain.

 

Input/output

The input output graph shows how the hearing aid applies different gain to different input levels at a single frequency. This describes the compression characteristics of the hearing aid. It measures dB SPL output as a function of dB SPL input in reference test gain.

 

Attack/recovery time

The attack and recovery time is measured to show how quickly the hearing aid compressor reacts to an increase and decrease in input signal. It measures dB SPL output as a function of time in milliseconds using the reference test gain.

 

Battery and telecoil measurements

In this section, we’ll discuss battery drain measurements and telecoil measurements that you can include as part of your technical HIT, including:

  • Battery current drain
  • Coil frequency response and full-on gain response

We’ll describe each of these tests briefly. You can also learn more about these tests by watching the video below, which includes demonstrations of each.

 

 

Battery current drain

For the battery current drain measurement, we're looking to measure the milliamps the hearing aid draws from the battery. It measures milliamps as a function of frequency with the hearing aid in reference test gain.

 

Coil frequency response and full-on gain response

For the coil frequency response and full-on gain response, the purpose is to measure the response of the hearing aid when using a telephone magnetic field simulator or TMFS coil. We're measuring the output in dB SPL as a function of frequency. The hearing aid can be set to the reference test gain or the full-on gain depending on which measurement you are running.

 

Aidapter™

The Aidapter (Figure 2), which is a rather new accessory, is used for testing receiver-in-the-canal type technologies of hearing aids or thin tube hearing aid technologies where you can't use standard adapters like the BTE adapter that we've been showing in other parts of our videos.

 

Figure 2: HIT box setup using the Aidapter and the Affinity Compact.

 

You can learn more about testing with the Aidapter in the video below.

 

 

FM systems (radio aids)

For most children with a mild to moderate hearing loss, they will perform reasonably well in a classroom setting that is small or quiet. If however, that classroom setting becomes more busy, noisier, or a particularly large room, they may find it much more difficult.

For these cases, there are systems that are used to improve the signal-to-noise (SNR) ratio. These systems are often referred to as FM systems or radio aids. Generally, they will work in the same way, the teacher or target speaker will wear a transmitter, usually as close as possible to the mouth and the child will wear a receiver.

To learn more about how to verify and use FM systems, please refer to the video below.

 

 

The underrated notion of HIT box measurements

If you’re reading this far, you hopefully now understand the importance of HIT box measurements and how they can play a large role in making sure your clients walk out of your clinic with hearing instruments that are working as they should.

If you’re considering adding HIT box measurements to your test battery, then visit our line of hearing aid fitting systems to learn more.

 

Related course

Getting started: Hearing Instrument Testing (HIT)

Presenter

Michael East
Michael is an Audiologist and Clinical Scientist from the UK with many years of experience working in senior clinical audiology positions within the National Health Service, specialising in both paediatrics and vestibular assessment/rehabilitation. He holds undergraduate and postgraduate degrees from the University of Southampton and Aston University respectively and completed the UK’s National School of Healthcare Science “Scientist Training Programme”; leading to his registration as a Clinical Scientist.

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