How do you measure hearing aids? In this short video, we demonstrate how to test hearing aids using SoundCheck with standard ANSI S3.22 measurement techniques. You’ll see how to set up an anechoic test box and coupler, and configure SoundCheck for key measurements including frequency response, input/output linearity, OSPL 90, and release time. This method can be used to accurately evaluate hearing aid performance and ensure compliance with industry standards.

Watch this Video Demo of Measuring Hearing Aids

Learn More about Hearing Aids

• • Hearing Aid Input vs Output Test Sequence: https://www.listeninc.com/resources/test-sequences/hearing-aid-input-vs-output-test-sequence/

Video Transcript: How to Measure Hearing Aids

Today,  I’ll demonstrate a hearing aid measurement using SoundCheck with some of our standard hearing aid test sequences. We’ll use some of the tests outlined in the ANSI S.22 hearing aid test standard – frequency response, input/output linearity, OSPL 90, and release time. 

First let’s take a look at our hardware setup.

It’s a little more involved than something like a loudspeaker measurement because hearing aids have some unique test requirements. For a start, we need an anechoic test box  – this one’s a B&K 4232 but you could use anything similar. It has a speaker integrated inside, which we’ll play our test signal out of. We’ll put our hearing aid in this coupler with a ½” pressure microphone. We also have a SCM reference free-field microphone which is also going to go in the test box. The ½” mic needs LEMO power, so it’s connected to a SoundConnect 2 power supply, and the mic input then goes into the AmpConnect 621 Audio Interface. The AmpConnect also provides power for the SCM mic and amplification for the speaker in the test box. 

If I were running the complete suite of ANSI S.22 tests, I would also need a DC power supply and current monitor but since the tests we’re running today don’t require voltage regulation or current monitoring, we’ll just power the hearing aid from its internal battery.  

I’ve already calibrated the source speaker located inside the test box using my SCM reference microphone pointed at the source speaker.  The coupler is configured with a ½” microphone capsule, and we’ll attach our hearing aid to the coupler’s input – we have some tack in here to get a good seal. Now we’ll put it in the test box so its microphone port is located at the same position I had the reference microphone.  Let’s close the box, and we’re ready to test.

Let’s start with Frequency Response. The hearing aid should be set to its reference test gain. We’ll play an equalized stepped sine sweep from 10 kHz – 100 Hz at a level of 60 dBSPL through the test box speaker, and analyze the output of the hearing aid with the Heterodyne algorithm to produce a frequency response. Next, the HFA,  or High Frequency Average is calculated by averaging the response values at three frequencies – 1000, 1600, and 2500 Hz. We then subtract 20dB from the HFA, and two post processing steps are used to find the upper and lower frequency points at which the response curve intersects the HFA -20 value. These are the high and low frequency cutoff points. Limits can be applied to all of the curves and values of this sequence. Let’s run it.

And here you see the Fundamental curve falls outside of the upper and lower limits at a few frequency points, the low frequency cutoff is 217 Hz and the high frequency cutoff is a little over 5 kHz.

Our next test is input/output linearity. We’ll leave the hearing aid at reference test gain, and use an amplitude sweep from 50-90 dBSPL played through the test box speaker. This is repeated at four frequencies (1 kHz – 4 kHz) in 1 kHz increments, and the output of the hearing aid is plotted vs. the input on the graph. This test is useful for characterizing any automatic gain control or compressor in the hearing aid,  and is used to attenuate the hearing aid’s output above a certain level so as to not further damage the user’s hearing. Let’s run it.

The effect of the automatic gain control is clearly visible at all four frequencies.  The x-axis represents the stimulus level and the y-axis the output level from the hearing aid.  We can see that the gain limiting kicks in when the input is in the 60-75 dB range, depending on the frequency and note how the overall level of the 4 kHz curve is 10 dB lower than the others because the frequency response of the hearing aid is around 10 dB lower at that frequency relative to the others.

The OSPL 90 test measures the hearing aid’s performance with an input signal of 90 dBSPL.  For this test we need to set the hearing aid gain to its maximum. An equalized stepped sine sweep from 8 kHz – 200 Hz is played at a level of 90 dBSPL, and a broadband rms response curve which includes harmonics as specified in the standard, is analyzed from the recorded time waveform of the hearing aid’s output. Next, the HFA (High Frequency Average) is calculated by averaging the values at three frequencies (1000, 1600, 2500 Hz), and the Max OSPL is calculated by finding the maximum point on the broadband response curve. Let’s take a look.

The OSPL-90 curve looks much like the frequency response curve from our earlier measurement and considering that the input to the hearing aid is a calibrated sweep at 90 dB, the amount of gain in our measurement is between 25 and 40 dB across most of the sweep range. The high frequency average from this measurement is 126.7 dB and the maximum value on the curve is 129.7 dB.

Lastly we’re going to measure the release time of the hearing aid. Again, we want the gain set to max, and we’ll play a  2 kHz sine tone at 90 dBSPL for 1 second, then immediately drop it to 55 dBSPL for 2 more seconds. A band limited time envelope to improve the measurement signal to noise ratio – from 1.5-2.5 kHz –  is calculated and then run through a post processing step, to calculate the release time – that’s the time the coupler SPL remains within 4 dB of the steady value for the 55-dB input. [run test]

Here we can see that the hearing aid output initially drops down to around 85 dB but after 45 ms the release time has stabilized and the hearing aid maintains a steady output level of 115 dB.

So that’s four quick examples of tests from the ANSI S.22 hearing aid standard. Of course, once you have the equipment for these, there are many other hearing aid tests that you can run using the same test setup, and you can configure tests for the complete ANSI S.22 standard in SoundCheck. In fact we’ve already done it, and we sell the test sequence package with all 20 measurements as an add-on to SoundCheck so you don’t even have to program it yourself! Please contact us to learn more.