Hearing Aids Practical Exam

Which of the two microphones is factory-calibrated?

Coupler microphone because they have a bigger diaphragm, so they will have a higher sensitivity compared to the noise floor. It is calibrated to have a flat frequency response and a specific reference level.

If this microphone is defective, will Verfit catch it?

Yes, if there is a mismatch between the two microphones, Verifit will not allow us to proceed. This line being shown is negative, so the reference mic is picking up less dB SPL than the coupler microphone. Both microphones need to be picking up the same dB SPL.

The Verfit will not catch it if both mics are off at the same frequencies in the same direction during the same calibration.

What is happening during this procedure?

Calibrating the reference microphones to the coupler microphone

Verfit creates a correction curve for the non-factory-calibrated reference microphone by comparing it to the factory-calibrated coupler microphone. The system is measuring the sound mea-sured by the coupler microphone (measurement 1) and the reference microphone (measurement 2). This correction curve is used to compensate for the reference microphones in test box measurements. So the line is the mathematical difference between the output coupler microphone and the reference microphones. So verifit stores the difference and flips it, and establishes a curve before the reference microphone.

Cross-check of the two microphones.

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Leveling involves playing a known sound level out of the speaker(s) and placing the two microphones that are used for measurements (in the case of the text box, this is the coupler microphone and the reference microphone) as close to each other as possible so that they are receiving the exact same sound (same distance from the loudspeakers). Given that the micro-phones are essentially in the same space, they should both measure the same sound pressure level. If they do not, the system will adjust sensitivity internally to make a correction. This allows the system to “trust” what the reference microphone is measuring during your testing. The reference microphone will monitor the level being produced by the loudspeakers and the system will make internal adjustments to ensure that the speaker is producing the correct level of sound so what you believe you are presenting is, in fact, the level that is being presented.

What is producing this line at “0” dB SPL given what you know about the goal of leveling? (Hint: If one sound is produced from the loudspeaker and the two microphones are measuring the exact same level of sound, if you subtract those two results, the difference would be 0 dB SPL across frequency.)

What does the line on the graph physically measure?

The mathematical difference between the reference microphone minus the coupler microphone (picking up the dB SPL of the reference microphone compared to the coupler microphone)

What is the first way that Verifit uses this information?

A correction curve for the uncalibrated reference mic to the calibrated coupler microphone. The Verfit stores the correction curve and applies the inverse of the correction curve. If the line is negative, the reference mic is picking up less dB SPL than the coupler mic. If the line is positive, it is the opposite.

What is the second way that Verifit uses this information?

A cross-check of the microphones to make sure the equipment is working properly. If the difference between the mics is too large, either mic is likely defective, so the software will lock you out of performing test box measures.

Can you show me that what you see is correct?

Yes, you will go to the help menu, and it will take you to the calibration test box reference microphone to show that the line is flat and has a good response. It should be fairly flat around 0 dB for Verifit 2 (around 20 dB for Verifit 1), with no significant peaks or dips.

The calibration monitors for improper curves and will flag errors, warning the user that the calibration is invalid.

After the calibration expires, you'll be prompted to calibrate whenever you attempt to test with the test box microphones.

How far away should you hold the probe when calibrating the microphones?

5-6 inches away from the loudspeaker. You want to stand away from it because you do not want to be a reflective surface. We want to play a broadband noise and compare the dB SPL at the probe microphone that has a tube connected compared to the dB SPL at the control microphone.

What is happening during the on-ear calibration?

The probe microphone responses are compared to the control microphone, which establishes a correction curve

What does it mean to be acoustically transparent?

The probe tube does not influence or change the sound that is being measured. This is due to the software correcting for and removing the tube's original resonances.

Where is the probe mic?

Inside the module

Where is the control microphone?

The control microphone is on the outside of the module.

What is the control microphone's purpose during this and all on-ear measures?

The control microphone controls the dB SPL out of the loudspeaker to create an equalized soundfield at the control microphone. When we are doing on-ear measures with the hearing aid, it will apply the microphone location effect, so it will equalize the sound field at the hearing aid microphone. (maintain stimulus level at a constant SPL at all frequencies at the HA microphone location + MLE correction)

What does the line on the graph physically measure for on-ear measures?

It measures the frequency response difference between the probe microphone and the control microphone.

Why is the line not flat?

The line is not flat because the probe tube is connected to the probe microphone. It has specific resonances due to the length of the tube, so it does not have a natural flat frequency response and will have peaks on the curve. Shorter tubes have higher resonance frequencies

Why does Verfit use this information for on-ear calibrations?

The Verifit uses the correction curve as a correction factor. The Verifit subtracts the probe tube resonances to future on ear measures, to allow the Verfit to show the true dB SPL at the TM. Also, uses it as a cross-check to make sure the equipment is working properly.

Could you please confirm that what you see is correct?

Yes, click help to show you what the calibration curve should look like

How will the line change if I cut the tube and recalibrate it?

The tube resonance will change because resonance relies on the length of the tube, so the calibration curve would be shifted to higher frequencies, thus making the previous calibration curve's information insufficient

Does it matter where the patient is positioned when measuring the real ear

Response for probe tube?

Yes, it matters where the patient is positioned when measuring real ear responses to get accurate measurements. The patient should be positioned in front of the loudspeaker at least 12-24 inches away to improve the signal-to-noise ratio at low-level intensities, while high-level intensities may overdrive the speaker of the system.

When calibrating the transducer, does it matter where the coupler is in the box?

No because the RECD transducer is plugged into the coupler microphone and the sound is coming from the transducer, so it does not matter where the coupler is in the box

What is the green line, and why is it not flat for RECD?

The 0.4 cc coupler measurement. The green line is not flat because it has a high frequency roll off due to the size of the coupler.

How deep should you put the probe tube in the adult ear canal and why?

In an adult ear canal, the probe tube should be inserted to a depth of 28-30 mm past the inter-tragal notch. So this means that the probe tube is typically placed within 5 mm of the tympanic membrane. This is important because it helps

minimize any measurement errors caused by the standing waves in the ear canal. When sound is going into the ear will bounce off the TM. So when the sound coming in and the sound coming out interfere with each other which creates standing waves. Having the probe tube within 5 mm of the tympanic membrane, it makes the measurements more accurate and consistent and helps us not overamplify when programming a patient's hearing

aid.

Does it matter where the patient is positioned when measuring the real ear response?

No because the RECD transducer is connected to the EM, no sound coming out of the loudspeaker

Where on the graph is the critical information?

The patient's personal RECD (real ear minus coupler) blue line

Real ear measure (pink)

0.4 cc Coupler response (green)

How will smaller-than-average ear canals affect this measurement?

The RECD measurement will be higher than average due to smaller ear canal volumes, so it will have a higher dB SPL

What if your patient has a larger-than-average ear canal?

The RECD measurement will be lower than average due to the larger ear canal volume

What are the acoustic effects of vents and slit leaks?

Will have a negative RECD and will see a low frequency roll-off below 1000 Hz

What will happen if you change the RECD coupling to foam tip?

The RECD will have higher resonant frequencies. The higher resonance was observed in higher frequencies when using foam tip. The foam tip has higher resonance frequencies due to it being a shorter tube so the Verfit will undo this when you select foam tip for the RECD coupling to show what you would have seen if you used an EM

What are the acoustic effects of a TM perforation or PE tube?

Will see a negative RECD value for the low to mid frequencies. The lowest point will be between 500-800 Hz, and then it will not continue to drop

What is the first way the measured RECD is applied when fitting hearing aids (speechmapping)

Provides a correction factor as a function of frequency that we can apply to our coupler measurements to estimate the real ear response.

What is the second way the measured RECD is applied when fitting hearing aids (speech mapping)?

Correction factor to convert dB HL to dB SPL at TM

What is your rationale for selecting the order for running the tests?

-Run average speech, MPO, soft speech, and loud speech. Average speech because it manipulates the compression ratio. If average speech is below targets, the soft and loud speech will also be under target, so all 3 would need to be adjusted.

What is the dotted line near the bottom of the speech map?

Reference for normal hearing

What are the units on the y-axis?

dB SPL at the TM

Why is the y-axis important?

It is important because the thresholds and hearing aid output are both expressed in dB SPL at the TM, so I can determine how much of the amplified speech is audible to the patient

What do the asterisks indicate?

patient's measured or estimated LDL's

What is your rationale for positioning the hearing aid in the box?

The front and back microphones of the hearing aids should be level in the horizontal plane. Need to put reference micropjone next to the front microphone.

How is the reference microphone being used during these measurements?

It controls the dB SPL coming out of the loudspeaker so that it is at the equalized sound field at the HA microphone to get the right dB SPL at HA microphone

What is happening during the on-ear measure procedure?

The output of the dB SPL of the probe mic with a probe tube attached is compared to the output of the dB SPL of the factory calibrated control mic and the differences are subtracted for all on-ear measures. A correction curve with this information will be created and a cross check to make sure both the reference mics and the coupler microphone are working properly

How far past the sound bore should you put the probe tube and why?

Your probe should be extended at least 6mm past the sound bore. This is important because when sound is leaving the hearing aid or earmold, it travels as radial waves. The radial waves will cancel, resulting in a lower dB SPL at high frequencies, which creates pressure nodes and antinodes within the first 5 mm of the sound outlet. If the probe tube is positioned too close to the sound bore, it will measure these acoustic artifacts rather than the actual sound pressure level in the canal.

Within the context of Boyle's law, explain how/why the RECD changes with age and why conducting an RECD regularly with all pediatric fittings is critical.

Within the context of Boyle's law, smaller volumes create higher sound pressure levels. Because infants and young children have smaller ear canals, hearing aids can become louder in their ears, leading to a larger RECD. SPL for the same sound can be 15-20 dB greater in an infant's canal than in an adult's. As children grow, so do their ear canals. This leads to decreases in sound pressure levels, making the RECD smaller. This is why it is important to measure the RECD regularly and during the greatest change in the first few years of life. It is recommended to measure every 3 months until age 2, then every 6 months until age 5, and then annually if still necessary. Not testing regularly puts the patient in danger of over-amplifying because the RECD dB decreases as the patient ages. Regularly performing RECD will make sure we are not programming the hearing aids too loud or too soft. If we were to under-amplify the hearing aid, we would be assuming that the patient has a longer ear canal, and this would lead to targets not being met on the hearing aid. If the hearing aid is under-amplified, the patient would not be accessing speech sounds that are critical for speech development.

Describe how the RECD for an earmold with a vent/slit leak/poor seal will differ from the average RECD.

When an earmold has a vent, slit leak, or a poor seal, the RECD will be more negative than the average RECD at lower frequencies. There will be a gradual roll-off of less than 1000 Hz. This is because sound will leave through the vent or leak instead of building up in the ear canal. The sound leaking will reduce the measured sound pressure level at the eardrum.

Describe how an RECD performed in an ear with a perforation/P.E. tube in the tympanic membrane will differ from the average RECD.

The RECD performed in an ear with a perforation/P.E. tube in the tympanic membrane will be mostly negative compared to the average RECD. It differs from a vent or poor seal, because the decrease in the low-frequency response will not continue to drop. It will reach a minimum of 500-800 Hz.

Describe how changing the on-ear RECD coupling method from “Earmold” to “Foam Tip”

These changes happened because the verifit accounts for the foam tip. When the foam tip is selected, the measured RECD becomes larger in the higher frequencies, especially 4000 Hz, because the foam tip creates a smaller residual ear canal volume and shifts the resonances. The Verifit uses this larger RECD to convert audiometric thresholds into ear canal SPL and to predict the real ear output during speech mapping. The larger RECD means the calculated thresholds in dB SPL become higher, and the Verifit will give less gain during speech mapping to avoid amplification. This will result in lower speech mapping outputs compared to the earmold at higher frequencies, where the RECD differences are the greatest.

Describe how changing the on-ear RECD coupling method from “Earmold” to “Average”

overamplified the speech mapping as well as the thresholds at dB SPL at the TM

These changes could be because averages do not account for variability among normal ears or earmold acoustics. When switching the Verifit from earmold to average, the Verifit is no longer using the patient's ear canal acoustics but instead using a general estimate to compute thresholds in speechmapping. When using earmolds, the Verifit uses the measured RECD. If the measured RECD is larger, there will be less gain resulting in a lower speech-mapping output. If the measured RECD is smaller, it will increase the gain to compensate, and speech-mapping will show more amplification.

Verifit 2 uses a 0.4 cc coupler, why is it negative?

The 0.4 cc coupler reference volume is smaller than the ear canal, which will produce a higher SPL in the coupler than the SPL at the tympanic membrane. The real ear will now have a smaller SPL than the coupler, meaning that the RECD will be negative.

In acoustic terms, explain the basic concept behind how the RECD can be used to fit hearing aids.

The RECD is a correction factor that helps us understand how sound is measured in a test box versus how the sound is measured at the patient's ear drum. It is important to have the correction factor because everyone's ear canal has a different shape and size. The RECD allows us to program hearing aids within the test box while making sure the sound is accurate for the patient's ear. This will make sure the hearing aid is not too loud or too soft.

What can you tell me about your measurement?

If it is negative, below average, larger ECV, and less dB SPL, low frequencies that are negative to a slit leak. If it is positive, it is above average, indicating a smaller ECV and a higher dB SPL