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Today, we bring you a discussion by our Hearing News Watch editor, Brian Taylor, AuD, on using clinical judgement to determine when hearing aid circuit noise is a problem. Today’s post is part of Brian’s “Signal & Noise” series, which is a bimonthly column at Hearing Economics.
Hearing Aid Circuit Noise: Using Clinical Judgment to Know When it’s Really a Problem
Recall the last installment of Signal & Noise discussed the clinical implications of a recent study from research audiologists at Vanderbilt University Medical Center. If the numbers in their study held true across the entire profession, clinicians would send back virtually every hearing aid because they fail to meet the ANSI specification for equivalent input noise (EIN).
For the industrious clinician, who systematically evaluates every device in a hearing test box with the appropriate coupler, sending a lot of hearing aids back to the manufacturer and rescheduling patient appointments is ruinous to the business for obvious reasons. Good judgment and some attention to research can help address this problem and keeping patient’s happy & clinic flow efficient.
Standards vs. Specifications
At the heart of this issue is the difference between an industry specification (what we call an ANSI “standard”) and a hearing aid performance standard. Let’s take a more careful look at this difference, and how it relates to the practice “good audiology.”
A performance standard is typically developed according to a specific set of rules and procedures with broad agreement among many interested groups. For example, hearing aid manufacturers, clinicians, and other stakeholders could convene a group and create a hearing aid performance standard. Once a performance standard is created, it is published by a neutral party for everyone to follow. To the best of my knowledge this has not been done for hearing aids, however, any of the scientifically-derived prescriptive fitting formulae, like the NAL or DSL family of targets, are, in a sense, a performance standard.
On the other hand industry specifications – like the ANSI hearing aid “standards” (I would argue they are specifications and different than a standard per the definition above) – have a different purpose. Industry specifications are used as a reference for design or product criteria. They provide a common set of definitions and tests that allows agreement throughout the industry. Further, the ANSI hearing aid “standards” provide a level playing field with which to test hearing instruments produced by all manufacturers.
When clinicians take the hearing aid out of the package, place it into the hearing aid test box in test mode and compare it to the printed specifications that accompany the device, they are answering the basic question: Is this hearing aid performing within a group of defined specifications?
The Peculiarities of Measuring Circuit Noise in the Clinic
Audiologists must be involved with following both specifications and standards, because at the end of the day, no matter how sophisticated or expensive the hearing aid, there are three things we need to get right:
- Do no harm. Ensure the amplified incoming sound is not unintentionally or undesirably altered by the hearing aid.
- Restore audibility. Ensure the amplified sound is above the patient’s threshold,
- Control the output. Ensure the amplified signal does not exceed the patient’s loudness discomfort level
It is point #1 – do no harm – that warrants our attention to circuit noise. The Holder et al study tells us that many modern hearing aids fail to meet the ANSI equivalent input noise (EIN) specification. But, a high failure rate on the EIN specification may not reflect a problem with the hearing aid fitting. According to the engineers I contacted, high EIN numbers might be artifact resulting from measuring a nonlinear hearing aid with its expansion feature activated. Recall that the EIN specification requires that the high frequency average gain for a 50 dB input be subtracted from the output when no signal is being delivered. This assumes the gain of the hearing aid for a relatively soft 50dB input is a valid reflection of gain needed for everyday listening. Thus, the EIN measure may not be a valid measure of circuit noise in typical listening situations.
The dilemma for the clinician is determining when circuit noise exceeding the EIN specification by more than 3 dB is really a problem that requires the hearing aid to be sent back to the manufacturer or more of a situation that does no harm to the patient. To address this dilemma let’s turn to some research.
A 2010 JASA study published by James Lewis and colleagues at the University of Iowa, investigated circuit noise in modern hearing aids. They evaluated the circuit noise (EIN) of hearing aids from six manufacturers and found that circuit noise is generated from components other than the microphone, and this circuit noise is often dependent on the input level of the incoming signal. They also found that certain adaptive features could make circuit noise worse. Maybe some of those adaptive features that provide additional benefit have the opposite effect of causing harm (annoying & bothersome circuit noise)?
One important consideration from the Lewis et al research and perhaps the one point that is most clinically relevant is the EIN measure by itself is really not all that important. It’s the relationship between the EIN measure and the patient’s threshold that really matters. For example, if you have a patient with relatively poor thresholds, say worse than 40dBHL, even high amounts of circuit noise won’t be audible. On the other hand, even if a hearing aid had low EIN, if the patient has normal or mild loss thresholds, it is possible that even relatively low levels of circuit noise could be audible.
It takes sound clinical judgment by the audiologist to know when an EIN measurement outside the specification might be a problem for the patient.
Here is another important consideration that requires good judgment: Just because circuit noise is audible doesn’t mean it’s annoying or bothersome to the patient. As Agnew (1997) demonstrated on a group of eight subjects with moderate loss, hearing aid circuit noise became audible at frequencies between 500 and 1500 dB when the noise reached intensities of 20 to 35dB. The difference, however, between audible and bothersome circuit noise ranged from 4 to 15dB. Since even small increases in audible circuit noise can make it bothersome for some individuals, audiologists need to carefully monitor its presence.
Based on these two papers and a couple of conversations and e-mails exchanges with industry engineers, here are a few things you can do in the clinic to ensure that potentially high levels of circuit noise are not doing patient harm.
- Using your hearing aid test box, measure EIN, but as Jim Jonkman, an engineer at Audiscan shared with me via email, “The challenge in the clinic has always been to make a solid EIN measurement and that’s due to ambient sound contaminating the measurement. Even if only one impulsive sound gets into the EIN averaging window, it can easily dominate the average and give a higher EIN than that produced by the device alone. The ambient sound can come from many sources, but a significant one often overlooked is vibrational pickup into the test chamber. Vibration picked up from a laptop on the same table, a mouse being moved on the desk, or even a door closed in an adjacent office can all become sources of unwanted sound potentially coupled into the test chamber. Any sort of impulsive sound occurring during the EIN measurement will easily skew the average and that will show up as a variation in the repeatability. To confirm that the setup is suitable for an EIN test, it should be run in the calibration condition with no device attached. It should be able to achieve a repeatability of 1 dB at the rated value for the equipment.”
- During the fitting appointment, listen for audible circuit noise. This is a good practice for all fittings, but especially for patients with normal low to mid frequency hearing or a mild to moderate loss through 1500 Hz. It can be done if your fitting room is in a sound-isolated space and you have a pair of high quality earphones that can be plugged into your probe mic system. With the hearing aids turned on and without an input signal, while wearing your earphones in your quiet room, listen for circuit noise. Ask your patient if they can hear it. If they can hear it, don’t panic, audible circuit noise in quiet doesn’t necessarily translate to being bothersome. This observation is something that needs to be monitored over time. (Back in the pre-digital era, when circuit noise came almost exclusively from the microphone, with the hearing aid turned on in the patient’s ear, you could plug the mic port with putty and if the noise remained the circuit noise was a likely culprit for poor sound quality. Given that circuit noise in a non-linear device with other automatic processing schemes may be generated from components other than the microphone, this technique of plugging the mic port and listening for circuit noise may no longer be quite so effective)
- After you’ve conducted this routine measurement on several hearing aids from different manufacturers, you might be able to identify makes or models of hearing aids that have consistently low circuit noise. Stick with these products for patients with normal hearing and mild hearing loss through 1500 Hz.
Identifying annoying & bothersome circuit noise from audible circuit noise which may be outside the ANSI specification requires a skillful audiologist. Although technology is rapidly becoming more automated, the good judgement of an audiologist to first do no harm is something that never goes out of style.
Brian Taylor, AuD, is Senior Director, Clinical Affairs, for Turtle Beach/Hypersound. He continues to serve as Editor of Audiology Practices, the quarterly publication of the Academy of Doctors of Audiology. During the first fifteen years of his career, he practiced clinical audiology in both medical and retail settings. Since 2005, Dr. Taylor has held a variety of leadership & management positions within the hearing aid industry in both the United States and Europe. He has published over 50 articles and book chapters on topics related to hearing aids, diagnostic audiology and business management. Brian has authored three text books: Fitting and Dispensing Hearing Aids(co-authored with Gus Mueller), Consultative Selling Skills for Audiologists, and Quality in Audiology: Design & Implementation of the Patient Experience. His latest book, Marketing in an Audiology Practice, was published in March, 2015. Brian lives in Golden Valley, MN with his wife and three sons. He can be reached at [email protected] or [email protected].
*feature image courtesy of Cambridge in Color