The romantic side of a 10 kohm resistor

As readers of this blog are already aware, most modern hearing aids do a poorer job of reproducing music than many of the hearing aids in the late 1980s.  The limitation with all digital hearing aids is the analog-to-digital (A/D) converter that converts analog sound and music from the microphone into a series of numbers for the digital hearing aid circuitry.  There are a number of reasons for this, but the primary one is the inherent limitations of a 16-bit system.  Modern hearing aids simply cannot transduce inputs that are in excess of about 96 dB SPL unless something else is done to allow this.  While this is not an issue with speech, it frequently is an issue with music.

A simple strategy that I have not discussed before is to use the direct audio input (or alternatively the telecoil inductive input).  It’s actually not that simple, but the solution is rather trivial.

The idea is based on the same idea as if you have access to the level of the music that you are listening to- simply turn down the volume of the input and, if necessary, turn up the volume of the hearing aid to compensate.  This lower level input is more likely to be within the operating range of the A/D converter so that there will be minimal distortion.  When you are driving in a car, turn down the radio volume and adjust the volume of the hearing aid to compensate; this allows for the desired output in the ear, but with minimal distortion since the A/D converter has not been over driven at the front end of the hearing aid.  This is much better than the converse, where the volume of the car radio is increased.

If you can use the telecoil or the direct audio input to receive the sound, again you can still benefit from a volume control reduction on the assistive listening device.  For example, if you are using an FM system, simply turn down the volume on the FM system and, if necessary, turn up the volume control on the hearing aid.  Since the hearing aid volume control occurs in the hearing aid circuitry after the A/D converter, there should be an optimal sound.

However, what if the assistive listening device that is being used with inductive coupling or via the direct audio input jack does not have a volume control?  This may be the case with any number of external microphones that hard-of-hearing people may use in more adverse listening environments.  Like the normal microphone route, the telecoil and direct audio input systems do have to go through an A/D conversion, so unless something special is done, you may not be better off.  This is especially the case if the use of the direct audio input does not disable the hearing aid microphone.  If there is an option, when the direct audio input or inductive input is used, the hearing aid microphone should be disabled, or at least set to a setting that is less sensitive.

The first “trick” is then to use an M/T setting on your hearing aid when listening to or playing music.  The setting of the M/T combination should be programmed so that the microphone is about 10-12 dB less sensitive.  This may solve the music problem.

However, this is not always a possibility with hearing aids.  This leads us to the second “trick.”  A simple circuit can be made by any hearing aid manufacturer that can connect to the audio input cord or the inductive loop or silhouette.  This circuit will reduce the input by about 10-12 dB and this will “fool” the A/D converter into thinking that the input is within its optimal operating range.

This is what you tell the manufacturer:  “I would like a 10-kohm resistor in series and a 1-kohm resistor to ground.  This should be put into a boot or a shell that will have a typical 1/8” mini-jack input and will have the correct output to plug in to a neck loop or a direct audio input cord.”

Although this is one of the most romantic things that a hearing aid tech will ever hear, I assure you that you will get exactly what you want.  This “10 kohm in series and 1 kohm to ground” trick will drop the input by about 10-12 dB and provide the A/D converter with exactly the type of input it can handle.  And who knows, this may be the start of something big?  I met my wife-to-be with such a romantic opening line. We’ll be married next June.

About Marshall Chasin

Marshall Chasin, AuD, is a clinical and research audiologist who has a special interest in the prevention of hearing loss for musicians, as well as the treatment of those who have hearing loss. I have other special interests such as clarinet and karate, but those may come out in the blog over time.