When I’m talking with my patients, I like to compare eyeglasses and hearing aids. They are familiar with bifocals and understand how they work for different viewing situations: the upper lens is for distance, the lower lens for close-up work.
Similarly, hearing aids have multiple programs that are adjusted for specific listening situations. The first program, often called the default program, is usually a broadband response that is most helpful in places where there is little or no background noise. Another common hearing aid program is designed for use in noisy environments such as restaurants, airports, and parties. Programming hearing aids so patients can understand speech in noise is our topic for this week.
SOUND SCIENCE
Let’s start with a little science. Sound is created by vibrations. The more vibrations there are per second, the higher the frequency of the sound they produce will be. Knowing this helps us understand the nature of background noise, especially the noise found in crowded restaurants, in large gatherings of people, and on busy streets.
If we understand the frequency distribution of typical background noise (outlined below), we can, with caution and skill, reduce the amount of amplification in those frequencies where background noise is the highest.
Most active people spend considerable time in noisy places. Here is a simple chart giving frequencies for various levels of typical noise. Note: These terms should be used only for teaching purposes and general guides:
FREQUENCY SOUND LEVEL
- 250 Hz HIGH
- 500 Hz MEDIUM HIGH
- 1000 Hz MEDIUM
- 2000 Hz LOW
- 4000 Hz VERY LOW
As this chart shows, higher noise levels are often found in the lower frequency zones. We can minimize the amount of noise a person will hear by controlling how much amplification we give the patient in the lower frequencies.
Let me walk you through a simple teaching example that illustrates this point. Suppose Joan, your patient, has a flat 60-dB hearing loss and you give her 20 dB of use-gain at all frequencies (a flat response on a flat hearing loss).
When Joan goes into a noisy restaurant, she will find the high level of background noise in the lower frequencies bothersome. Chances are she will turn down the gain setting of the hearing aids so the amplified sound in the lower frequencies won’t be too loud.
Unfortunately, when Joan makes this adjustment, she reduces the gain in all frequencies, including the higher ones. This is bad because 78% of the speech information is in these higher frequencies. So, by making the amplified sound in the restaurant more comfortable for her, Joan is at the same time making it very difficult or impossible for her to understand what her dinner companions are saying.
We do not want our patients to turn down their hearing aids because of excessive noise in the lower frequencies and as a result miss most of the speech information in the higher frequencies. It is important to note that for Joanie and for other patients with hearing losses like hers, there is no need to reduce the sound in the higher frequencies. Only the loud low-frequency sound bothers them.
A CASE IN POINT
Let me illustrate this concept with a true story. Years ago I taught the Industrial Audiology class at San Diego State University. I had my students go around with a Bruel & Kjaer octave band noise analyzer and measure sound in many different environments.
Their first test point was on a footbridge connecting a parking lot to the university. The bridge crossed over a very busy road. The students came back saying, “We have the values in the lower frequencies, but we don’t have any measurement at 4000 Hz.” That didn’t seem right so I went back to check their measurement technique. To my surprise, they were measuring correctly. The cars and trucks generated a lot of low-frequency noise, but almost none in the octave band at 4000 Hz.
The lesson here is to make sure your patient is not overloaded with low-frequency noise. You want to preserve all the speech information in the higher frequencies, but this can’t happen if the patient has to reduce the volume substantially.
In summary, I like to use a broadband program (lots of good gain in the lower frequencies) for Program 1, the default program. For Program 2, the “noisy environment program,” I substantially reduce the low-frequency gain. In other words, I make Program 1 big (fat), just not too big; and make Program 2 “little” (low cut), just not too little.
You can often push these two programs psychoacoustically farther apart. I like to push the default program toward a true broadband response. Then, for the noisy environment program, I use the “low-frequency cut” strategy to increasingly aggressive degrees, depending upon the patient’s hearing.
