Today's advanced compression circuits make yesterday's attitudes out of date
Robert L. Martin · The Hearing Journal · 2001
FigureI have a question: “Is the compression of amplified sound helpful or harmful?” How you answer this question likely depends on: (1) how long you have been working in this profession and (2) your view of amplification. Some people use amplification to restore the ear's normal loudness function. Others see amplification primarily as a tool to enhance word understanding. Old-timers, like me, who focus on word understanding tend to think that the compression of amplified sound is a necessary evil, a type of temporal distortion that can reduce word understanding. But things have changed, so keep reading. HOW COMPRESSION USED TO BE Many years ago, compression was used simply to keep amplified sound from getting too loud. Most compression hearing aids operated in a linear mode for soft and medium loud inputs. The compression circuit kicked in at a fairly high level (70 dB to 80 dB) with an aggressive compression ratio (e.g., 6:1) that markedly limited any increases in output. Consider the Siemens 284PPAGCI BTE as an example. This wonderful old aid has four control pots: low-cut, high-cut, automatic gain control (AGC), and power. The AGC control lets you set the compression kneepoint between 60 dB and 90 dB. I usually set it about 75 dB. The compression ratio for this aid is fixed and quite aggressive. If you set the AGC kneepoint to 60 dB, the amplification is notably muffled and distorted. When you look at the research concerning dynamic range of hearing, UCLs (uncomfortable loudness levels), and recruitment, you have to conclude that all hearing aids should have some type of compression circuit. Yet, during almost all of the last 40 years, linear amplification, not compression, has been the most frequently fitted circuit. Why? I believe most dispensers listened to the various hearing aids on the market and chose the instruments that sounded best to them. They selected linear amplification because it sounded better than aggressive compression. One manufacturer studied compression and decided we needed Adaptive Compression, a smart compression protocol. So a circuit was created that adjusted compression depending on the listening environment. Another manufacturer did some research to find out which type of amplification (linear or compression) produced the best word understanding. These results showed linear amplification to be the best, so the company built a two-channel amplifier. Compression was used in the lower band to control loudness. Linear amplification was used in the higher band to enhance word understanding. These developments—Adaptive Compression and multiband compression—simply reinforced my bias and viewpoint that compression is deleterious and best avoided if possible. LOWER KNEEPOINTS, RATIOS Then, several years ago, the K-AMP and ReSound circuits came along, and we were introduced to a new way of thinking about compression amplification. These circuits have much lower compression kneepoints (as much as 20 dB lower) than previous circuits and also much lower compression ratios (e.g., 2:1). The amplified sound from these aids is almost always compressed because the kneepoint is so low. Yet the sound is not distorted or muffled. Then single-band amplifiers, which had dominated most of the history of hearing aids, started being replaced by multiband amplifiers. As a result, the issue was no longer whether or not to use compression, but instead how to adjust compression knee-points and ratios in four contiguous bands in two different programs for both ears. Next, digital amplification was introduced, and the complexity of compression increased further. We are now asked to choose linear compression vs. curvilinear compression and syllabic vs. dual compression, as well as how to set the kneepoints and ratios. And we can also set the active digital signal processing (ADSP) levels to off, minimum, medium, or maximum in each of eight contiguous bands in both programs, for both ears. Wow! Double wow!! And now we are hearing about changing the compression kneepoints to even lower levels, i.e. the 20-dB range. SHEDDING NEW LIGHT ON COMPRESSION Last week at an advanced training class, I asked, “Is it necessary to have such low kneepoints? Is all this emphasis on compression of soft signals really important?” One of my classmates, a woman, who, like me, has been in the profession for 100 years, helped me see compression in a new light. She said, “Bob, you're used to thinking of compression as a tool or mechanism used to control loud sounds. But don't think of a compressor circuit as a unit that squeezes or pushes a lot of sound into a small space. “That is outdated. In the past, highly compressed sound was not desirable because it hindered rather than helped word understanding. But now that compression ratios are no longer very high, you should think of compression as a gain adjustment. Compression lets you provide the patient with much more gain in very specific areas, e.g., between 1700 Hz and 2800 Hz. This is possible because compression in today's hearing aids occurs in separate, independent bands and all aspects of the compression are under your control. “The new multiband independent compression settings allow you to help the patient hear very soft speech sounds without distortion in almost all listening conditions. Set correctly, compression settings facilitate word understanding. Remember, you can't understand words unless you can hear them.” My classmate had a better understanding of compression than I did. And she helped me see compression as a tool that can markedly help, rather than hinder, word understanding. This is especially important for patients who wear hearing aids in high-level noise environments such as a noisy street. When a high-tech hearing aid is used, the intense low-frequency ambient noise no longer affects compression activity in the high-frequency zones. Good word-understanding ability can be maintained. Things change. I'm learning to change with them.