How is directionality affected by hearing aids utilizing wide dynamic range compression?
The short answer is, it isn't, but I'll go on. Most of today's hearing aids that employ directional technology also have WDRC, so this is a reasonable question, especially when you think about how WDRC works. We all know that when WDRC is employed, greater gain is applied as the signal becomes softer. So consider this example: Your patient's WDRC hearing aid (2:1 compression ratio) is programmed to apply 20 dB of gain for a 70 dB input, and he is in a room where there is a 70 dB input originating from in front of him (the person he's talking to) and there is also 70 dB originating from behind him (party noise). With an omnidirectional instrument, the amplified S/N ratio would be 0 dB. But you wisely fit him with a directional instrument, and we'll say that it reduces the input from the back by 8 dB, so now the S/N ratio would be +8 dB (70 + 20 = 90; 62 + 20 = 82; 90 minus 82 = 8 dB). Remember, however, that as the input goes down, gain goes up for a WDRC instrument. This makes it tempting to think that the noise from the back would receive more gain than 20 dB (e.g., for a 2:1 compression ratio, a 62 dB input would receive 4 dB more gain than a 70 dB input, and hence the gain would be 24 dB, not 20 dB. In other words, the WDRC is adding back in ½ of the gain that the directional microphone so diligently took away. This doesn't sound like a good thing. This effect, however, only occurs when the sound in question occurs in isolation, with no other sounds present; an unlikely event in the real world, and a situation when directional amplification isn't needed (This situation does exist, however, when probe-mic measurements are conducted, which is why it is necessary to turn off the WDRC during this testing). It's possible to conduct polar plots with both the signal of interest and competing noise on at the same time. When this method is used, the polar plot is the same for both the WDRC and linear settings of the hearing aid.
If you need more convincing, or just find this topic really interesting, I suggest you check out a recent article from Todd Ricketts (Vanderbilt) and George Lindley (Towson State). These authors compared four different models of directional hearing aids that could be switched from WDRC to linear processing. Testing was conducted in moderately reverberant rooms, and uncorrelated cafeteria noise from five loudspeakers surrounding the subjects was the competing stimulus. The subject's performance for both the HINT and the CST was not significantly different between WDRC and linear processing for any hearing aid model.
H. Gustav Mueller, Ph.D. has an audiology consulting practice in Castle Pines Village, Colorado. He is the senior audiology consultant for Siemens Hearing Instruments, Contributing Editor for The Hearing Journal, and teaches audiology classes at Vanderbilt, Central Michigan, Nova Southeastern and Northern Colorado Universities. His publications related to probe-mic measures include:
- Mueller HG, Hawkins DB, Northern JL. Probe Microphone Measurements. Singular Publishing. 1992
- Mueller, HG. Probe-microphone measurements: Yesterday, today, and tomorrow. The Hearing Journal, 1998, 51, 4: 17-22.
- Mueller HG, Hall JW. Audiologists' Desk Reference. Singular Publishing, 1988.
- Mueller, HG. Probe-mic assessment of digital hearing aids? Yes you can! The Hearing Journal, 2001, 54, 1: 10-17.
- Mueller, HG. Probe microphone measurements: Twenty years later. Trends in Amplification, 2001 (in press).