HEARING AIDS: DIRECTIONAL MICROPHONES & DIGITAL NOISE
REDUCTION
The main challenge for hearing aid wearers is background noise. In the last insert
(Hearing Aids & Their Digital Programs) we looked at how a “listening in noise”
program addresses this problem by reducing low-frequencies and increasing high
frequencies. This however, is only a partial solution. There are two more things employed
by virtually all digital hearing aids today that specifically work to increase the
intelligibility of speech while in noisy situations: Directional microphones (Dmics) &
Digital noise reduction (DNR).
Microphones simply change sound into electricity; speakers change electricity back into
sound. In hearing aids, microphones are placed inside tiny little holes on the outside of
the hearing aid. They pick up and change incoming sounds into electrical current, which
is sent on to the digital signal processor (DSP) inside the hearing aid. The DSP, which is
really the heart or main command centre of the hearing aid, changes the current into
digital bits and bytes; this is where the amplification and shaping of sound takes place.
The results from the DSP are sent on to a small speaker, which is usually placed at the
end of the hearing aid in the ear canal.
Most microphones are “omni-directional.” This means they pick up sounds from all
directions. In other words, they are equally sensitive to sounds from any direction. Dmics
however, are more sensitive to sounds from some directions than from other directions.
Normally we face those with whom we are having a conversation. To make speech more
intelligible in noise, it would thus make sense to have the Dmic be most sensitive to
sounds coming from the front of the listener, and less sensitive to sounds coming from
the sides and behind.
Figure 1 shows what are known as microphone “polar plots.” These are used to illustrate
the sensitivity patterns of various types of microphones. The “00” is the front and the 1800
is the rear Note that the omni-directional microphone has a round polar plot; this means
the microphone is equally sensitive to sounds coming from all directions. In contrast,
Dmics do not have rounded polar plots. In fact, one type of Dmic has a “cardioid” polar
plot because its sensitivity pattern is shaped like a heart. This Dmic is most sensitive to
sounds from the front and not at all sensitive to sounds coming from the rear. There are
many types of Dmics; two more depicted here are “super-cardiod” and “hyper-cardioid.”
Note that these have the same frontal sensitivity, less side sensitivity, but more rear
sensitivity.
With today’s digital hearing aids, Dmics go even further. They can automatically change
from acting as omni-directional microphones while the hearing aid wearer is in quiet
listening situations, to Dmics when the wearer moves into noisy situations. What’s more,
they can also adapt the shape of their polar plots so as to best increase speech
intelligibility in changing noisy conditions!
The story does not end here; background noise is also addressed with DNR. The sound
waves (acoustics) produced by speech are very unique and different from those produced
by noise. DNR is a kind of artificial intelligence that uses these acoustic differences in
order to classify or separate incoming speech from noise. To accomplish this separation
of speech from noise, DNR uses what is called “amplitude modulation.”
Figure 2 shows two sound waves. The top wave is from a tone that does not change or
modulation in its amplitude, or intensity, over time. Most noises like fans or air
conditioners, share this same characteristic. Even the background speech at a cocktail
party stays fairly consistent in loudness over time (that’s why they call it “babble” or
“hubbub!”). In contrast, the bottom sound wave is from typical speech from someone
right in front of the listener. Note how it fluctuates rapidly in loudness over time. It is this
speech sound that the DNR of the hearing aid attempts to increase when the listener is in
a noisy environment.
Dmics & DNR are very different from each other. Research with statistics has shown
time and time again, that Dmics objectively increase the intelligibility of speech in noisy
environments. The same cannot be said for DNR; instead, it has been shown to
subjectively enhance listening comfort while in noisy environments. One can think of
Dmics as the science and of DNR as the art. Dmics are the “head,” while DNR is the
“heart.” Although they are very different from each other, Dmics & DNR work well
together as a team.