Noise, Reverberation, and Distance: Directional Microphones and ALDs
Editor: People with hearing loss often make the statement that they
can hear someone speaking, but they can't understand what is being said.
Recent technology has addressed this situation and provides some
improvement. Here's Cheryl Heppner's report on the SHHH workshop by
Matthew H. Bakke, Ph.D and Mark Ross, Ph.D.
Dr. Bakke is with the Department of Speech Language Pathology at
Gallaudet University. Dr. Ross is with the Rehabilitation Engineering
Research Center at the Lexington Center. He is also a technology
columnist for Hearing Loss: The Journal of SHHH.
Interference from background noise, reverberation and distance from a
speaker is the biggest problem for people with hearing loss using
amplification. Dr. Bakke and Dr. Ross gave an explanation of directional
microphones and then discussed the benefits, limitations and problems in
their use. Here are my notes:
Dr. Matthew Bakke
- The ability to understand speech depends on how much of a signal
can be heard in background noise.
- Signal to Noise Ratio (SNR) in a typical environment is 10 dB to 0
- People with hearing loss require 5-15 dB better SNR than those with
normal hearing to understand speech in noise.
- Noise reduction in digital signal processing hearing aids has not
yet been proven to increase SNR or intelligibility, although it has been
proven to reduce noise through filtering.
- Dr. Bakke has been working in hearing aid research for 18 years. In
those early years we would have needed to wear something the size of a
refrigerator to do what we can now put in an ear.
- Efforts to solve removing noise from speech with one microphone are
not yet successful.
- Why use directional microphone hearing aids? The goal is to reduce
the signal level of sounds arriving from other than the desired
location. The result of directional microphones is an increase in SNR
based on spatial relation of sounds. This is especially beneficial when
you only want to hear one source of interest at a time and certain
listening conditions are right.
- Some new hearing aids enable you to automatically switch
directional microphones. This is not always good because the hearing aid
decides, you don't. Dr. Bakke likens it to using Microsoft Word; when
you go to format a paragraph, the whole document changes.
- Similar directional benefits are possible for analog, digital
programmable and true digital hearing aids, but analog may not have the
logic to turn the directional microphone on and off.
- Placed in a hearing aid or used with an assistive listening device
(ALD), a directional microphone rejects sounds not from the front. SNR
improvements are 3-10 dB depending on test materials and conditions.
- Directional microphones were used prior to the 1930s; they became
available in the 1970s in behind the ear hearing aids. They've only
recently become popular due to increased directivity and miniaturization
that makes it possible to place them in hearing aids that fit within the
All microphones have diaphragms that move back and forth in response
to changes in air pressure across it. Air is trapped in the back of the
diaphraghm since the rear of the microphone is closed except for a small
air leak. This type of microphone has equal output (sensitivity) from
Types of Directional Microphones
- A single diaphragm has two ports or entryways. Its design is like
that used back in the 1970s and it doesn't require a computer. Internal
and external time delays are how it gets directionality. The secret is
in the delay you introduce when a sound comes from one direction vs.
when it comes from another direction.
- Arrays may have dual microphones or simple arrays or three
Do Directional Microphone Hearing Aids Work?
Many reports from researchers say that they work for both adults and
children. But with many on the market, how do we compare them? Dr. Bakke
is going to be looking and information published on these microphones. A
directional index has been developed to score performance. Reported
performance can vary depending on how the hearing aid's directional
characteristics are measured. You must be a wise buyer. Much of what we
know has only been lab-measured and not human-measured, and the test
results claimed by the manufacturer can depend on the lab setup.
- Directional microphones in the in the ear hearing aids can improve
the SNR by 2.9 to 6.3 dB.
- The SNR improvement from an in the ear hearing aid directional
microphone (brand name called D-MIC) is 3.5 to 6.7 dB better than an in
the ear hearing aid with common omnidirectional microphones.
- Potential benefit of binaural directional microphone hearing aids
gives an additional advantage of 3 dB.
- Directional microphones in hearing aids won't help when sound is
from a direction other than the front. They also won't help in an
environment where there is reverberation and the listener is far from
the source of the signal (e.g., gym, cafeteria, senior center) because
you get sound bouncing all over the place.
- Directional microphones reject sound from the sides and behind you.
Assistive listening devices use a remote microphone to catch desired
sound at its source, before it crosses a room and becomes attenuated and
distorted due to distance and poor room acoustics.
- Assistive listening devices that reach out and grab the sound are a
solution - they improve SNR 16-20 dB. You can mate an assistive
listening device with a directional microphone and get even better
The RERC on Hearing Enhancement is doing research with the U.S.
Access Board on the use of assistive listening devices in public places.
They have requested funding for an additional five years; if granted
they will be working more on this. A common complaint is that when
people with hearing loss go to theaters, public buildings, etc. they
have trouble borrowing receivers to use the listening system, or the
receivers do not work. Many try to solve the problem by buying their own
receiver, only to learn that it won't work with the system because the
transmitting signal doesn't match the receiver or has different
characteristics than the receiver. Some kind of FM and infrared
transmitter standard will be examined, with the goal of a universal
Dr. Mark Ross
- A low tech solution is available now: the audioloop. It's an old
technology getting a rebirth. If you are using a telecoil, this system
provides great benefits. There's a bridge for something like Bluetooth
to make it wireless. Dr. Ross noted that we should stop using the term
"telecoil" and use the term "audiocoil" to better
describe how it works.
- Inductive coupling requires a telecoil with sufficient sensitivity.
The telecoil has to be oriented correctly within the case of the hearing
aid. Adequate magnetic field must come from the assistive listening
device, telephone, etc. There must be little or no magnetic interference
from things like fluorescent lights.
- The problem with telecoils is with people who want tiny hearing
aids that don't show. You need a certain size to fit the telecoil in a
- A good source is the Tibbetts Telecoil (www.tibbettsindustries.com)
- We should be asking for AMPLIFIED telecoils because they give us an
- Positioning of the telecoil is also a problem. For phone use, you
want it horizontal. For loop use, you want it vertical. If you intend it
for both uses, you'd have to compromise for one positioned like a /.
That would give you a bit of disadvantage for both uses.
- David Myer has a Let's Loop America campaign. When he started
getting churches looped in Michigan, attendance went up.
Q: Will you find variations in a neckloop even if it is from the same
A: Yes. The same is true of the telecoils in hearing aids. Dr. Ross got
new ones and had to tilt his head to pick up the loop in his house that
he set up to watch his television.
- In reality, companies make the telecoil vertical because you can
change the orientation of the phone, but you can't stand on your head
when using a neckloop.
- FM and infrared receivers are excellent in some situations.
Receivers are the problem and have been since the beginning. At
performance venues, you get receivers that are dirty, have dead
batteries, etc. Why? Because the people who are supposed to take care of
them are not committed to taking care of them.
- The advantage to using a telecoil as an assistive listening system
receiver is that there is nothing to check in or out. There is less
reluctance to using the listening system, an important issue when people
don't want to broadcast their hearing loss. Since the hard of hearing
person is responsible for the upkeep, they are committed to it. The
signal processing of the hearing aid is preserved as well.
- Installing a loop correctly does require some technological savvy.
You have to know the field strength and other things.
Q: Is there a problem with induction loops interfering with
A: Manufacturers have said that they have never heard of a neckloop
interfering with a pacemaker.