Overcoming Noise, Reverberation, and Distance: Directional Microphones and ALDs
by Cheryl Heppner
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 dB.
– 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 ear.
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 all directions.
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 microphones.
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 performance. Research
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 receiver.
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 hearing aid.
– A good source is the Tibbetts Telecoil (www.tibbettsindustries.com)
– We should be asking for AMPLIFIED telecoils because they give us an extra boost.
– 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 manufacturer?
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 pacemakers?
A: Manufacturers have said that they have never heard of a neckloop interfering with a pacemaker.