Wednesday, January 29, 2014

Advanced Signal Processing in Today’s Hearing Aids

Written by Courtney Morrison, M.Cl.Sc.

Today’s hearing aids have come a long way from the big beige bananas that may first come to mind.  Not only do they amplify sound, they work to enhance meaningful signals (i.e. speech) in order to compensate for a damaged auditory system. When you realize how much your little hearing aid is actually doing second by second, you may find you have a new appreciation for the tiny battery that keeps it going for 100 hours or more!

Acoustic Environment Classification
Almost any new hearing aid you will try is constantly analyzing the sounds around you in order to classify the listening environment into one several categories. More basic hearing aids are generally determining between a ‘quiet’ or a ‘speech in noise’ environment, whereas more advanced hearing aids are often sorting the incoming signal between quiet, speech in noise, noise alone and music categories.
The hearing aid makes this classification based on various characteristics of the incoming sound including the overall level and the frequency and variations in amplitude of the sound waves that are picked up at the microphones.




By classifying the acoustic environment, the hearing aid can then automatically enable or disable other features which reduce noise and are designed to make speech audible and understandable.

Noise Reduction
One of the main goals of signal processing in hearing aids is to make listening comfortable and noise less bothersome. Imagine sitting in a café, in conversation with a friend, when suddenly the loud humming sound of a blender cuts in. Your hearing aid detects this hum, determines it is an unwanted sound and then uses an algorithm to reduce the noise. Many of today’s hearing aids can reduce noise levels by up to 10 dB! Noise attenuation is particularly effective for what we call ‘steady-state’ sounds – think of the fairly constant sounds from fans, refrigerators or vacuum cleaners.  Studies have shown that noise reduction techniques employed in hearing aids improve listening comfort, reduce listening effort and offer preferred signal quality for hearing aid users.

Directional (Beam-forming) Microphones
One of the biggest issues for new hearing aid users is that unwanted and distracting background sounds are amplified, along with the things that you are trying to hear.  Directional microphones have been proven to improve speech understanding in noise, reducing some of the background noise, thereby boosting the signal-to-noise- ratio (SNR). They work by changing the direction of microphone sensitivity, generally making the microphones more responsive to sounds coming from the front and less responsive to sounds from the sides and rear. Essentially, they act like a spotlight, directing the focus to the front and minimizing other distractions from your surroundings.
Imagine you are in the café when several groups of people enter and sit at tables nearby. In this case, the background noise that is interfering with your ability to communicate with your friend is the babble and chatter coming from the people around you. Noise reduction as discussed above is likely to be less effective in this case, as the babble of speech is much more dynamic then steady-state sounds; however, the directional microphones of the hearing aid have kicked in and are now focusing their beam of sensitivity towards your friend sitting across from you, and dampening sensitivity to the noisy table behind you.

Feedback Cancellation
At one time or another, you have likely heard a whistling hearing aid, perhaps from the person sitting next to you in church or when you’ve gone in to hug your great-aunt Myrna. That whistling sound, termed feedback, occurs when amplified sounds leak out around a hearing aid and are then picked up by the microphone and re-amplified. Having a properly fitted earmold or custom aid is the first step in ensuring feedback is minimized; however, advanced hearing aids also use various algorithms to stop the whistling quickly or before it occurs. As with most digital processing strategies, exactly how and when this is done varies between hearing aid manufacturers. Certain adaptive feedback managers will generate a sound wave opposite to that of the whistle, essentially cancelling it out, while others will change the output frequency slightly so that it is no longer troublesome. These advances in digital feedback management have allowed receiver-in-canal (RIC) style aids to be possible!




This is just some of the advanced digital processing that goes on inside of hearing aids. Other notable technologies include frequency compression, frequency lowering, wind noise and impulse noise reduction. Certainly there is a lot more going on in today’s tiny hearing aids than meets the eye!

Wednesday, January 8, 2014

Noise Induced Hearing Loss

Written by: Kelsey Spurrell, Clinical Audiologist


We all know that working in a loud environment can impact your hearing. However few people actually understand the impacts of noise induced hearing loss.

Hearing loss can be caused naturally with age but this doesn’t ordinarily happen until later in life. If you are experiencing hearing loss earlier and have been exposed to loud sounds, it is more than likely noise induced. NIHL (noise induced hearing loss) is caused by two factors working together: the INTENSITY of the noise and the LENGTH OF TIME a person is exposed to it. Repeated exposure in combination with these two factors gradually produces NIHL a form of SNHL (sensorineural hearing loss). SNHL is permanent and this is due to the death of hair cells within the inner ear.

NIHL normally presents itself as a high frequency hearing loss. This means that high pitched sounds, such as the /t/, /k/, /s/ sounds in speech, become more difficult to hear. This results in the perception of muffled speech. Other common symptoms of NIHL are tinnitus (ringing in the ears), the inability to understand speech, and difficulty following conversations when there is background noise. It’s important to know that NIHL can progress over time, and can eventually impact low pitched sounds, such as vowel sounds in speech.

But what is noise? Noise is any loud unwanted sounds. In fact, anything that is 85 dBA or more is considered dangerously loud. For instance, a lawn mower, motorcycle, jackhammer, and even a blow dryer can all produce sounds that can cause hearing loss.

The cause of NIHL may be most often work related, but the effects are felt far beyond the workplace. According to a 2000 Board of Health Report to the City of Toronto: there is evidence indicating that noise can effect general health, which include but are not limited to an increased risk for cardiovascular disease, chronic fatigue, and damages to mental health such as anger and anxiety. The report also states that “more people are affected by noise exposure than any other environmental stressor.”

While there are a variety of negative side effects associated with NIHL, it is somewhat preventable. It`s your duty and your right to protect yourself from any dangers in the workplace.

HOW TO PROTECT YOUR HEARING FROM NOISE
1) WEAR HEARING PROTECTION
2) TURN DOWN THE VOLUME
3) LIMIT EXPOSURE TIME TO LOUD NOISE
4) MONITOR YOUR HEARING PROFILE


Noise level exposure is regulated by both federal and provincial legislatures. Occupational exposure limits for noise can be described as the maximum duration of exposure based on different levels of noise. Two important factors for calculating this are the criterion level and the exchange rate. The criterion level is defined as the level of noise permitted for an 8 hour work shift. In N.B., regulations state the criterion level to be 85 dBA, that’s the level of a power drill. The exchange rate states that when the noise levels increase, exposure to the noise must decrease. In N.B., regulations define the exchange rate to be 3 dBA. So for every 3 dBA increase in noise level, the exposure time must be reduced by half. This means if the noise level at your workplace increased to 88 dBA then you should only be exposed for 4 hours!!

To protect your hearing, try to stay away from the source of the loud noise but most importantly use hearing protective devices. A variety of these exist including foam ear plugs, custom ear plugs and ear muffs.

The following infographic provides a great loudness scale for reference: