Audiology 6- Hearing aids Flashcards
(30 cards)
what is gain in a hearing aid?
-gain is amplification
- the difference between the output and input of the hearing aid
- typical maximum gain for low powered hearing aid is 50dB
- for high powered aids the maximum gain is approx. 75dB
what is maximum output of a heairing aid?
- as loud as the amplifier will go
- also known as saturation sound pressure level (SSPL)
- typically the SSPL for low powered hearing aids (mild/moderate loss) is 120 dBSPL or less
- for high powered aid (severe/ prodound loss) the SSPL is 135dBSPL
- does not exceed the uncomfortable loudness level of the wearer
what is frequency response of a hearing aid?
- range of frequencies over which the amplifier amplifies
- most aids amplify over a range of about 200Hz to 4000Hz +
- Difficult to push anything above 4000 Hz through the tube of a behind the ear hearing aid
what is distortion of a hearing aid?
- how much does it distort the output signal as compared to the input signal
What is a behind the ear/ power aid (receiver- in- the- aid) ?
- has a microphone, battery, processor, receiver all housed within hearing aid case, shaped to fit behind the pinna
- output through tube via conventional earmold or open fitting
what are the two types of hearing aid styles
- in the ear
- behind the ear
What is a behind the ear with convention earmold hearing aid?
- have a fitted earmold which almost completely fills the outer ear and ear canal
- separates sound output from the microphone and so reduces feedback
- may be easier to handle as they’re bigger
What are the benefits of a behind the ear open fit hearing aid?
- open fitting has a number of benefits:
comfort
reduction of occlusion effect
uses normal ear resonance
cosmetic benefits (smaller and can’t been seen) - but cannot be used for severe or profound loss as they aren’t powerful enough
What is a receiver- in - the canal hearing aid and what are the benefits?
- similar to open fit aids but receiver is at the end of a wire and situated in the ear canal
benefits over a receiver in the aid fitting
- receiver in the canal means it is smaller
- can be used for mild to severe loss
- offers more gain in high frequencies
- smoother frequency response in the mid-high frequencies
- reduction of occlusion effect
what are in the canal, completely in the canal and invisible in the canal hearing aids?
- their components are all housed in a case which is custom made to the individuals’ ear
- suitable for mild to moderate/ severe losses
what is compression in a hearing aid?
- a signal processing technique
- typically talking about amplitude compression
- used for sensory hearing losses that exhibit recruitment (loss of outer hair cells)
- so you need high gain for low intensity sounds
- and low gain for high intensity sounds
- gain must vary depending on the input signal
- a compression aid turns down its gain as the input to the aid increases
- also helps to reduce distortion
what is signal processing
- number of different strategies to:
- make the hearing aid more comfortable for the users
- offers more benefit in terms of speech perception for the user
e.g. compression, multiple channels, multi-programme, noise suppression, directional microphone
what is frequency compression in a hearing aid?
Some hearing aids offer frequency compression
- Useful for people with severe/ profound or moderate losses with little or no usable hearing in the higher frequencies
- Take the high frequencies and squash them to a lower frequency where hearing is better
- Affects the pitch of the sound but because it is where the hearing is better, the person will understand better than if you try and deliver it where they have no useful frequencies
what is a multi-channel hearing aid?
- you can divide the amplification across different frequency bands so different amounts of gain and different compression strategies can be applied across these bands
- some hearing aids have 20 channels or more
- means a hearing aid can be tuned to match an individuals audiogram configuration
- beneficial for people with moderately or steeply sloping hearing loss
- can lead to improved speech perception and sound quality
what is a multi-programme aid?
- two or more different processing set-ups which can be saved to an individual aid and allows the patient to switch between them depending on the listening situation
- e.g. second program may be implemented for noisy situations or music
what is a noise reduction/ cancellation aid?
- the hearing aid listens to the environment and tries to amplify speech and not amplify noise/ background noise and process the two signals differently
- works better for speech in background noise that is not speech noise e.g. traffic
what is a directional/ multi- microphone hearing aid?
- most effective method of improving the signal- noise ratio
- pick up the sounds from in-front and not voices that are coming from behind
- can be fixed or adaptive
- uses dual/ multi-microphones the hearing aid can be set to amplify sounds from one direction
- works on time of arrival of sound. sounds that reach the front mic first are amplified, those that reach the rear mic are not
what is acoustic feedback?
- caused by amplified sound leaking from the ear canal back to the microphone
- occurs when earmould is poor acoustic fit
- occurs when an ear canal is occluded or partially occluded by wax
what are the aims of amplification?
- to restore audibility of sounds (amplify) and customise frequency response to compensate for hearing loss across frequencies.
limitations of hearing aids
Today’s hearing aids cannot:
- restore clarity
- completely restore normal hearing thresholds
- eradicate acoustic feedback
-eradicate background noise whilst preserving speech
what is signal to noise ratio?
a comparison that is useful for estimating how understandable speech is in a room
the sound level of the teacher’s voice in dB, minus the background noise level in the room in dB= the S/N in dB
a large S/N means better speech perception
what is the recommended signal to noise ratio in a classroom
+15 dB
for some children with hearing impairments it needs to be +20dB
where is the lowest signal to noise ratio in the classroom?
either:
- at the back of the classroom, where the teacher’s voice has fallen to its minimum value
- near the noise source, where the noise is at its maximum
what is a sound- field amplification system
- works by sending an FM or infrared signal from the teacher’s transmitter to an amplifier which is connected to one or more loudspeakers in the classroom so all students benefit
- rooms require reasonably good sound insulation to prevent amplified sound transmission to adjacent rooms as well as low reverberation
