Introductory Concepts Flashcards
Describe the acoustic era
involved speaking tubes, horns, trumpets.. at most these can result in an increase of around 10-15dB in the mid-frequencies Reasonable BGN reduction Popular in the 1800s
Describe the Electric-Carbon Era
1899 first started Diaphragm compressed carbon granules which changed voltage Suitable for mild-moderate hearing losses Poor quality sound, little control over frequency response
Describe the Vacuum Tube Era
1920, first wearable model 1934 Three components A) Ear receiver B) Microphone,Amplifier C) Batteries
Describe the Transistor Era
First in 1952 Hearing aids could now be much smaller than before and slightly less power hungry
Describe the digital era
The incoming voltage or current from the mic is an analogue signal (variations in voltage are analogous to variations in the sound source), we can sample that incoming signal at discrete time intervals converting it into a set of binary (number) representations. We can then perform all sorts of mathematical operations to try to get the signal to come out the way we want it. Hearing aids are almost all digital these days
Basic Terminology: Gain
Amplification at each frequency We can apply different gains in different channels or bands
Basic Terminology: Linear Amplification
Apply the same amount of gain regardless of the input level
Basic Terminology: Compression/Non-Linear Amplification
Different levels of gain for different input levels
Basic Terminology: Input/Output Diagram
Typical way of describing how much amplification
Basic Terminology: Average Gain
Gain averaged over several frequencies
Basic Terminoloy: Frequency Response
Shape of gain curve (gain plotted against frequency)
Usually shown graphically
Sometimes called the gain curve or gain frequency response
Important to specify measurement conditions and input level
To fully describe the gain of a linear amplifier it is necessary to state its gain at every frequency within the freq range of interest - that is the gain freq response
Basic Terminology: Saturation SPL
All amplifiers become non-linear when the input or output signals exceed a certain level.
The highest value of SPL that can be produced is called the Saturation Sound Pressure Level, or SSPL. As with gain, the SSPL varies with frequency, and a useful measure is the SSPL response curve.
Terms closely related to SSPL are Output Sound Pressure Level for a 90 dB SPL input level (OSPL90), and Maximum Power Output (MPO)
SSPL90 is the SSPL measured with a 90dBSPL input, we assume that an input of 90dB is enough to cause any aid to saturate
Problems faced by hearing impaired people and how hearing aids can help
Inaudibility of sounds
- Speech is loud enough but not clear enough
- Hearing aids can:
- amplify different frequencies by different amounts
- there are some limits to this, for example feedback
Reduced Dynamic Range and Recruitment
- “Speak up” followed by “no need to yell”
- Hearing Aids can:
- use compression amplifiers to amplify soft sounds and not loud sounds
Reduced Frequency Discrimination
- frequency smearing
- not a lot hearing aids can do
- there are some aids that can take information from dead regions and present them to hearing regions
Reduced Temporal Discrimination
- People with sensorineural hearing loss can lose their ability to extract snippets of speech information during the weaker moments in the background noise effectively.
- Loud sounds that occur immediately before or after soft sounds can mask them (essentially blocking them out). This makes it difficult to understand speech, especially in competing background noise.
- This is a difficult problem to overcome with hearing aids
- Fast acting compression hearing aids
- can increase the loudness of speech signals that occur between the noise
- however this has the disadvantage of also increasing the softer components of the noise, as the hearing aid cannot distinguish between the signals that we want to hear and the signal that we don’t.
- Fast acting compression hearing aids
Overlapping/Masking of Noise
- Difficulty in BGN
- People with normal hearing have an SNR ability of -10, this is when the average noise level above speech level is 10dB
- People with SNHL often need the SNR of +4 - +10 dB
All these problems means someone could have a perfectly fitted hearing aid and still have difficulty understanding speech
Difference between bilateral and unilater vs. binaural and monaural
Monaural = sound presented to one ear Unilateral = fitting is the fitting of only one aid Binaural = sound presented to both ears Bilateral = fitting is the fitting of two hearing aids
Advantages of Binaural Hearing
Localisation
- Horizontal
- Brain uses differences between intensity of a sound and the time it takes the sound to reach each ear to determine the location of the sound
- Timing and Level
- Vertical
- Pitch (spectral) intensity cues are used
- Pinna and ear canal alter the spectral intensity of the frequency components of the sound (by reflecting them differently)
- Adding a hearing aid changes the physical properties of the ear and can effect vertical localisation
- We need good localisation for speech in noise, multiple sound sources can become confusing without it, when hearing aids are worn for the first time this can impair localisation ability, possibly leading to the sensation that wearing hearing aids makes it harder to hear in noise
Head Shadow Effect
- Brain can listen to the ear with the best SNR
Binaural Squelch
- Bgn is suppressed. Requires central processing
- Requires that the signal reaching each ear is different
- Brain is able to attend to each ear separately and to attend to some signals whilst suppressing others
Binaural Redundancy
- Brain receives the same information from both ears independently
- Appears to results in a small advantage over monaural listening
- Effects are smaller than for binaural squelch
Loudness summation
- around 5dB, or 3-10dB so less gain needed
- Sound that is heard binaurally rather than monaurally is perceived as being twice as loud
- Improves speech perception in quiet and noise
- When fitting hearing aids, less gain is needed when two hearing aids are fitted than with just one
