what are some problems that people face when they have sensorineural hearing loss?
- decreased audibility
- decreased dynamic range
- decreased frequency resolution
- decreased temporal resolution
auditory processing disorders
disorders of the brainstem, mid-brain, or auditory cortex that can exist independently of any peripheral HL or can be the direct consequence of an impaired cochlea sending deficient signals to the brainstem
spatial processing disorder
a deficit that occurs even when sounds are made comfortably audible by individually prescribed amplification
- does fall under auditory processing disorder (is a type)
- hearing impaired have and EXTREMELY hard time separating the sound from noise/masker
frequency
- cycles per second
- Hz
period
1/frequency
-seconds or milliseconds
phase
timing of a sound, or 1 component of a sound, relative to some other aspect of the sound/or relative to another sound.
-360 degrees
wavelength
distance a sound wave travels during 1 period
diffraction
the way the sound is altered by an object
pressure
force per unit area
-pascals
sound pressure level
number of dB by which any sound pressure exceeds the arbitrary, but universally agreed reference sound pressure of 2x10 (^-5)
RMS
root mean square:
represents the strength of a fluctuating signal over a certain time.
waveform
how the pressure of a sound wave varies from moment to moment in time.
spectrum
added mixture of puretones that produce a complex sound over a portion of time
fourier transform
enables the spectrum to be calculated if the waveform is known
impedance
how easily a medium vibrates when a sound pressure is applied to it.
linear amplifier
everything gets bigger by multiplying the input signal by a fixed amount
gain-frequency response
the gain of a linear amplifier is necessary to state its gain at every freq w/in the freq range of interest
essential components for hearing aids
- 1 or more mics (sound –> electrical signal)
- an amplifier to increase the strength of the electrical signal; in the process it provides balance of the sound, usually giving more emphasis to HF sounds and weak sounds than it does to LF sounds and intense sounds
- a mini loud speaker (receiver), to turn electricity back into sound
- a means of coupling the amplified sound into the ear canal
- battery to provide the power needed by the amplifier
RITE
receiver in the ear canal
RICA
receiver in the aid
microphone
aka transducer
-takes an electrical waveform and turns it into an acoustic waveform
diaphragm
-inlet port that is thin and flexible
-sound pressure causes movement to move the diaphragm towards/away from the electret. this change in distance changes the electrical force btwn opposing charges means that the voltage btwn the backplate and diaphragm changes.
sound wave to electric wave
back-plate
small air space behind the diaphragm that separate it from the rigid metal plate
-on the back plate is the “electret”
what are other names for microphone amplifier
aka FET: field effect transistor, buffer amplifier, follower
what are other names for completely electronic microphones
silicon microphones, solid-state, integrated or micro-electro-mechanical system (MEMS)
why does resonance occur?
it occurs btwn air in the inlet port (acoustic mass) and volume of air next to front of diaphragm (acoustic compliance)
Helmholtz resonance
mechanical compliance of diaphragm and air behind diaphragm.
this resonance can cause a 5dB gain at about 4-10kHz in the frequency response
what are microphone imperfections?
- major imperfection: mics breakdown if exposed to adverse chemicals (sweat)
- random electrical noise (sometimes audible in HA in quiet) [greatest in mic that use internal acoustic path to roll-off LF]
- sensitive to sound AND VIBRATIONS: when mic is shaken (like running) any vibe gets amplified
- feedback loop when mechanical transmission of vibes from receiver to the mic are strong enough they make an oscillation
- mic will overload & distort sound with increased pressure
- bad design/construction of the HA
- subject to wind noise (turbulence increases the pressure, and mic converts that to sound
Internal feedback
can be detected w/coupler or by an oscillation.
-indicated by bumps in the frequency response at increased volume, but disappear at lower volume.
how does a directional microphone work?
- sound comes from rear and hits the front part later
- sound in rear is delayed as it passes thru internal low pass filter
- if internal and external delays are equal, sound from rear will hit both sides of diaphragm at the same time so there will be no net force on the diaphragm = insensitive to sound from rear
- if internal delay is less than external delay, mic will be insensitive 2 sounds from other directions
maximum external time delay
time btwn sounds arriving from front to rear to the other is equal to the distance btwn the ports receiving sound after they diffract around the head to the other ports.
internal time delay
rear port has acoustic damper (resistor). the resistor and cavity at the back make a low pass filter (passes most freq w/o attenuation, but w/some delay is apparent to all filter.)
why are directional microphones important/
they suppress noise coming form some direction while keeping sensitivity to sounds from 1 direction.
-good for HAs cuz it processes signals to improve SNR and increase intelligibility
polar sensitivity pattern:
directional sensitivity of mic on a circle graph.
-by changing the ratio of internal/external delay we can change the pattern.
omni directional microphone
opposite of directional microphone
- a single port with a circle polar pattern
- non-directional
Directivity index (DI)
ratio of sensitivity for frontal sounds relative to sensitivity averaged across all other directions.
2D-DI = horizontal plane
3D-DI = all directions
front-to-back ratio
ratio of frontal/rearward sensitivity.
-says nothing about effectiveness of HA in suppressing noise arriving from directions other than precisely behind the aid wearer
dual microphone technique
2 omni mics both w/1 inlet port.
the output of 2nd mic is electronically delayed and subtracted from the 1st
total harmonic distortion
power of all distortion products are added and expressed relative to power of wanted output signal component
intermodulation distortion
distortion products arise from modulation (mutual alteration) of every component in the input signal
compression amplifier
aka Automatic gain control (AGC) or automatic volume control.
