ict 1 outer hair cell functionality and new models for noise damage ntaf’s etterutdanningskurs...
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1ICT
Outer Hair Cell functionality and new models for noise damage
NTAF’s etterutdanningskursHell, October 2007
Olav KvaløySINTEF ICT
2ICT
’State of the Art’ of understanding of damage risk
At levels from 85dBA to <130dBA:
Damage = dBlevel * time (dose)
At higher levels >150dB instantaneous damage (legislation states max 130dBCpeak)
Our model works on the continuous noise integration
3ICT
’State of the Art’ understanding of OHC
OHC is damaged by continuous high noise
OHC hearing loss is often accompanied by hyperacusis
OHC represents +57dB gain at threshold (Moore)
OHC is not active above 90dB (Moore)
4ICT
’State of the Art’ questioned
Why is OHC damaged at high levels if they are inactive above 90dB?
We is know these are the dangerous levels.
5ICT
Beyond ’State of the Art’
OHC is damaged by high noise
OHC hearing loss is often accompanied by hyperacousis
OHC represents +57dB gain at threshold
New hypothesis: OHC is active above 90dB
6ICT
OHC, Gain and Protection
If OHC work against or dampen the movement at high levels this would explain:
Why OHC are damaged at high levels (they work harder!)
Hyperacousis when OHC damage (no OHC to restrict the level)
8ICT
‘Diffusion’ model for HOC damageAssumptions in the model: OHC work to protect cochlea during high noise and are
‘worn out’ by to much work
Some substance ‘X’ is needed for the cells to work Some substance ‘Y’ needs to be carried away from cells If the X or Y concentration goes beyond certain limits the
cells will be damaged
These substances must be transported to / fro the cells by diffusion
10ICT
The Cochlea with diffusion model
http://en.wikipedia.org/wiki/Image:Cochlea-crosssection.png
Iros
Vg
11ICT
Diffusion model of HOC damage
Literature shows that Reactive Oxygen Specimen (ROS) is produced in the cells and will destroy them
Antioxidants protect the ear against noise (Vitamin C will reduce ROS concentration and protect)
The model will give credit to resting periods between noise
Literature shows destructive levels start <70dB if no rest
12ICT
Diffusion model of HOC damage
x(t) is instantaneous sound pressure
h(t) is the ROS production function
Vg is antioxidant source
We assume TTS is a sign increased ROS concentration (by this we can calibrate the model)
15ICT
Protocol for OHC hypothesis test: Gather young healthy subjects (students?) Expose them to high noise (produce TTS)
concerts or brass-band practice as exposure source (students own choice!) Measure Threshold and UnComfortable Level (UCL)
before and after sound exposure
Threshold levels and/or OAE to document OHC activity ’Loudness Scaling’ and/or ’Reflex Measurement’ used to measure
UCL
Lower OHC activity and lower UCL after exposure proves hypothesis
Main Problem: Ethics in exposing subjects to potentially hazardous sound levels
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