INTRODUCTION TO NON-LINEAR HEARING AIDS
A Coggle Diagram about TIME CONSTANTS (DYNAMIC CHARACTERISTICS)
, WHAT IS BEST?.• Compression or Linear Amplification?
• Which Attack & Release times to choose?
• the # of channels to use?Yes to compression, but: For low freqs, or high freqs? & for low or high intensities & also: what response times?
, NUMBER OF CHANNELS TO USE (EFFECT ON SPEECH INTELLIGIBILITY)
, COMPRESSION CIRCUITS
(COMPRESSION CIRCUITSThese reduce the gain when either the lP to the device → or the OP from the device → exceed a certain predetermined kind / value.AGCi (input controlled compression)AGCo (output controlled compression).
- the circuits shown here are analogue. The same effect can be achieved with DSP by altering the algorithms.
), RECAP:• GAIN = The Difference between the output & input of a hearing aid. ie. How much the HA is amplifying input sounds.• This is plotted either as an IP /oP curve for each frequency or as Gain as a function of frequency (the Frequency Response) .•The HA wearer can adjust the amount of Gain by adjusting the volume control.• MAXIMUM OUTPUT = The level at which the Hearing Aid limits its output to a certain input signal. The HA is said te be in SATURATION & any further increase in input will not result in any increases in output .• SATURATION = the loudness & distortion for the listener.
It is important to have enough HEAD ROOM to prevent the HA from saturating for high input levels .
If insufficient headroom the user may turn down the volume to avoid distortion . But: this means → they will be under-utilising their HA , beneath its optimum setting for optimum speed discrimination.• OUTPUT LIMITING
Aim: output limiting aims to limit / restrict the amount / level of amplification to prevent head input sounds from exceeding the HA user's uncomfortable listening levels.• PEAK CLIPPING:
this clips the peaks from the response in order to reduce the Maxinum 0utput.
, PROGRAMMABLE SETTINGS:This involves different programmes (with different frequency responses and or different features ) to be set up for different listening situations.Modern Devices are multi-channel and multi-programme.
, RECAPLINEAR AMPLIFICATION =
As the input increases the output increases by the same amount . ie. There's No Compression.NON-LINEAR AMPLIFICATION ie. Compression =
the slope of the IP/ OP function is <45 degrees .LINEAR COMPRESSION =
Once activated there is a linear relationship between the IP & the OP .CURVILINEAR COMPRESSION =
This is non-linear compression where the compression Ratio (CR) changes constantly as a function of the input.
, (CT) or (CK) Compression knee point or Threshold=ANSI defines the CT as the input level that corresponds to the point of the IP/ OP function that departs from the linear amplification slope by 2dB.COMPRESSION RATIO =
The change in Input divided by the change in output when the HA is in compression.eg-a CR of 2:1 indicates that the op increases by 5 dB fer every 10 dB increase in input.*A higher # on the left hand side indicates more compression .
and EXPANSION=Expansion is the opposite of compression.
Expansion avoids the unwanted amplification of how level sounds. (in the expansion region).The Expansion Threshold AT or TK exp=
The IP level below which expansion occurs.The Expansion Ratio (XR) is the extent to which the signal is expanded. (XR<1 )