Amplification Test 1

Point where cmopression begins.
Where the input/output function becomes nonlinear.
AGCo usually around 95-115 dBSPL (closer to MPO)
AGCi usually around 35-55 dBSPL (lower); if it gets higher than 55 db, the fitting is no longer WDRC

Once engaged, it's the agressiveness of compression.
degree of "squash" effect
AGCO usually about 10:1 (more aggressive)
AGCI 1.5:1 to 3:1, common is 2:1 (normal)

Relates to duration of syllables in general conversation

Time required for output to reach compression after input has passed CK

· time required for circuit to respond to a decrease in input and readjust to lower compression than previously
- Fast: 150 msec or less
- Slow: exceeding 1 sec
- Common: 150 msec- 1000 msec

Change in the output envelope following a change in the input envelope

· squashes what is below the CK
- follows the spectrum of soft speech
- often set around same place as CK for a given freq
- goal is to reduce gain for low level noise without reducing gain for important speech signals

1) Prevent sounds from being uncomfortably loud
2) Rearranging soft, avg, loud sounds into right place of dynamic range

AGCo and AGCi

Automatic Gain Control Output (AGCo) aka compression limiting
- Output maintained at appropriate level
- MPO not affected by VC setting
- Gain is shaped by AGCo
- LDLs have variability between people (Bentler and Cooley, 2001)
- Specific LDLs to set MPO
- Different channels can have different kneepoints
- Most common method to control MPO

- Loud sounds squashed
- Speech distortion
- Wont have necessary dynamics; peaks clipped; music dull
- Loudness percept limited (avg and loud sounds close in dB)
- SNR will be worse because speech will bump the ceiling

- Gain decreased, can’t hear soft/avg
- Only will use aids when s/he knows there are no loud sounds (missing amp for most listening situations)
- Will have negative experiences leading to not wearing them

- Not enter individuals LDLs and take the company guess based on PT thresholds or predicted values from prescriptive fittings
- Enter LDLs for a couple frequencies; company will select respective MPO based on fitting formula (MPO can vary 10-15 dB btn companies)
- Conduct LDLs and set AGCo CK yourself (use Cox Contour Test/Chart for loudness levels)

1) Use Cox chart
2) Convert LDLs into RETSPLs (2cc coupler formula)
-RETSPLs should be provided by audiometer
3) 2 cc coupler value/formula = LDL + RETSPL

Ø Automatic Gain Control Input (AGCi) aka Wide dynamic range compression (WDRC)
- Compresses the input signal before it reached the amplifier.
- Goal is compressed input + gain fits the pts residual DR
- Often not effective to control MPO because it is influenced by VC
- Nearly always compressed a wide range of incoming speech signal
- You can have AGCi without WDRC, but you can’t have WDRC without AGCi
- Used to control the MPO if kneepoints are low enough and the ratios are big enough
- Makes loudness perceptions of soft, avg, loud inputs closer to that of normal hearing individuals
- Allows soft sounds to be more audible (avg inputs compressed and pt turns up gain to make them comfortable, in turn it increases gain for soft)
- If programmed right, the need for VC should be minimized
- Leave CK alone and change CR
- For ballparking CR= range of sounds (~ 100 dB) / LDL – threshold of pt
- For WDRC, softer the signal the greater the gain-therefore, when input goes up gain goes down