Title: Should Hearing Aids Make Speech Normally Loud or Make Speech Clear
1Should Hearing Aids Make Speech Normally Loud or
Make Speech Clear?
- Gitte Keidser and Frances Grant
- National Acoustic Laboratories
2Procedures for fitting non-linear hearing aids
- LGOB, IHAFF, FIG6, DSLi/o, ScalAdapt Loudness
Normalisation - NAL-NL1 Maximise Speech Intelligibility make
overall loudness equal to, or less, than normal
3International Long-term Average Speech Spectrum
4Loudness perception
5IHAFF NAL-NL1
- Contour test loudness functions
- VIOLA targets
- Input/output curves Compression ratio and gains
for 65 dB input (speech)
- NAL-NL1 formula Threshold data as input -
Frequency shaping and compression ratios as
output
Wideband Compression threshold of 46 dB SPL Fast
time constants (2.5, 40) msec. MPO - half way
between the two prescriptions
Both rationales are implemented in two-channel
schemes
6Subjects (24)
7Prescribed gain, 65 dB input(NAL-NL1 - IHAFF)
Flat losses, N16
Sloping losses, N8
8Prescribed CR
LF
HF
9Laboratory Tests(KEPAR from Knowles)
- Paired Comparison Test (preference with respect
to clarity and ease of understanding speech) - Speech Recognition Test (BKB sentences)
10Paired Comparison
Speech in quiet (avg. level)
11Paired Comparison
NAL-NL1 IHAFF
12Paired Comparison
13Gain difference at 65 dB input after level
adjustment (NAL-NL1 - IHAFF)
IHAFF preferred
NAL-NL1 preferred
14Speech recognition test
15Summary, laboratory tests
- In general, more subjects preferred NAL-NL1 than
IHAFF - especially in noise listening to an
average input level - Of the subjects who rated IHAFF higher than
NAL-NL1, a majority had mild flat losses - On average, subjects performed significantly
better with NAL-NL1 than with IHAFF when
listening in traffic noise
16 Field test (JUMP-1 devices from
Oticon)
- Test period four to six weeks
- Three to seven individually selected everyday
listening situations - Satisfaction (rating) and performance
(descriptive) of each program
17Field test
18Field test
Difference
Distinct
Slight
Hardly any diff.
No difference
NAL-NL1 IHAFF
19Field test observations
- IHAFF sounds soft, muffled, dull, and echoey -
comfortable and less noisy - NAL-NL1 makes speech clearer, and sounds more
natural - loud in high level background noise - Low level noise intrusive on both programs
20Conclusion
- When there is a sufficient difference between
targets, hearing impaired people prefer the
rationale of Maximising Speech Intelligibility
(NAL-NL1) to the rationale of Loudness
Normalisation (IHAFF)
21The end