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Odd (?) PTC data
With pure-tone maskers and signals, beats provide
a powerful detection cue when the signal
frequency is close to the masker
frequency. Beat detection results in elevated
masker levels. The added narrowband noise
probably interfered with beat detection via the
phenomenon of modulation detection interference
(MDI). Hence the masker level required to mask
the signal became much lower. See:
Moore, B. C. J., Alcántara, J. I., and Dau, T.
(1998). "Masking patterns for sinusoidal and
narrowband noise maskers," J. Acoust. Soc. Am. 104, 1023-1038.
Kluk, K., and Moore, B. C. J. (2004). "Factors
affecting psychophysical tuning curves for
normally hearing subjects," Hear. Res. 194, 118-134.
A few of us are reviewing a Zwicker (1974) paper
describing the relationship between
psychoacoustic and neural tuning curves. PTC?s
in the paper were obtained using pure tone
target and masking stimuli. Of interest is a
comparison between 2 kHz PTCs obtained in quiet
with the 2kHz target presented at an absolute
level of 31 dB SPL (Figure 1). The masker level
around 2 kHz is about 45 dB SPL.
A PTC at 2 kHz was also derived with an
additional 500 Hz narrowband noise (100 Hz BW;
overall level of 71 dB SPL) present (Figure 5c).
The level of the CF tone (2 kHz) was set to 5 dB
SL in this condition which happened to also be
31 dB SPL (as in Figure 1). Here is the
confusing part (for me). The level of the pure
tone masker around CF in this condition was ~26
dB SPL or almost 20 dB less than the masker
level needed in quiet when the CF tone was at 31 dB SPL.
In all cases the PTC?s were derived using a
simultaneous masking paradigm so any suppression
effects should be occurring on both the signal
and masker (as I understand it). Given the 500
Hz noise (71 dB SPL) produced enough excitation
to shift threshold to 31 dB SPL (at 5 dB SL)
[Threshold for the CF tone in quiet was 6 dB
SPL.], why would such a lower (26 versus 45 dB
SPL) masker level be needed to mask the CF tone?
I imagine I?m missing the obvious but thought
I?d put this out to any interested. I?ve
attached the relevant pages from the Zwicker article if you are interested.
Zwicker, E. (1974). On a psychoacoustical
equivalent of tuning curves. In Facts and Models
in Hearing. Edited by E. Zwicker and E.
Terhardt. (Springer-Verlag, Berlin). pp. 132-141.
Brian C. J. Moore, Ph.D, FMedSci, FRS,
Professor of Auditory Perception,
Department of Experimental Psychology,
University of Cambridge,
Cambridge CB2 3EB,
Tel. +44 (0) 1223 333574
Fax. +44 (0) 1223 333564