[LAU] Rolling off high frequencies when mastering?

[Niels Mayer]

On Sat, Apr 24, 2010 at 7:50 PM, nescivi <nescivi at gmail.com> wrote:
> yes, and in fact for higher frequency signals it is generally understood
in
> psycho-acoustics that we distinguish location more by level differences
(due
> to masking of the head) than by phase differences, as the latter have
become
> quite irrelevant, as the wavelength of these higher frequency is generally
> much smaller than our ears are apart from each other. OTOH, the high
frequency
> waves have a harder time bending around our heads, and thus create level
> differences based on whether the sound source is to the left or right of
us.

Thank you for providing some useful perspective and attitude. (Is this your
work? http://www.aes.org/events/122/papers/session.cfm?code=P5  "Reproduction
of Arbitrarily Shaped Sound Sources with Wave Field Synthesis—Physical and
Perceptual Effects" -- Very interesting!!)

What is the frequency at which these diffraction level effects produce level
differences in the ears?? Are the results taken from
http://www.aes.org/e-lib/browse.cfm?elib=14003 ? (The Effect of Head
 Diffraction on Stereo Localization in the Mid-Frequency Range—Eric
Benjamin, Phil Brown, Dolby Laboratories - San Francisco, CA, USA) -- IMHO,
that paper explicitly focused on the mid-frequency range -- doesn't mean
that is the extent of our hearing, nor is it clear they deny high-frequency
effects, as they'd be in contradiction to a huge body of academic literature
that states the exact contrary. Further, by limiting to "stereo
localization"  and mid-range they're implicitly ignoring pinna effects and
throwing their experimental results in the direction they planned to prove
in the first place: which is pretty obvious basic mixing -- you can use a
pan-pot to place a midrange source in a stereo field.  Also, i very much
doubt they did they experiments to the same standards demanded by academic
psychology research -- are their findings Post hoc ergo propter
hoc  scientific justification for existing product features or limitations?
Has anybody in psychology accepted their findings as valid? Or reproduced
the results independent of Dolby's financial interests??

The part that doesn't make sense is that we can also localize sound
up/down/front/back and certainly for up/down, level differences between the
ears couldn't explain our ability to localize -- pinnae and high-frequency
response does. Which is what the animal and perception psychology literature
tells us. Fortunately, there *ARE* animal studies very similar to the ones i
suggested we do, before declaring -- apriori -- what sound matters and what
sound doesn't.

One thing you might find interesting is that there is at least
counterargument to your statement for mammals  -- with a 33khz hearing
limit, requiring 40Khz bandwidth, and performing worse on a sound
localization task as the bandwidth reduces under 20Khz: "like humans,
chinchillas use the upper octave of their hearing range for sound
localization using pinna cues." And yet we allocate very little resolution,
at 44.1Ksamples/second, to hearing from which we derive positional cueues:
"In humans, frequencies as high as 15 kHz have been shown to be necessary
either for optimal discrimination in the lateral field or in elevation
(Hebrank and Wright, 1974; Belendiuk and Butler, 1975). As the 60-dB upper
limit of human hearing is at most 20 kHz, this means that the
highest octave of the human hearing range is used to derive pinna locus
cues."

http://linkinghub.elsevier.com/retrieve/pii/S0960982209011701
Sound localization in chinchillas. II. Front/back and vertical localization
Rickye S. Heffner *, Henry E. Heffner, Gimseong Koay

...

Effect of low-pass filtering

The role of high frequencies in discriminations of eleva-
tion was systematically explored by determining the ability
of chinchillas to localize the 4 different low-pass signals
presented from two speakers located 45 ° apart, again an
angle chosen for its moderate difficulty. As shown in Fig.
6, the performance of the 2 animals declined steadily,
reaching the 50% discrimination level at the 10-kHz low-
pass setting and falling to chance at the 5-kHz setting.
   To determine the effect of increasing the angle of
separation on performance, chinchilla P was also tested at
an elevation of 75 ° . As shown in Fig. 6, increasing the
angle of separation resulted in only a small increase in
overall performance at the 40-kHz setting, but a much
larger increase at the lower filter settings. As at the 45 °
separation, performance declined steadily as the high-
frequency content of the signal was reduced, although the
degree of the decline was not as great. Thus the impor-
tance of high frequencies for vertical localization can be
demonstrated at either angle. However, the size of the
effect and the low-pass filter setting at which performance
falls to chance is dependent on the angle of separation.
...

 For comparison with the results of the two previous
 left/right localization. Testing was conducted at 30 ° sepa-
 ration, the smallest angle supporting sustained performance
 of approximately 80% and was thus of comparable diffi-
 culty to the angles used in the tests of front/back and
 elevation localization.

Fig. 7 shows that there was little difference in the ability
of the chinchillas to localize the broadband (i.e., the
40-kHz low-pass signal) and the filtered stimuli. Although
performance declined slightly with the 5-kHz low-pass
noise, there was no systematic reduction in performance as
seen in the two previous tests (cf., Fig. 7 with Figs. 4 and
6). Thus, it appears that chinchillas do not require the
presence of high frequencies to perform a left~right
sound-localization discrimination, a result which suggests
that the binaural time cue is adequate to maintain their
level of acuity in this task. This point is discussed in more
detail below.
..........

4.3. Role of high frequencies

   In order to be effective, pinna cues require the presence
of high frequencies in the signal to be localized. In hu-
mans, frequencies as high as 15 kHz have been shown to
be necessary either for optimal discrimination in the lateral
field or in elevation (Hebrank and Wright, 1974; Be-
lendiuk and Butler, 1975). As the 60-dB upper limit of
human hearing is at most 20 kHz, this means that the
highest octave of the human hearing range is used to
derive pinna locus cues.

    Like humans, chinchillas require high frequencies to
localize sound using pinna cues. The results of both the
f r o n t / b a c k and vertical localization tests demonstrate that
performance declines as high frequencies are removed
from the signal and eventually falls to chance (Figs. 3, 4
and 6). The range of frequencies necessary for pinna cues
to be effective depends, o f course, on the angle of separa-
tion (cf., Fig. 6). In order to perform consistently at 50%
or better at the angles of separation used here, vertical
localization requires frequencies o f 10 kHz or higher
whereas f r o n t / b a c k localization requires frequencies of 5
kHz or higher (cf., Figs. 4 and 6). It should be noted that
performance in both tasks improved as higher frequencies
were added to the signal, even for the addition of frequen-
cies above 20 kHz. As the 60-dB upper hearing limit of the
chinchilla is 33 kHz (Heffner and Heffner, 1991), this
suggests that, like humans, chinchillas use the upper oc-
tave of their hearing range for sound localization using
pinna cues.

4.4. Implications for the evolution of high-frequency hearing

A m o n g vertebrates, mammals are unique in possessing
good high-frequency hearing, i.e., the ability to hear above
 10 kHz. Some time ago it was suggested that the selective
advantage of mammalian high-frequency hearing is to
enable even small mammals to make use of binaural
spectral-difference cues to localize sound (Masterton et al.,
1969). That is, interaural spectral-difference cues are avail-
able to an animal as long as it can hear frequencies with a
wavelength short enough to be shadowed by its small head
and pinnae. This view is supported by research that has
shown that acuity for left/right sound localization in rats
and mice is decreased when high frequencies are filtered
out (Heffner, 1989; Mooney, 1992; Heffner and Donnal,
1993).

    Over time it has become apparent that not all mammals
need high frequencies for optimum acuity for left/right
localization (Heffner and Heffner, 1992). For example,
frontal locus acuity in humans is not adversely affected by
the removal of high frequencies from an acoustic signal
because the time cues available in the low-frequency por-
tion of the signal provide sufficient locus information
(Musicant and Butler, 1984). Similarly, left/right localiza-
tion acuity in horses is not significantly affected by re-
moval of high frequencies and, indeed, there is evidence
that horses have lost the ability to use the binaural spectral
difference cue (Heffner and Heffner, 1986,1988c).
    As with humans, the ability of chinchillas to perform a
left/right localization discrimination is not affected by the
filtering out of high frequencies (Heffner et al., 1994). It
should be noted that this finding does not mean that
chinchillas do not use the binaural spectral-difference cue.
On the contrary, their ability to localize high-frequency
tones demonstrates that they readily use binaural
intensity-difference cues and, presumably, the spectral-dif-
ference cue (Heffner et al., 1994). However, it does
demonstrate that their frontal locus acuity can be sustained
by the binaural time cue alone. Thus, for chinchillas, the
primary selective advantage of high-frequency hearing may
be to enable them to use binaural spectral cues a n d / o r
pinna cues to localize sound sources located away from the
midline and to make vertical locus judgements. Accord-
ingly, we might conclude that the appearance of pinnae
accompanied by an extension of the high-frequency hear-
ing range combined to improve the sound localization
ability o f mammals within the lateral and vertical auditory
fields. Thus it is not surprising that the only bird known to
have good acuity throughout auditory space is the barn
owl, an animal which has evolved a pinna-like structure
and has pushed its upper limit of hearing to just over 12
kHz, higher than that of most birds (Knudsen, 1980;
Konishi, 1993).

-------------------

-- Niels
http://nielsmayer.com
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