7/31/2019 Auditory Time Illusions
1/8
Richard Nyonyi
Dr. Eric Mandell
PHYS 3500
26 April 2012
AUDITORY TIME
Scientists over the years have been amazed with the human ability to perceive reality. It
has been proven that most of the time what human perceives is not fully what it is out there. Take
an example of the visual perception and how it is limited in different ways, we all see the sky to
be blue, but the sky is never blue, just because of our limited visual ability that is what we
perceive (Gibbs). This simply means we do not see everything, we only see what we are able to
see.
Similar to how our visual perception is limited in different ways, so is our auditory
perception. Sound is said to be coming from pulsating (vibrating) motion of air molecules in the
air (Hass), but why is that when someone waves a hand we dont hear any sound? Waving a
hand creates vibrations in the air so we should expect to hear something. We shall find out the
answer later. We are going to see how among others limitations, time has been playing a major
role in our auditory system. First, we shall see what time has to do with our hearing system that
causes all the illusions, and later we going to see few applications in sound processing, and see
how the time factor has helped scientists and artists to fool the human perception.
Our hearing system is such a complicated system that requires different knowledge from
physical sciences to health sciences. As we have already defined sound, the vibrations of air
7/31/2019 Auditory Time Illusions
2/8
molecules reach the outer ear which is well shaped to collect useful information like detecting
direction of the incoming sound and amplifying it (Nave).
The collected and amplified air vibrations reach the ear drum and other structures inside
the ear and are converted to human neural impulses and the physics of sound marks its end. The
brain receives the impulses and translates them in a very complex mechanism that scientists are
still working on.
The good news is that we already have enough information that will help us appreciate
the role of time in this particular study. The range of human hearing is generally considered to be
20 Hz to 20,000Hz (W.Smith). In plain English, we can hear vibrations in the air that are coming
to us at the rate of 20 to 20,000 complete vibrations per second. This already answers our
question of waving a hand since there is by no means a human being will be able to wave a hand
at a rate of at least 20 complete cycles in one second, which is the just the lowest frequency we
can hear. To show that we are still talking about time, frequency is simply a count of number of
events in given unit time, in this case number of air vibrations in one second. As if this frequency
7/31/2019 Auditory Time Illusions
3/8
7/31/2019 Auditory Time Illusions
4/8
original wave. Hope you can see straight away that this new wave wont be a good exact
representation of the original wave , the new wave seems to have the same ups and downs but we
cant ignore it`s boxed shape, and we can imagine how that will sound. Fortunately our time
factor comes to help, it can be proven mathematically that if we decrease the time interval
between snapshots, hence increasing the number of snapshots, technically equivalent of saying
increasing sampling rate we can represent the exact original waveform digitally. The question is
how much we need to increase the sampling rate to be able to do that? Now since the intended
audiences are humans, scientists agreed to take the snapshot at the rate that human hearing
system will not be able to tell the difference. Simply this means that, since our hearing range
goes to maximum of 20,000Hz they decided to take the snapshots/samples at least at the rate
twice of our maximum hearing range (Repetto). Using what is called Nyquist theorem, the
normal sampling rate of sound is never less than 40,000Hz.Usually compact discs are sampled at
a rate of 44,100Hz (Schulzrinne). So since we cannot detect differences in the sampling rate this
high, time has beaten our auditory system and if we send our sampled sound to a speaker we hear
the same sound we heard before it was processed electronically.
Let us also check on more fun side of this, let us pay attention to what musicians call
delay based effects. Of course whenever you hear a word delay, you know for sure we are
talking about time. So in simple English we are talking about time based effects. To achieve
these effects, simply they take a copy of a sound and play simultaneous with the original sound
but delaying the copy of the sound by a certain amount of time (Cousins). So the final output
contains the two equal sounds but one being delayed.
Our hearing system does detect repeated sounds that occur after 30 milliseconds
(Pattern). This is when we can unquestionably hear the two distinct sounds. Most delays that are
7/31/2019 Auditory Time Illusions
5/8
above this time range is what we refer to as echo. For the repetitions of sound, below 30 ms of
course with other factors such as lowering the amplitude (volume), these repetitions of sounds
are not detected as different/distinct sounds and our brain get fooled .This addition of depth and
fullness in the sound gives our brain a sense of space. So using this technique, we can fool our
brains to assume different sizes of the surroundings. We can make the sound feel like it is being
played in the cathedral while it was just recorded outdoors. Again notice the role of time and our
ability to hear the difference.
Also, when it comes to flanging and chorusing effects no one will argue the sweet sound
the two produce. In simple English flanging is that robotic sound, while chorusing effect is the
emulation of a number of performers performing together in unison.
Basically for the chorusing effect as seen in the diagram below, not only is the copied
sound delayed between 20-30 ms but also using what is called low frequency oscillator (LFO),
the delay time is gradually shifted up and down within few milliseconds and this simulate small
variations of pitch in that small amount of time (Cousins). Simple math is that if the
pitch/frequency changes once in every 20 ms then, in one second we hear 3,000 variations of
7/31/2019 Auditory Time Illusions
6/8
pitches, this simulates the idea of chorus very well and our brains are being fooled to assume that
there is a big group of performers performing the sound together.
.
For the case of flanging, the major differences with chorusing are that the delay is set
from 0 to 10 ms instead of 20 to 30, also the LFO will do its work the same way like in
chorusing but the delayed sound is simultaneously allowed to go to the output but a replication is
sent back to the delay machine (Cousins). In other words the copy of the delayed sound is sent
back to input, allowed to mix with original sound, the combination goes into the delay machine
and the loop goes on and on as the big curved arrow shows. The end result is what our brain
perceives as the metallic/robotic sound.
So it is obviously observed that our auditory system has limitations relating to time and
as a result of that what we perceive in our hearing system is not exactly what has reached in our
ears. Our brains do some very high sophisticated translations of the resulting sound. Other
information in sound that our hearing system is not capable of capturing is lost or ends up
forming a different effect as we have seen. At some point it is beneficial to have limitations. It is
a confusing just to imagine what life would be if we could hear infinitely everything. It is known
that bats make very high frequency sound above level of human hearing, and they use it to locate
their prey (Richardson), if we could hear everything, being close to bats will be truly noisy. The
same is for our visual perception, if we could see everything in the air from bacteria, pollen, and
7/31/2019 Auditory Time Illusions
7/8
all microscopic particles in the air the term clear sky might not exist in a way. So obviously these
limitations are advantageous to our way of living and if we play smart enough with them, there is
still a lot more we can do to fool the human brain.
7/31/2019 Auditory Time Illusions
8/8
Bibliography
Cousins, Mark. "Chorus ,Flange and phase." MusicTechmagazine (2007): 56.
Gibbs, Philips. "Why is the sky blue." 05 1997.
www.math.ucr.edu/home/baez/physics/general/bluesky/bluesky.htm. 25 04 2012.
Hass, Jeffrey. "What is sound." 2003. www.indiana.edu/~emusic/acoustics/sound.htm.25 04 2013.
Nave, Dr.Rod. HyperPhysics. 2010. web. 26 04 2012.
Pattern, John. "Diffeence Between Echo and Reverberation." 2012.www.ehow.com. 22 04 2012.
Repetto, Douglas. "Sampling theory." 2011.
music.columbia.edu/cmc/MusocandComputers/frontmatter.php. web. 22 04 2012.
Richardson, Phil. "The Secret life of Bats." 2002. www.fathom.com/course/21701775/index.html. 26 04
2012.
Schulzrinne, Henning. "Explanation of 44.1 kHz CD sampling rate." 09 01 2008.
www.cs.columbia.edu/~hgs/audio/44.1.html.25 04 2012.
W.Smith, Steven. "Human Hearing." W.Smith, Steven. The Scientist and Engineer`s Guide to Digital
Signal Processing. California Technical Publishing, 2011.
Top Related