Introduction Psycho-Acoustics Definition Psycho-acoustics is ...
• Psycho-acoustics is the study of the perception of
– How humans hear
– How humans can separate different sounds.
• Psychoacoustics is the study of subjective human perception of
• Alternatively it can be described as the study of the
psychological correlates of the physical parameters of
Computer Vs Human Brain
• Human brain does not work exactly like a
• A computer records a series of samples that
form the sound.
• Human ear sends information to the brain
based on the spectrum of the sound.
• Studying the key differences aids us with
digital audio compression.
Limits of Perception
• Sounds in the range of 20Hz to 20,000Hz are
typically audible to the human ear.
• As age increases the range decreases, most adults
unable to hear about 16,000Hz.
• Study has shown that although sounds below
20Hz cannot be heard, these sounds do have
affects on the listeners.
• This is called the ‘Hypersonic effect’, and studies
are on going to show brain activity when these
sounds are present.
• Does the hypersonic effect occur via the "ordinary" route
of sound travelling through the air passage in the ear, or
in some other way?
• A peer-reviewed study in 2006 seemed to confirm the
second of these options, by testing the different effect of
high-frequency components when presented via
loudspeakers or via headphones
• The hypersonic effect did not occur when the high-
frequency components were presented via earphones.
– T. Oohashi, et al, Inaudible high-frequency sounds affect brain activity: Hypersonic effect.
Journal of Neurophysiology, 83(6):3548–3558, 2000.
– T. Oohashi, et al, The role of biological system other than auditory air-conduction in the
emergence of the hypersonic effect. Brain Research, 1073:339–347, February 2006.
• The study of human sound perception is highly
important in the development of audio codecs
• Lossy audio data compression is based in the
principle that some audio information is not
perceivable and is therefore discardable
• Psychoacoustics is a complex problem that
involves physiological and psychological factors.
• A codec is a device or program capable of encoding
and/or decoding a digital data stream or signal (Audio
– The word codec may be a combination of any of the
following: 'compressor-decompressor', 'coder-decoder', or
• In a distributed environment or hand held device
based environment image and sound files remain a
major bottleneck within systems
• Much of the slowness of a website's downloading is
due to the presence of non-optimized sound and
• Compression solutions today are more portable
due to the change from proprietary high end
solutions to accepted and implemented
• Two categories of data compression algorithm
can be distinguished:
– lossless and
• Lossy techniques cause imagesound quality
degradation in each compression/decompression
• Careful consideration of the human perception
ensures that the degradation is often
unrecognisable, though this depends on the
selected compression ratio.
• In general, lossy techniques provide far greater
compression ratios than lossless techniques
Lossless coding techniques
• Lossless coding guaranties that the decompressed
image is absolutely identical to the soundimage
• This is an important requirement for some
– e.g. medialscientific applications, where not only high quality
is in demand, but unaltered archiving is a legal requirement
• Lossless techniques can also used for the
compression of other data types where loss of
information is not acceptable
– e.g. text documents and program executables.
Codecs and Compression quality
• Lossy codecs
– Many of the more popular codecs in the software world are
• Lossless codecs
– There are also many lossless codecs which are typically used
for archiving data in a compressed form while retaining all of
the information present in the original stream
– If preserving the original quality of the stream is more
important than eliminating the correspondingly larger data
sizes, lossless codecs are preferred
– Especially if the data is to undergo further processing (for
example editing) in which case the repeated application of
processing (encoding and decoding) on lossy codecs will
degrade the quality of the resulting data such that it is readily
identifiable (visually, audibly or both
Codecs and Compression quality
• Codecs are often designed to emphasise certain aspects of
the media to be encoded
– For example, a digital video (using a DV codec) of a sports
event, such as baseball or soccer, needs to encode motion well
but not necessarily exact colours, while a video of an art exhibit
needs to perform well encoding colour and surface texture.
• E.g audio codecs for cell phones need to be very low
latency between a word being spoken and that word
being heard; while audio codecs for recording or
broadcast can use high-latency audio compression
techniques to achieve higher fidelity at a lower bit-rate
Psycho-acoustics and digital audio processing
• Psycho-acoustics effects are important in digital
– Haas effect
– Masking effect
– Missing fundaments/Phantom effect
• A psycho-acoustic effect related to the ability of
listeners to accurately localize sounds coming
from around them.
• When two sounds originate from two sources at
different distances from the listener, humans
localize the sound source based upon the first
arriving sound, if the arrival times of the sounds
are within ~30 ms.
– A phenomenon that might be described as
"involuntary sensory inhibition" in that one's
perception of later arrivals is suppressed
Applications of Haas Effect
• Our well-being and insanity.
– Sound wave bounce off of all walls, if we could hear
each individual sound wave we would hear the same
thing over and over.
– Echo's/reverbs are caused when the sound is delayed by
more then ~30ms
• 3D sound
– Haas effect can be used with other effects to create 3D
sound from only two speakers.
– Duplicating sound and delaying by 30ms causes a more
– Utilized by surround sound technologies such as Dolby
Applications of Haas Effect
• Public Address Systems
– Most commonly used application for the Haas Effect
– Used to ensure that the perceived location or direction
of the original signal remains unchanged.
– Large audiences may demand multiple loud speakers
that need to be placed away from the stage and closer
to the ear.
– Sound must be delayed to these speakers so that the
sound from the stage is heard first.
– This serves to ensure that the sound is perceived as
coming from the point of origin rather than from a
loudspeaker that may be physically nearer the listener
• When two pure tones are close in frequency and are
largely different in amplitude, the louder one drowns out
the weaker one.
– Simultaneous masking
– Temporal masking
• Simultaneous masking:
– When close tones are produced at the same time humans have
difficulty perceiving each as a unique or separate tone
– A perceptual phenomenon that relates to unconscious filtering
• Temporal masking:
– When both tones are produced with a small difference in time
humans have trouble hearing both
– So if a loud sound and a quiet sound are played simultaneously,
you cannot hear the quiet sound
– This distance, or threshold, turns out to be 5ms appox when
working with pure tones, though it varies up and down in
accordance with different audio passages
Applications of Masking Effect
• An important concept for audio
– Frequency components can be discarded or
compressed more when a sound is masked.
– A fundamental element of data transfer over
• The way pitch is computed from tones with
multiple harmonics can be used to construct
a number of illusions
– Continuously Ascending and Descending
Series of Pitches
– Missing fundamentals
Missing Fundamentals Effect
• A sound is said to have a missing fundamental, suppressed
fundamental, or phantom fundamental when its overtones suggest a
fundamental frequency but the sound lacks a component at the
fundamental frequency itself
• For some people the brain perceives the pitch of a tone not only by
its fundamental frequency, but also by the ratio of the higher
harmonics. Thus, we may perceive the same pitch (perhaps with a
different timbre) even if the fundamental frequency is missing from
• Thus a sound with a “missing fundamental” refers to a sound that
due to its overtones suggest a fundamental frequency but the sound
lacks a component at the fundamental frequency itself.
• Thus we perceive the same pitch even if the fundamental frequency
is missing or encapable of being produced by the speaker.
– Note: It is possible that one may not experience this effect.
Applications of Missing
• Used to create an illusion of bass.
• By processing certain overtones selectively
a rich bass effect can be created using small
• The processed bass overtones compel the
brain to replace the missing fundamental
bass signals creating the illusion of bass.
• Many different audio fields are interested in
– Home theatre (designs, components, etc…)
– Audio compression
– Speaker design (loud and bookshelf speakers)
– Public Address Systems
– And many more…