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Six Qualities of Sound Perception Kitchen
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An auditory perception primer.

The study of sound perception is called pschoacoustics. Any time you hear or feel a sound, there are some amazing things that happen. For every sound your ear-brain system processes, you get information about:

Pitch - (also associated with frequency), the perception of a high or low sound.

Loudness - (also called amplitude), the intensity of a sound.

Phase - the increase and decrease in pressure cycle any single vibration.

Direction - (hearing with two ears creates left/right, high/low, front/back qualities), first come first heard by one ear or the other.

Distance - (also associated with reverberation time), perception of how near or far away a sound's source is.

Timbre - (also called tone color), the perceived quality of any sounds' multiple frequencies changing through time.

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What is Pitch?

Pitch is the term we use to describe the psychological sensation or perception of a sound by people.

Pitch is sometimes confused with frequency, the term we use to describe the physical phenomena of sound energy created by a series of vibrations.

These are two different but complementary aspects of sound perception.

Typically, the first concept we introduce to beginners is the idea that sounds can be low or high, and that these sounds exist in a sound spectrum. Some animals hear lower than humans and some hear higher. Other natural things like ocean waves, earthquakes create ultra-low sounds (we feel it!) and a bat's sonar create ultra-high sounds. So this spectrum covers a wide range from .01 vibrations per second to over 10 million vibrations per second!

A science term for vibrations per second is Hertz, abbreviated Hz. This term is named after the German physicist Heinrich Rudolf Hertz (1857 - 1894).

What is the human range of hearing? - approximately 15-20,000 vibrations per second. It differs in everyone.

What is a musical sound? - regular vibration rate (periodic vibration / harmonic sounds).

What is human speech and noise? - irregular vibration rate aperiodic / inharmonic sounds).

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What is Loudness?

Loudness is a quality defined as the intensity of sound energy as it comes in contact with an eardrum or other surface. Another term describing loudness is amplitude.

How do we measure sound intensity? - we use a system called decibels based on ratios. One bel is the ratio between the differences in intensity of two sounds of 10 to 1, and a decibel is one-tenth of a bel. The bel was named after Alexander Graham Bell, (1847 - 1922) though we don't know why the last 'l' was dropped.

Decibels are logarithmic in nature - a 10 decibel increases energy tenfold, so the difference between 60 decibels and 100 decibels is not 40 times, but 10,000 [10 to the 4th] times as much energy!

What happens as sound travels away from its source? - sound loses energy as it travels away from its source so it gets gradually softer.

How does the head effect sound waves? - the head's acoustic shadow blocks frequencies above 1,000 vibrations per second further reducing their loudness as they travel across the head. For example, play your stereo and put a record jacket in between your head and speakers. What happens? It blocks high frequencies, the sound is duller.

What's the difference between high and low frequencies in terms of loudness? - low ones (approximately 15 - 100 vibrations per second) travel further with the same amount of initial energy vs. high ones (approximately 1,000 - 20,000 vibrations per second).

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What is a Sound's Phase?

Every sound consists of an increse/decrease cycle. This cycle effects air pressure, increasing it, then decreasing it.

Scientists use positive numbers to describe the pressure increase and negative numbers to describe the decrease.

Your brain is sensitive to a sound wave's phase. Based on the subtle differences in a sound wave's phase between your two ears, your brain can tell what direction a sound came from.

Phase can be a regular pattern, like in a musical tone, or it can be irregular, like in a waterfall.

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How Do We Know What Direction a Sound Comes From?

How does the loudness of a sound effect the way we locate the direction of that sound's origination? Our auditory pathway rapidly calculates the difference in loudness between the sound as it enters both ears. In other words, a sound coming from your left side has less energy and is slightly softer by the time it reaches your right side.

What other phenomena effect the way we perceive sound?

the ambient noise level of any space or room - the mix of all of the random sounds (called masking).

preceding - following sounds (expectation vs. surprise).

wanted vs. unwanted sounds - our minds filter out a large portion of sound deemed unimportant. For example, a parent seemingly ignores their screaming baby while you may notice it.

concord vs. discord (harmony vs. bad notes).

pitch (frequency rate\vibration rate).

head position in relation to sound source (called phase angle) - effects low frequency perception (moreso than loudness).

first come, first heard - we can distinguish a 1/10,000th second difference between the same sound entering one, then the other, ear (called precedence).

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What Does Distance Have to Do with Sound Perception?

Sounds have different charactoristics based on how near or far they are heard from their source.

What is reverberation? All sound has a unique echo pattern created and shaped by its physical environment, whether it's an ocean, subway, or phone booth.

What is reverberation time? It is the amount of time for a sound to decrease 1\1,000,000 times [or 60 decibels] from its original level. Reverberation time is approximately .6 seconds in an average living room, 1-2 seconds in an average concert hall.

How do we know how big a room or space is based on its sound characteristics? Any space or listening environment has its own unique sound characteristics based on reverberation time and timbre. For example, why does a honking horn sound different in an open field vs. a subway tunnel? Why does a voice or door slam sound differently?

Each sound has a unique echo signature based on the size and shape of the space it occurs in. For example, a gymnasium or the Grand Canyon will have much longer reverberation times than a living room because the sound bouces back and forth over a much larger area.

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What is timbre (pronounced tam-ber)?

One caveat before we start! Timbre is the most complex quality of the six to understand, much less teach to someone. It combines the concepts of frequency and loudness through time. There is also an entire set of mathematics that supports these concepts related to the harmonic series. With this in mind here's a go at it...

The perceived quality of any sound, such as bright or dull, wooden or metallic, etc. is called timbre. Musicians use the term 'tone color' to describe the way instruments sound together or separately. An 18th century scientist, named Joseph Fourier (1768 - 1830), proved mathematically that any sound is actually made up of a set of multiple frequencies called the harmonic series.

That's right, just as you can break white light into component colors of the rainbow (ROYGBIV), so too can any sound be analyzed as a series of sounds (the one exception is the laboratory-produced pure sine wave tone).

The tones of the harmonic series

A Cello plays a low C - note that in practice all of these tones sound at the same time!

The lowest sounding tone is called the fundamental partial, in this example a Cello's low C. It is the loudest tone of the series and the one we usually identify in music.

The other higher sounding partials are almost always softer, and our ear/brain system blends them together into our perception of a single tone, just as our eye/brain system blends differennt colors of light into our perception of white light.

The point of all of this is that the difference in the loudness through time of these frequencies that we perceive as timbre.

For example, a flute and violin playing a middle C, are both vibrating at the same rate (261.63 times per second). However, the effects of the air column in the flute vibrating vs. the string on the violin vibrating produce different sets of frequencies with variable loudness that make each uniquely identifiable.

In a musical sound (or periodic vibration), these tones are related by whole number ratios for example, 2:1, 3:2, 4:3, 5:4, etc. In spite of this fact we may perceive a musical passage as pleasant or unpleasant because music is a complex set of tones and tonal relationships.

In noise (or aperiodic vibration), the harmonic series are not based on whole numbers, but fractional number ratios for example, 2.2:1.1, 3.3:2.2, 4.4:3.3, etc. In spite of this fact we may consider some noises musical, like a crash cymbal or tubular bell! A dog barking or baby crying may be considered pleasant or unpleasant based on a larger set of psychological and social factors.

What is Fourier analysis? - the graphic representation of the multiple frequencies of any musical sound or noise:

its component frequencies, in terms of low to high in Hertz,
the loudness of all its component frequencies, in terms of decibels
the frequency and loudness through time, in terms of seconds.

Fourier analysis is used by scientists for a range of research in the field of acoustics.

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but not any simpler."

Albert Einstein


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