Title: Location K-4

5-8

9-12 Description Discipline

Six Qualities ofSound Perception Kitchen - X X An auditory

perception primer.

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

Pitch - (also associated with frequency), the perception of a high orlow sound.Loudness - (also called amplitude), the intensity of a sound.Phase - the increase and decrease in pressure cycle any singlevibration.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 hownear or far away a sound's source is.Timbre - (also called tone color), the perceived quality of any sounds'multiple frequencies changing through time.

[ Go to the Top ]

What is Pitch?

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

Pitch is sometimes confused with frequency, the term we use to describe thephysical 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 soundscan be low or high, and that these sounds exist in a sound spectrum. Someanimals hear lower than humans and some hear higher. Other natural thingslike ocean waves, earthquakes create ultra-low sounds (we feel it!) and a bat'ssonar 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 isnamed after the German physicist Heinrich Rudolf Hertz (1857 - 1894).

What is the human range of hearing? - approximately 15-20,000 vibrationsper 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).

[ Go to the top ]

What is Loudness?

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

How do we measure sound intensity? - we use a system called decibelsbased on ratios. One bel is the ratio between the differences in intensity of twosounds of 10 to 1, and a decibel is one-tenth of a bel. The bel was named afterAlexander 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, sothe 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 energyas it travels away from its source so it gets gradually softer.

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

What's the difference between high and low frequencies in terms ofloudness? - low ones (approximately 15 - 100 vibrations per second) travelfurther with the same amount of initial energy vs. high ones (approximately

1,000 - 20,000 vibrations per second).

[ Go to the top ]

What is a Sound's Phase?

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

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

Your brain is sensitive to a sound wave's phase. Based on the subtledifferences in a sound wave's phase between your two ears, your brain cantell 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.

[ Go to the top ]

How Do We Know What Direction a Sound Comes From?

How does the loudness of a sound effect the way we locate the direction ofthat sound's origination? Our auditory pathway rapidly calculates thedifference 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 thetime 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 therandom sounds (called masking).preceding - following sounds (expectation vs. surprise).wanted vs. unwanted sounds - our minds filter out a large portion ofsound deemed unimportant. For example, a parent seemingly ignorestheir 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) - effectslow frequency perception (moreso than loudness).

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

[ Go to the top ]

What Does Distance Have to Do with Sound Perception?

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

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

What is reverberation time? It is the amount of time for a sound to decrease1\1,000,000 times [or 60 decibels] from its original level. Reverberation timeis approximately .6 seconds in an average living room, 1-2 seconds in anaverage concert hall.

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

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

[ Go to the top ]

What is timbre (pronounced tam-ber)?

One caveat before we start! Timbre is the most complex quality of the six tounderstand, much less teach to someone. It combines the concepts offrequency and loudness through time. There is also an entire set ofmathematics that supports these concepts related to the harmonic series. Withthis 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 wayinstruments sound together or separately. An 18th century scientist, namedJoseph Fourier (1768 - 1830), proved mathematically that any sound isactually made up of a set of multiple frequencies called the harmonicseries.

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

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

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

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

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

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

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

In noise (or aperiodic vibration), the harmonic series are not based on wholenumbers, 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 acrash cymbal or tubular bell! A dog barking or baby crying may be consideredpleasant or unpleasant based on a larger set of psychological and social factors.

What is Fourier analysis? - the graphic representation of the multiplefrequencies 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 decibelsthe frequency and loudness through time, in terms of seconds.

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

[ Go to the top ]

"Things should be made as simple as possible,but not any simpler."

– Albert Einstein

Send your comments to: webteam@cafemuse.com

© 1996 – 2005 Wildfire Media, All Rights Reserved.