01 Physics of Sound Ver 2

Acoustic & Psychoacoustic Phenomenon

Computer Sound Production

COSC2190 | Computer Sound Production

sound is fifty percent of the motion picture experience

George Lucas


Broadway, NYC, 1941 Citizen Kane released

A journalist asked:Mister Welles, What do you prefer the most, the radio or the cinema?

Orson Welles answered:.. radio, the screen is wider


sound influences how we react to a picture and/or becomes image. cf. director, Michael Haneke

if you are watching a film or television with the sound off and then with it on usually you will find that more information comes from the sound than the picture and that two different sound tracks for the same picture will produce two different meanings.

creating sound design is often tantamount to defining the productions conceptual and emotional intent



What is sound ?& How do we hear ?

Sound is the aural perception of vibrations.

Two types of audible sound:

Noise is defined as all sounds that are not organized or harmonious that are surround us constantly.

Music is organized & harmonious.


wavelength amplitude frequency period loudness pitch bandwidth dynamic range Hertz (Hz) decibel (dB)

masking equal loudness

principle frequency

response

concepts & vocabulary


production - creation of the sound by a sound emitter

propagation - movement of the sound through a medium

perception - hearing of the sound


sound is propagated through a medium (such as air) as a compression wave.

if were to look at one microscopic sample of the air through which a sound travels for one frozen it could be visualized as below


Properties of a sound wave

amplitude

wavelength

frequency

f = wavelength/time


the physics of sound (acoustics) is often confused with the way in which we perceive it (psychoacoustics).



Sound & Perception

structure of the human ear


amplitude is not loudness

frequency is not pitch

loudness and pitch are how our brain perceives and interprets the physical properties of amplitude and frequency



Amplitude & Loudness

amplitude is measured in decibels

dB = 10log(P1/P0)

the decibel is a unitless ratio of two quantities, usually a reference sound level (ie. P0) to another (P1)

the ratio of the softest to the loudest sound for human hearing is 120 decibelsthis is known as the dynamic range


PresenterPresentation Notestwo sounds with an amplitude of 100dB-SPL when added is about 103dB-SPLMost people do not perceive a sound level as having doubled until it has increased anywhere from 3 to 10 dB! Alten p 15

intensity of sound

dB Units of Energy Example

130 10,000,000,000,000 threshold of pain

120 1,000,000,000,000 jet taking off

100 10,000,000,000 shouting, jackhammers

80 1,000,000,000 loudest TV sounds

60 1,000,000 ordinary conversation

30 1,000 whisper

0 1 threshold of hearingCOSC2190 | Computer Sound Production

due to this logarithmic relationship changes in loudness are not additive unlike changes in amplitude

doubling the sound pressure intensity yields a 3dB increase in sound

doubling the loudness of a sound yields an exponential increase in sound pressure intensity


PresenterPresentation Notestwo sounds with an amplitude of 100dB-SPL when added is about 103dB-SPLMost people do not perceive a sound level as having doubled until it has increased anywhere from 3 to 10 dB! Alten p 15

acoustic sound pressure is measured in terms of sound pressure leveldB-SPL

sound that has been transduced/converted into electrical energy is measured in decibelsdBv



Frequency & Pitch

the pitch of a sound is how the brain perceives the physical phenomena of a sounds frequency

unit of measure for pitch is Hertz (Hz)

the frequency range (bandwidth) of human hearing is 20Hz to 20,000Hz


pitches of commonly heard sounds

Hz Example20,000 highest range of human hearing

10,000 hiss of spoken consonants (s, ch, z, f, th)

4,186 highest note on a piano

1,000 high range of singing voice (fundamental tone)

400 high range of child's or woman's speaking voice

263 middle note on a piano

80 low range of male speaking voice

50 low range of singing voice

27 lowest note on a piano20 lowest range of human hearing10 earthquake


Range EQ Range

Hi

brilliance 6 kHz-20 kHz

presence 4 kHz-6 kHz

Mid-range

hi-mid 2 kHz-4 kHz

low-mid 250 Hz-2 kHz

Lo

bass 60-250 Hz

sub-bass20-60 HzCOSC2190 | Computer Sound Production


The Perceptual Nature ofHuman Hearing

the human ear does not respond to frequency and sound pressure in a linear manner


PresenterPresentation Notesit is not a linear receiver with respect toits sensitivity changes with:frequencyamplitude levelagetirednessabuse

equal loudness principle - the threshold of hearing is frequency dependent


PresenterPresentation Notesthe response of the human ear is not equally sensitive to all audible frequencies. Depending on loudness we do not hear sounds of low and high frequencies as well as we hear middle frequencies.

the compression effect that takes place when we listen to music when it is loud is why loud music seems to sound better


PresenterPresentation Noteshttp://blogs.msdn.com/audiofool/Louder Sounds BetterAbove is an example of the Fletcher-Munson Equal Loudness Curve. It is one of the most recognized graphics in audio engineering. The horizontal axis is frequency of tones, and the vertical axis is actual sound pressure in dBSPL.Each point on a curve has about the same subjective "loudness" to the human ear. The low parts of the curves are the frequencies where the ear is more sensitive. Conversely, the high parts are where the ear is less sensitive and it takes more pressure to get the same 'loudness'. The dotted line at the bottom represents the threshold of hearing. Any frequency below that line can't be heard at all by the average human. Consider an arbitrary waveform coming out of a speaker. That waveform has the frequency response shown in red on the diagram to the right. The blue line is the same waveform amplified by 20dB, and the orange line is amplified by 80dB. Notice that the louder signal aligns better with the flatter Fletcher-Munson curves at the top. There is less variation in sound at different frequencies, and the result is that the louder signal has a richer, fuller sound.The opposite is also true.For quieter sounds,certain frequencies to which the ear is less-sensitive can seem to drop out of the signal, especially at very high and very low frequencies.The orange linehas a very powerful bass, with low frequencies staying close to the sameloudness asthe middle. The blue line'sbass is much less 'loud' than the middle frequencies. Many frequencies on the red line have completely dropped out, below the threshold of hearing.TheFletcher-Munson curves illustratean audio engineering example of why, to the human ear, louder sounds better. The higher the signal amplitude, the more frequencies are present,making the signal richer and fuller.You can also read this article, which discusses it from an audio production and musician's point of view.Apracticalresult of this is that when a person tries to compare two sounds, the louder one will often subjectively sound better regardless of their relative signal quality. Any time you want to compare sounds by ear objectively, you have to make sure they're the same level.One final note: I hope this post does a little bit to explain why people are drawn to loud concerts, or to turning headphones up to extremely high levels, but there's a very dangerous flip-side. The orange line in my figure above peaks out at about 125dBSPL. A concert at that level may indeed sound good, but it is alsoat or abovethe threshold of pain, depending on the listener, and any prolonged exposure will likely cause permanent hearing damage. There are any number of guidelines about maximum safe listening levels, and I recommend you heed them. That set of headphones may sound mighty fine cranked up to 90dBSPL, but they are also doing irreparable damage to one of your most important and finely tuned sensory organs. Perhaps someday I'll do a post on exactly what happens when you destroy your auditory receptors. Trust me, it isn't fun.

Clipping in popular musicAside from the distortion artifacts, one of the biggest problems thatresults fromclipping is a loss of dynamic range. Remember that the dynamic range of a signalis effectively the difference between the maximum output level and the noise floor. When you clip a waveform, you lower the maximum sample value, which lowers the output level and reduces dynamic range.This can happen especially when you amplify a signal. Amplification increases all parts of a signal, including its noise floor and its maximum value. Normally,these two values increase by the sameamount, so theratio between them stays the same anddynamic range is unaffected. Unfortunately, a signal that is amplified too much has to clip at the top end, and that's whenDR suffers.So the moral here is don't overamplify your music, right? That seems easy enough. The problem is that popular recording studios don't listen. Recall that, superficially, louder music sounds better. Better sounding music sells more discs, and so it pays for the studios to master their music as close to the clipping level as possible. "But wait," the studio says,"most people are tone-deaf anyway. We can get it just a little louder than our competition if we clipa little bit off of the peaks." The competition responds in kind, clipping just a little more, and the result (as Larry scooped me on) is why popular music has terrible dynamic range.The following articles tell the sad story far better than I can. I encourage you to give them a read.http://www.mindspring.com/~mrichter/dynamics/dynamics.htm(mirror)http://en.wikipedia.org/wiki/Loudness_warhttp://www.prorec.com/prorec/articles.nsf/articles/8A133F52D0FD71AB86256C2E005DAF1Chttp://www.austin360.com/music/content/music/stories/xl/2006/09/28cover.htmlAnd one of my favorite illustrations: http://recforums.prosoundweb.com/index.php/mv/msg/4286/0/0/0/And now you know why serious audio aficionados stick to the classics. :)Published 07 February 07 02:44 by RyanBemrose Filed under: Audio

the ratio of the softest to the loudest sound for human hearing is 120dBin recording a dynamic range of 90dB is considered high fidelity.

our inner ear has a dynamic range of 50dB.

loud external increases in sound pressure yield smaller changes in the levels in our ears

our ears compress sound ie. a 4dB external change results in a 1dB increase transmitted to the inner hair cells


masking

the hiding of some sounds by other sounds when each is a different frequency and they are presented together

generally louder sounds mask softer ones, and lower pitched sounds tend to mask higher pitched sounds



Advanced Physical Properties of Sound

phase (additive properties of waves) timbre envelopes

PresenterPresentation Notesit is not a linear receiver with respect toits sensitivity changes with:frequencyamplitude levelagetirednessabuse

the simplest wave shape is a sine wave


PresenterPresentation Noteshttp://hem.bredband.net/bersyn/VCO/sine.jpg

when two sound waves of the same frequency combine with one another their amplitudes are added. If they are in phase the resultant wave shape is the same but the amplitude if different


when two sound waves of the same frequency but 180 degrees out of phase combine with one another their amplitudes are added with an unusual result.


timbre is that unique combination of fundamental frequency, harmonics, and overtones that gives each voice, musical instrument, and sound effect its unique coloring and character.


Fourier was able todemonstrate that complex periodic wavescan be simplified to their constituent sine wave components.


PresenterPresentation Noteshttp://hem.bredband.net/bersyn/VCO/sine.jpg

When an object vibrates it propagates sound waves of a certain frequency.

This frequency, in turn, sets in motion frequency waves called harmonics.


PresenterPresentation NotesIn all but some esoteric cases, the first harmonic (the fundamental, called f) is the pitch that you'll perceive when you listen to the sound of the plucked string. The second harmonic (also called the first 'overtone') is half the wavelength of the fundamental and therefore twice the frequency. In isolation we would perceive this as a tone exactly one octave above the fundamental.The third harmonic has a frequency of 3f (which is the perfect fifth, one and a half octaves above the fundamental) and the fourth harmonic, with a frequency of 4f, defines the second octave above the fundamental. The next three harmonics then lie within the next octave, and the eighth harmonic defines the third octave above the fundamental. And so it goes on...This is the information we need to understand Pythagoras's observation. The shorter of the two strings in the 1:2 relationship is producing a fundamental at the same frequency as the second harmonic of the longer one. It's exactly one octave higher. In the example of the 2:3 strings, the third harmonic of the longer string is at the same frequency as the second harmonic of the longer one. In other words, the harmonic structures of the two strings are closely related to one another, and we hear this as musically 'pleasing'.

http://www.soundonsound.com/sos/may99/articles/synthsec.htm?session=6335dc1f8b3440bc7c52f9f7e250c1f7

The basic frequency and its resultant harmonics determine the timbre of a sound. The greater the number of harmonics, the more interesting is the sound that is produced. Pleasing sounds have harmonic frequencies that a possess an integer relationship to the fundamental pitch (ie. 1f, 2f, 3f, 4f)


sound envelopes

(1) Attack sound begins at A and reaches its peak at level B.

(2) Sustain it drops slightly in level and remains steady until C.

(3) Decay when the sound source is removed at C, the sound decays to a point of silence D.


directionality of sound

Imaging Staging


wavelength amplitude period frequency pitch decibel (dB) Hertz (Hz) range of hearing phase

timbre harmonics fundamental pitch envelope rhythm monoaural stereo multi-channel

concepts & vocabulary


One of the main points to consider when placing speakers is the fact that their directionality is frequency dependant.

Low frequency sounds are pretty much omni-directional, being able to diffract around obstacles (including the speaker cabinet) quite readily.

High frequencies, however, are highly directional with only limited diffraction capacity. The speech band (the frequencies in which we are most often interested) occupies the mid-frequencies.


response of human ear

frequency response ~20 Hz - 20 kHz. range drops with age ear sensitivity is:

o frequency dependento drops with increasing sound level (effect reduces

overload due to very high sound levels)

Volume

Loudness


http://www.jhu.edu/signals/listen- new/listen-newindex.htm


Chapter 1 & 2 in:

Alten, Stanley R. (2002) Audio in Media. Wadsworth.

Chapter 1 in:

Cancellero, Joseph (2005) Sound Design for Interactive Media.

Introduction to Computer Music: Volume One http://www.indiana.edu/~emusic/etext/acoustics/chapter1_i ntro.shtml


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01 Physics of Sound Ver 2