Revision as of 22:46, 1 July 2007

Speed of Sound

The Speed of sound is measured in meters per second.The speed of sound is affected by several factors: temperature, humidity, and elasticity.

SOS ~ 344 m/s or 1130 f/s

NOTE: for class we will assume the speed of sound in air @ 20 degrees Celsius an 75% humidity is 344m/s

Speed of sound Calculator http://www.measure.demon.co.uk/Acoustics_Software/speed.html

Frequency/Period

Sound is a periodic event. This means that there is a fixed amount of time between different events.The Frequency component of a sound wave affects the pitch of the sound.

Frequency if the measure of how many cycles a wave complete per second.

Frequency (f) is measures in Hertz

Period (T) in the inverse of infrequency is is measured in seconds

Frequency = Cycles/Second Period = Seconds/Cycle

f = 1/T T = 1/f

Different types of frequency components periodic / aperiodic.

Periodic

sound that has a measurable frequency

Aperiodic

sound that is mostly random

It is assumed the humans can hear frequencies from 20Hz to 20,000 Hz.

20-20KHz Sweep

http://iam.colum.edu/sound/media/sweep20-20.wav 41kb,16 bit 3.7megs

Resonance

Resonance

Wavelength

Wavelength is measured in distance (ft. of m)

The actual physical length of a single cycle of a wave

Wavelength = speed of sound / Frequency in Hz

λ = SOS/f

wavelength = speed of sound * period

λ = SOS*T

The wavelength of a sound wave varies greatly with the frequency of the wave. A 20 kHz wave has a wavelength of 1.72 (0.7 in)cm while a 20 Hz wave has a wavelength of 17.2 m (~56 ft.).

Examples http://iam.colum.edu/MTD2/lectures/week1_examples.htm

Perception of Amplitude and Frequency

Humans do not hear all frequencies equally. We perceive different frequencies with equal energy to have different amplitudes. The Fletcher Munson Curves show the phons scale, how amplitude is perceived by frequency

Fletcher Munson Curves 1 http://arts.ucsc.edu/EMS/Music/tech_background/TE-02/AcNumbers/AcNumbers.html

Fletcher Munson Curves 2 http://www.allchurchsound.com/ACS/edart/fmelc.html

Fletcher Munson Curves 3 http://hyperphysics.phy-astr.gsu.edu/hbase/sound/eqloud.html

Interference and Beats

adding sound waves

• Interference and Beats

http://www.school-for-champions.com/science/soundbeat.htm

Difference between two notes is called interval

Frequency and Pitch

Frequency if the measure of how many cycles a wave complete per second.

Frequency (f) is measures in Hertz

Period (T) in the inverse of infrequency is is measured in seconds

Frequency = Cycles/Second Perdiod = Seconds/Cycle

f = 1/T T = 1/f

The perception is frequency is know as pitch.

Intervals

An octave is doubling of the frequency.

ie 440 Hz is an A

the next A after 440 Hz is 880 Hz

In westen music the octave is divided into 12 semi-tones

http://thinkzone.wlonk.com/Music/12Tone.htm

http://en.wikipedia.org/wiki/Interval_(music)

http://www.musicalintervalstutor.com/

http://www.keystrokepublishing.com/html/sight_singing_tips.html

Phase

Phase in measured in degrees.

The phase of a sound has to do with the time domain.

Adding two Simple Harmonic waves -Applet

Adding two Simple Harmonic waves2 - Applet

http://library.thinkquest.org/19537/java/Wave.html

Timbre/Harmonic structure

Timbre is a descriptive word used to help describe the 'color' and envelope of a sound.

All sound is made up of simultaneous sounding tones. In the 1700 Joseph Fourier a French mathematical physicist proved that all sound can be synthesized be adding sine waves. The way these sine waves are added together make things sound different.

http://www.gac.edu/~huber/fourier/

Different sound don't sound the same due to different fundamental frequencies, harmonics, complexity, and envelope.

Timbre is made up of Harmonics and Envelope. The Harmonics define the different frequencies present in a a sound and the envelope defines the amplitude through time.

Harmonic Structure

http://www.rane.com/par-t.htm Harmonics defined @ rane

Understanding Harmonics Fundamental Frequency and Harmonics Instruments

   * Guitar string
   * Open-End Air Columns
   * Closed-End Air Columns

Natural harmonics are multiple of the fundamental/

Understanding Harmonics @ Harmony Central

http://ptolemy.eecs.berkeley.edu/~eal/eecs20/berkeley/scale/demo/timbre.html

http://www.umanitoba.ca/faculties/arts/linguistics/russell/138/sec4/source.htm

http://library.thinkquest.org/19537/cgi-bin/showharm.cgi