Sound

Sound is Energy in Waves

• Specifically, longitudinal waves

• Different from transverse waves

Transverse Waves

• Not sound but easier to describe.

• Also known as sine waves

-1.5

-1

-0.5

0

0.5

1

1.5

Wavelength

• The length of the wave (duh) between the arrows.

-1.5

-1

-0.5

0

0.5

1

1.5

Amplitude

• The height of the wave (zero to one—arrows below).

-1.5

-1

-0.5

0

0.5

1

1.5

Period (T)

• The time for one cycle (up and down).

• The reciprocal is called frequency, waves per second or Hertz.

Sound is a Longitudinal Wave

• Like pulses on a slinky.

• Compressed sections are called, er, compressions, and stretched sections are called rarefactions.

• A wave train of compressions and rarefactions in air travels to your ear and you hear sound.

Speed

• The speed of a waves is f times , (frequency times wavelength), c.

• The speed, c, of a wave in a medium is a fixed quantity.

• For instance, the speed of sound in sea level air at 50% humidity at 20oC is 345m/s, and nothing can make sound go faster under these conditions.

• In water, steel, etc, c is different.

Three Terms of Physics and Music:• Physics: frequency, Music: pitch

• Physics: amplitude, Music: volume

• Physics: quality, Music: timbre (TAM` ber)

The Ear

Frequency

• Human range from 20 (bass) to 20,000 (treble)• Higher than 20K: ultrasonic. Lower than 20:

infrasonic.• Most sensitive from 1000-5000 on average.• Damage for overexposure to loud sounds or

diving reduces high frequency sensitivity.• Women more sensitive to highs than men.• Young generally hear highs better than old.

Amplitude

• How loud (not how high) a sound is: volume.• Measured in deciBels (dB)• Quietest sound for humans is 0dB, loudest band

on record is 120dB=747 takeoff.• Damage to your ear comes for repeated exposure

to loud sounds.• Cilia become damaged and a ringing develops.

Quality

• Not good or bad sound!• Timbre is the quality of different sound that

allow the ear to distinguish one from another.• It is the addition of harmonics with amplitudes

unique to a particular sound.• For instance, a clarinet has only odd harmonics

(1,3,5,7 etc. times the fundamental pitch) with decreasing amplitude.

• A bell has harmonics based on the square root of multiples of the fundamental pitch.

Harmonics

• Multiples of the fundamental wave frequency (e.g. middle C is about 256 Hz)

• The fundamental wave frequency is determined by the resonance of the mechanical system.

• For instance, a guitar string will vibrate at a certain frequency depending on its length, weight, and tension.

Continued…

• However, the string also vibrates in halves (meaning twice the fundamental), thirds (thrice the fundamental), in fourths and so on.

• When you add all these secondary, tertiary, quaternary etc. vibrations it causes a unique harmonic signature which sings “guitar!”

Resonance

• Pipes have their own resonant conditions for resonance, depending on their length, diameter, and if they are open at both ends or not.

• Bars have their own resonant conditions depending on their length and density.

• Your own voice depends on the length of your vocal tract and your vocal cords (and degree of intoxication).

Doppler Shift

• The “zzzzoooooOOOOOMMMMM” as a car goes by.

• When a sound source approaches you (or you approach it) its sound waves are squeezed up because they can’t go any faster in a particular medium.

• This results in a higher pitch (don’t think louder, think higher).

Continued…

• As the source retreats, the sound waves are stretched out, and have a lower pitch (think lower, not softer).

• When the sound source is traveling at the the speed of sound itself—about 700mph—the sound waves pile up in a big bunch causing a sonic boom.

• The Shuttle or a whip both break the sound barrier and cause a boom.

Echo and Reverb

• Discreet reflections of sound are called echoes.

• Can be used to range distances (dolphins and SONAR).

• Reverb is a set of continuous reflections too numerous and close together in time for the ear to separate.