ALL sounds are produced by vibrations.

Sound waves travel as longitudinal (compressional) waves.

Compression – region of high molecular density and high air pressure.

Rarefaction – region of low molecular density and low air pressure.

Sound MUST have a medium to travel through. It will not travel through a vacuum.

The speed of sound is determined by the temperature, elasticity,

and density of the medium through which it travels.

Frequency of sound waves = frequency of vibrating object

pitch – subjective perception of a sound wave’s frequency

The average human hearing range of frequencies is from 20 Hz to 20,000 Hz.

* infrasonic - below 20 Hz

* ultrasonic - above 20,000 Hz

Sound Intensity and Loudness

Intensity is proportional to the square of the amplitude.

Sound intensity is objective (can be measured) Loudness is related, but is subjective

When a sound’s intensity increases by a factor of 10, its loudness increases by a factor of 2. So if one sound is 10 times more intense than another, it will sound twice as loud as the other.

Sound Intensity and LoudnessThe Decibel scale

-measures relative intensity

-A logarithmic scale

-An increase of 10 decibels (dB) means the sound intensity increases by a factor of 10. So 40 dB is ten times more intense (and therefore twice as loud) as 30 dB.

-60 dB is 100 times more intense (and 4 times louder than) 40 dB

-Named after: Alexander Graham Bell0 dB threshold of hearing

10 dB rustling leaves

20 dB quiet whisper

50 dB normal conversation

70 dB vacuum cleaner

90 dB lawn mower

115 dB loud music

120 dB THRESHOLD OF PAIN

130 dB nearby jet engine

The Speed of SoundThrough the same medium, sound waves of all frequencies will travel at the same speed.

Since v = f

high f = short

low f = long

The speed of sound in dry air at 0 degrees Celsius is 331 m/s.

The speed of sound increases by 0.6 m/s for each degree rise in the air temperature above 0 deg C.

So speed of sound = 331.6 m/s at 1 deg C = 332.2 m/s at 2 deg C = 332.8 m/s at 3 deg C, etc

Or, v = 331 m/s + (0.6 m/s/C)(Temp oC)

Beat Frequency•When two sound waves with slightly different frequencies are heard together.

•A series of alternating constructive and destructive interference.

•Heard as a rise and fall in volume.

Beats - interference in time• Superposition of two waves of similar

frequency

+

=

Musical instrument tuning• When two strings or a string and tuning fork

are close in pitch, beats may be heard.

• The number of times per second that the volume rises and falls is the beat frequency.

• The beat frequency is just the difference in the two frequencies

• fbeat = | f1 – f2 |

• To tune the instrument, the string is tightened or loosened until the beat frequency decreases and goes away.

Sound Quality

Timbre – a term referring to the general quality of a sound.

Assonance – when multiple frequencies combine to produce a pleasant sound.

Dissonance – when multiple frequencies combine to produce an unpleasant sound.

Forced VibrationsWhen one vibrating object causes another object to vibrate. For example, vibrating guitar strings will cause the air in the guitar and the guitar box itself to vibrate.Natural Frequency – the frequency at which minimum energy is required to produce forced vibrations.

Resonance – when the frequency of forced vibrations equals an object’s natural frequency.

-a dramatic increase in amplitude occurs due to constructive interference

-Example: pushing someone on a swingset

Resonance• Two tuned tuning forks coupled by

sound in air exhibit a marvelous resonance - excite one with a hammer and the sound will excite the other.

Resonance has been blamed for the collapse of some bridges!

• 1831 – marching soldiers accidentally matched a bridge’s natural frequency, causing it to fall.

• 1941 – wind caused resonance in Tacoma Narrows suspension bridge, until it came crashing down.