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Sound

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Sound Waves

Sound waves travel as compression waves.Another name for compression waves is longitudinal waves

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Factors affecting speed of sound Temperature – Sound moves through hot air faster than cold air due to the increased speed of the particles.Type of medium – Sound moves well through dense material and material that returns quickly to its original position.

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Factors affecting speed of soundThus, sound moves fastest in a solid and slowest in a gasSound travels more quickly through solids and liquids because the individual molecules are closer together than the molecules in gas Sound waves CANNOT move through a vacuum – they need a medium!

Bells in a bell jar…

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Decibels

The decibel is the unit used to measure sound intensity or loudness.Loudness corresponds to the amplitude of a wave.

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DecibelsSounds greater than 120 dB can cause pain in human ears.Sounds with an amplitude between 0 dB and 120 dB are called audible.Anything below 0 dB is considered subaudible.

Decibel drag races

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Frequency and PitchPitch – how high or low a sound is.High pitch = high frequencyLow pitch = low frequency

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Frequency and PitchHumans can hear pitches that have a frequency between 20 Hz and 20,000 HzPitches above 20,000 Hz are called ultrasonic.Pitches below 20 Hz are called subsonic.

Phone Tone Generators

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Doppler EffectThe Doppler effect is a change in the frequency or pitch of a sound that is caused by either the movement of the source or the observer of the wave.Example: The sound from an ambulance siren changes as it approaches the listener

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Doppler Effect

First observed in 1842 by Christian Andreas Doppler The Big Bang Theory - Doppler Costume

Video explaining the Physics of the Doppler Effect

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ResonanceResonance is the vibration of an object at its natural frequency.Example: Windows rattle when the sound from a passing truck matches the window’s natural resonance.

Resonance of different things

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Nodes and Anti-nodes

Node – A place where two waves meet and destructively interfere so that the displacement is zero

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Nodes and Anti-nodesAnti-node – the point of largest amplitude when two waves interfere constructively

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Music – Natural FrequencyA natural frequency exists without any driving source. It is a natural frequency if its waveform has nodes that match up with the ends of the object. The lowest frequency at which this occurs is the fundamental, or the 1st Harmonic.

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Music - HarmonicsHarmonics – a sound wave with a pitch that is a multiple of the natural frequencyOvertone – has a higher frequency than the fundamental Octave = ½ or double the frequency of a sound; 8 notes on the musical scale

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MusicConsonance – multiple waves combining to form a pleasant soundDissonance – multiple waves combining to form an unpleasant soundAcoustics – the control of noise and the vibrations that cause noise

Toccata Fugue in D Minor

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Music – open pipe resonators

Open pipe resonators – both ends are openExamples: brass instruments, flutes, saxophonesIt reflects an inverted wave

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Music - Closed pipe resonatorsClosed pipe resonators – have one end enclosedExample: pan-flute, blowing across a bottle top, hanging pipes under marimbas, xylophones