Physics of Sound Part 1 Sound waves How they are generated and travel

Sound Waves Generation and Propagation Sound wave = changes in pressure caused by vibrating object Compression = High pressure Rarefaction = Low pressure Sound needs a medium to “vibrate” Usually air, but could be anything Speed of sound depends upon the medium Air = 1130 ft/secWater = 5000 ft/secSteel = ft/sec

Measuring sound waves Sound waves are longitudinal waves Vibrating object compresses the air around it. Pushes air away leaving an area of low pressure Vibrating object then compresses more air to create a “chain”

Measuring methods Cycle A single push and pull of the vibrating object One are of compression followed by one area of rarefaction An initial increase in atmospheric pressure from the norm, followed by a drop below the norm and then a return to normal Mathematically displayed by a sine curve  Pressure on Y axis  Time on X axis

Measuring methods Period (T) and Frequency (f) Period - The time it takes to create one cycle Frequency - The number of cycles in one second Measured in Hertz (Hz) or cycles per second

Measuring methods Example It takes ¼ sec to create one cycle. What is the sound wave’s frequency?

Measuring methods Frequency will determine pitch High frequency = high pitch Low frequency = low pitch Octave – a doubling of halving of the frequency

Measuring methods Human hearing range Low range between 15 to 30 Hz With enough power lower than 15 Hz can be felt, buy not heard as “sound” High range varies with age and gender Women - up to 20 kHz Men – between 15 to 18 kHz High frequency range will lower with exposure to high levels of sound and age

Tuning Traditional orchestra would tune First Chair Violin A first. Remaining instruments would tune relative to that A above middle C was tuned to about 420 Hz As halls grew larger it was found to be desirable to tune sharper 1939 A was established to be 440 Hz Corresponds to the 49 th key on a full size piano Tuning is not a science. The relative frequency difference is what is important

Measuring methods Wavelength The distance from one area of compression to the next or one area of rarefaction to the next l= wave length V = velocity of sound in medium usually 1130 ft/sec f = frequency

Measuring methods Amplitude How high the pressure goes above and below normal atmospheric pressure Corresponds to how loud the sound is “loudness” is relative to frequency and dependant on the listener.

Timber and Harmonics Harmonics – multiples of a base frequency Timber – the characteristics of a particular sound or instrument Different harmonics combined in different levels White Noise Contains all frequencies at equal power Equivalent to white light Pink Noise Contains all frequencies at equal power per octave Homework Read