Sound Waves What Causes Sound VIBRATIONS Sound Waves

Sound Waves What Causes Sound? VIBRATIONS

Sound Waves Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.

Longitudinal Waves vs. Transverse – Longitudinal waves follow the same “rules” as the transverse waves we have dealt with previously. – However, because cohesive forces between particle are necessary for transverse wave propagation, transverse waves only travel on the surface of fluids. – Where as, longitudinal waves may propagate through the “bulk” of a fluid, because they rely on pressure forces, not cohesive forces.

The Fundamentals of Sound Waves Sound waves are longitudinal waves n Frequency – number of oscillations in pressure per second n Wavelength – distance between each successive region of high or low pressure.

The Speed of Sound n Molecules of the medium collide, allowing the wave to be transmitted through it. – The speed of sound depends on elasticity of the medium. – The more elastic the medium, the faster the sound. – Elasticity – the ability of a solid to return to its original shape after external forces have been exerted on it.

The Medium n What if there is no medium? – A vacuum perhaps… n Waves in different media – In what medium will sound travel fastest… Steel, wood, or you? What does the speed of sound depend upon? The Medium

Echo Location and Sonar

Loudness n Loudness - depends on the amplitude of sound wave – Again, the amplitude of a wave indicates its energy. The greater the amplitude, the greater the energy. On the graph to the right Where is the sound loudest?

Sound Levels Loudness is measured in Decibels (d. B) (Data Table on page 394 of text) – Breathing – Normal Speech – Concert – Pain Threshold 10 60 115 120 d. B.

Sound and Pitch n Pitch - the frequency of a sound wave – The human ear is not equally sensitive to all frequencies n Most people cannot hear frequencies below 20 Hz or above 16, 000 Hz n Most people are most sensitive to frequencies between 1, 000 and 5, 000 Hz Musical Scale n Example “middle C” has a frequency of 262 Hz.

Natural Frequency & Resonance n Natural frequency – An object’s own set of frequencies – Depends primarily on elasticity and shape vs

Resonance When the frequency of a forced vibration matches an object’s natural frequency, a dramatic increase in amplitude occurs

Tacoma Narrows Bridge n Unfortunately, resonance is not always a good thing…

Noise & Noise Reduction n Noise – sound of any kind (especially unintelligible or dissonant sound) n Noise Reduction – LORD Corporation – Aerospace Engineering – The use of destructive interference to eliminate noise

Doppler Effect n Doppler Shift - Change in sound frequency due to the relative motion of either the source or the detector. n example: a passing car

DOPPLER EFFECT n n Refers to the change in frequency when there is relative motion between an observer of waves and the source of the waves Doppler with Sound

Sonic BOOM n An aircraft traveling through the atmosphere continuously produces air-pressure waves similar to the water waves caused by a ship's bow. n When the aircraft exceeds the speed of sound, these pressure waves combine and form visible shock waves

Sub & Super Sonic Subsonic - slower than the speed of sound Supersonic - Faster than the speed of sound n Mach Number = speed of object speed of sound

Bow waves V-shaped pattern made by overlapping crest

Shock Waves n Produced by supersonic aircraft, three-dimensional cone shaped n Sonic boom – sharp crack heard when conical shell of compressed air that sweeps behind a supersonic aircraft reaches listeners on the ground below.

The Physics of Music n What is the source of sound, again? VIBRATIONS n The 3 Types of Musical instruments All create Vibrations in air Type n String n Wind n Percussion Origin Plucked String Mouthpiece or Reed Stretched Membrane

Sound Quality Fundamental and Harmonics n Dissonance and Consonance n Intervals and Octave n – Beat Notes – Superposition of waves Demo