Sound Waves Sound Wave Properties Sound Waves are

Sound Waves

Sound Wave Properties

Sound Waves are Longitudinal Waves The air molecules shown below are either compressed together, or spread apart. This creates alternating high and low pressure.

Frequency • The frequency of a sound wave (or any wave) is the number of complete vibrations per second. • The frequency of sound determines its pitch. • Human Hearing ranges from • …. 20 Hz to 20 KHz

The higher the frequency, the higher the pitch

Wavelength • Wavelength is the distance between two high pressures, or two low pressures. This property is dependent on the velocity of the sound and it’s frequency.

Amplitude The Amplitude of a Sound Wave Determines its loudness or softness

Velocity of Sound

The velocity of sound depends on • The medium it travels through • In air at room temperature sound travels at 343 m/s (~766 mph) • v = 331 m/s + (0. 6)T – v: velocity of sound in air – T: temperature of air in o. C

Relationship between velocity, frequency, and wavelength • V = f • V = velocity of sound • = wavelength of sound • f = frequency of sound

• Sound travels faster in liquids than in air • Sound travels faster in solids than in liquids • Sound does not travel through a vacuum – there is no air in a vacuum so sound has no medium to travel through

Echoes: REFLECTION

Echoes are the result of the reflection of sound Sound waves leave a source, travel a distance, and bounce back to the origin.

Things that use echoes. . . • • • Bats Dolphins/ Whales Submarines Ultra sound Sonar

REFRACTION OF WAVES

Refraction of Sound • Refraction is the bending of waves when they enter a medium where their speed is different

DIFFRACTION: THE BENDING OF WAVES THROUGH A SMALL OPENING

BENDING OF A WAVE

Beats

A beat occurs when sound waves of two different (but very much alike) frequencies are played next to each other. The result is constructive and destructive interference at regular intervals.

Sound waves move out like this:

But when they move, the front of the wave gets bunched up (smaller wavelength) and the back of the wave starts to expand (larger wavelength):

Observer C hears a high pitch (high frequency) Observer B hears the correct pitch (no change in frequency) Observer A hears a low pitch (lower frequency)

Police use the Doppler Shift when measuring your speed with radar • A frequency is sent out of the radar gun • The sound wave hits the speeding car • The frequency is changed by the car moving away from the radar and bouncing back • The amount the frequency changes determines how fast you are going • The faster you are going, the more the frequency is changed.

When the source goes faster, the wave fronts in the front of the source start to bunch up closer and closer together, until. . .

The object actually starts to go faster than the speed of sound. A sonic boom is then created.

Resonance

• Resonance happens with closed pipe resonators and open pipe resonators • Resonance occurs when there is a standing wave in the tube • Closed pipe resonators – open end of tube is anti-node – closed end of tube is node • Open pipe resonators – both ends are open – both ends are anti-nodes

Closed pipe resonator

Open pipe resonator

SOUND INTENSITY: THE LOUDNESS OF SOUND

DECIBEL • MEASURES THE LOUDNESS OF SOUND • RELATES TO THE AMPLITUDE OF THE WAVE • EVERY INCREASE OF 20 d. B HAS 10 x GREATER AMPLITUDE

150 d. B JET TAKING OFF 115 d. B CHAIN SAW 100 d. B POWER MOWER 80 d. B NOISY 75 d. B VACUUM CLEANER 50 d. B AVERAGE HOME 25 d. B PURRING CAT 20 d. B 15 d. B 0 d. B RESTAURANT RUSTLING LEAVES WHISPER FAINTEST THAT CAN BE HEARD

A SOUND 10 TIMES AS INTENSE IS PERCEIVED AS BEING ONLY TWICE AS LOUD

NOISE POLLUTION · Prolonged exposure to noise greater than 85 -90 d. BA (decibels measured on the A-scale) may cause hearing loss · Brief exposures to noise sources of 100 -130 d. BA can cause hearing loss · A single exposure to a level of 140 d. BA or higher can cause hearing loss