Sound Waves Chpt 14.

What is sound? Sound waves are in all fluids –Fluids are both gases and liquids People can hear from 20 Hz- 20,000Hz The process of hearing is an intricate procedure.

The Amazing Ear

There are sounds outside the limits of human hearing Infrasonic: sounds with frequencies lower than 20 Hz –Ex: Earthquakes, wind, weather, elephants, cattle Ultrasonic: sounds with frequencies higher than 20,000 Hz. –Dogs <45,000 Hz –Cats <70,000 Hz –Bats <100,000 Hz

Test Your Range!

Speed of Sound in Different Materials Materialm/s Air (0°ͦ)331 Air (20°)343 Helium (0°)972 Water (25°)1493 Seawater (25°)1533 Copper (25°)3560 Iron (25°)5130

Find the wavelength in air at 20° C of an 18 Hz sound wave, which is one of the lowest frequencies that is detectable by the human ear.

What is the wavelength of an 18 Hz sound wave in seawater at 25°C?

Find the frequency of a sound wave moving through iron at 25°C with a wavelength of 1.25m.

A 2280 Hz sound wave has a wavelength of 0.655m in an unknown medium. Identify the medium

Think of a string attached to a clamp on one end and the other moves up and down, like an oscillation. If the string is vibrated at the right frequency a standing wave appears –A standing wave looks like it’s not moving

Standing Wave Patterns The area where the wave is “tall” are areas of constructive interference The areas where it looks like a “point” are areas of deconstructive interference

When complete destructive

What Does a Standing Wave Look Like? The Tacoma Narrows Bridge was a bridge that

Open Air Columns Many musical instruments are made up of air column in metal tubes. Extra long tubes are bent back on top of each other Flutes and organs are examples

If each end of the pipe is open at both ends, each end is an antinode.

Natural Frequencies Natural frequencies are know as harmonics of the instruments There are standing wave patterns

If both ends of the tube are open it is a open- pipe resonator –Ex: Trumpet,

Speed of Sound The speed of sound can change based on the medium and the temperature it is propagating through. v = ( T c ) V = velocity (m/s) T = the air temperature in degrees Celsius

Ex: On a cool October afternoon (air temp = 15 degrees) your seat on the centerfield stands 113m from first base, you watch a play at the World Series. You see the runner’s foot tag the base, half a second later you hear a baseman’s glove hit the base. Is he safe or out?

Sound is Intense! Intensity: the rate of energy transfer for a given time over a given area. I = P/A = P/ (4ΠR 2 ) The intensity of a point source of sound is inversely proportional to the square of the distance from the source

With this in mind: Sound intensity is perceived as loudness.

Threshold of Hearing (I 0 ): a sound wave at 1KHz with an intensity of W/m 2 Threshold of Pain (I p ): a sound wave with an intensity of 1 W/m 2. At this point the sound is uncomfortably loud and may be painful to the ear Decibel (0.1 bel) named after A. Graham Bell

Sound Intensity or decibel level (β): β = 10 log I I 0 where I 0 = W/m 2 What unit have you typically heard of for sound? See pg 488 Example 14.4

Logarithmic Sound Intensity Levels

Sound Levels and Damage Exposure You can damage hearing by excessive noise Turn to page 491 and look at table 14.2 for exposure time and intensities

Ex: A 10 W speaker emits sound at full power. What is the intensity 5.5m away? Find the relative intensity of the sound.

If a rock band is playing at 92 dB, find the intensity of the sound.

Ex: What is the distance from a lightning strike in 20C weather of you hear thunder 3.5 s after the strike?

Hmwk! Pg 508 #1, 10-13, 16, 20, 29, 30, 32, 35,36, 37b, 38