Sound Bites

Basics Sound is a mechanical, longitudinal wave. The medium usually associated with sound is air, but sound can travel through both liquids and gases as well. As sound travels through a medium, it alternately compresses and expands the medium the same way a slinky behaves when you create a longitudinal wave in it. compressed areas - high pressure area expanded areas- low pressure areas

Drawing a sound wave It is somewhat difficult to draw a longitudinal wave in terms of high and low pressure areas. Instead you will draw a graph of the pressure changes themselves. Once you have done that, you will have a picture that looks like a transverse wave, which is easier to describe.

Speed of Sound The speed of sound depends on the medium that it is passing through. Generally speaking, the denser the medium, the greater the speed of sound. As a result of this, sound travels much faster in solids than it does in gases. Material Speed (m/s) Air343 Water1440 Wood4000 Glass4500 Steel5000 explains the ear to the train track!

The speed of sound also depends on the temperature. For sound travelling through air, the relationship between the temperature and the speed is given by: v ≈ ( T) m/s v = speed of sound in air T = temperature in degrees Celsius

Give it a go What is the speed of sound at room temperature (20° C)? Use this answer if the temp. is not given

Example 2 The speed of sound in air is 350 m/s. To the nearest degree, what is the air temperature?

Echoes All the properties of waves apply to sound waves. reflection, superposition, refraction diffraction Reflected sound waves are given a special name, echoes. The wave equation also applies to sound. v= f

Example Some cameras use very high frequency sound to determine the distance from the object being photographed. If an object is 2.0 m from such a camera, how long will it take for the sound to return to the camera? You yell into a mine shaft and hear your echo 3.00 s later. How deep is the well?

Pitch With sound we use specialized terms for the general wave properties. pitch - frequency of sound. The human ear responds to sounds in the range of 20 Hz to Hz. ultrasonic – sound with freq above Hz Bats can hear sounds with frequencies as large as Hz. Infrasonic - sound with freq below 20 Hz created by earthquakes and machinery

Interference with Sound Waves Sound waves can interfere just like other waves. An interesting phenomenon occurs if the two sound waves that are interfering are fairly close in frequency.

Beats The top two green waves have slightly different frequencies. When they interfere with one another there will be times when they are nearly in phase and a loud sound will be heard and at other times they will be nearly out of phase and very little sound will be heard. The red wave represents the result of the interference. Notice that the change in the amplitude has a frequency of its own. The frequency at which the amplitude changes is called the beat frequency. The beat frequency is simply the difference between the frequencies of the original two waves.

Example tuning fork #1 = 256 Hz. tuning fork #2 = ? Hz Beat freq = 4 Hz The two possible answers for the second fork are 260 Hz AND 252 HZ. How can this property be used to tune musical instruments?

Doppler Shift You have probably noticed the frequency (or pitch) of a siren change as it approached and passed you. If either the source of a sound, the observer of a sound or both are moving, the "observed" frequency of the sound will change. Of course, the actual frequency of the sound source remains the same. When the source and observer are moving towards one another, the observed frequency will be higher and when they are moving away the frequency will be lower.

Doppler Shift This is a stationary sound source emitting waves at a constant frequency. Notice that the wavelengths are equal. Now the sound source is moving to the right at a constant velocity. As the source emits sound waves, it "catches up" to the waves in front of it and pulls away from the waves behind. This has the effect of decreasing the wavelength of the waves in front and decreasing the ones behind.

Doppler Shift As sound moves away form you ↑ then f ↑ (low pitch) As sound moves towards you  then f ↑ (high pitch)

Calculating Doppler Shift f' = observed frequency (Hz) f = frequency of the sound source (Hz) v = speed of sound (usually 343 m/s) vo = velocity of the observer (m/s) vs = velocity of the sound source (m/s)

Example An ambulance carrying Ms. Ryan (coffee overdose) is approaching you at 30 m/s. If the ambulance's siren emits sound at a frequency of 1500 Hz, what frequency will you hear as you stand in shock at possibly losing your beloved physics teacher?

Give it a go A tornado siren goes off so you jump on your bicycle and drive away from the siren at a speed of 20.0 m/s. If the siren has a freq of 500 Hz, what freq will you hear? (471 Hz)

Sonic Boom An interesting thing happens when an object (such as a plane) travels with a velocity equal to the speed of sound. The sound waves in front of the plane can't move away and build up in one very large pressure wave. It actually takes a burst of extra energy to break through this barrier. Once the plane has broken through, it will be outrunning its own sound waves and is said to be supersonic. The sound waves will all line up to constructively interfere and to form what is known as a shock wave. This shock wave is commonly called a sonic boom.