1 Sound Chapter 11

2 The Nature of Sound Remember: -every sound is produced by an object that vibrates. -sound waves are compressional waves, which are made up of 2 regions: compressions & rarefactions. -when a radio speaker vibrates outward, the nearby molecules in the air are pushed together to form rarefactions

3 -when the speaker moves inward, the nearby molecules in the air have room to spread out, and a rarefaction forms. -As long as the speaker continues to vibrate back and forth, compressions and rarefactions form

4 Most sounds travel through air (a gas) to get to your ear, it can also travel through solids & liquids. -the material a wave travels through is a medium -sound cannot travel through a vacuum (outer space) The speed of a sound wave through a medium depends on the substance the medium is made of and whether it is solid, liquid, or gas.

5 In general, sound travels fastest through solids and slowest through gases -individual molecules in a liquid or solid are closer together than the molecules in a gas. However, the speed of sound doesn’t depend on the loudness of the sound. -loud sounds travel through a medium at the same speed as soft sounds.

6 In which picture is the bucket passed the fastest? -the second Why? -the molecules (students) are closer together, making the ‘transfer’ quicker

7 The speed of sound is not only dependent on the medium through which it travels, but is also dependent on the temperature of the medium. -as the temperature of a substance increases, its molecules move faster, allowing more collisions between molecules. -the more collisions between the molecules, the faster the sound waves can transfer energy ~at 0 o C in air: 331 m/s 20 o C in air: 343 m/s

8 The Nature of Sound Review Write the Q & A 1. What type of wave is a sound wave? 2. In which of the following environments would sound waves not travel? 3. Describe the two things that affects the speed of sound. 4. Why does sound travel better through a string than through air? 5. Why does sound travel better through a hot air than cold air?

9 Human Hearing Human hearing has 4 stages: 1. The outer ear gathers sound waves, passing them through the ear canal to the eardrum 2. The vibrating eardrum passes the sound to three tiny bones in the middle ear: hammer, anvil, stirrup -this amplifies the sound wave

10 Human Hearing Human hearing has 4 stages: 3. The stirrup vibrates and transfers sound to a membrane in the oval window, then to the inner ear’s cochlea, a spiral shaped structure that contains hair cells 4.As the hair cells in the cochlea vibrate, nerve impulses are sent through the auditory nerve to the brain.

11 Human Hearing Review 1. Which region of the ear a. amplifies sound waves? b. gathers sound waves? c. sends the nerve pulses to the brain? d. vibrates and transfers the sound to the cochlea? 2.What are the three regions of the ear?

12 Properties of Sound What happens to the sound waves from your radio when you adjust the volume? -The notes sound the same, but something about the sound changes: ~quieter sound waves do not carry as much energy as louder sound waves do. -The energy a wave carries corresponds to its amplitude. ~For a compressional wave, amplitude is related to the density of the particles in the compressions and rarefactions.

13 Properties of Sound When an object vibrates strongly with a lot of energy, it makes sound waves with tight, dense compressions. When an object vibrates with low energy, it makes sound waves with loose, less dense compressions. Which wave has more energy? -second: the compressions are tighter

14 Properties of Sound Remember energy, not matter, is transported during the compression and rarefaction of a compression wave -Matter compresses and expands as the wave of energy passes through the matter. Intensity is the amount of energy that flows through a certain area in a specific amount of time. -lower the volume, the lower the intensity -influences how far away a sound can be heard -in a whispered conversation, the sound waves have low intensity and do not travel far.

15 Properties of Sound Loudness is the human perception of sound intensity. -When sound waves of high intensity reach your ear, they cause your eardrum to move back and forth a greater distance than sound waves of low intensity do. -The bones of the middle ear convert the increased movement of the eardrum into increased movement of the hair cells in the inner ear, producing a louder sound The greater the intensity, the louder the sound.

16 Properties of Sound Intensity (& loudness) can be described using a measurement scale. -each unit on the scale for sound intensity is called a decibel, abbreviated dB.

17 Properties of Sound On this scale, the faintest sound that most people can hear is 0 dB Sounds with intensity levels above 120 dB may cause pain and permanent hearing loss.

18 Properties of Sound If you were to sing a scale, your voice would start low and become higher with each note. -Pitch is how high or low a sound seems to be. ~related to the frequency of the sound waves. ~For a compressional wave, frequency is the number of compressions that pass by each second. -high frequency waves cause many compressions to hit your eardrum each second. -Your brain interprets these fast vibrations as a sound with a high pitch.

19 Properties of Sound A healthy human ear can hear sound waves with frequencies from about 20 Hz to 20,000 Hz. -The human ear is most sensitive to sounds in the range of 440 Hz to about 7,000 Hz. This figure shows different notes and their frequencies.

20 Properties of Sound Most people can’t hear sound frequencies above 20,000 Hz, which are called ultrasonic waves. -Even though humans can’t hear ultrasonic waves, they use them for many things. ~used in medical diagnosis and treatment. ~used to estimate the size, shape, and depth of underwater objects.

21 Properties of Sound Infrasonic, or subsonic waves, have frequencies below 20 Hz  too low for most people to hear. -produced by sources that vibrate slowly, such as wind, heavy machinery, and earthquakes. -Although you can’t hear them, you may feel them as a rumble inside your body.

22 Properties of Sound The change in pitch or wave frequency due to a moving wave source is called the Doppler effect. -occurs when the source of a sound wave is moving relative to a listener. As a race car moves, it sends out sound waves in the form of compressions and rarefactions. -The race car creates a compression, labeled A. -Compression A moves through the air toward the flagger standing at the finish line.

23 Properties of Sound By the time compression B leaves the race car, the car has moved forward. -the car has moved since the time it created compression A, compressions A and B are closer together than they would be if the car stayed still. -As a result, the flagger hears a higher pitch. -The faster the change in position, the greater the change in frequency and pitch.

24 Properties of Sound The Doppler effect also occurs for other waves besides sound waves. -the frequency of electromagnetic waves, such as radar waves, changes if an observer and wave source are moving relative to each other. -radar guns use the Doppler effect to measure the speed of cars. -weather radar also uses the Doppler shift to show the movement of winds in storms, such as a tornado.

25 Properties of Sound Review 1. Each unit on the scale for sound intensity is called a __________. 2.Sound frequencies above 20,000 Hz are called __________ waves. 3. Describe the Doppler effect. 4. What is the relationship between intensity and loudness? 5. What is the relationship between frequency & pitch? 6. What is the pain threshold for humans?