Chapter 10 Sound

Sounds are caused by waves Longitudinal Waves The direction of the wave is also the direction of the particle movement Travels through air It is a mechanical wave Needs matter to travel through Energy is transferred when one particle collides with another In the compression part of the wave, the particles are pushed close together In the rarefaction part, the particles are further apart

Mediums Sound can travel through any medium Air is the most obvious In water, sound travels faster but is less clear In solids, sound travels even faster Recall that the different forms of matter have properties based on how the atoms are arranged

Where can’t sound be transmitted? In the absence of matter, sound waves can not transfer energy. This means that, in space, there can be no sound Astronauts must use transmission equipment to talk to, and hear one another

Speed of Sound Sound travels at different speeds in different mediums Air, liquids, solids It also travels different speeds through each medium depending on the temperature, density, and elasticity of the matter

Temperature As the temperature of any matter increases the particles in the matter move faster. More collisions=more energy transfer=more heat This increase in collisions also affects the transfer of sound waves If particles are moving faster, they are bumping into each other more, and therefore transferring sound faster If the matter is cooled, the particles move slower and sound is slower to transfer. The speed of sound in air at room temperature (21 degrees C) is 346m/s At 0 degrees C, the speed falls to 330 m/s

Density Density=mass/volume If the particles in an object are closer together, the object will be more dense In a dense object, particles bump into one another more often This will transfer energy (sound) more quickly

Elasticity Elasticity-the tendency of an object to rebound to its original state when deformed Rubber bands have high elasticity Paper does not (It stays deformed) Sound waves travel more quickly through elastic objects Solids tend to be more elastic (air and liquids do not stretch)

How do we hear sound? Human ears have the ability to collect sound waves and transfer signals to the brain The brain reads the signals There are 3 sections of the ear The outer ear The middle ear The inner ear

The outer ear The part that you see (cartilage) along with the ear canal and Tympanic membrane The shape of the ear helps gather sound waves and direct them into the ear canal. The ear canal is 2-3cm long, and about 0.7 cm in diameter The sound waves travel down this canal until they reach the ear drum The Tympanic membrane (a.k.a. Ear Drum) is a tough membrane that vibrates when sound waves reach it It is about 0.1 mm thick

Middle Ear As the Tympanic Membrane vibrates, it passes the sound waves into the middle ear Consists of 3 bones Malleus (hammer) Incus (Anvil) Stapes (Stirrup) As these bones vibrate, they amplify the sound waves The Stapes is connected to the oval window, and causes it to vibrate

The inner ear Fluid-filled portion of the ear The oval window vibrates, transferring the sound waves to fluid waves that travel through the inner ear The inner ear contains the cochlea Filled with fluid and contains tiny hair cells, which transmit signals to the auditory nerve (cochlear nerve), through which they travel to the brain The brain reads the signals and deciphers them

Hearing loss Damage to the ears can result in hearing loss Loud noises can damage or destroy the hair cells in the cochlea These do not grow back and cannot be replaced Research is looking into ways to fix this The easiest way to avoid hearing loss is to refrain from loud sounds

Amplitude of Longitudinal Wave Amplitude is the amount of disturbance of a wave I.E. How much the matter is disturbed by the wave In a longitudinal wave, it is how close the particles are pushed together Higher amplitude=more compressed This occurs when more energy is added Amplitude is related to intensity

Intensity The amount of energy that passes a point in a given time Measured in decibels (dB) If you increase the volume of a sound-emitting device, the amplitude increases, as does the intensity More energy is carried past a point in a given amount of time The opposite happens if you decrease the volume If intensity is higher, the sound travels further because it has more energy

Distance and Intensity Sound does not travel forever Eventually, the energy is lost to the environment as it converts to other forms of energy (thermal energy) Also, the wave spreads out This spreads the energy out, lowering the intensity until the sound is no longer audible

Loudness The human perception of sound volume Depends on intensity Higher volumes (i.e. louder sounds) have more energy and cause your eardrum to vibrate farther back and forth This is what you perceive to be “LOUD!” Lower volumes cause the eardrum to vibrate less This is what you perceive to be “quiet”

The Decibel Scale The unit of intensity is the decibel (dB) Defined as 1/10 of a bel A unit named after Alexander Graham Bell, inventor of the telephone The bel was too large of a unit, so the decibel was used instead Deci=1/10 It is a logarithmic scale Based on Powers of 10 of the number of bels Going from 10 dB (1 B) to 20 dB (2 B) results in 10x the intensity Going from 10 dB (1 B) to 30 dB (3 B) results in 102 =100x the intensity

Example Problem How much more intensity is there in the sound of normal conversation (60 dB) than in whispered conversation (20 bB)? 60 dB-20 dB=40 dB 40 dB=4 B 104=10,000 times as much intensity

You Try It The intensity of the classroom on a typical work day is 90 dB. I ask that it be brought to 30 dB. How much more intense is the sound than it should be? 1,000,000 times as intense...

The Decibel Scale The lowest (theoretically) that we can hear is 0 dB Sound can be less intense, and other organisms can hear it, but we cannot The pain threshold for humans is about 120 dB Sustained sound at 90 or more dB can cause hearing loss Ear protection is recommended if exposed more than 15 minutes per day Even seconds of 120+ dB sounds can cause hearing loss

Pitch How high or low a sound seems to be Related to frequency Frequency is the number of waves that pass a point each second As frequency increases, the pitch gets higher As frequency decreases, pitch gets lower https://www.youtube.com/watch?v=qNf9nzvnd1k The pitch/frequency a human can hear varies, and decreases with age. You can probably hear from 20 Hz to 20,000 Hz https://www.youtube.com/watch?v=VxcbppCX6Rk Other animals can hear sound at much higher frequencies Dogs can hear to 35,000 Hz, and bats to 100,000 Hz

The Doppler Effect The change in wave frequency due to the movement of the source of the wave or the observer https://www.youtube.com/watch?v=a3RfULw7aAY When an object is emitting a sound and moving towards an observer, the sound waves are pushed closer together, increasing the frequency of the sound As the object moves away from a stationary observer, the waves are pulled apart, decreasing the frequency of the sound https://www.youtube.com/watch?v=h4OnBYrbCjY Also occurs in light, as will be discussed next chapter

Noise vs. Music Noise has random patterns and pitches https://www.youtube.com/watch?v=CZuiyRFIJ-Q Music is any collection of sounds that are deliberately used in a regular pattern https://www.youtube.com/watch?v=Yq2rH7mhUT4

Natural Frequencies and Resonance All objects have natural/fundamental frequencies Particular set of frequencies at which an object vibrates Musical instruments use these to create the sounds that we know The sounds created by instruments are amplified by resonance The air vibrates as the instrument is played This makes the sound louder

Beats Occur when different frequencies combine Sound like little breaks in the sounds This is a type of interference https://www.youtube.com/watch?v=TCXgvG_Jt6E

Using Sound: Acoustics The study of sound The size, shape, and materials present in a room can greatly affect sound quality Acoustic engineers study these to determine the best way to make effective concert halls

Echolocation Some animals use sound waves to locate objects

Sonar A system that uses underwater reflection of sound waves to find things

Ultrasound Using sound waves that cannot be heard by humans Above 20,000 Hz Used in medicine to examine body parts Including fetuses in the uterus