Sound PreAP Physics Chp. 25

What is Sound?Sound Longitudinal wave –Caused by a vibration that travels away from the source through solids, liquids, or gases –CAN NOT travel through a vacuum. A vacuum has no air molecules (not what you use to clean your house) Needs a medium to transmit energy from a sound wave

Speed of Sound Independent of the pressure, frequency, and wavelength of the sound. In a gas it is proportional to the temperature T. Equation to determine the speed of sound in air v = T v = velocity of sound at a particular temperature in (m/s) 331 = the speed of sound in m/s at 0°C T = the temperature in °C

Speed of Sound Continued Travels at different rates depending on the materials.materials –Depends on the density of the material More dense = faster speed of sound The speed of sound in air at room temperature (20ºC) is 343 m/s.

Pitch & Loudness Pitch –How high the sound is –Measured by the frequency High frequency = high pitch Low frequency = low pitch Loudness –Measure of energy (amplitude) in a sound wave –Unit of measure in decibels Logarithmic scale

Threshold of hearing0 dBMotorcycle (30 feet)88 dB Rustling leaves20 dBFoodblender (3 feet)90 dB Quiet whisper (3 feet)30 dBSubway (inside)94 dB Quiet home40 dBDiesel truck (30 feet)100 dB Quiet street50 dBPower mower (3 feet)107 dB Normal conversation60 dBPneumatic riveter (3 feet)115 dB Inside car70 dBChainsaw (3 feet)117 dB Loud singing (3 feet)75 dB Amplified Rock and Roll (6 feet) Threshold of Pain 120 dB Automobile (25 feet)80 dBJet plane (100 feet)130 dB Typical average decibel levels (dBA) of some common sounds. Decibel Scale

Human Hearing Average human audible range –20 Hz to 20,000 Hz. Ultrasonic sounds –Frequencies above 20,000 Hz Infrasonic sounds –frequency is lower than 20 Hz. Mosquito Tone Test

HUMAN EAR: The ear consists of three basic parts: Outer ear: serves to collect and channel sound to the middle ear. Middle ear: serves to transform the energy of a sound wave into the internal vibrations of the bone structure of the middle ear and transform these vibrations into a compressional wave in the inner ear. Inner ear: serves to transform the energy of a compressional wave within the inner ear fluid into nerve impulses which can be transmitted to the brain.

Hearing: A compression forces the eardrum inward and a rarefaction forces the eardrum outward, thus vibrating the eardrum at the same frequency of the sound wave.

INTERFERENCE OF SOUND WAVES: BEATSBEATS If two sources are close in frequency, the sound from them interferes and what we hear is an alternating sound level. The level rise and falls. If the alternating sound is regular, it is called beats.

The beat frequency equals the difference in frequencies between the sources. This is a way to tune musical instruments. Compare a tuning fork to a note and tune until the beats disappear. CI = Constructive Interference DI = Destructive Interference

DOPPLER EFFECT DOPPLER EFFECT – Change in frequency due to movement. Either the source of the sound moves or the observer. EX.) When a source of sound waves and a listener approach one another, the pitch of the sound is increased as compared to the frequency heard if they remain at rest. If the source and the listener recede from one another, the frequency is decreased. This phenomenon is known as the Doppler Effect. Doppler Effect. Cop with sound Moving Targets

Light waves also exhibit the Doppler effect. The spectra of stars that are receding from us is shifted toward the longer wavelengths of light. This is known as the red shift. Measurement of the red shift allows astrophysicists to calculate the speed at which stars are moving away. Since almost all stars and galaxies exhibit a red shift, it is believed that the universe is expanding.

Resonance Forced vibration matches the natural frequency of an object causing it to vibrate. Examples –Wine GlassesWine Glasses –Breaking GlassBreaking Glass –Tacoma Narrows BridgeTacoma Narrows Bridge

SHOCK WAVES AND THE SONIC BOOM Speed of a source of sound exceeds the speed of sound Sound waves in front of the source tend to overlap and constructively interfere. The superposition of the waves produce an extremely large amplitude wave called a shock wave. Sonic Boom Applet Sound wave

The shock wave contains a great deal of energy. When the shock wave passes a listener, this energy is heard as a sonic boom. Is heard only for a fraction of a second; Sounds as if an explosion has occurred and can cause damage.

Diffraction When any wave bends around a boundary or opening Wave doesn’t go through the rigid medium, it bends around it instead Example –Water waves bend around the rocks or other obstacles –Sound bends out door ways and windows

Radios and Cell phones Voice or music signals are transmitted via electromagnetic waves to the radio or cell phone Speakers turn those signals into sound (longitudinal) waves

Echos & Sonar Sound waves reflect off of surfaces Time between the initial sound and the return of the echo are recorded –Time is for the sound to hit the object and bounce back Used to figure out how far away objects are located Examples –Bats, Dolphins –Submarines & Ships –Back up sensors on cars

Speed of Waves Remains constant if in the same medium Changes when wave enters a new medium