Ch. 13 - Sound The Nature of Sound Speed of Sound Human Hearing Doppler Effect Seeing With Sound

Production of Sound Waves begins with a vibrating object source of the disturbance vocal chords string on a violin, guitar, etc tynes of a tuning fork diaphragm of a radio speaker

Production of Sound Waves air vibrates, causing changes in air pressure high pressure = compressions crests correspond to compressions low pressure = rarefactions results in a longitudinal wave see Fig. 13-2

Frequency determines pitch highness/lowness of sound higher frequency = higher pitch

Human Hearing ultrasonic waves infrasonic waves human ear can hear between 20Hz and 20,000 Hz. < 20Hz is infrasonic > 20,000Hz is ultrasonic higher f = shorter λ small wavelengths can “see” much smaller objects ultrasonic waves infrasonic waves

Application: Ultrasonic waves are used in medical testing because the wavelength is short enough to produce an image. Most ultrasonic devices are around 10MHz. This allows for the emitting crystal to recapture the sound and produce the outline. see Fig. 13-3 Echolocation works in the same manner.

“Sound Navigation Ranging” Seeing with Sound Ultrasonic waves - above 20,000 Hz Medical Imaging SONAR “Sound Navigation Ranging”

Speed of Sound can travel through solids, liquids and gases constant for a given medium faster in solids particles are closer together can vibrate the next particle faster

Speed of Sound higher temperatures in gases also increase the speed particles collide more often not much difference in solids and liquids particles are closer together

Motion of Sound vibrates out in a spherical wave called wave fronts distance between wave fronts is wavelength sound travels in three dimensions rays can be drawn show direction of wave motion very far waves appear as a straight line called a plane wave

Doppler Effect relative motion creates an observed change in frequency if sound is on a moving object also occurs in light red shift blue shift

Doppler Effect as object moves toward a stationary observer: waves pass by more frequently than if standing still frequency increases higher pitch as object moves away from a stationary observer: waves pass by less frequently than if standing still frequency decreases lower pitch

Sound Intensity the rate at which energy is transferred through a unit area of the plane wave a.k.a. power per area of plane wave intensity decreases as distance increases spread over larger area

Sound Intensity intensity and frequency both determine if sound is audible frequency determines if “hearing” is possible intensity determines the loudness (volume) measured in decibels (dB) see Table 13-2 on pg. 490

Human Hearing threshold of hearing the softest sounds that can be heard by the average human ear frequency of about 1000Hz intensity of 1.0x10-12 W/m2 threshold of pain intensity beyond 1.0W/m2 120 DECIBEL SCALE 110 100 80 70 40 18 10

Practice… What is the intensity of the sound waves produced by a trumpet 3.2m away when the power output of the trumpet is 0.20W? Assume that the sound waves are spherical.

Forced Vibration and Resonance a vibrating object is attached to another object attached object experiences “forced vibrations” sympathetic vibrations increase the sound if at the object’s “natural frequency” it will vibrate much better this is called resonance

Application: The human ear is divided into three sections: outer, middle, and inner. Hearing involves sound waves that travel down the ear canal in the outer ear and terminate at a thin flat piece of tissue called the eardrum. The eardrum then vibrates and transfers these vibrations to the three small bones of the middle ear. These bones in turn transmit the vibrations to the inner ear, which contains a snail shaped tube called the cochlea. The small hairs and nerve fibers in the cochlea send impulses to the brain. The brain then interprets these waves.

converted to nerve impulses in cochlea Human Hearing sound wave vibrates ear drum amplified by bones converted to nerve impulses in cochlea

Harmonics harmonics account for sound quality, or timbre based on a standing wave, frequencies can be produced by musical instruments through strings or columns of air. harmonics account for sound quality, or timbre each instrument has its own mixture of harmonics at varying intensities gives it its own unique sound more harmonics, richer sound

Standing Waves when a wave is produced it reflects back produces nodes and antinodes frequency of the string determines more/less nodes and antinodes fundamental frequency corresponds to the first standing wave determines pitch

String Instruments (guitar, violin, etc.) strings can vibrate in different ways (different frequencies) changes the number of nodes and antinodes 13th frequency is an octave above the 1st n= harmonic number l=length of the string

Columns of Air (flutes, piccolos, etc) open column of air (flute, recorder, pipe organ) see diagram pg. 496 n= harmonic number l=length

Columns of Air (woodwinds, brass, etc) closed column of air (coke bottle, trumpet, clarinet) see diagram pg. 497 n= harmonic number l=length only odd harmonics are present n is odd

Beat interference between two waves of slightly different frequencies interference pattern varies alternation between loudness and softness called beat see pg. 502 # of beats per second corresponds to the difference between frequencies. more beats  further off