9-May-15 Physics 1 (Garcia) SJSU Origin of Sound Sound is a wave that is produced by the vibrations of material objects. Tuning fork Guitar string Drumhead
Nature of Sound in Air Sound in air is a longitudinal wave created by compressions and rarefactions.
9-May-15 Physics 1 (Garcia) SJSU Demo: Sound is not Wind With sound, air molecules oscillate in place. With wind, air moves from place to place. Smoke rings are not sound because the air moves from place to place.
9-May-15 Physics 1 (Garcia) SJSU Demo: Light & Sound Sound waves can only travel through a material, such as air, water, etc. Light and radio waves can travel through vacuum. See the cell phone ringing inside vacuum chamber but don’t hear any sound. Radio Wave
9-May-15 Physics 1 (Garcia) SJSU Check Yourself Do light waves have energy? What do we call the type of heat transfer that occurs when light transfers energy? Do sound waves have also have energy?
9-May-15 Physics 1 (Garcia) SJSU Media That Transmit Sound Sound travels better through elastic liquids and solids, such as water and rocks, than through air. This is due to the close proximity of the atoms as they vibrate. Hear richer, louder sound transmitted by string
9-May-15 Physics 1 (Garcia) SJSU What Your Voice Sounds Like Your voice sounds different to you when you hear it from a recording. This is because when you are speaking aloud, most sound waves reach your ear traveling through the solid flesh and bone of your skull. Leave yourself a voice-mail
Human Ear Pressure variations of sound waves push the eardrum, whose vibrations are transmitted by the ossicles (ear bones) to the cochlea (hearing canal)
Cochlea Vibrations transmitted by the ear bones create oscillations in the fluid with the cochlea (snail in Latin), which is a spiral-wrapped tube. These oscillations within the cochlea cause the basilar membrane to ripple, like a waving flag.
Organ of Corti The organ of Corti forms a ribbon of sensory epithelium that runs lengthwise down the entire cochlea. The hair cells of the organ of Corti selectively transform the oscillations of the basilar membrane into nerve signals.
9-May-15 Physics 1 (Garcia) SJSU Loudness & Amplitude Loudness depends on amplitude of pressure and density variations in sound waves.
Decibels Loudness of sound depends on the amplitude of pressure variations in the sound waves. Loudness is measured in decibels (dB), which is a logarithmic scale (since our perception of loudness varies logarithmically). From the threshold of hearing (0 dB) to the threshold of pain (120 dB) the pressure increase is a million times higher. At the threshold of pain (120 db) the pressure variation is only about 10 Pascals, which is one ten thousandths atmospheric pressure.
9-May-15 Physics 1 (Garcia) SJSU Demo: Make Some Noise Let’s experience the loudness of sound like by clapping at various decibel levels. Sound Meter Start clapping softly and slowly increase or decrease loudness, as I direct you using the sound meter.
Hearing by Age & Sex Hearing acuity decreases with age, especially in the high frequencies. In general, women have greater acoustic sensitivity than men. Absolute thresholds of hearing by age in males and females Male, Age 20 Male, Age 30 Male, Age 40 Male, Age 50 Male, Age 60 Female, Age 60
Hearing Loss Hair cells that respond to high frequency sound are very vulnerable to destruction, and loss of these neurons typically produces difficulty understanding human voices. Much of this type of permanent hearing loss is avoidable by reducing exposure, such as to loud music. The hair cells that line the cochlea are a delicate and vulnerable part of the ear. Repeated or sustained exposure to loud noise destroys the neurons of the Organ of Corti. Once destroyed, the hair cells are not replaced, and the sound frequencies interpreted by them are no longer heard. What?
9-May-15 Physics 1 (Garcia) SJSU Speed of Sound in Air Speed of sound in air is about 340 m/s. Sound travels about one kilometer in three seconds, about one mile in five seconds. Light is a million times faster than sound.
9-May-15 Physics 1 (Garcia) SJSU Demo: Helium Voice Sound speed in helium is higher than speed in air. Wavelength of sound unchanged (size of vocal cords is unchanged). Frequency of voice is higher since He Talk like me! (Wave speed) (Wavelength) (Frequency) = Breath Helium…
9-May-15 Physics 1 (Garcia) SJSU Reflection of Sound Sound reflects strongly from rigid surfaces. Softer surfaces absorb sound. Quiet after a fresh snowfall because the soft, irregular surface of the snow absorbs sound instead of reflecting it.
9-May-15 Physics 1 (Garcia) SJSU Check Yourself When crowded, which restaurant will be quieter?
9-May-15 Physics 1 (Garcia) SJSU Singing in the Shower Multiple reflections from the hard walls create reverberation. Hear your voice from several sources, slightly shifted in time. Reverberation extends each note and smears (smoothens) the pitch. Your voice sounds better when singing in the shower
9-May-15 Physics 1 (Garcia) SJSU Refraction of Sound Sound speed can vary by material or conditions. This causes the sound to bend in direction, in the same way that light bends when it passes through a glass lens. Fig. 20.8
9-May-15 Physics 1 (Garcia) SJSU Ultrasound Ultrasound is high frequency (Megahertz), short wavelength (0.1 mm) sound. Reflections and refractions of ultrasound by flesh and bone allow “seeing” inside the human body.
9-May-15 Physics 1 (Garcia) SJSU Forced Vibrations Vibrating guitar strings force the vibration of the guitar’s body, producing most of the sound. 553 Hz 731 Hz Circular rings indicate where the surface is vibrating up and down
9-May-15 Physics 1 (Garcia) SJSU Demo: Tuning Fork & Sound Box Tuning fork by itself is not very loud. Sound is much louder if it is held against a sound box, such as the body of a guitar or any similar rigid surface. The tuning fork forces the surface into oscillation at the same frequency.
9-May-15 Physics 1 (Garcia) SJSU Natural Frequency Metal wrench and wooden bat sound very different when dropped to the floor. Different materials and shapes vibrate at their own natural frequencies.
9-May-15 Physics 1 (Garcia) SJSU Demo: Singing Rod Stoking an aluminum rod with rosin-covered fingers induces loud vibrations at the rod’s natural frequency.
9-May-15 Physics 1 (Garcia) SJSU Resonance Resonance occurs when forced vibrations match an object’s natural frequency. Oscillations grow in amplitude due to synchronized transfer of energy into the vibrating object.
Acoustic Resonance Sound at an object’s natural frequency can produce resonant vibrations. If the amplitude of the sound is sufficiently large, resonant vibrations can shatter a wine glass. As shown by Myth Busters, this may even be achieved by exceptionally powerful singers (and by average singers using electronic amplifiers).
9-May-15 Physics 1 (Garcia) SJSU Tacoma Narrows Bridge In 1940, the first Tacoma Narrows bridge was destroyed by resonance. First Bridge Second Bridge