Sound
Sound Waves Caused by vibrations Longitudinal or compressional waves Mechanical waves - require a medium Travel fastest in solids, then liquids, and slowest in gases
Characteristics Loudness --> Amplitude Pitch -->Frequency Sound Intensity -->Energy
Range of Hearing in Humans Average range is 20 – 20,000 Hz Infrasonic - frequencies below 20 Hz or below the range of hearing for humans Ultrasonic - frequencies above 20,000 Hz or above range of hearing for humans
Speed of Sound in Air The speed of sound in air changes with temperature. The speed at 0° C is 331m/s for dry air It increases by 0.6 m/s for each degree Celsius above zero.
Speed of Sound con’t at three times the speed of sound
Echoes An echo is a reflected sound wave An echo travels to a barrier and then reflects back
Echo Problems es
Beats Beats occur due to interference patterns between sound waves of slightly different frequencies. Hear alternating loud and soft sounds. Humans can only hear beat frequencies of 7 Hz or less. Δf = beat frequency, beats/sec or Hz f1= frequency of one sound, Hz f2 = frequency of another sound, Hz
Resonance A condition that exists when the frequency of a force applied to an object matches the natural frequency of vibration of the object. The object will then begin to vibrate with sympathetic vibrations. An extreme example of resonance is the collapse of the Tacoma Narrows Bridge.
The Doppler Effect A frequency shift that is the result of relative motion between the observer and the source of sound. The Doppler effect occurs with any type of wave. For sound, if the source of sound is approaching an observer, a higher pitch (or frequency) will be heard. If the sound is going away from an observer, a lower pitch (or frequency) will be heard.
Doppler Effect f ' = apparent frequency (detected freq), Hz f = frequency of source (emitted freq), Hz v = velocity of sound in air, m/s vo = velocity of observer or detector, m/s vs = velocity of source of sound, m/s Note: Use if v is not given.
Doppler - continued Use if the observer and source are getting closer together (hear higher pitch) Use if the observer and source are getting farther apart (hear lower pitch)
General Wave Formulas
Sound Formulas
Remember units! Frequency -- Hertz (cycles/sec) Period -- seconds Wavelength -- meters Wave speed -- m/s Sound Intensity -- W/m2 Relative sound intensity -- decibels (dB)