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Chapter 12 - Sound.docx

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McGill University
PHYS 101
Kenneth Ragan

Chapter 12 – Sound 12.1 – Characteristics of Sound: Loudness: is related to the intensity (energy per unit time crossing unit area_ in the sound wave Pitch: refers to whether the sound is high (i.e., piccolo or violin) or low (i.e., bass drum or string bass)  The physical quantity that determines pitch is the frequency  The lower the frequency, the lower the pitch Audible Range: frequencies from 20 Hz to 20, 000 Hz (human ear)  Frequencies above 20, 000 Hz are called ultrasonic  Frequencies below 20 Hz are called infrasonic (i.e., thunder, volcanoes) Sound waves are longitudinal waves, also known as pressure waves. 12.2 – Intensity of Sound: Decibels: Intensity of a Wave: the energy transported by a wave per unit time across a unit area perpendicular to the energy flow 12  Human range of intensity = spanning a factor of 10 from lowest to highest Sound Level: specified on a logarithmic scale  1 bel = 10 dB  x2 loundness = x10 intensity 𝐼 𝑝 𝛽 𝑖𝑛 𝑑𝐵 = 10log OR 𝛽 𝑖𝑛 𝑑𝐵 = 20log 𝐼0⬚ 𝑝0 I0= 1.0 x 10-12W/m 2 Example: If an increase in 3dB means "twice as intense" what does an increase of 6dB mean? 3dB = 10 0.= ~2 6db = 10 0.= ~4 Intensity Related to Amplitude: intensity I of a wave is proportional to the square of the wave amplitude A Chapter 12 – Sound 12.3 – The Ear and Its Response; Loudness The function of the ear is to transform the vibrational energy of waves into electrical signals, which are carried to the brain by way of nerves. Loudness Level: the number labeling each curve represents (units are called phons) which is numerically equal to the sound level in dB at 1000 Hz Threshold of hearing: the softest sound that is just audible by a very good ear  Most sensitive to sounds of frequencies between 2000 and 4000 Hz Threshold of pain: sounds above this level can actually be felt and cause pain (> 120 phons) Example #2: If you double your distance from a source of sound that is radiating freely in all direction, how does the intensity that you hear change? By how many dB does the sound level change? I=P/4𝜋r2 1. Double the distance, therefore doubles the radius 2. Distance is ¼ the Intensity 3. Therefore, if you double the distance, the intensity will increase by ½ 12.4 – Sources of Sound: Vibrating Strings and Air Columns The source of any sound is a vibrating object. Stringed Instruments: standing waves are the basis for all stringed instruments  Pitch is normally determined by the lowest resonant frequency (t
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