Sound is defined as vibrations that travel through the air or another medium as an audible mechanical wave. It is produced from a vibrating body. The vibrating body causes the medium (water, air, etc.) around it to vibrate thus producing sound.
1. What Is Sound? Sound is a pressure wave which is created by a vibrating object. This vibrations set particles in the sur- rounding medium (typical air) in vibrational motion, thus transporting energy through the medium. Since the particles are moving in parallel direction to the wave movement, the sound wave is referred to as a longitudinal wave. The speed of a sound pressure wave in air is The result of longitudinal waves is the 331.5+0.6Tc m/s , Tc temperature in Celsius creation of compressions and rarefactions within the air. The particles do not move down the way with the wave but osciallate back and forth The alternating configuration of C and about their individual equilibrium position. R of particles is described by the graph of a sine wave (C~crests, R~troughs)
2. Wavelength, Amplitude, Frequency of a Wave The amount of work done to generate the energy that sets the particles in motion is reflected in the degree of displacement which is measured as the amplitude of a sound. The frequency f of a wave is measured as the number of complete back-and-forth vibrations of a particle of the medium per unit of time. 1 Hertz = 1 vibration/second f = 1/Time Depending on the medium, sound travels at some speed c which defines the wavelength l: l = c/f
3. Measuring the Intensity of Sound • The softest audible sound modulates the air pressure by around 10-6 Pascal (Pa). The loudest (pain inflicting) audible sound does it by 102 Pa. • Because of this wide range it is convenient to measure sound amplitude on a logarithmic scale in Decibel [dB]. • Decibel is not a physical unit - it expresses only a ratio for comparing the intensity of two sounds: 10 log10 (I/Io) where I and Io are two intensity/power levels (I~P2 , P is sound pressure) • One can say e.g. a channel is amplifying the sound by 3 dB, meaning the output is 3 dB louder than the input. • In order to make it interpretable as a real unit, a fixed pressure P0 = 2*10-5 Pa is defined (the reference of 0db corresponds to the threshold of hearing) and the absolute sound pressure P in Decibel is defined as: 20 log10 (P/P0) • Thus +20 dB means an increase in pressure by a factor of 10
4. Examples for Sound Levels in Decibel Threshold of hearing 0 dB softest audible 1000 Hz sound 6 dB quiet living room 20 dB soft whispering 25 dB refrigerator 40 dB soft talking 50 dB normal conversation 60 dB busy city street noise 70 dB passing motorcycle 90 dB somebody shouting 100 dB pneumatic drill 100 dB helicopter 110 dB loud rock concert 110 dB air raid siren 130 dB pain threshold 120 dB gunshot 140 dB rocket launch 180 dB Instant perforation of eardrum 160 dB 1) TOH: One-billionth of a centimeter of molecular motion 2) The most intense sound (without physical damage) is one trillion times more intense
5. Humans vs Machines on similar tasks (2001) Tasks Vocabulary Humans Machines Connected digits 10 0.009% 0.72% Alphabet letters 26 1% 5% Spontaneous telephone 2000 3.8% 36.7% task WSJ with clean speech 5000 0.9% 4.5% WSJ with noisy speech 5000 1.1% 8.6% (10db SNR) Clean speech based on 20000 7.6% 4.4% trigram sentences • Humans are at least 5 times better than machines, and far more robust • In the last experiment humans and machines have the same syntactic and semantic model > the difference disappears (Experiments by Microsoft, 2001)
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