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LIN228H1F 2012 Week 7 Kochetov-1 Introduction to Acoustics The way we have described speech sounds so far was in terms of how they are produced by manipulating the vocal apparatus and how they can be transcribed with phonetic symbols on paper. However, we can also describe sounds in terms of how we can hear them. How we can hear a sound depends on its acoustic structure. Acoustics: the science of the physical properties of sounds. Acoustic Phonetics: the science of the physical properties of speech sounds. We want to be able to describe the acoustics of speech for many reasons: explanation for why certain sounds are confused with other sounds certain details of speech are not explainable if only articulation is considered vowels are better described in acoustic terms than in articulatory terms vital information for designing automatic speech recognition devices (speech synthesis by computers) insight into sound recognition by humans (perception) audio data of speech is the easiest to obtain. Propagation of sound A line of people waiting to buy tickets to a concert is useful analogy for a sound wave (based on Johnson, K., 2003, Acoustic and Auditory Phonetics). The movement of the person at the front of the line creates a gap (rarefaction); this gap travels through the line. Then the first person is shoved back into the second person, creating more crowdedness (compression); the crowdedness also travels through the line. Time 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1 1 1 1 1 1 1 1 2 2 2 2 2 2 X 1 1 1 1 1 1 * 2 2 3 3 3 3 3 3 X 2 2 2 2 2 3 3 3 4 4 4 4 4 4 X 3 3 3 3 4 4 4 4 5 5 5 5 5 5 X 4 4 4 5 5 5 5 5 6 6 6 6 6 6 X 5 5 6 6 6 6 6 6 7 7 7 7 7 7 X 6 7 7 7 7 7 7 7 7 Figure 1.Wave motion in a line of seven people (1-7), from time 1 to time 15 in arbitrary units Figure 2 shows a pressure waveform at the location indicated by the asterisk in Figure 1. The wave fluctuates with time, having peaks of rarefaction and compression. Sound waves are similar to the wave in Figure 2, to the extent that they also show amplitude fluctuations as they travel past a particular point in space. LIN228H1F 2012 Week 7 Kochetov-2 A Pressure Waveform 1.5 1 0.5 0 -0.5 Crowdedness (arbitrary units) -1 -1.5 0 5 10 15 Time (arbitrary units) Figure 2. A pressure waveform of the wave shown inFigure 1. Time is on the horizontal axis and the distance between people is on the vertical axis. Sound waves A sound wave is a traveling pressure fluctuation that propagates through a medium. Pressure fluctuations impinging on the eardrum produce the sensation of sound. Pressure fluctuations are often cyclic, repetitive. The waveform of the pressure variation has the same shape as that of the movement of an air molecule. It is represented with the time dimension on the horizontal axis and the amplitude dimension on the vertical axis. Pure tones, when represented on a waveform, will appear as sine waves simple periodic waves. Sine waves are smooth and symmetrical s- shaped waves. Figure 3 gives some examples of sine waves that differ in amplitude and/or frequency. Amplitude and intensity are physical properties of sound waves that correlate with the psychological property of loudness. The greater the amplitude of a sound wave, the louder the sound. The greater the intensity of a sound wave, the louder the sound. amplitude: degree of variation in air pressure from neutral to higher and lower (measured in hPa [hecto-Pascal] or mb [millibar]) intensity: power transmitted by the wave (measured in dB [decibels]) Frequency is a physical property of sound waves that correlates with the psychological property of pitch. The greater the frequency of a sound wave, the higher the pitch of the sound. Frequency is measured in terms of number of cycles per second (measured in Hertz (Hz)). number of cycles Frequency in Hz = time in secondsLIN228H1F 2012 Week 7 Kochetov-3 For example: If a cycle of a wave (= period duration) is completed in 6.25 milliseconds, the frequency of the wave is 1/0.00625s = 160 Hz. If a wave has a frequency of 250 Hz, then each cycle lasts 1/250 seconds, or 4 ms. Simple Periodic Wave, 100Hz 1 0.5 0 Amplitude -0.5 -1 0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 Time (sec) Simple Periodic Wave, 1000Hz 1 0.5 0 Am-0.5ude -1 0 0.002 0.004 0.006 0.008 0.01 0
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