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Since a transmission line behaves like a four terminal model, one cannot really define or measure the impedance of a transmission line component. One can however measure its input or output impedance. It depends on the cross-sectional area and length of the line, the sound frequency, as well as the
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material, the influence of air velocity on wavelength (Mach number), etc. This approach also circumvents impractical theoretical concepts, such as acoustic impedance of a tube, which is not measurable because of its inherent interaction with the sound source and the load of the acoustic component.
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characteristic impedance of the sound propagating medium within the duct. Only in the exceptional case of a closed end tube (to be compared with electrical short circuit), the input impedance could be regarded as a component impedance.
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Where a transmission line of finite length is mismatched at both ends, there is the potential for a wave to bounce back and forth many times until it is absorbed. This phenomenon is a kind of
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A high impedance load (e.g. by plugging the end of the line) will cause a reflected wave in which the direction of the pressure wave is reversed but the sign of the pressure remains the same.
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load (e.g. leaving the end open in free air) will cause a reflected wave in which the sign of the pressure variation reverses, but the direction of the pressure wave remains the same.
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to vary in the direction of propagation, which causes a pressure gradient to travel perpendicular to the cross section at the
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A load that matches the characteristic impedance (defined below) will completely absorb the wave and the
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which can be used to generate periodic acoustic signals such as musical notes (e.g. in an organ pipe).
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Acoustics: An
Introduction to its Physical Principles and Applications,
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The application of transmission line theory is however seldom used in
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When this resonance effect is combined with some sort of active
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Other examples include the rear passage in a transmission-line
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which is intended to be guided, but the dimensions of its
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and will tend to attenuate any signal fed into the line.
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16:Physical structure for guiding sound waves
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266:- a type of technology used in some
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318:McGraw Hill, 1981, NYC, NY.
311:McGraw Hill, 1948, NYC, NY.
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286:Pierce, A.D., Ch. 7 (1981)
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