Acoustic waveguide

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An acoustic waveguide is a physical structure for guiding sound waves, i.e., a waveguide used in acoustics.

Contents

Examples

One example is a speaking tube used aboard ships for communication between decks.

Other examples include the rear passage in a transmission-line loudspeaker enclosure, the ear canal, and a stethoscope. The term also applies to guided waves in solids.

A duct for sound propagation also behaves like a transmission line (e.g. air conditioning duct, car muffler, etc.). [1] [2] The duct contains some medium, such as air, that supports sound propagation. Its length is typically around a quarter of the wavelength which is intended to be guided, but the dimensions of its cross section are smaller than this. Sound is introduced at one end of the tube by forcing the pressure to vary in the direction of propagation, which causes a pressure gradient to travel perpendicular to the cross section at the speed of sound. When the wave reaches the end of the transmission line, its behaviour depends on what is present at the end of the line. There are three generalized scenarios:

A low impedance 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.

A load that matches the characteristic impedance (defined below) will completely absorb the wave and the energy associated with it. No reflection will occur.

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.

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 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.

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 resonance and will tend to attenuate any signal fed into the line.

When this resonance effect is combined with some sort of active feedback mechanism and power input, it is possible to set up an oscillation which can be used to generate periodic acoustic signals such as musical notes (e.g. in an organ pipe).

The application of transmission line theory is however seldom used in acoustics. An equivalent four terminal model which splits the downstream and upstream waves is used. This eases the introduction of physically measurable acoustic characteristics, reflection coefficients, material constants of insulation 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.

See also

Related Research Articles

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<span class="mw-page-title-main">Waveguide</span> Structure that guides waves efficiently

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<span class="mw-page-title-main">Loudspeaker</span> Converts an electrical audio signal into a corresponding sound

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<span class="mw-page-title-main">Impedance matching</span> Adjusting input/output impedances of an electrical circuit for some purpose

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<span class="mw-page-title-main">Horn loudspeaker</span> Loudspeaker using an acoustic horn

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In radio-frequency engineering and communications engineering, waveguide is a hollow metal pipe used to carry radio waves. This type of waveguide is used as a transmission line mostly at microwave frequencies, for such purposes as connecting microwave transmitters and receivers to their antennas, in equipment such as microwave ovens, radar sets, satellite communications, and microwave radio links.

<span class="mw-page-title-main">Loudspeaker enclosure</span> Acoustical component

A loudspeaker enclosure or loudspeaker cabinet is an enclosure in which speaker drivers and associated electronic hardware, such as crossover circuits and, in some cases, power amplifiers, are mounted. Enclosures may range in design from simple, homemade DIY rectangular particleboard boxes to very complex, expensive computer-designed hi-fi cabinets that incorporate composite materials, internal baffles, horns, bass reflex ports and acoustic insulation. Loudspeaker enclosures range in size from small "bookshelf" speaker cabinets with 4-inch (10 cm) woofers and small tweeters designed for listening to music with a hi-fi system in a private home to huge, heavy subwoofer enclosures with multiple 18-inch (46 cm) or even 21-inch (53 cm) speakers in huge enclosures which are designed for use in stadium concert sound reinforcement systems for rock music concerts.

<span class="mw-page-title-main">Acoustic transmission line</span> Acoustic waveguide used to transmit sound

An acoustic transmission line is the use of a long duct, which acts as an acoustic waveguide and is used to produce or transmit sound in an undistorted manner. Technically it is the acoustic analog of the electrical transmission line, typically conceived as a rigid-walled duct or tube, that is long and thin relative to the wavelength of sound present in it.

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A quarter-wave impedance transformer, often written as λ/4 impedance transformer, is a transmission line or waveguide used in electrical engineering of length one-quarter wavelength (λ), terminated with some known impedance. It presents at its input the dual of the impedance with which it is terminated.

<span class="mw-page-title-main">Waveguide filter</span> Electronic filter that is constructed with waveguide technology

A waveguide filter is an electronic filter constructed with waveguide technology. Waveguides are hollow metal conduits inside which an electromagnetic wave may be transmitted. Filters are devices used to allow signals at some frequencies to pass, while others are rejected. Filters are a basic component of electronic engineering designs and have numerous applications. These include selection of signals and limitation of noise. Waveguide filters are most useful in the microwave band of frequencies, where they are a convenient size and have low loss. Examples of microwave filter use are found in satellite communications, telephone networks, and television broadcasting.

A transmission line loudspeaker is a loudspeaker enclosure design which uses the topology of an acoustic transmission line within the cabinet, compared to the simpler enclosures used by sealed (closed) or ported designs. Instead of reverberating in a fairly simple damped enclosure, sound from the back of the bass speaker is directed into a long damped pathway within the speaker enclosure, which allows far greater control and use of speaker energy and the resulting sound.

Transmission loss (TL) in duct acoustics describes the acoustic performances of a muffler-like system. It is frequently used in the industry areas such as muffler manufacturers and NVH department of automobile manufacturers, and in academic studies. Generally the higher transmission loss of a system it has, the better it will perform in terms of noise cancellation.

References

  1. Pierce, A.D., Ch. 7 (1981)
  2. Morse, P.M. (1948)

Bibliography