Acoustic transmission lines

Acoustic transmission lines

An acoustic transmission line is the acoustic analog of the electrical transmission line, typically thought of as a rigid-walled tube that is long and thin relative to the wavelength of sound present in it. Pipe organs, woodwinds, and the like can be modeled as transmission lines.

Applications to loudspeaker systems

A type of loudspeaker enclosure was proposed in October 1965 by Dr A.R. Bailey and A.H. Radford in "Wireless World" (p483-486) magazine. The article postulated that energy from the rear of a driver unit could be essentially absorbed, without damping the cone's motion or superimposing internal reflections and resonance, so Bailey and Radford reasoned that the rear wave could be channelled down a long pipe. If the acoustic energy was absorbed, it would not be available to excite resonaces. A pipe of sufficient length could be tapered, and stuffed so that the enegy loss was almost complete, minimizing output from the open end. No broad consensus on the ideal taper (expanding, uniform cross-section, or contacting) has been established.The difference between a transmission line loudspeaker and a vented loudspeaker system is clear: the rear wave is audibly absorbed and the cavity does not form a tuned system.

Transmission Line Loudspeakers have:
# minimal acoustic output from the enclosure except from the driver;
# accurate bass, without 'boom';
# excellent transient response;
# typically, extended bass below a half wave frequency of the reflected line length e.g. 30Hz=8 foot length;
# typically, smoooth impedance curves, possibly from a lack of frequency-specific resonances;
# low efficiency.

Many popular and conventional enclosure designs have a large cavity behind the driver, and a back wall that is partially acoustically reflective. This allows a portion of the energy to be reflected back to (and through) the driver after travelling 300 to 900 mm, inducing an identifiable re-radiated spike of energy 1 to 3 milliseconds later than the original, and causing narrow dips and peaks in the frequency response. Transmission lines lack a large rear cavity, so reflections, while still present, occur at short distances, typically 300 mm, shifting the interference caused by the delayed signal upweards in frequency to the range above 500 Hz.

Commercially successful folded transmission lines have been built, although some have suffered from reflections at the bends, unless properly designed.

One example of a transmission line enclosure design was by Vivan Capel called the 'Kapellmeister' published in "Electronics Today International", circa 1975. This was a double-folded line which placed the first, third, and fifth harmonics of the line's resonant frequency at the bends and the exit, where they would cause least movement. The Kapellmeister suffered from poor deep bass, and was designed around a low power driver. Capel, like Bailey, believed that the pipe's cross-sectional area needed to be equal the driver's cone area.

In 2003, a new design, SUHTL, using a low resonance enclosure material (not wood), and a single drive unit, has provided improved bass, which, although it requires some equalisation to provide 'full-range' response, improves on some the modest output level ability of previous designs.

A duct for sound propagation also behaves like a transmission line (e.g. air conditioning duct, car muffler, ...). The duct contains some medium, such as air, that supports sound propagation.Its length is normally of a similar order to the wavelengths of the sound it will be used with, but the dimensions of its cross-section are normally smaller than one quarter of a wavelength.Sound is introduced at one end of the tube by forcing the pressure across the whole cross-section to vary with time. A plane wave will travel down the line at the speed of sound. When the wave reaches the end of the transmission line, behaviour depends on what is present at the end of the line. There are three possible 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 air displacement 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 air displacement 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 portion of the energy to be reflected until it is absorbed by frictional losses. This phenomenon is a kind of resonance and will tend to exaggerate the duration and level of a pulse or tone. This phenomena is exploited most effectively in pipe organs, where a specific column of air, with a specific sized exit port is tuned to resonate at a specific frequency, as a Helmholtz resonator.

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 unpractical 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.

"Transmission line" is also the name of a specialized audio speaker enclosure topology, in which sound from the back of the bass speaker chassis passes along a long (generally convoluted) path within speaker enclosure and emerges from the open end of the path "in-phase" with the sound radiated from the front of the driver, enhancing the output level at low frequencies.

See also

* Loudspeaker acoustics
* Voigt pipe
* Loudspeaker measurement

External links

* [http://www.quarter-wave.com/ Quarterwave loudspeakers] -- Martin J King, developer of landmark TL modeling software
* [http://www.suhtl.com/ 'S'ingle Drive-unit 'U'ltra-low-resonance 'H'alf-wave 'T'ransmission 'L'ine Speaker Page & forum]
* [http://www.t-linespeakers.org/ Transmission Line Speakers Pages] -- TL projects, history & more
* [http://www.geocities.com/rbrines1/Articles.html Brines Acoustics Articles] -- Application, tips, essays
* [http://www.diracdelta.co.uk/science/source/q/u/quarter%20wave%20tube/source.html Quarter Wave Tube - DiracDelta.co.uk] - description of operation, equation and online calculation.


Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • Transmission line — This article is about the radio frequency transmission line. For the power transmission line, see electric power transmission. In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating… …   Wikipedia

  • Remote data transmission — (RDT) was a term used in the 1980 s, primarily in Germany, for the transmission of data between computers over a medium using a communications protocol. At the time, the most widespread form was RDT over the telephone network. Other transmission… …   Wikipedia

  • Voigt pipe — The Voigt pipe is a type of loudspeaker enclosure that embodies a combination of transmission line, ported enclosure and horn characteristics. It is highly regarded by some speaker designers, as evidenced by established manufacturers such as… …   Wikipedia

  • Impedance matching — In electronics, impedance matching is the practice of designing the input impedance of an electrical load (or the output impedance of its corresponding signal source) to maximize the power transfer and/or minimize reflections from the load.… …   Wikipedia

  • Metamaterial antenna — This Z antenna tested at the National Institute of Standards and Technology is smaller than a standard antenna with comparable properties. Its high efficiency is derived from the Z element inside the square that acts as a metamaterial, greatly… …   Wikipedia

  • Loudspeaker enclosure — A loudspeaker enclosure is a cabinet designed to transmit sound to the listener via mounted loudspeaker drive units. The major role of the enclosure is to prevent the out of phase sound waves from the rear of the speaker from combining with the… …   Wikipedia

  • Metamaterial — Negative index metamaterial array configuration, which was constructed of copper split ring resonators and wires mounted on interlocking sheets of fiberglass circuit board. The total array consists of 3 by 20×20 unit cells with overall dimensions …   Wikipedia

  • Waveguide — This page is about waveguides in the most general sense. For ordinary metal pipe waveguides, Waveguide (electromagnetism). For optical waveguides, see Waveguide (optics). A section of flexible waveguide with a pressurizable flange …   Wikipedia

  • Loudspeaker — For other uses, see Loudspeaker (disambiguation). An inexpensive, low fidelity 3½ inch speaker, typically found in small radios …   Wikipedia

  • Skin effect — Skin depth redirects here. For the depth (layers) of biological/organic skin, see skin. Skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor with the current density being largest near the… …   Wikipedia

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”