Phase plug

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A phase plug of the first type is a device used with horn loudspeakers . A compression chamber is connected upstream of the driver for the highest demands on the efficiency ( PA ) and the upper limit frequency .

Basics

The compression chamber narrows the cross section from the driver membrane to the horn neck, which optimizes the acoustic impedance transformation of the horn while maintaining the same membrane area. At the upper end of the transmission range, the dimensions of the compression chamber are similar to the wavelengths in air. The geometric differences in transit time from the various areas of the membrane to the horn neck therefore lead to superimpositions that are not in-phase, the frequency response becomes wavy and the upper frequency limit drops. Specially shaped, concentric rotating bodies enable sound to be guided, which has an almost identical path from all areas of the membrane to the horn neck, with uniformly increasing compression. These bodies are called 'phase plug' assembled as one component.

differences

This differs from phase plugs of the second type, which are used in some free-radiating conical loudspeaker chassis instead of a dust cover . While the dust protection cap is firmly connected to the membrane , the phase plug sits in the same place, but is firmly connected to the pole core of the loudspeaker and therefore does not move. Proponents believe that the phase plug prevents or reduces a reflection of the sound waves emitted by the cone on the pole core. It is also claimed that, without a plug, phase shifts between the direct sound and reflected components lead to cancellations, which become noticeable as a drop in the frequency response. As a pleasant side effect, the use of a phase plug significantly increases the thermal load capacity of the loudspeaker chassis. This is why this element is often made of a material with good thermal conductivity (e.g. copper or aluminum ). As a result, the power loss is better dissipated from the voice coil and released into the room air as heat via the phase plug. However, the characteristic sound pressure is slightly reduced because the dust protection cap is no longer part of the moving membrane, so the effective membrane area is smaller.

criticism

The phase plugs of the first type fulfill their purpose, namely to achieve a maximum acoustic load on the membrane. However, non-linear effects inevitably occur:

  • The diaphragm can be deformed by the forces that occur.
  • Air behaves non-linearly at high compression.
  • The reduced cross-section can generate flow noises. Clinking and background noises are therefore no less with such horns than with direct radiators.

However, those can reach considerably greater volumes than these. In view of the acoustic performance required for loudspeaker transmissions outdoors, one must therefore accept the disadvantages.

There is criticism of the introduction of phase plugs of the second kind. Critics object that

  • the highest frequency that a piston diaphragm should emit is the one whose wavelength in the diaphragm material is roughly equal to the diaphragm circumference. At higher frequencies there are always other solutions for the equations of motion of the membrane in which partial surfaces oscillate out of phase ( partial oscillations ), which leads to very irregular frequency response ( resonances ) and strongly frequency-dependent directivity. If the wavelength is greater than the circumference of the membrane, it is certainly much greater than the structure of the pole core or the phase plug . In such conditions, the - although obvious - observation analogous to ray optics is not permissible, since it is not about reflections , but rather diffraction phenomena , which can only be determined by laboriously solving the wave equations. Thus, the streamlined shape of the plugs appeal more to aesthetics and common sense , in that they offer a benefit in terms of physics.
  • There can be no thermally close coupling between the voice coil and the phase plug / pole core (air gap), the greatest thermal resistance is the air gap itself, and the influence of the phase plug as a cooler is therefore not effective.
  • the shape of most phase plugs (small surface, shiny) does not meet the requirements of a heat sink (large surface, plate or mushroom shape, ribs, matt black).
  • However, there are high-performance bass chassis that have large winding surfaces and thus strong heating of the pole core. With a membrane surface that remains as constant as possible, these have a correspondingly shaped cooling mushroom that is only connected to the pole core via a stem of small diameter. Under these special conditions, the maximum thermal load capacity can be almost doubled, which in the best case only allows an increase in the sound pressure by a factor of 1.4 (3 dB).
  • In particular, other structural elements of the loudspeaker chassis such as the basket, magnetic structure and centering spider are still affected by the sound components radiated backwards, which leads to diffraction, resonance and reflections. The interfering sound waves generated in this way can deflect the membrane or penetrate it and thus reach the listener. The power radiated to the rear of a membrane is similar to the useful power radiated to the front.

It is therefore quite possible for manufacturers to use phase plugs of the second type without there being any clear improvements that can be proven by measurements, so that this variant would be a kind of fad.

literature

  • Berndt Stark: Loudspeaker manual. 7th edition, Richard Pflaum Verlag GmbH & Co.KG, Munich, 1999, ISBN 3-7905-0807-1
  • Siegfried Wirsum: Practical sound technology, device concepts, installation, optimization. 1st edition, Franzis Verlag GmbH, Munich, 1991, ISBN 3-7723-5862-4
  • Wolfgang-Josef Tenbusch: Basics of the loudspeakers. 1st edition, Michael E. Brieden Verlag, Oberhausen, 1989, ISBN 3-980-1851-0-9