Acoustic phase inverter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Acoustic phase inverter. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Speakers

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The implementation of high-quality reproduction of the lower frequencies of the audio range presents known difficulties, due to the shortcomings inherent in loudspeakers in this frequency range. The main ones are an increase in the harmonic distortion coefficient of the loudspeaker at lower audio frequencies, the presence of a resonant peak of the moving system, and a drop in radiation efficiency with decreasing frequency.

If you install the loudspeaker on a shield or in a large box, then the loudspeaker output at low frequencies increases significantly. However, when working on a large shield, the loudspeaker has a pronounced natural resonance of the moving system and rather large non-linear distortions.

The large value of the nonlinear distortion of the loudspeaker at low frequencies is due, as is known, to the fact that with the same power supplied to the loudspeaker, the amplitude of the oscillations of the moving system increases with decreasing frequency. Since the compliance of the corrugations is limited, this puts a limit on the increase in the amplitude of oscillations - non-linear distortions occur.

The negative role of the self-resonance of a mobile system has been repeatedly considered in the literature.

To improve the performance of the loudspeaker at lower frequencies, you can use an acoustic phase inverter.

Let us consider in general terms the principle of operation of a phase inverter. The loudspeaker is installed in a box tightly closed on all sides. The box has a hole for installing a loudspeaker and an additional hole with which the pipe section ends, as shown in the figure.

Acoustic bass reflex

The elasticity of air in the volume of the box and the mass of air in the pipe form an oscillatory system that has its own mechanical resonance at the frequency f0. This oscillatory system, when the loudspeaker is operating, loads the movable system.

In such a device, the radiation from the pipe opening coincides in phase with the radiation from the front side of the diffuser at a frequency above f0.

Typically, the frequency f0 is chosen so that it coincides with the frequency of the mechanical resonance of the moving system of the loudspeaker.

The phase inverter reduces harmonic distortion at lower frequencies and smoothes the resonant peak of the loudspeaker. In addition, the efficiency of radiation of the lower frequencies of the audio range increases.

The tuning frequency of the phase inverter depends on the volume of the box, the area of the hole (pipe) S and the length of the pipe L.

In acoustics, there is an expression that allows you to calculate the volume of a box when all other quantities are known:

In this expression, c is the speed of sound, and ω0 is the angular frequency equal to 2πf0.

For practical calculations, the formula is more convenient:

here all dimensions are expressed in centimeters, and the volume occupied by the pipe is also taken into account. The volume of the box calculated in this way should be increased by the volume occupied by the loudspeaker and the absorbing cover.

The box for the phase inverter does not have to be rectangular, it can also be made in the form of a trihedral prism for an angular structure or made in some other suitable shape. However, it should not be made too high compared to the transverse dimensions.

The area of the pipe opening is taken equal to the area of the radiating opening of the diffuser, which can be defined as the area of a circle drawn in the middle of the width of the corrugations.

Two loudspeakers can be installed in a phase inverter if their parameters differ little from each other.

The use of two loudspeakers will allow you to get a more uniform frequency response. In addition, the efficiency of the system is improved, since the power delivered to the loudspeakers is doubled, while the radiated power is almost quadrupled if the loudspeakers are placed close enough together.

When two loudspeakers are used, the area of the pipe opening is taken to be . the sum of the areas of the radiating openings of both loudspeakers.

The phase inverter in this case is calculated for a frequency equal to the square root of the product of the resonant frequencies of the loudspeakers.

The bass reflex box must be made of solid wood or plywood with a thickness of 15-20 mm, otherwise the reproduction quality will deteriorate.

In order to avoid parasitic resonances inside the box in the region of medium sound frequencies, it is necessary to cover the side and rear walls of the box, as well as the upper and lower walls with a layer of absorbing material (felt, fiberglass, etc.) 20-40 mm thick. Sponge rubber 10-15 mm thick is also suitable, which does not have a continuous film on the surface facing the inside of the box. However, the result will be somewhat worse than in the case of using felt.

In conclusion, it should be noted that equipment (receivers, amplifiers) should not be placed inside the phase inverter box, since, firstly, there will be poor cooling conditions for the equipment in a closed volume and, secondly, it will be necessary to further increase the volume of the box.

See other articles Section Speakers.

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