Created on 28.10.2019 11:30.
Last updated on 17.04.2020 08:45.
Author: Shorov V.
One of the reasons for the poor response of a loudspeaker in the low-frequency range is the interaction of radiation from the front and back of the diffuser. To combat this phenomenon, it is necessary to design the loudspeaker, which, providing an optimal acoustic load, separates these emissions.
From this point of view, a phase inverter is of interest, in which the radiation from the back side of the diffuser is used to increase the output at low sound frequencies. However, a conventional bass reflex, operating at frequencies of the order of 40 Hz, must have a significant volume and therefore has not become widespread.
The search for a more successful solution to this problem led the Moscow radio amateur A. G. Presnyakov to create an acoustic unit, which he called a “horseshoe” (Fig. 1).
The unit was demonstrated at the 17th All-Union Exhibition of Radio Amateurs’ Creativity. Like a horn, it serves as a waveguide for sound vibrations propagating through it and has an increased efficiency at low sound frequencies. Along with its great advantage, this unit has a significant drawback. The loudspeaker installed in it turns out to be loaded on a pipe tapering towards the middle, so that a large-volume pre-blocking chamber is formed behind the diffuser.
As a result, a number of bursts and dips appear on the frequency response of the loudspeaker sensitivity, which worsen its uniformity. Obviously, it is more expedient to make the acoustic unit not in the form of a horseshoe tapering towards the middle, but in the form of a horn rolled into a horseshoe (Fig. 2).
The forming horn, as in the A.G. Presnyakov unit, has only side walls, its upper and lower covers are parallel. A loudspeaker installed in the narrow part of the horn is then loaded onto the expanding tube.
As a result, not only unwanted resonances are eliminated, but the matching of the high radiation impedance of the loudspeaker to the low medium impedance is improved.
The author has made several such units of various sizes. Two of them are shown in Fig. 3; at the top there is a “small horn phase inverter” with a volume of 50 dm3operating with a 5GD-1 loudspeaker, and at the bottom there is a “large horn phase inverter” with a volume of 140 dm3working with loudspeaker 6GD-1.
Both units can be used with other loudspeakers. As shown by measurements carried out in the electroacoustics laboratory of NIKFI, the units have satisfactory frequency response characteristics.
One of them is the characteristic of a small phase inverter with a 5GD-1 loudspeaker with and without an acoustic impedance panel (PAS) is shown in Fig. 4.
The frequency response of the sensitivity of a large horn phase inverter with a loudspeaker 6GD-1 was given in the magazine “Radio” N 4, 1969, p. 28, Fig. 4.
The sound of the horn phase inverters has a pleasant peculiar timbre, which is explained by the high radiation efficiency at low sound frequencies. Jazz music played especially well by small ensembles. For high-quality reproduction of symphonic music, the units can be damped with PAS panels (Fig. 3). The PAS is mounted in a cover covering the large bell of the unit.
Holes with a diameter of 10-30 mm or louvres 10 mm wide and over the entire cover must be evenly distributed over its entire area. PAS, like any other damping of a mobile loudspeaker system, reduces its efficiency, so their use depends on the taste of the radio amateur and cannot be recommended as mandatory. For comparison, the table shows the values of the efficiency of the loudspeaker 4A-28, measured by the method of recording polar radiation patterns for various types of design.
As can be seen from the table, the PAS panel reduces the efficiency at low frequencies, however, when working with a horn phase inverter, it remains quite high. Almost a horn phase inverter allows using one loudspeaker to sound a hall that can accommodate 50-70 people, for example, a cafe, restaurant, club or school assembly hall.
In a small room (foyer, hall), the horn phase inverter can operate from a standard single-ended bass amplifier with a 6P14P lamp at the output.
The own loudspeakers of the device used (tape recorder, radio) must, of course, be turned off. In a living room, you can get a significant sound volume by connecting even a transistor radio receiver of the “Speedola” type to the horn phase inverter without an additional amplifier.
Despite the rather complex configuration, the manufacture of the unit does not require special skills and is available to every radio amateur. To do this, you must have two standard sheets of thick (12-15 mm) and two or three sheets of ordinary thin three-layer plywood. For a lid for a large bell, you will need an additional piece of thick plywood; a lid for a small bell can be made from the trim remaining after cutting out the upper or lower base of the phase inverter. You will also need casein glue and 5-6 rolls of elastic bandage (rubber band sold in pharmacies).
Work begins by marking the upper and lower bases. Base marking is the most critical operation. Previously, this can be practiced on a piece of paper. Then, putting a sheet of thick plywood on the table, the overall dimensions are applied from the right near corner – the diameter and depth (height) of the loudspeaker that is supposed to be used in the unit. Leaving a margin of 15 mm on each side, start marking (Fig. 2).
After a slight taper, immediately following the loudspeaker, there should be a gradual expansion of the base, ending in a characteristic bell in the near left corner of the plywood sheet. It is desirable that the shape of the sockets be symmetrical. Having marked out one base, the resulting shape is transferred to another sheet of plywood. After that, both bases are cut out and knocked together with nails. It is advisable to place the nails as shown in fig. 5, then the holes can be reused.
When hammering in bases, nails should not be driven in all the way so that they can be easily pulled out. It is better to finish the highlanders with a broom file, but so that there are no chips of the upper layers of plywood. After processing, the bases are disconnected.
|h, at frequencies,%||100 hz||160 hz||315 hz||5000 hz||h,% average|
|Large horn bass reflex without PAS||3.36||3.03||2.16||0.6||2.29|
|Large Horn Bass Reflex with PAS||1.08||2.14||1.86||0.53||1.40|
|Closed box with a volume of 120 dm3||0.66||0.73||0.94||0.68||0.75|
The side walls are made of three layers of thin plywood, …