Created on 05/11/2015 11:55 AM.
Author: Pavel Sayk.
Usually in the amateur and professional literature, acoustic systems are considered in terms of obtaining the widest possible bandwidth of reproducible frequencies and minimum distortion, that is, from the point of view of improving sound. These aspects are also important to us, but we also need the maximum output of the acoustic system, that is, the maximum efficiency of converting electrical power into acoustic power.
A dynamic head without acoustic design has a very low response and very poorly reproduces the lower frequencies of the audio spectrum. The explanation is simple: when the diffuser moves forward, the air is simply distilled from the front of the diffuser to the back and vice versa, a so-called acoustic short circuit occurs, and only a small fraction of the energy is converted into a sound wave.
The easiest way to eliminate acoustic short circuits is to mount the head on a flat screen or “reflective board” of a fairly large size. Which ones? Its size should be at least a quarter of the wavelength at the lowest sound frequency. Knowing the speed of sound in air (v = 330 m / s), the wavelength is easy to calculate: λ = v / f. Even for the lowest frequency of 100 Hz, the size of the acoustic screen will be about a meter. Now it becomes clear why small transistor receivers cannot reproduce bass frequencies!
The ideal acoustic baffle is a partition between two rooms or between a room and a veranda. If you insert a dynamic head there, then both rooms will be sounded, and with good quality. True, the acoustic power will be divided in half between the rooms and the sound will be quieter. More often it is necessary to sound only one room, then it is advisable to install the screen under the ceiling, as shown in Fig. 3.1. There it will not interfere with anyone, and a significant volume can be created around the head, which will improve the reproduction of low frequencies, in addition, the ceiling and walls form a kind of horn.
The screen can be made of plywood, chipboard and even hardboard. It should fit tightly to the ceiling and walls, without gaps (you can seal the joints with foam rubber or cotton wool), this will significantly reduce all sizes, except for one – from the head to the hole.
It forms a bass reflex – a system that allows you to improve the response at low frequencies, using radiation from the back of the diffuser.
Having passed the path l, the sound wave will change its phase, and under the condition l = λ / 2, the phase will change to the opposite (inverted) and add up with the wave emitted by the front side of the diffuser. It is advisable to choose the area of the opening not less than the area of the diffuser of the head.
If the edges of a flat rectangular screen are bent back, you get the well-known body of a radio receiver, TV, etc. This is a speaker with an open back. It also sounds better in the corner of the room, and it is advisable to choose the distance from the walls for the best volume and sound quality.
It is very easy to make an open speaker from the body of an old TV – you only need to make a reflective board from chipboard, install from 2 to 8 heads on it, cover it with rare cloth and replace the front panel. And such a product looks and sounds very good (Fig. 3.2a). It is better to place the heads on the reflective board asymmetrically, then the peaks and dips of the amplitude-frequency characteristic (AFC) are somewhat leveled.
It makes sense to install several heads in the speaker, even different ones, for several reasons: the sound pressures of individual heads add up, so the output of the speaker increases, but the peaks and dips in the frequency response of individual heads do not coincide, just as the mechanical resonance frequencies do not coincide, and the overall frequency response is leveled … The circuit for switching on the heads must be selected so that the power released on them is proportional to the nameplate.
The polarity of switching on the heads is also very important: when a constant voltage is applied to the AC (for example, from a galvanic cell), all diffusers must move in one direction, which corresponds to in-phase switching. At least one head, operating in antiphase, sharply reduces the recoil.
As an example, Fig. 3.2b shows a circuit for switching on seven speakers: two 4GD-35 (4 W, 4 Ohm), three 1GD-40 (1 W, 8 Ohm) and two high-frequency (“beepers”) 2GD-36 (2 W, 8 Ohm). To calculate the power released at each head, mentally apply some voltage convenient for calculations, for example 8 V.
Then the current in the circuit of powerful heads will be 1 A, in the circuit of three one-watt 1/3 A and in the high-frequency circuit (only at high frequencies) – 1/2 A. The power released on each head (P = I2R) will be 4 W for high-power ones, 0.9 W for single-watt ones and 2 W for high-frequency ones, which is quite acceptable. The capacitance of the filter capacitor, which passes only high sound frequencies to the “tweeters”, is found by the formula C = 0.16 / f • R, where f is the cutoff frequency, R is the total resistance of the high-frequency heads. Having set the parameters of 6.5 kHz and 16 ohms, we get C = 1.5 μF. The total speaker impedance is 6 ohms, slightly decreasing at high frequencies.
If it is possible to make speakers of different designs, then it is better to place several speakers vertically, one above the other, in order to concentrate the radiation in the horizontal plane, at the level of the listeners’ heads (Fig. 3.3a). It is better to place tweeters at this height, in the middle, and powerful low-frequency ones – at the edges of the column, since the radiation directivity at the lower frequencies is less.
Placing the column in the corner of the room increases the recoil (horn effect) and allows only one baffle board to be produced.
In the corners of the ceiling and floor, you can try to install triangular sheets of plywood or plastic – “acoustic mirrors”, reflecting the radiation from the back of the speaker to the listeners (Fig. 3.3b).