Mechanical damping of diffusers of domestic speakers

It is not the first time that Radio magazine has raised the problem of improving the properties of diffusers of mass loudspeaker heads by impregnating them with vibration-absorbing mastics. Several years ago, V. Shorov’s article “Improving loudspeaker heads” was devoted to this topic. The published article acquaints readers with the work on damping of speaker diffusers by V. Zhbanov, who gives recommendations on the use of correcting devices to improve the shape of the frequency response of the heads in terms of sound pressure. With this publication, the editors once again want to draw the attention of manufacturers to this very simple and affordable way to improve the parameters of loudspeaker heads, which, unfortunately, has not found application in production.

In household radio equipment produced by the domestic industry (televisions, receivers, tape recorders), broadband dynamic heads of low power are most often used, such as 2GD-40, 3GD-38, etc.

Along with the advantages (low price, good output, wide frequency range), these heads have significant disadvantages: significant irregularity of the frequency response of the radiation at medium frequencies; the presence of extraneous overtones (for some loudspeakers) when playing a sinusoidal signal in the range of 500-2000 Hz; significant scatter of parameters between individual instances.

All of these shortcomings are caused by one reason – the formation of resonating surfaces in small areas of the diffuser, corrugation or collar. The areas of these surfaces may be small, but due to the high quality factor of the resulting resonance process, they emit very intense acoustic waves. The resonance frequencies of individual sections of the diffusers are different, which leads to uneven frequency response of the radiation from the head and its directional pattern.

The influence of such local resonances is so great that often the reproduction of a sinusoidal signal by the head, even of a relatively low power, is accompanied by audible extraneous sounds. With an increase in the input signal power to the rated value, the likelihood of extraneous overtones increases sharply. The part of the diffuser, “guilty” of the appearance of extraneous overtones, can be detected by a sharp change in the timbre color of the sound (amplification or disappearance of overtones) with a light touch to its surface.

In the absence of audible overtones, the resonating areas can be localized using a small microphone capsule (for example, DEMS) connected to an oscilloscope. The capsule is placed over different parts of the diffuser at a distance of 1–3 mm and the signal shape is observed when rectangular oscillations with a frequency of 50–100 Hz are brought to the head (Fig. 1).

When the capsule is above the resonating surface, the response observed on the oscilloscope screen appears to be modulated by a slowly decaying sinusoidal signal (Fig. 2).

If you remove the capsule from the surface of the diffuser at a distance of 20 … 30 cm, then on the screen you can observe the total response from the entire surface of the diffuser, which, as a rule, has a complex shape (Fig. 3).

In the course of the tests carried out by the author, it was found that resonant oscillations of large amplitude most often occur in individual segments of the corrugation or in small areas of the collar. The sections of the diffuser resonate with a lower amplitude, but since the area of ​​the sections themselves is very significant, their contribution to the formation of the frequency response of the head radiation is quite large.

To eliminate the indicated sound distortions, various methods of changing the mechanical characteristics of the resonating sections were tested, as well as the refinement of the speakers according to the method described in [1]… In the course of these works, it was found that increasing the rigidity of the material of the resonating sections does not give stable results, and mechanical damping of the diffuser and corrugation with liquid viscous liquids (glycerin, castor oil) is also ineffective.

Application of Guerlain on the diffuser collar according to the[1] in the technique eliminates the resonances of the collar, but almost does not affect the local resonances of the corrugation segments and sections of the diffuser. Local resonances of the corrugation and diffuser are suppressed by impregnating the “affected” areas with a Guerlain solution in gasoline.

The author has worked out and proposes to the readers the following method of reworking the heads, which makes it possible to obtain good and stable results.

First of all, it is necessary to prepare solutions of Guerlain in gasoline of two consistencies No. 1 and No. 2. After lowering the brush into solution No. 1, thick drops should come off from it. Solution No. 2 – twice diluted solution No. 1. Then, through the windows of the basket, using a thin brush, smear the gap between the collar and the basket with solution No. 1.

When the solution is dry, repeat the operation. Then on the back and outer sides of the diffuser and corrugation it is necessary to apply solution No. 2 so that the diffuser is completely saturated with it, but there are no sagging on its surface. After it dries, a thin layer of solution No. 1 2-3 cm wide is applied on both sides of the corrugation and the adjacent part of the diffuser (a drop of solution is “stretched” over the surface, as in dyeing). The head parameters can only be measured the next day after rework.

According to the above method, the heads 2GD-40, ZGD-42, ZGD-Z8, 4GD-53, 4GD-8 were modified. For the first three types of heads, the irregularity of the frequency response of radiation and directional patterns in the range of 500–6000 Hz has significantly decreased, and on their transient characteristics, parasitic emissions have practically disappeared (Fig. 4).

In the latter two, the effect of refinement is less pronounced (due to the increased thickness and rigidity of the corrugation and peripheral sections of the diffuser), but it is also quite noticeable.

The influence of the mechanical damping of the diffuser on the parameters of the heads was tested on the example of the modification of 18 heads 2GD-40 (ZGDSH-2). Before revision, extraneous overtones were heard in six of them when a sinusoidal signal with a power of 1 W was applied to them. The sound was heard at one or two discrete frequencies in the range of 600-1500 Hz.

The sources of overtones in five cases were corrugation segments located along the major axis of the diffuser, and in one – the collar.

In four more heads, side-tones appeared when a 3 W signal was applied to them (source – collar). All heads had irregular radiation frequency characteristics and directional patterns; parasitic emissions were observed on their transient characteristics (Fig. 3).

After the heads were refined, their frequency response of radiation became smoother. In fig. 5 shows the scatter zone in which the frequency response of the radiation of all 18 heads fit (the characteristics were normalized relative to the values ​​at a frequency of 250 Hz).

When applying a sinusoidal signal with a power of 8 W to the modified heads in the frequency range above …