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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Mechanical damping of diffusers. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers Household radio equipment produced by the domestic industry (TVs, receivers, tape recorders) most often uses low-power broadband dynamic heads, such as 2GD-40, ZGD-38, etc. Along with the advantages (low price, good return, wide band of reproducible frequencies ), these heads have significant drawbacks: significant uneven frequency response of radiation at medium frequencies; the presence of extraneous overtones (for some loudspeakers) when playing a sinusoidal signal in the range of 500 ... 2 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 resonant process, they emit very intense acoustic waves. The resonant frequencies of individual sections of the diffusers are different, which leads to uneven frequency response of the head radiation and its directivity pattern. The influence of such local resonances is so great that often the reproduction by the head of a sinusoidal signal of even a relatively low power is accompanied by extraneous overtones that are noticeable to the ear. With an increase in the input power of the signal to the passport value, the likelihood of extraneous overtones increases sharply. The section of the diffuser, "guilty" of the appearance of extraneous overtones, can be detected by a sharp change in the timbre color of the sound (intensification or disappearance of the overtone) when lightly touching its surface. In the absence of audible overtones, resonating sections can be localized using a small microphone capsule (for example, DEMSh) connected to an oscilloscope. The capsule is placed above different sections of the diffuser at a distance of 1...3 mm and the shape of the signal is observed when rectangular oscillations with a frequency of 50...100 Hz are applied to the head (Fig. 1, a). When the capsule is above the resonating surface, the response observed on the oscilloscope screen is modulated by a slowly decaying sinusoidal signal (Fig. 1b). If the capsule is removed from the diffuser surface at a distance of 20...30 cm, then on the screen one can observe the total response from the entire diffuser surface, which, as a rule, has a complex shape (Fig. 1, c).
In the course of the tests carried out by the author, it was found that resonant oscillations of large amplitude most often occur on separate segments of the corrugation or on small sections of the collar. The diffuser sections resonate with a smaller 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 these sound distortions, various methods were tested to change the mechanical characteristics of the resonating sections, as well as to refine the heads according to the method given 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 the mechanical damping of the diffuser and corrugation with liquid viscous liquids (glycerin, castor oil) is also inefficient. The application of Guerlain to the diffuser collar according to the method described in [1] eliminates the resonances of the collar, but has almost no effect on the local resonances of the corrugation segments and diffuser sections. Local resonances of the corrugation and diffuser are suppressed by impregnating the "affected" areas with a solution of gerlen in gasoline. The author has worked out and proposes to readers the following technique for refining the heads, which allows obtaining 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 and solution No. 1, thick drops should come off from it. Solution No. 2 - half diluted solution No. 1. Then, using a thin brush, smear the gap between the collar and diffuser holder with solution No. 1 through the windows in the diffuser holder. When the solution dries, repeat the operation. Then, solution No. 2 should be applied to the back and outer sides of the diffuser and corrugation 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 to the corrugation and the adjacent part of the diffuser on both sides (a drop of the solution is “stretched” over the surface, as in dyeing). Head parameters can only be measured the next day after refinishing. According to the above methodology, the heads 2GD-40, ZGD-42, ZGD-38, 4GD-53, 4GD-8 were finalized. For the first three types of heads, the uneven frequency response of radiation and directivity patterns in the range of 500 ... In the last two, the refinement effect is less pronounced (due to the increased thickness and rigidity of the corrugation and the peripheral sections of the diffuser), but it is also very noticeable. The influence of the mechanical damping of the diffuser on the parameters of the heads was tested on the example of the revision of 18 heads 2GD-40 (ZGDSh-2). Before revision, six of them had extraneous overtones when a sinusoidal signal with a power of 1 W was applied to them. The overtone was heard at one or two discrete frequencies in the range of 600...1500 Hz. The sources of overtones in five cases turned out to be corrugation segments located along the major axis of the diffuser, and in one case - the collar. Another four heads had overtones when a 3 W signal was applied to them (the source was the collar). All heads had jagged frequency response of radiation and directivity diagrams, parasitic emissions were observed on their transient characteristics (Fig. 1, c). After finalizing the heads of their frequency response, the radiation became smoother. On fig. Figure 2 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 a sinusoidal signal with a power of 8 W was applied to the modified heads in the frequency range above 500 Hz, no extraneous overtones were found in any of the heads, which indicates a significant reduction in nonlinear distortions introduced by the diffuser. Due to the high repeatability of the shape of the frequency response of the radiation of the modified heads, it becomes possible to further equalize it over the entire range of reproducible frequencies by correcting the frequency response of the amplifier. Schematic diagram of the corrector with frequency response shown in fig. 2 (curve 1) is shown in Fig. 3. When any of the 18 modified heads was connected to a corrected amplifier, the unevenness of their frequency response of radiation in the range of 150 ... 12000 Hz did not exceed ±3 dB, and at a frequency of 18 kHz, some heads had a decrease of no more than -6 dB. A typical frequency response of the head radiation with a corrector is shown in fig. 4, and its typical transient response - in fig. 1, d. The frequency response of the corrector has a significant rise at higher frequencies, but this does not lead to a noticeable decrease in the dynamic range of the UMZCH, since the amplitude of the high-frequency components in the spectrum of the musical signal is small. In some cases, it is possible to confine ourselves to correcting the frequency response of the head radiation to a frequency of 14 ... 16 kHz, then the rise in the frequency response of the corrector at the highest operating frequency will be much lower. The same corrector can also be used to correct the frequency response of the ZGD-42, ZGD-38 and 4GD-53 heads (the author modified two heads of each of the listed types, the frequency response of all the modified heads fit into the scatter zone shown in Fig. 2). The refinement described above also affects other parameters of the heads: the natural resonant frequency increases by 5 ... 10%, the acoustic quality factor decreases by 20 ... 40% (while the total quality factor remains practically unchanged), due to a slight increase in the diffuser mass by 1 ...2 dB the return is reduced, the mechanical strength of the diffuser and corrugation is significantly increased. I would like to draw the attention of radio amateurs to the fact that a decrease in the sensitivity of the head by 1 ... 2 dB is equivalent to a decrease in its efficiency by 20 ... 37%. This imposes certain restrictions on the applicability of the above modification: in designs where there is no UMZCH power reserve, and also where the efficiency of the power source is important, one should limit oneself to the modification recommended in [1]. It should also be noted that the corrector improves the sound of heads modified not only according to this method and according to the method [1], but even unfinished heads. In all cases, the frequency response of the radiation of the heads is noticeably equalized, and during comparative listening, experts note a more pleasant sounding of heads with a corrector (it became "soft", "juicy", more "soft" in comparison with high-frequency heads, higher sound frequencies were reproduced). The sound of the modified heads noticeably wins when reproducing a high-power signal, so they can be recommended for use in stationary equipment, as well as in two-way loudspeakers as mid-frequency and high-frequency emitters. The author compared the sound of a loudspeaker in which two heads 2GD-40 are installed with a turn in the horizontal plane by 45 °, and a two-way loudspeaker with heads 15GD-11A and 10GD-35. All listeners noted that the sound quality of a single-way loudspeaker is not inferior to a two-way speaker, and some even preferred a single-way speaker. The method of improving the parameters of broadband heads by means of mechanical damping of the diffuser material is also very effective in relation to high-frequency heads with a paper cone (ZGD-31, 2GD-36 and 1GD-3). Moreover, the heads ZGD-31 and 2GD-36 are first recommended to be modified according to the method given in [2] (felt can be replaced with strips of Guerlain). After disassembling the head, two layers of solution No. 1 should be applied to the inner surface of the corrugation and the adjacent part of the diffuser with a width of 1.5 ... 1 cm, and after assembling it, the outer side of the diffuser should be treated similarly. For heads 1 GD-3, the solution is recommended to be applied to the outer edge of the diffuser with a width of 3 ... 4 mm. Such a refinement of the high-frequency heads smoothed out their frequency response and transient characteristics while maintaining the same sensitivity, made it possible to significantly reduce the acoustic quality factor and the nonlinear distortions introduced by the diffuser in the lower part of the frequency range they reproduce (the last two factors make it possible to reduce the requirements for the crossover filter used in conjunction with the high-frequency head). In conclusion, it should be noted that all the advantages that mechanical damping of the diffuser, corrugation, collar and centering washer of the head can only be fully realized in the factory. Indeed, due to a significant increase in the mechanical strength of the corrugation and diffuser, it becomes possible to make them thinner, which would make it possible to maintain or even increase the sensitivity of the heads, reduce their resonant frequency to 50 ... 80 Hz, and smooth out their frequency response by optimizing the impregnation of diffusers. However, these possibilities can only be realized at the design stage. References:
Author: V.Zhbanov
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