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362 Posts

Posted - 2020/08/05 :  14:04:01  Show Profile Send adhoc a Private Message  Reply
Jag saxar in en del text från boken Acoustic Absorbers and Diffusers av Trevor Cox och RPGs Peter D'Antonio. (Fetningen är min)

"Figure 12.14 shows the random incidence absorption coefficient for a planar binary surface with and without the perforated mask. This shows that the hybrid surface is behaving like the Helmholtz absorbers discussed in Chapter 7. It is possible to predict the absorption characteristics using the transfer function matrix method. Problems arise, however, because the hole spacing is not regular and many holes are too close together for the normal assumptions used when modelling sound propagation through a perforated sheet. Nevertheless, it seems possible to at least predict the trends of the absorption." -Man får alltså en hyfsad indikation av absorbtionen hos
BAD-paneler med samma öppna andelsarea och skivtjocklek som hos en "traditionell helmholtzabsorbent" där alla hålen är på samma avstånd från varandra.

The amount of added mass in the holes determines the increase in absorption at bass frequencies. For the BAD panel, if additional bass absorption is required, it is possible to reduce the open area (this will not be true for all geometries), as shown in Figure 12.14. Alternatively, thicker mineral wool layers can be used, or the panel can be spaced from the wall to effectively increase the backing depth and so lower the resonant frequency. The effect of changing the backing depth is shown in Figure 7.9, giving the expected trends. The drop-off in high-frequency absorption can be explained by the open area of the panel. In a simplistic analysis, a surface with a 50% open area would be expected to have an absorption coefficient of 0.5 at high frequency. Figure 12.14 shows the effect of reducing the open area on the absorption coefficient. Boundary element modelling (BEM) predictions show that the absorption coefficient response is similar for ternary diffusers, if a little less smooth with respect to frequency. It is assumed that this is due to reflections from the wells providing out-of-phase reflections when compared to other parts of the surface, and therefore, waves combine to put energy into the reactive field."

"Usually, the absorption coefficient of the soft patches is not 1 at low frequency because there is insufficient depth of mineral wool to cause complete absorption. At mid and low frequencies, mutual interactions across the panel render the Kirchhoff boundary conditions inaccurate."

För minskad loobverkan när flera skivor med samma borrade hålmönster används bredvid varandra: Vrid den ena 90 grader: While not as good as the modulation using two base shapes (graph b), one base shape modulation still gives a better result than the periodic arrangement." samt: "The only way to deal with this (=loobning) is to introduce some possibility of phase cancellation, for example, by curving the surface or using a ternary sequence."

Moving from a flat hard surface to a plane hybrid device increases the dispersion, as was also found in the measurements reported in Figure 12.17. However, curving the hybrid surface produces more dramatic improvements. The quality of dispersion for the curved
hybrid surface is only a little worse than that of the rigid optimized curved surface, which is four times as deep.
This seems to indicate that a hybrid curved surface is a good methodfor generating more diffuse reflections from a restricted depth, provided partial absorption is acceptable or wanted. It should be remembered that practical hybrid surfaces are often mainly absorptive up to about 2 kHz, and consequently for these surfaces, it is only at mid to high frequencies that dispersion needs to be considered.

Den sista fetningen kan vara viktig. -Om man redan har en hel del absorbtion i rummet från säg, 500 Hz och uppåt, kan det vara olämpligt att dämpa mellanregistret ytterligare. med stora ytor BAD-paneler.
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Trädgårdsmästarn, 100.000-klubben

7939 Posts

Posted - 2020/08/05 :  16:53:31  Show Profile Send Björn-Ola a Private Message  Reply
Jag kan inte riktigt säga att jag förstår, man pratar om diffusion med en kurvig yta men här har vi väl en plan yta och ingen diffusion? Jag vet inget om denna typ av panel men tyckte att det såg ut som en kombination av helmholtz resonator och ljudabsorberande mineralullsskiva för att få en bättre dämpning i de lägre frekvenserna.
Själv har jag bara några tubetraps hemma som en kompis byggde en gång i tiden. Jag har gipsväggar så jag vet inte om de gör så mycket nytta egentligen. Har man betongväggar så finns det kanske anledning till att fundera på basdämpning.

"Innan man är riktigt säker är man ofta tvärsäker"
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531 Posts

Posted - 2020/08/05 :  22:23:44  Show Profile Send nisseW a Private Message  Reply
Tack adhoc.
Intresant läsning.

Blufföretag är ingen bluff, de finns i verkligheten!
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