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Natural Science Forum / Physics / Acoustics / December 2003acoustic absorbing Wedges, pyramids and anechoic chambers
Does anyone know of a complete reference(or an ISO norm) that provides detailed material of the calculations of the length, the tip angle of the wedges for a desired absorption coefficient and a lower cut-off frequency. Hello Erick I don't know everything that exists, but I doubt you'll find what you're looking for. Producers protect their very costly know-how. Standards define the properties of anechoic rooms, not how the wedges should be made. It should be MUCH to easy for competitors to copy those things. Anyhow they must be measured anyhow. The Fraunhofer institute in Germany has now developed a flat version in foam, with a limp mass in-between (combined membrane/absorption system). This relates to a previous question of Angelo designing flat alternatives in order to save space in measurement rooms. But the real data how exactly those values are obtained is not published. 1 or 2 companies now work with them, bring them on the market, under some kind of license. But you will not find enough data to know how exactly they are build-up. In the groups archive, you'll notice that there are previous threads about anechoic room wedges. There are lots of shapes. Testing in impedance tubes will qualify them. Those wedges are often mounted at a small distance from the wall (ca 5 cm = 2 inches) with minor gaps in-between (additional Helmholtz and whatever effect). Regards Eric > Does anyone know of a complete reference(or an ISO norm) that provides > detailed material of the calculations of the length, the tip angle of the > wedges for a desired absorption coefficient and a lower cut-off frequency. > The Fraunhofer institute in Germany has now developed a flat version in > foam, with a limp mass in-between (combined membrane/absorption system). > This relates to a previous question of Angelo designing flat alternatives in > order to save space in measurement rooms. The nasty little secret (not-so-secret) is that the wedges account for half, perhaps less than half, of the frequency range of iportance. Out of the 80Hz -10kHz, that from perhaps 500 Hz upward is controlled by the wedges. Below that frequency, and especially below 200 Hz the compliant nature of the permiter "walls" is of highest important, whereby such surfaces act as panel absorbers, broadbanded by some very clever juxtapostions. It has long been my opinion that wedges, in the final analysis, are not necessary when an appropriately graded impedance change from air to something heavier can be achieved for "500Hz" upward. I suspect that the overall wall thickness will be equal to or less than that for wedges. So, if any of you are of the inventive bent, this will be a good area in which to practice. It is my impression that this has been known throughout the ages (well, since circa 1940), but the reasonable feasibility of wedge construction combined with the "sexy" appearance of same has made the wedges king in our times. Angelo Campanella > in the final > analysis, are not necessary when an appropriately graded impedance > change from air to something heavier can be achieved for "500Hz" upward. > I suspect that the overall wall thickness will be equal to or less than > that for wedges. That's the basic idea of the design, making the absorption MUCH thinner than the current Wedges. And indeed the low surface/entrance impedance takes cares of mid and high sound penetration. In fact it's based on this typical property of Melamine Resin Foam of BASF (sold under lots of different brand names now) with it's extreme low density ( ca 9 to 11 kg/m³ = 0.56 to 0.69 pcf). The entrance impedance of the foam is very low, but the gasflow resistance also causing some low frequent absorption limitation. So the design combines a few things, using the foam at the backside of the membrane as spring (dynamic stifness) and as absorbtion (cavity absorption). The membrane takes care of the lows - low/mids, and the foam on top for the mids and highs. And the membrane is a limp mass with very high internal loss. The idea is simple, I just think it's a lot of experimenting to make sure one reaches unit (tolerance <= 0.01) over the whole spectrum. Regards Eric > That's the basic idea of the design, making the absorption MUCH thinner than > the current Wedges. > And indeed the low surface/entrance impedance takes cares of mid and high > sound penetration. It is an idea "whose time has arrived"; a very good opportunity for increasing the working volume of all hemi- and fully- anechoic chambers, with no sacrifice in performance. Ang. C. |
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