Helmholz Spreadsheet Questions

Hi, I want to build a Slot Resonator like this one I use this to calculate http://www.johnlsayers.com/phpBB2/viewtopic.php?t=1363 1. When I want to calculate the depth from wall I was told to take the "avarege depth" what means i add the maximum with the minimum depth and divide it with 2. In this exampel I don't know wich side to use... a= (95.4 + 5)/2 = 50.2 cm b= (56.9 + 5)/2 = 30.95 cm What's right? Or should I go on? ...c=(a+b)/2 = 40.575 cm 2.When I am searchin the absorption Frequency by changing Slat Depth.. Width and Slot Width i am not shure wich one i got in this exampel. depth from Wall= 600 mm Slat depth= 20 mm Slot depth= 2 mm a= Slat width 200 mm = absorbtion freaquency = 45 Hz b= Slat width 100 mm = absorbtion freaquency = 64 Hz Wich freaquency will be absorbed with this slot? djoki

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These traps, being non-parallel, are pretty wide-band; this means that they are quite forgiving, and rarely make a room sound worse than without :? I'd use 30 cm for depth, and make sure your absorbent and cloth is TOUCHING the rear side of the slats; that earlier drawing is too general. 50 to 100mm of absorbent (48 kG/m^3 is ideal) with dark cloth in front will work well. More like this - http://www.johnlsayers.com/phpBB2/download.php?id=4421 HTH... Steve

knightfly wrote:

These traps, being non-parallel, are pretty wide-band; this means that they are quite forgiving, and rarely make a room sound worse than without :? I'd use 30 cm for depth, and make sure your absorbent and cloth is TOUCHING the rear side of the slats; that earlier drawing is too general. 50 to 100mm of absorbent (48 kG/m^3 is ideal) with dark cloth in front will work well. More like this - http://www.johnlsayers.com/phpBB2/download.php?id=4421 HTH... Steve

Thanks for the answer

make sure your absorbent and cloth is TOUCHING the rear side of the slats

ok

50 to 100mm of absorbent (48 kG/m^3 is ideal)

Ok...you say 45 kg/m^3 is ideal whats with Rockwool up to 100 kg/m^3? I need to absorb from 45-100 Hz. To get in this freaquency range I need to make slots with a width of about 2 mm. I thought about to use always the same slot width and change only the slat width to change the absorbing freaquency. if i have a... avarege depth of 40 cm a slat depth of 2 cm and a constant slot depth of 2 mm than I get by changing the slat width (cm)

width	  Hz

30	   46
25	   50
20	   56
15	   64
10	   78
8	    87
6	   101

Will this work? In my first post I asked a question about calculating the avarage depth can you please give me a answer? Lets say i build a box with 2 slats with a diffrent width in front. That means the Box has only one slot. Whats the absobing freaquency? depth of Box 600 mm slot width 2 mm slat depth 20 mm Slat 1 width 200 mm Slat 2 width 100 mm Thanks and sorry for my hardheadedness.... djoki

OK, first of all as I said this type corner absorber is fairly WIDE band; this means that any small variations in slat or slot width will NOT be noticeable, so it's mostly a waste of time. The varying depth of the air space will have more effect than the changes you propose. I would be very surprised if this absorber had less than about a 2 OCTAVE peak absorption; even a constant depth slat absorber typically will have an octave range. In addition to that, unless your absorber is HUGE, it will AVERAGE it's acoustic result because the wavelength of the proposed frequencies is 'way larger than the device. Under these circumstances, only the percentage of slot vs. total surface area counts, so different width boards/slots (assuming small slots/low frequency target as you said) will not make a difference, only the AVERAGE percentage of perforation. Higher density insulation isn't optimum for acoustic work, especially for traps; some use as high as 100 kG stuff for absorbent type corner traps, but this would depend on how much high frequency you want to absorb; I wouldn't use this high density stuff anywhere you might get early reflections, because the denser material has more HF reflections due to the closer spacing of its interstices (fibers) Also, virtually all helmholtz calculators that don't ask you for a specific gas-flow resistance (ours included) ASSUME that you'll be using Owens Corning 703 inside; this has a 48 kG density (3 PCF) and was the mainstay during the times when F. Alton Everest wrote about these traps - it's only been in later years that other absorbents have been used. Another thing; using these traps across corners is done mainly for breaking up flutter, etc - unless you're willing/able to put dividers inside the trap so that each slot becomes its own SEPARATE helmholtz trap, there is yet another averaging happening; the volume behind the slats, for anything other than direct incidence, tends to widen the bandwidth too. Generally, if you use the average depth of the trap from inside surface of slats to the boundary (usually a wall) in the calculator, and don't try to over-think things too much, you can't go wrong. If you find you need really thin slots (and you will for that low frequency) you can use a caliper to measure some washers - they aren't always the same thickness, so you can find a matched pair that are close to the thickness you need and maybe adjust the slat width to make up the difference; you would use these washers as temporary spacers while you fasten each slat to the frame with screws. ONce the screws are in, you remove the washers with a pair of pliers and use them for the next slat. HTH... Steve

knightfly wrote:

OK, first of all as I said this type corner absorber is fairly WIDE band; this means that any small variations in slat or slot width will NOT be noticeable, so it's mostly a waste of time. The varying depth of the air space will have more effect than the changes you propose. I would be very surprised if this absorber had less than about a 2 OCTAVE peak absorption; even a constant depth slat absorber typically will have an octave range.

Ok If i build a Cornerslotresonator with a constant slat width, depth and a constant slot width wich is pointed at 60 Hz what will be the absorbing freaquency range? 40-80 Hz?

knightfly wrote:

In addition to that, unless your absorber is HUGE, it will AVERAGE it's acoustic result because the wavelength of the proposed frequencies is 'way larger than the device. Under these circumstances, only the percentage of slot vs. total surface area counts, so different width boards/slots (assuming small slots/low frequency target as you said) will not make a difference, only the AVERAGE percentage of perforation.

Ok I understand ...more holes (slots) more absobtion... :mrgreen: ( in this case)

knightfly wrote:

Higher density insulation isn't optimum for acoustic work, especially for traps; some use as high as 100 kG stuff for absorbent type corner traps, but this would depend on how much high frequency you want to absorb; I wouldn't use this high density stuff anywhere you might get early reflections, because the denser material has more HF reflections due to the closer spacing of its interstices (fibers)

Yes i saw this in the product data of diffrent Rockwool products. But if I am gonna build a slot resonator wich is pointed at lower freaquencys what means that i have to take a very small slot depth and i higher slat width, isnt it irrelevant if I use absobant that is weak in the highs? I thought it would be more important that the matirial is stronger in the lows.

knightfly wrote:

Also, virtually all helmholtz calculators that don't ask you for a specific gas-flow resistance (ours included) ASSUME that you'll be using Owens Corning 703 inside; this has a 48 kG density (3 PCF) and was the mainstay during the times when F. Alton Everest wrote about these traps - it's only been in later years that other absorbents have been used.

Aha ok... :wink:

knightfly wrote:

Another thing; using these traps across corners is done mainly for breaking up flutter, etc - unless you're willing/able to put dividers inside the trap so that each slot becomes its own SEPARATE helmholtz trap, there is yet another averaging happening; the volume behind the slats, for anything other than direct incidence, tends to widen the bandwidth too.

ok

knightfly wrote:

Generally, if you use the average depth of the trap from inside surface of slats to the boundary (usually a wall) in the calculator, and don't try to over-think things too much, you can't go wrong.

ok

knightfly wrote:

If you find you need really thin slots (and you will for that low frequency) you can use a caliper to measure some washers - they aren't always the same thickness, so you can find a matched pair that are close to the thickness you need and maybe adjust the slat width to make up the difference; you would use these washers as temporary spacers while you fasten each slat to the frame with screws. ONce the screws are in, you remove the washers with a pair of pliers and use them for the next slat. HTH... Steve

I am gonna use metal angles that are used for wood. They small ones have standard thikness of 2 mm here in germeny. Steve, thanks for your reply :wink: djoki

knightfly wrote:

In addition to that, unless your absorber is HUGE, it will AVERAGE it's acoustic result because the wavelength of the proposed frequencies is 'way larger than the device.

Steve, Just info: Since a Helmholtz absorber, which is what you're describing here, is a mass-spring system, the waves being huge is a condition to work as Helmholtz absorber. Being small and it isn't or doesn't work as an Helmholtz absorber anymore. This is a gradual thing and depending on the authors, the size of the Helmholtz should be within 1/12 to 1/8 of the wave length. As such an easy to rember figure is 1/10, while for design I shouldn't surpass 1/12. Best regards Eric

Eric_Desart wrote:

This is a gradual thing and depending on the authors, the size of the Helmholtz should be within 1/12 to 1/8 of the wave length. As such an easy to rember figure is 1/10, while for design I shouldn't surpass 1/12. Best regards Eric

Hi, what you mean with size? The depth? For exampel 60 Hz = 571 cm Depth of Helmholtz= 1/10 = 57,1 cm

djoki wrote:

Hi, what you mean with size? The depth? For exampel 60 Hz = 571 cm Depth of Helmholtz= 1/10 = 57,1 cm

Djoki, Steve gave you very good explanation about this stuff. I only wanted to point out for Steven that this condition of HUGE wavelengths versus resonator measures is a condition on which the principle works. It is a mass spring system, were the enclosed air volume works as a spring, and the air enclosed by the slots as a mass. What I gave was the MAXIMUM size such a cavity should have (in order for the air volume to strictly behave as a spring, indepent of wavelength behavior), NOT the size defining the tuned frequency. In fact it should be good that Steve enters a link in that post containing that adjusted Helmholz calculator, to the post it originated from. It explains a bit the principles behind it, compares it to panel traps, and gives credit to anyone involved who found, discussed and checked this original error. http://www.johnlsayers.com/phpBB2/viewt ... ight=#9891 Simplified what you're looking for as depth is the equivalent of the triangular volume translated in a standard rectangular volume behind a normal Helmholtz resonator in front of a wall. This is because that volume defines the stiffness of that air layer. As a matter of speach one could substitute this depth by a volume in those formulas (and calculate internally the equivalent depth), to prevent those "what depth to take" questions with irregular shaped cavities. (but that becomes a question too with large ceiling applications, where the average depth is not completely true anymore = related to law of inertia). Possible here are some mini corrections needed, but I wonder if anyone measured that. The principle has proved here to work, and some assumptions are made. The most defining factor is that people seem to have used them with success.