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Stand alone DIY slat resonator, first attempt
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Author:  msikio [ Wed Jul 19, 2006 10:34 pm ]
Post subject:  Stand alone DIY slat resonator, first attempt

Here a few pictures of a stand alone DIY "panel" slat resonator that I made the other day.

According to the revised formula it should be tuned around 406 Hz.

The internal cavity is damped with 60mm (2 1/3 inches) high density rock wool (density equiv. to O/C 705)

It's only 120 x 60 cm x 10cm (4 x 2 x 1/3 ft), made of light construction wood, fairly lightweight and easily portable. There's a string at the back and it can be affixed to the wall as a big picture. There is no finish yet, I might use oil, varnish or paint.

It bear the acoustic panel standard size, but I'm wandering if it's worthwhile to built such a small slat absorber. I suppose I would need a good few ones in a 20 x 13 ft room to make any significant improvement to the sound ?

Any thoughts ?

Author:  Ro [ Wed Jul 19, 2006 10:55 pm ]
Post subject: 

well, for starters, is LOOKS superb!

I've been wondering, indeed, does size matter. and even more, does the choice of WOOD contribute to the tuning formula? (it's not mentioned in the XL sheets)

Author:  msikio [ Thu Jul 20, 2006 12:29 am ]
Post subject:  Thanks :)

Thanks Ro,

I'm quite happy with the look too ;)

I don't think the type of wood makes a significant input, as the slates are not supposed to vibrate, but are only there to provide a slot for the Helmholtz resonator to work. In this case the slates are 50 mm x 10 mm, the slot width is 3 mm and the (sealed) cavity depth is 86 mm.

Michel

Author:  knightfly [ Thu Jul 20, 2006 3:06 am ]
Post subject: 

Very nice looking indeed; and, as in other things :wink: size DOES matter; at that frequency, the wavelength is around 1 meter so the device should be larger than that. Making more of them, and positioning as one larger array would help.

Also, as a (VERY!!) rough guess, figure 70-75% absorption at resonance, with about 1-1/2 to 2 octaves bandwidth centered at resonance, and dropping to around 40-50 % absorption at the upper and lower part of the absorption curve; so at resonance, you'd have your square footage of surface times 70% for a rough guess at the number of sabins of absorption.

Your absorber, minus whatever amount (I don't know the answer to this) caused by small size relative to wavelength, would produce about .7 x 2 x 4 sabins @ resonance, or around 5.6 sabins.

Positioning is also important; these are RESONANT traps, they work on sound PRESSURE instead of VELOCITY so will need to be placed at a high SPL point for the frequency they are tuned in order to do much of anything (besides LOOK nice :wink: )

HTH... Steve

Author:  msikio [ Thu Jul 20, 2006 8:39 am ]
Post subject:  thanks

Thanks Steve,

I like the visual aspect too :)

Technically, that's more or less what I was guessing.

As the rockwool is flush to the facing of the panel, I'm also wandering if a panel like this would act also as a low-mid bass trap. There is 60 mm (2 1/3") of high density rockwool with a 26 mm (1") air gap behind. As low frequency diffract around even small objects, I'm wandering if the 3mm (0.12") slot is big enough to let the mid-bass frequencies going through to be tamed by the rockwool.

As I'm more familiar with absorption coef than Sabins, could you point me to a web site that would help me to find the number of Sabin needed for a room like mine to achieve a RT between 0.3-0.5s ?

My room is not rectangular but is roughly 18.30 x 13 x 8.80 ft. The room is intended for both tracking and mixing. At the moment I'm planning to use 3" 705 bass traps in the corners, a big dual membrane absorber on the side wall, a broadband absorber for the rear wall, a few 705 panels on the walls and half of the ceiling covered with 80 mm (3") 705 and 25mm (1") 705.

Thanks again,

Michel

Author:  Ro [ Thu Jul 20, 2006 7:35 pm ]
Post subject: 

Michel, could you upload more "detailed" photos of the inside/back of the thing, might be interesting to actually see what you mean :)
(and to show off ofcourse, hahah)

Author:  msikio [ Thu Jul 20, 2006 9:46 pm ]
Post subject: 

Here a picture of the back. I don't have pictures of the inside, and it's now sealed. Sorry :?

Author:  knightfly [ Fri Jul 21, 2006 1:12 am ]
Post subject: 

Sorry, buried in projects these days so not much time for freebies -

1 sabin at any given frequency is = to a 1 foot square "black hole" - IOW, an actual 12" x 12" hole in your wall leading to the great outdoors would have an absorption coefficient of 1.0 at ALL frequencies, supposedly. In actual use, I don't think lower frequencies would see it that way because of their long (up to 35 feet) wavelengths.

If you get enough of your 2x4 foot panels together (like, at least three) to work at your specific frequency, you'd have 8 x 3 sq. ft. or 24 sq. ft. of surface with an absorption coefficient of approx. 0.7 at that frequency, rolling off to about 0.4 by one octave either side of center frequency.

I don't think you'd see much low bass absorption with this, it's not tuned to that and can't be tuned that low (John's tried on this, with little or no success) so for really low freq's you'd need to use panel absorbers tuned and placed correctly.

A better approach would be if you were to post a sketch of your room so I can see what you mean by "not rectangular", and maybe a pic or two. For one thing, 400 hZ range isn't looking like it should be a problem in your room - by the time the modes get that high in frequency, they're all tangential and oblique and WEAK. I ran a Bonello chart on those dimensions, and it shows pretty even distribution (Thank,s Jeff Szymanski)

One thing - 705 won't work as well for even absorption as 703 does, except at 90 degree incidence; this may be a good thing or bad, but it's something to keep in mind.

Also, what is a "big dual membrane absorber"??!?

Another - using a room for both mixing and tracking can work, but I'd be thinking about some removable absorbers around the mix position while mixing, to get the RT shorter.

Gotta run for now... Steve

Author:  msikio [ Fri Jul 21, 2006 4:53 am ]
Post subject: 

Thanks Steve for the great amount of valuable info.

Quote:
I don't think you'd see much low bass absorption with this, it's not tuned to that and can't be tuned that low (John's tried on this, with little or no success) so for really low freq's you'd need to use panel absorbers tuned and placed correctly.


I appreciate this, and I know you have much more experience than me.... but, how comes we are all building massive walls and/or carefully sealed mass/spring/mass systems if the bass frequencies can be blocked with a few 0.39" thick wooden slats spaced apart ? I must have missed something along the line :?

I targeted the 400 Hz range with the tracking use in mind. To "work on sound PRESSURE" I suppose the musicians should play in front of a this type of panel ?
Quote:
A better approach would be if you were to post a sketch of your room so I can see what you mean by "not rectangular", and maybe a pic or two. For one thing, 400 hZ range isn't looking like it should be a problem in your room - by the time the modes get that high in frequency, they're all tangential and oblique and WEAK. I ran a Bonello chart on those dimensions, and it shows pretty even distribution (Thank,s Jeff Szymanski)


We've been discussion my room in this thread. I think I should treat the ceiling and the rear wall and do another set of measurements before we can discuss any further. That's because there is probably as much comb filtering as mode ringing.
Quote:
One thing - 705 won't work as well for even absorption as 703 does, except at 90 degree incidence; this may be a good thing or bad, but it's something to keep in mind.

The stuff I'm using [url=http://www.rockfon.fr/sites/fr/download/downloadfile.asp?file=/sites/fr/uploads/ProductCeiling\5776_datasheetpdf.pdf&FILETO=Rockindus]is Rockwool Rockindus 70kg/m3[/url]. I'm using two 30mm layers of it.
From the reading on books and specs I understood the denser stuff might work better for the low frequencies but not as well for hi frequencies. What you are saying is the denser stuff (70kg/M3) doesn't work as well on axis ?

Quote:
Also, what is a "big dual membrane absorber"??!?
More info on this in a thread to come :)

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