John Sayers' Recording Studio Design Forum

Florian, other than just verifying the difference in results, I've not had time for much of anything; this is likely to be the case until winter sets in and I can't work outside anymore. I'm so far behind on home repairs and similar projects that I must take advantage of every minute of daylight/dry weather, or I'll be setting buckets under the buckets when the rains come.

I bought Cox/D'Antonio's absorber book several months ago, and have barely had time to thumb through it - when the weather gets nasty, I'm going to budget some time to go thru that and this thread, and probably write 1 or 2 new spreadsheets for better calculation of both helmholtz and panel absorbers (it wouldn't hurt my feelings a bit if someone were to beat me to it though - hint hint ) Steve

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Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...

Still pretty much buried in work, but would like to interject this -

Will, you stated that the cavity behind the slats was NOT filled with any mineral wool - only the cloth at the slats? According to Everest, on P. 221 of the Master Handbook of Acoustics, adding absorbent inside a Helmholtz resonator would change the results quite a bit - to quote - "More damping e.g. with mineral wool would primarily have an influence on the bandwidth and the absorption coefficient." -

All this does for me, til I find more conclusive information, is to further muddy the waters.

I don't contest the fact that your spreadsheet accurately predicts the results of a NON-filled trap, but will need more info before telling people who build half-filled traps to use your formula, especially since on the same page of MHOA there is a chart of actual absorption of a half-filled trap (which I've yet to compare, maybe in a few days)

Just some thoughts and a gentle "bump"... Steve

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Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...

Hi Steve,

the resonance frequency will become slightly lower and the bandwidth will become broader by adding soft mineral wool (e.g. 5 kPa*s/m²) into the air cavity.
The resonance frequency will become only higher if you increase the flow resistance of the mineral wool (e.g. wool with higher density).

Best wishes
Will

Thanks Will - I'm not familiar with acoustic resistance vs. density of commonly used insulation materials - I started out here just moderating the construction section and kind of got "volunteered" for the rest when another mod had no more time to donate. I doubt I'll ever have enough time to become versed enough in this aspect to be able to design custom treatments, so I tend to offer up what's worked for others (mainly John) and continue to try to find time to read as much as I find on the subject...

Do you have any sources of info on the relationship of resistance vs. density vs. material type vs. thickness?

For example, a commonly used material for absorbers would be either mineral wool or rigid fiberglass in 3 PCF (48 kG/m^3) - if this were used inside a Helmholtz trap, what would the effect be on Fo for (say) half fill and complete fill as compared to an empty trap?

I appreciate any help you may offer in this area, and I'm sure our other members will also... Steve

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Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...

Hi Steve,

steel wool could have a higher density but a lower flow resistance than mineral fibre wool.
It’s not advisable to connect density with flow resistance in an absolute way. I just stated an example!
Many other parameters which define the flow resistance have to be considered as well, e.g. the type of capillaries, the open area, the type of tortuosity…etc.

I wrote an excel sheet in order to your other questions.

(Note: Analysis ToolPak needs to be installed and activated: Menu  Tools  Add-Ins  Analysis ToolPak)

Best wishes
Will

Calculated elements of slitresonator depicted:

Interface:

Nice drawing! Thanks.

Andre

You're welcome!

Here is the Formula for the wall impedance of the slotted plate (Zsp). Most of it is used in the second Excel sheet.
The wall impedance is the difference of the sound pressure on the plate divided by the velocity of the plate. This formula is a parallel connection of the slotted plate theory and the plate theory. The plate theory considers the mass, the stiffness and the loss factor of the plate. The real part of the slotted plate is a series combination of the viscous losses in the neck by considering the viscosity of air and the resistance R of the fabric or fleece material at the neck.

so.... what's the verdict
(okay, it's been a year, but still)




** jup, I'm reading through the whole darn forum here.

The verdict should preferably have been written by someone other than me.
From my point of view it would be best to disregard the recommended calculating spreadsheets.

Here is a summary why this should be done:
1. Calculating a Helmholtz type of resonator without a mouth- or end- correction would be totally wrong, since air particles are not only moving inside the neck as a mass, but around the neck as well.

2. A mouth- or end- correction should not only be considered geometrically dependant but also frequency dependant. This was already derived and mentioned 50-60 years ago by Cremer, Ingard, Smits, Kosten, and others. Therefore a fixed mouth- or end- correction would be wrong.

3. The resonant frequency can also be shifted by the viscous losses of porous materials. However, the viscous losses by the friction of air at the edges of the neck, the viscous resistance of the hole surface and the viscous resistance at the neck needs to be considered as well.

4. Finally there should be a proof by comparing measured results vs. calculated results, which I have done before. This can also be found in the book of Cox and D’Antonio (Acoustic Absorbers and Diffusers) on page 178, though this article is considered very superficial/shallow.

Will

Thanx for replying. Let's keep this threat active then. (whish I had the/some knowledge to challenge the formulas)

Take the slatted helmholtz formula from Everest's MHOA, page 224 (variables explained on p. 225), and move the part of the calculation (inside the radical) that converts decimal to per cent (X 100) to the OUTSIDE of the radical by changing the multiplier of 216 to 2160, and you have the formula used in the spreadsheet found in this forum. Only other difference I see is that the forum version also includes mouth correction; however, this is a fixed value of 1.2 which I agree with Will isn't likely to be optimum in all cases... Steve

Oh, Will, I cropped your pic to forum specs so we didn't have to scroll sideways. (moved your copyright in a bit to fit)

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Soooo, when a Musician dies, do they hear the white noise at the end of the tunnel??!? Hmmmm...