Add wood slats to face of bass trap to retain some reflectivity?

[666666]

A few times I've seen guys place wood strips (like a "grill" made of 1"X3"s for instance) directly on the face of their bass traps, the idea being to add a bit of extra reflectivity to the face of the bass trap.

So does anyone know (even a good speculation), if you had a good 2'X4' bass trap, say 6" thick of 703, but then, as an example, you added strips of 1"X3" pine wood to the face of it (1"X3" is 3/4" thick and 2.5" wide, add strips of this so that the wide dimension faces the room), and add these 2.5" strips parallel to each other every 2.5 inches.... like a grill. Could you describe the extra reflectivity that would take place due to the addition of this new pine wood grill made of 1"X3"s? What freq range might this reflect? (I'd imagine it would be a very narrow, upper range) Is this an acceptable method of modifying a bass trap to yield a bit of high-end reflectivity in addition to bass trapping?

If this wood strip / slat idea is not good, then IS there an "approved" method of adding something to the face of bass traps to increase reflectivity in the upper range (but not too much)?

[Jens Eklund]

http://www.gearslutz.com/board/6471630-post32.html

The reflection is related to the perforation percentage, download this calculator to play around with numbers:

Porous Absorber Calculator V1.58

[Glenn Kuras]

Originally Posted by Jens Eklund

[URL]

Porous Absorber Calculator V1.58

I don't see a link on that website to download. Can you email it to me or post the download link?

[Jens Eklund]

Originally Posted by Glenn Kuras

I don't see a link on that website to download. Can you email it to me or post the download link?

Just below the headline:

”The following versions of this Excel spreadsheet are available for Microsoft Office:
Office 2007
Office 2003/97”

Do you not already have this sheet?

[Glenn Kuras]

Thanks Jens, I see it now.

[666666]

Thanks!

Unfortunately for me, this calculator will not run on my computer for assorted reasons, one of them being that my copy of Excel is too old, etc. At some point I'll see about trying to download this to another person's computer if they'll allow me.

In the meantime, would anyone like to take a quick stab at giving me a ballpark idea about what the reflection behavior would be in the hypothetical example from my initial post?

Again, with more detail:

Take a 2'X4' bass trap (6" thick of 703), then add strips of 1"X3" pine wood to the face of it, with wide side facing out, running the short length of the trap... so the strips will be 24" long. Place 10 of these 2.5" wide strips on the face of the trap, parallel to each other and evenly spaced apart. Adding 10 along a 4' trap creates a spacing of about 2.5" between each strip. So, we have ten 2.5" wide wood strips, and 2.5" spacing between them, across the 2'X4' bass trap. These strips are 3/4" thick by the way. So, the FACE of the trap is now about 50% pine wood (3/4" thick), and 50% open to the 703 insulation.

My question is... let's assume for a moment that a 6" thick 703 trap offers 100% absorption for all frequencies (I know it doesn't, but to make this simple at the moment, let's say it does). So now we add the ten 1"X3" strips as described above, what is now getting reflected by the strips? Regardless of % of perforation of the face of this trap, I'd think that individual 2.5" wide wood strips that are 3/4" thick are only going to reflect a certain window of frequencies (higher ones of course). Like, for instance, maybe a meter would show that frequencies from 5k to 9k are getting reflected back from this trap, but all others are being absorbed by the 6" of 703. Or...?

I'm not even concerned with the amount or degree of reflections occurring (verses the theoretical maximum from the total area of the trap face), but I'm just interested in the actual frequency figures.

Finally, regardless of the % of perforation of this hypothetical trap face, a certain degree of frequencies will be reflected off of a 3/4" thick piece of pine wood, and a certain amount will pass through it. I wonder if this figure is constant for all sizes of 3/4" pine wood, or if the figure changes as you make the face area of the wood smaller.

I did some research but could not find: a long time ago I had seen a chart that showed all common materials (wood, brick, etc), and if I remember correctly, it showed, for a common thickness of each material, at which point reflection begins to occur. For instance (just making this up now for sake of example), for 3/4" plywood maybe everything from 500Hz and below would pass right through it and almost everything from 500hz and up would be reflected off it, and so on. If such a chart actually exists (and I'm not just dreaming it), what is this chart called? (so I can go find it, been looking to no avail so far)

Finally, I'd like to think that if I had a trap and desired to achieve a bit of scattered, partial upper-frequency reflection off the face of it, from say 5kHz and up, what type of material would I want to place on the front of it (and how thick)? Above I ask about 1"X3" pine strips because they're very cheap and easy to deal with, a grill of 1"X3" pine strips can be made very easily and quickly. Though I realize there may be a much better way to approach this. Or maybe wider slats spaced evenly would be better and reflect to a lower freq??? Or maybe slats faced with ceramic tile would reflect to a higher freq than just bare pine wood slats? And if so, what freq range would this reflect? These are the things I'd like to look at. Just curious.

Thanks!

[SAC]

You are aware that there is a version of the spreadsheet specifically for older versions of Office, aren't you?

And if you have a version of Office earlier than 2003 or 1997, what are you running...Windows 3.1 or Windows 95??? And how are you accessing this forum? The string must be very long attached to your tin can...

Please check out the version for Office 97/2003 - I think you will find that works...

[Jens Eklund]

Originally Posted by SAC

Please check out the version for Office 97/2003 - I think you will find that works...

It does.

If you’re running a non-English version of Excel, you also need to run the translation tool (or manually translate the sheet).

[666666]

Ok, I see, I tried again and am having another problem now related to security, digital signatures, etc... I'll keep messing with it, I think I need to loosen up my firewall a bit or something.

The program does open though and I can see it. Just looking at it, it appears very confusing to me. I'm wondering if this is the correct tool needed to figure out the answer to my specific question. Again, I just want to know approximately what frequency range would be reflected by my hypothetical bass trap with pine wood strip grid attached to the front. Just the frequency range, not the amplitude or anything else. Would these wood strips reflect say 5k to 8k, or 9k to 12k, or 700 to 15k, or ....???? General, approximate idea. Just curious.

Hmmm.... I do not have any audio measuring tools, but maybe I should look into getting a measuring mic and appropriate software etc and actually just do my own tests. I'd think that taking a careful measurement is more accurate than doing math. In a hands-on example, there will likely always be some variables that the math may not account for, etc.

Or, I can also just do experiments and merely LISTEN and decide if it sounds good to me, regardless of the specs and math etc. That is indeed the very best test. It is either pleasing to the user or not. Only problem with this is that there is then no data to help decide how to make adjustments, if adjustments are needed. Then it becomes a guessing game which of course is no way to do anything.

But gee, doesn't anyone here have any idea about the reflectivity of a 3/4" piece of pine wood? I'd think that this would be something that pro acoustic people would be dealing with all the time when dealing with general studio construction. But I'm sure I'm just overlooking something here, I do realize that acoustics are often not what they seem and often way more complex than one might initially expect. And after all, I'm just a musician , not an acoustician. That's why I'm here asking questions.

[SAC]

I am a bit surprised to note that those so loudly eschewing science and acoustics aren't jumping in by the droves to 'just provide an answer'....

Are you really interested in the "reflectivity of a 3/4" piece of pine wood", or do you want to know what frequencies a piece of reflective material, in this case wood, will reflect?

They are two different things.
I know this doesn't matter to some of the flatlanders, but to us science types, words mean things...sometimes...

If you have a particular width of wood slat and can provide us with the width, I can of course provide the lowest frequency that it will reflect.

To start, an individual piece of wood of the following widths will reflect:
3 inches wide: ~4500 Hz and up.
4 inches wide: ~3375 Hz and up...
5 inches wide: ~2700 Hz and up...

I hope that this is what you meant...if not, well I guess we can always guess again.

Longer wavelengths (lower frequencies) will diffract around it...

I hope this information doesn't offend our 'sound is my life' non-acoustic friends...

Of course, if it does, we can always explain how and why this can be determined. Until then... its magic. Booga booga (to quote Groucho...)

But as an aside, its funny, as the calculations don't even mention what the reflection will 'sound like', as apparently that is based upon the material with which the energy is incident and not the composition of the actual incident energy itself. But wood being wood, I guess that is why a clarinet sounds like an upright bass, as both are composed of wood. I guess those science types simply forgot to include that in the formula...

[Torea]

Originally Posted by 666666

A few times I've seen guys place wood strips (like a "grill" made of 1"X3"s for instance) directly on the face of their bass traps, the idea being to add a bit of extra reflectivity to the face of the bass trap.

So does anyone know (even a good speculation), if you had a good 2'X4' bass trap, say 6" thick of 703, but then, as an example, you added strips of 1"X3" pine wood to the face of it (1"X3" is 3/4" thick and 2.5" wide, add strips of this so that the wide dimension faces the room), and add these 2.5" strips parallel to each other every 2.5 inches.... like a grill. Could you describe the extra reflectivity that would take place due to the addition of this new pine wood grill made of 1"X3"s? What freq range might this reflect? (I'd imagine it would be a very narrow, upper range) Is this an acceptable method of modifying a bass trap to yield a bit of high-end reflectivity in addition to bass trapping?

If this wood strip / slat idea is not good, then IS there an "approved" method of adding something to the face of bass traps to increase reflectivity in the upper range (but not too much)?

Sounds like something DanDan posted a little while back

http://www.gearslutz.com/board/6339939-post4.html
http://www.gearslutz.com/board/6346355-post12.html



Yes? No? Maybe so?

[Torea]

Originally Posted by SAC

But wood being wood, I guess that is why a clarinet sounds like an upright bass, as both are composed of wood. I guess those science types simply forgot to include that in the formula...

Just like this carrot sounds like a clarinet - YouTube - Carrot clarinet


I wonder if they have an upright bass version...

[SAC]

Now we are talking two different things - of which the question never made clear.

Oh, the flatlanders will be angry today!

The individual slats reflect. Then these individual reflections (we apparently are not allowed to mention phase and edge diffraction, so we won't...) then interact to create a diffuse result.

And since we are not allowed to explain the components of reflection, we certainly are not allowed to address diffusion, lest the flatlanders object on the basis that logic obfuscates the issue. Not to mention that it also simply confuses them.

[666666]

Originally Posted by SAC

To start, an individual piece of wood of the following widths will reflect:
3 inches wide: ~4500 Hz and up.
4 inches wide: ~3375 Hz and up...
5 inches wide: ~2700 Hz and up...
I hope that this is what you meant...if not, well I guess we can always guess again. Longer wavelengths (lower frequencies) will diffract around it...

Thanks SAC! Yes, this is the info that I'm after!

So the width of the slats does indeed effect the "cut off" frequency in terms of reflectivity. I thought it might, figuring, as you said, that longer waves would diffract around skinnier slats.

But then I still wonder if the thickness of the slat comes into play here also. But.... now thinking about it, my wild guess is that, at least with slats that are a mere 3" wide for example, the thickness would not have any effect because a larger size, 3/4" thick piece of wood will reflect below 4.5kHz anyway. In other words, the WIDTH of the slat yields a higher cut-off than the thickness would, and is the limiting factor, in this particular case. But perhaps if the slat was 8 feet wide, then the opposite would be true and the thickness would be the more limiting factor instead... the thickness would then cut-off before the width. Just pure speculation of course from someone who has zero experience with acoustics other than purchasing some 703. And I do realize that there are a lot of other factors involved that I'm not even mentioning (resonance, etc, etc, etc)... but just making for some general discussion. That's how us acoustic nimrods slowly learn.

SAC, I appreciate the info. Thanks. I realize that I cannot draw any perfect end-all conclusions from this info, but even as just a rough guide, I find it interesting and educational.

Originally Posted by Torea

Sounds like something DanDan posted a little while back
http://www.gearslutz.com/board/6339939-post4.html
http://www.gearslutz.com/board/6346355-post12.html
Yes? No? Maybe so?

Thanks Torea! Also very interesting stuff! Yes, what DanDan is showing here, as well as the chart posted by avare (which I think I did see in a John Sayers article somewhere while I was researching earlier) is exactly what I'm interested in.

Great info. Thanks!

[SAC]

If it was 8 foot wide, it would reflect down to ~141 Hz.

Real world behavior will be slightly different than the ideal model, as reality does not so neatly isolate and deal with only certain variables to the exclusion of others and serve as what are generally called sources of error. - as distinct from mistakes - meaning factors conceptually not assumed to be significant but which nevertheless will tend to skew a result from the results predicted in an ideal simplified model.

An example of a source of error might be if is one measuring the motion of say, a pendulum, where a simplified formula for periodic motion may not take into consideration the effects of air resistance on the moving components. As such, the extrema of the pendulum's travel would be slightly less than if there were no air resistance. And as the resulting displacement from normal is less, than a calculated value for the accelerating effects of gravity might be exaggerated and the value of gravity rendered a bit larger than actual (as the effects of air resistance would be added to that actually due to gravity).
The larger significance being that understanding the various contributing sources of error can help one reasonably anticipate how such sources of error can effect outcomes.

[666666]

Originally Posted by SAC

Real world behavior will be slightly different than the ideal model....

I dig it.

If it was 8 foot wide, it would reflect down to ~141 Hz.

Got it.

[RKrizman]

Originally Posted by 666666

A few times I've seen guys place wood strips (like a "grill" made of 1"X3"s for instance) directly on the face of their bass traps, the idea being to add a bit of extra reflectivity to the face of the bass trap.

Something like this?

[666666]

Originally Posted by RKrizman

Something like this?

Yes! Cool!

Is that yours? If so, how do you like it?

Approximately what slat widths are being used there? Looks like the largest is about 3.5". Looks like the "3-4-5" scheme.

[RKrizman]

I like it. The proportions are proprietary at this point. You'd have to ask Jeff Hedback, who designed it. You can find him around here.

Thanks,

-R

[takman]

SAC, thanks for the info. I always wondered about the grills on bass traps and panels.

havent read the links yet...but i will delve a bit further into this subject, since i do like the grill look on absorbers....

probably covered in a thread already.....but im wondering is the gap between the slats random, or is that calculated as well?

[666666]

Originally Posted by takman

....but im wondering is the gap between the slats random, or is that calculated as well?

http://www.gearslutz.com/board/6346355-post12.html

".... the widths of the strips and openings are based on a numerical sequence that provides more or less equal area of absorbent and reflective surfaces, but without any simple regularity, which could lead to problems at certain frequencies having properties coinciding with regularity..."

Below is the suggested multiple... first number is a slat, second number is an opening, third number is a slat and so on:

534435453534435

It's easy enough to follow and looks cool too.

One easy way to achieve this if you are interested in reflecting frequencies above approximately 5k:

For the "3", use a standard 1"X2", which is 1.5" wide.

For the "5", use a standard 1"X3", which is 2.5" wide.

For the "4", you'd have to rip a 1"X3" down to 2.0".

This represents a quick, easy and cheap way to arrive at this type of slat sequence... minimum ripping, and no special, expensive wood stock needed. Standard grade pine 1"X2"s and 1"X3"s can be gotten at any Home Depot or Lowes, quite cheap. They're not stain grade really, will have knots etc, but you can always paint. Either that or get "select grade" (also available at Home Depot and Lowes), but you'll pay a few bucks more per each 8' length.

Following SAC's math, the slat widths I've proposed above would reflect above these frequencies:

1.5” slat – reflects above 9000 hz
2.0” slat – reflects above 6500 hz
2.5” slat - reflects above 5400 hz

This would seem to me to be a good reflection range for many situations.

The 534435453534435 sequence is supposed to provide close to 50/50 absorbency verses reflectivity. So then I wonder, what if one wishes to achieve a different balance, say for instance, 70% absorbency and 30% reflectivity? One thought is to follow the original sequence, but simply add a constant, extra bit of width to the openings only. For instance, add say 0.5" to every opening figure and keep everything else exactly the same. Well, this would be an "easy" way anyhow. I guess to really do it" right", you'd re-scale the openings by a percentage perhaps. But I have to question just how critical it is to stick exactly to this sequence anyway, it seems more like a good, general rule of thumb for reducing regularity, not necessarily a perfect "magic formula".

[avare]

Originally Posted by 666666

...it seems more like a good, general rule of thumb for reducing regularity, not necessarily a perfect "magic formula".

Exactly!

Precisely,
Andre

[Jeffrey Hedback]

The slat pattern I used in Rick's room is a .5 scale version of the "534". However, I specified (and supplied via a mill shop partner I work closely with) the slats to be 1" thick and of basswood. The slats were mounted onto custom made GIK 244 panels.

The reasons are mainly to do with the dynamic uses of Rick's space. He may go from a mix, to drum to single voice in the same day. The .5 scale spacing with lower volume uses addresses specular issues but sounds mostly like a reflective surface (in other words not absorptive or diffusive)...but at louder volumes the multiple edge diffraction benefits kick in and when combined with rotated arrays (2'x2') the result sounds like the air is compressed...spacious and detailed but compressed/controlled. It gives a lot a placement and micing options while serving the primary mix function well.

The purpose of the basswood is to reduce unwanted resonance by thinner typical wood stock. Basswood is an affordable tonewood. We did discuss ripped 3/4" MDF as an option. I'm glad Rick followed my recommendation of the basswood. The 1" thickness increases the edge diffraction benefit described above.

[666666]

Originally Posted by Jeffrey Hedback

...The .5 scale spacing with lower volume uses addresses specular issues but sounds mostly like a reflective surface (in other words not absorptive or diffusive)...but at louder volumes the multiple edge diffraction benefits kick in and when combined with rotated arrays (2'x2') the result sounds like the air is compressed...spacious and detailed but compressed/controlled... The purpose of the basswood is to reduce unwanted resonance by thinner typical wood stock...

Really nice, man. Thanks for sharing.

[Jens Eklund]

Just to give you an idea what to expect from different slat configurations:


Predicted (SoundFlow) absorption coefficient normal incident:


Predicted (SoundFlow) absorption coefficient diffused field:


Slat thickness: 1”. 120 mm wool (8,8 kPa*s/m², 30kg/m³) + 120 air gap = 240 mm total depth.


Sincerely Jens Eklund

[ritelec]

Originally Posted by 666666

http://www.gearslutz.com/board/6346355-post12.html

".... the widths of the strips and openings are based on a numerical sequence that provides more or less equal area of absorbent and reflective surfaces, but without any simple regularity, which could lead to problems at certain frequencies having properties coinciding with regularity..."

Below is the suggested multiple... first number is a slat, second number is an opening, third number is a slat and so on:

534435453534435

It's easy enough to follow and looks cool too.

One easy way to achieve this if you are interested in reflecting frequencies above approximately 5k:

For the "3", use a standard 1"X2", which is 1.5" wide.

For the "5", use a standard 1"X3", which is 2.5" wide.

For the "4", you'd have to rip a 1"X3" down to 2.0".

This represents a quick, easy and cheap way to arrive at this type of slat sequence... minimum ripping, and no special, expensive wood stock needed. Standard grade pine 1"X2"s and 1"X3"s can be gotten at any Home Depot or Lowes, quite cheap. They're not stain grade really, will have knots etc, but you can always paint. Either that or get "select grade" (also available at Home Depot and Lowes), but you'll pay a few bucks more per each 8' length.

Following SAC's math, the slat widths I've proposed above would reflect above these frequencies:

1.5” slat – reflects above 9000 hz
2.0” slat – reflects above 6500 hz
2.5” slat - reflects above 5400 hz

This would seem to me to be a good reflection range for many situations.

The 534435453534435 sequence is supposed to provide close to 50/50 absorbency verses reflectivity. So then I wonder, what if one wishes to achieve a different balance, say for instance, 70% absorbency and 30% reflectivity? One thought is to follow the original sequence, but simply add a constant, extra bit of width to the openings only. For instance, add say 0.5" to every opening figure and keep everything else exactly the same. Well, this would be an "easy" way anyhow. I guess to really do it" right", you'd re-scale the openings by a percentage perhaps. But I have to question just how critical it is to stick exactly to this sequence anyway, it seems more like a good, general rule of thumb for reducing regularity, not necessarily a perfect "magic formula".

Hi all.
The multiplier of, and which freq. to address? As I've removed some roxwool corner saddle traps (rear of speaker) and am replacing with Fluff corner traps, I would like to install slats over the rear of speaker wall traps and the new fluff corner traps while that area is open.
Will the lumber dimensions in 666666's post benefit my room, or should I use a different multiplier (as if I knew what I was multiplying).

Attached is freq. response of room with roxwool corner traps.

Thank you.
-rich

[Jens Eklund]

Originally Posted by ritelec

Hi all.
The multiplier of, and which freq. to address? As I've removed some roxwool corner saddle traps (rear of speaker) and am replacing with Fluff corner traps, I would like to install slats over the rear of speaker wall traps and the new fluff corner traps while that area is open.
Will the lumber dimensions in 666666's post benefit my room, or should I use a different multiplier (as if I knew what I was multiplying).

Attached is freq. response of room with roxwool corner traps.

Thank you.
-rich

Identify the cause of your peak at 50 Hz and the dip at 65 Hz, and treat accordingly. Also check your ETC for early reflections that might cause the problems seen in the 700 Hz range. How’s your decay times?

Acoustics/Treatment Reference Guide - LOOK HERE!

[666666]

Originally Posted by ritelec

...
Will the lumber dimensions in 666666's post benefit my room, or should I use a different multiplier (as if I knew what I was multiplying)...

Just a side note regarding the widths I came up with here:

For the "3", use a standard 1"X2", which is 1.5" wide.
For the "5", use a standard 1"X3", which is 2.5" wide.
For the "4", you'd have to rip a 1"X3" down to 2.0"

This is merely a representation of a lazy man's way of making slats. By using standard lumber (such as stock 1"X2"s and 1"X3"s etc), a lot of wood ripping work can be avoided. This was the main point of my post. This can be a helpful tip if very many slats need to be made. If you don't have many to make, then it doesn't matter.

The slat widths I've listed by no means represent any degree of precise acoustic science, they're just "average" widths that, for a typical, not so precise, DIY situation, should probably work ok.

I hate ripping wood. I have a good table saw and know how to use it, but... one time I had to rip a ton of strips, was ripping for hours, after that I swore I'd never do it again. So now whenever I "design" anything, I am very mindful to keep ripping to a minimum. I always look for ways of utilizing standard lumber sizes. For big projects, this can potentially mean the difference between almost entire extra day of just ripping, or hardly any ripping at all.

The last batch of bass trap frames I made, I used standard 1"X6" lumber (5.5")... then fitted with 4" of OC 703, with a 1.5" air space behind. I feel this is a pretty efficient design, no ripping at all, and having the extra 1.5" of air space behind the 703 only helps. I made a lot of frames, if I had chosen to rip all the lumber to 4" to fit the 703 exactly, I would have surely lost another entire day.

I realize that when it comes to precise acoustic science, there really is no place for "lazy man" tricks. If science and testing designates a specific design, then that design must be followed... no cutting corners. But there are still a lot of situations where budget comes first and some degree of compromise has to be made, this is where "efficiency tips" can be helpful.

[ritelec]

Jens, I believe the peak at 50Hz may be do to the concrete room I'm in??? That is why I have removed the roxwool traps and am replaceing with the Fluffy.

As far as the ETC. I've eliminated some (and yet created others ).

I'm hopeing that the fluff trap will attack the 50Hz, and I saw a post where some one just stuck rolls of regular insulation in the corners and it helped with his null at 65/70Hz.

666666- Not lazy here. Will rip and tear for proper widths.

Can you guys give me the widths I should need? Please?

I think I need something that would affect +/- 150, 750,2k,12k???

Enclosed, please find reverberation time.

Thank you for your help.

-rich

[Jens Eklund]

Looks like the same problem as in this thread:

Room Measurement After Consulting Gearslutz

Too long decay in the lows and too short in the mid/highs. Adding more velocity base absorbers will probably only make it worse.