Slot Resonator Slat Layout

[c7sus]

I'm looking for a formula or rule-of-thumb regarding layout and configuration of slats.

Specifically, I'm shooting for a 250-500Hz band.

I did all the calcs this afternoon and have a little better understanding now.

I understand I have more questions........

Anyway, I'm curious about a formula to determine which frequencies to shoot for along the way from 250 to 500......

Is a 6-slat pattern covering approximatly 50- cycles per slat acceptable?

The panel is effectively 4-1/2 inches deep by 59 inches tall, and 41 inches across. I prefer to run the slats vertically because I kinda painted myself into that corner, so to speak.

I calced the range between using 1in, 3/4in and 1/2in slat depths and it looks like I will need to vary the slat-width, slat depth, and the gap width to hit specific specific frequencies as closely as possible.

I saw another post about a prime number formula to determine cycle patterns and wonder if the same formula applies here, and what that formula might be.

Also, is it practice to use a pattern that can be repeated at least twice across a given area, rather than to lay out a single pass pattern to make a broadband resonator???




Also, something kinda curious occured to me while doing the calcs......... I got to thinking about all the old radios with the slotted wood fronts from the 30's and 40's, with the cloth pressed tight against the cabinet front. Were they using Helmholz's ideas then in radio construction???

Thanks,

Craig

[Eric_Desart]

Hello Craig,

I'll maybe go deeper later.

I saw a rather good message from barefoot somewhere about slat Helmholz resonators.

Still what I miss and could not find in any message is the fact that a Helmholz resonator is a mass-spring system exact similar to a panel resonator.

The air enclosed by the neck is the mass (corrected by a factor to add some of the surrounding air to the mass) and the air in the cavity is the spring.
The relation with wavelengths is only important as to make sure that one uses this airspring below the lowest modes (+ spare for extending effect),
Possible resonances (standing waves) in the slots itself are secundary side effects, which are not included in the calculation of the main mass-spring mechanisme defining the Helmholz resonator.

As such the air acts as a pressure room as a sound field does in a room below the lowest room modes.

So in function of inertia indeed one can broaden the absorption (lowering Q) by playing with panel widths and slot widths and depths, but that's relative limited. The whole system averages it self. So don't think that a slot with another width or depth next to another, will suddenly be tuned to 2 different frequencies. This is only true if clear separated from one another.
The mass is totalled and the spring is the total air cushion.
I'm not discussing dampening etc., just basics.

Just if spread over large surfaces than indeed law of inertia prevents equalizing pressure with same speed as mass-spring system causing a broadening in frequency range.

So the different slat widths mainly will cause reflection and scattering (which is good).

It's almost a system mounted in parallel. You can't see individual slats, slots and air volumes as independent things (if not separated inside)

It's as a panel trap in a corner. The bandwidth is not defined by the different distances to the wall but by the dynamic stiffnes of the air cushion.

Best regards
Eric.

PS: I'm just telling things never intented to harm or degrade anything or anybody. If I should give that, certainly unintended, impression I'm really sorry.

[Eric_Desart]

A bit for the maniacs,

http://educinno.rug.ac.be/oiproject1/overzicht.htm.

I can't help it, but this is a Dutch site.
All Data copyrighted (subsidized by Flemish government)

However if you press the links on the page one gets access to several animations, showing the physics of Helmholtz resonators.

There are also lots of formulas, but it's possible that you need to download a free IBM plug-in to read them.

So really only for the maniacs.

Kind regards
Eric

[John Sayers]

I'm just telling things never intented to harm or degrade anything or anybody. If I should give that, certainly unintended, impression I'm really sorry.

tell it as it is Eric - we are all self taught acousticians here - except Thomas perhaps. We are all here to learn

cheers
john

[Eric_Desart]

Hello John,

I have a serious meant question.
Knowing the SAE site, seeing your site and projects, it's clear that slat type Helmholtz resonators are common practice for you.

I know the principle and physics.

But since yesterday, I'm trying to imagine what is really happening when you combine different slat, slot and panel widths and depths.
I'm trying to vizualize this, but it's not clear to me.

Have you ever done some testing, where could isolate the effect of your suggestions (even when not 100 % perfect).
Can you in principle tell what happens with the Q (sharpness) and height of absorption + bandwidth.
I know it's not easy since you work with low frequencies difficult to measure, and in practical circumstances with all kinds of influencing parameters.
I just ask the question because the slat type approach seems very familiar to you.

Just want some practical experience (numbers only if possible).
But only tell things or trends you're sure they are related. Assumptions (if not defind as assumptions) can only add to confusion.

I really want to get the feel.
Imagine two masses (air in different slots), resting on a common spring (air in cavity). But this spring isn't infinite stiff (so a 100 % addition of the masses as being 1 mass isn't correct), but they also can't be seen as individual springs exclusively related to their own slots.
A lot of interaction happens.

So, maybe some practical data or experience can help me here.

Best regards
Eric

It's your fault I ask difficult questions.
Your warm welcome causes me to investigate your forum/site more in-depth and you're the slat type guy.

[John Sayers]

Eric - I first came across slot resonators when I owned a studio in Sydney and we asked the local professor of acoustics at Sydney University to design us an acoustic treatment for our studio.

He designed two wall treatments for us. The first was a wall of 2 x 2 frames with various panel ansorber fronts. i.e. the whole wall became a low frequency panel absorbers wall. The other wall was a slot resonator wall. It consisted of CinderBlock (8" concrete block) columns with angled slots between. Like the attached.

I started using slots because they work on the low mids which IMO is the main problem area in most studios AND they are still diffusive in the higher frequencies. Like most people I was sick of the really dead rooms that the 70's produced and I've worked on getting brighter sounding rooms and I've found that the slots work for me. I see so many rooms today where the walls are totally covered in 703. For me that's tooo dead and dull. That's why I use slots.

Left Bank is a really bright sounding studio. People who walk into it comment immediately that it doesn;t sound dead or dull. Engineers are able to record everything flat with out it sounding dull and need ing rthe proverbial high end lift. The control room is bright and clean. I really had to wind back the top end on the Genelecs. I've been into another studio locally where they have Dead walls (4" foam) and the top end on their Genelecs is flat yet the speakers sound duller than at Left Bank.

cheers
john

[Eric_Desart]

Hello John,

Thanks for responding.
I'm certainly not disputing the use of slat type Helmholtz resonators.

And I 1000% agree that lots of studios and control room have too much absorption in mids and highs.

And the slats not only take care of reflection in function of width versus frequency but also scattering and diffusion.

I also use them when scared that mids to highs are getting killed.

I'm in fact more looking to the physical effect now of your suggestions with different widths of slats and gaps, wedge like gaps and slats and so on.
What I'm sure is that applying the standard simplified formulas does not cover this anymore. Also those numerical series (as for Schroeder diffusers which are realy wavelength related) does not apply here.

Just trying to get the feel what really happens, and what effect it has on the resonance frequency(ies).

Should be nice if some comparative measurements should exist.

Warm regards
Eric

[John Sayers]

eric - I'd love to be able to measure them - as you say, like so many things in acoustics once you move away from the basic formula you can't predict it, damn it!! I've been to the National Acoustic lab in Sydney and they have the gear, the reverb chambers and the anaechoic chambers - but they cost a bundle and spend most of their time measuring STC for the aluminium window/door manufacturers. (what a waste)

The splayed slots at Left Bank were an total experiment.



When we were building the booths I would go in to the rooms and talk, listening closely to my voice sound and others. The ceiling treatment and the insulation on the wall really sucked up the highs and voices sounded dull and boooomy in the low mids. When the slots went in the improvement was quite dramatic - suddenly the highs were back and the low-mids were gone. Mate - I have no idea of the physics that makes them work but to my ear they work fine :)

I'm sure you can describe the physics concisely which is why we all want you to moderate the acoustics forum :)

cheers
John

[laptoppop]

Sometimes I wonder if there are secondary effects going on here. For example, if I understand it correctly, Barefoot's analysis seems to indicate that for fractions of a wave slot width just doesn't matter much. On the other hand the splayed slots of Left Bank seem to produce a good sound. Would non-splayed slots at the same average width work the same? Somehow I don't think so - I think the absorbtion peak would be sharper around a particular frequency. My *guess* is that the splayed slots help broaden the frequency absorbtions. By how much? ahhhh, that's the rub. I'd love to get a bunch of different slot absorbers in testing chambers to quantify some of these parameters.

This kind of reminds me of an old joke...

At a math/physics/engineer convention:

They decide to play a game where all the men are on one side of the
room and the women on the other. Every minute people are allowed to
move to half the distance between them and their partner on the other
side. At this point all the math people and physicists sit down and
give up. They realize they are *never* going reach their partner.
The engineers on the other hand are getting real excited because they
have calculated that in just a few minutes they will be close enough
for practical purposes.



-lee-

[rsb]

After a year of taking a math analysis course as part of a math major, that was all too close to home.

Ryan

[Eric_Desart]

This kind of reminds me of an old joke...

At a math/physics/engineer convention:

.......They realize they are *never* going reach their partner.
The engineers on the other hand are getting real excited because they
have calculated that in just a few minutes they will be close enough
for practical purposes.



-lee-

Maybe there are tooooooooo many (sound) engineers then to get rid of the magic in acoustics ????????

Eric

[giles117]

Thanks Lappoptop

Thgat is why I am an engineer....

ANyway. Hey Erik/John Got Questions for you...

Which do you like?? The angled slots of the vertical slots???

I am about to put up my rear wall (Yeah John It's Day 3 for me on this studio construction) And have decided I will do 1x2, 1x3 & 1x4 (inches) Slats.

Just havin' Fun

Bryan

Wow I did not realize I was dealing with so many issues where bass was concerned. I can hear a difference and I am not even done yet. Nothing against Auralex, but all it did was dull what I was hearing. Thank god for mastering.....My Monitors for the 1st time sound like they sounded when I 1st bought them. I had to turn the HF driver down cuz it was WAYYYYY to bright, but before er'thing seemd so dark.

[John Sayers]

I like Bruce's slots



cheers
john

[giles117]

Thanks John 1x4 is what those look like (25mmx100mm) with varied spacing. I think I will do 1x4 on the rear 2 walls and 1x2 on the 2 side walls.

Or Are those 1x6? I ran the numbers and I am looking at a LF of about 90Hz if I seperate a few at 1mm of spacing

Is this adequate or shoudl I look into doing a 1x12 which showed me a resonance of 72hz at my deepest depth.

Or am I just being anal and leave it at the 1x4, 1x2 combination. I Can get over meticulous, so I am asking.

Bryan Giles

[John Sayers]

1 x 6" Bryan - hey don't do your whole rear wall in slots - it'll sound awful with all that reflection behind you.

cheers
john