Can I use the shallow space behind my door for bass trapping?

[Max Dread]

Hi Guys

I'm in the process of measuring my room at the moment to determine what kind of treatments I need and where to put them....

In one of the corners of my room (a back wall corner) there is a door, so I am limited as to what I can do here. The dimensions of the area I could use would be:

Depth: 14cm
Width: 50cm
Height: 210cm

I know that the depth will mean that porous absorption won't reach very low in the frequency range. But I'm wondering about pressure based traps? Unfortunately, that's one thing I have not had time to look into yet and so have little knowledge. So I wondered if anyone could give me a rough idea of how low in frequency a pressure/panel trap in that area could help with.

It's just really so that I can be considering what my options are once I've finished measuring and begin the plans of what to do with the room.....

Cheers

Max

[Jens Eklund]

Originally Posted by Max Dread

Hi Guys

I'm in the process of measuring my room at the moment to determine what kind of treatments I need and where to put them....

In one of the corners of my room (a back wall corner) there is a door, so I am limited as to what I can do here. The dimensions of the area I could use would be:

Depth: 14cm
Width: 50cm
Height: 210cm

I know that the depth will mean that porous absorption won't reach very low in the frequency range. But I'm wondering about pressure based traps? Unfortunately, that's one thing I have not had time to look into yet and so have little knowledge. So I wondered if anyone could give me a rough idea of how low in frequency a pressure/panel trap in that area could help with.

It's just really so that I can be considering what my options are once I've finished measuring and begin the plans of what to do with the room.....

Cheers

Max

You could reach 20-30 Hz with that depth if you needed to but the Q will be high if shallow depth. Use this calculator to play around and see what happens with different configurations:

Porous Absorber Calculator V1.58

[Max Dread]

Hmmmmm interesting.... I will have a play with the Whealy calc. Just did not think I could get that low with such a shallow depth.

Basically, as things stand I have two main issues in the LFs - the first axial mode which causes some major ringing at around 40Hz. And a big broad dip from around 90Hz to 150Hz, the cause of which I'm still investigating, although I think it is needless to say that it will be a combination of factors at play.

I have the area behind the door as mentioned (which is quite restricted in space which is why I thought a pressure would be better)...... but also some quite large areas in the back of the room where I could trap using velocity or pressure based (including some free wall - ceiling areas).

Once I've taken a few more measurements and taken time to analyse them, I'll make a new post where (hopefully) I can get a bit of advice as to whether my novice conclusions are correct, and what's best to do with the free spaces I have.

Cheers

Max

[Jens Eklund]

The 40 Hz mode will probably need a pressure based absorber but the others might be treatable with velocity based. Just be aware that a uncovered bass trap using porous only is a broadband absorber:

Room Measurement After Consulting Gearslutz
Velocity based vs. pressure based absorbers

[Max Dread]

Yeah, I've been thinking that but have not been overly concerned. The way I see it, if the room becomes too dead it is quite an easy fix - some rigid card or thin wood or wooden slats etc on the face of the traps should help the problem.

And as these are quite easy to add/remove, I can keep on experimenting and measuring until I get the balance right (whilst also checking ETCs to ensure I'm not introducing unwanted reflections).

Does that seem a good logical way to approach it?

[Jens Eklund]

Originally Posted by Max Dread

Yeah, I've been thinking that but have not been overly concerned. The way I see it, if the room becomes too dead it is quite an easy fix - some rigid card or thin wood or wooden slats etc on the face of the traps should help the problem.

And as these are quite easy to add/remove, I can keep on experimenting and measuring until I get the balance right (whilst also checking ETCs to ensure I'm not introducing unwanted reflections).

Does that seem a good logical way to approach it?

Absolutely!

You’re on the right path.

[Max Dread]

OK, so I'm playing with the PAC and I must admit that the plethora of tweakable dimensions and features is making it really hard to come up with the goods!

Starting from the start, would a Perforated or Slotted panel be best? After that... could anyone just give me a pointer in the right direction, just generically, so that I'm at least playing in the right ball-park so to speak?

Cheers

Max

[Glenn Kuras]

Originally Posted by Max Dread

Yeah, I've been thinking that but have not been overly concerned. The way I see it, if the room becomes too dead it is quite an easy fix - some rigid card or thin wood or wooden slats etc on the face of the traps should help the problem.

And as these are quite easy to add/remove, I can keep on experimenting and measuring until I get the balance right (whilst also checking ETCs to ensure I'm not introducing unwanted reflections).

Does that seem a good logical way to approach it?

Right covering a broad band bass trap with rigid card board will work fine. Also I want to note that building a tuned trap right is pretty tricky. I have been working with them as of late and have found that if not built right then all you have is a big box. Also I want to point out that not all of the calculations for a tuned trap are right. I have seen some that spec out for say 60hz actually test out to around 90hz. All and all if you want something you KNOW is going to work then stick with broad band.

[Max Dread]

Originally Posted by Max Dread

OK, so I'm playing with the PAC and I must admit that the plethora of tweakable dimensions and features is making it really hard to come up with the goods!

Starting from the start, would a Perforated or Slotted panel be best? After that... could anyone just give me a pointer in the right direction, just generically, so that I'm at least playing in the right ball-park so to speak?

Cheers

Max

Thanks Glenn.

From my own post I've quoted here I can see things will be tricky. It's hard enough to make the calculation, so I dread to think how difficult making the panel will be!!!!

That being said.....

I will most likely be using a lot of broad band absorption in the room. But the area behind the door seems to shallow for broadband, so my thinking was pressure based or don't bother in that area.

But another thought. I have a really wide dip running from around 90 - 150Hz. As I said, I plan to add a lot more bass trapping to the room (I currently have SSCs in three corners). And hopefully that will help. But perhaps I should target that dip instead with the pressure based trap? As the area is quite broad, if I aim for the central frequency and end up getting it a bit wrong, at least it will still be helping!

While on that note, when trying to fix nulls rather than peaks, should I still be looking for areas where the frequency is at its highest and treating those? Is it the same method for nulls and peaks? (I might of asked this before but can't find the post?).

Cheers

Max

[Max Dread]

Any further thoughts guys? Cheers

Max

[John White]

Max,

I also have a small door in one of my corners in which I built a panel trap onto the door face.
I couldn't figure out how to deal with this area of the room until I woke up in the middle of the night with this easy solution.
It's a small second door in a tight area that doesn't allow the door to open completely. A panel trap is ideal for this difficult spot.
Using half your allowable depth and a sheet of luan would put you right in the middle of your dip.

ƒ=170/√d*m Caulk or glue liberally. Easy cheesy.




It is a solid mdf door I built for the room for a suitable backing.
Assuming your door opens inward, just make sure to use a door stop to prevent the panel from getting beat up.

[Liquid360]

You can use any space anywhere creatively to minimize sound waves.... Until your wife makes you stop.

[Liquid360]

Originally Posted by Max Dread

While on that note, when trying to fix nulls rather than peaks, should I still be looking for areas where the frequency is at its highest and treating those? Is it the same method for nulls and peaks?

Max

I think it is. Moving materials... sometimes even your speakers around.

[Max Dread]

Very helpful John - thank you very much.

Could I maybe pick your brains a little more?

Originally Posted by John White

Using half your allowable depth and a sheet of luan would put you right in the middle of your dip. ƒ=170/√d*m Caulk or glue liberally. Easy cheesy.

Is there any chance you can expand a little here or if not direct me to anywhere I can read up some more to make sense of this?

Also, when you say "right in the middle of your dip" are you referring to the 90-150Hz dip? Do you think I would be able to tackle the 40Hz room mode with this trap, with the dimensions available?

Cheers

Max

[John White]

Max,

I wrote a very comprehensive and perhaps self-indulgent post about my idea, but it got lost. I would still like to share my thoughts with you since I believe it may be helpful.

I'd rather not make any assumptions about your understanding or needs so I'll try to keep each post fairly concise. Eager to build a dialogue here-

In my limited understanding of acoustics, for a 40Hz problem, a Helmholtz resonator is supposed to be quite effectual at targeting and absorbing specific frequencies. This may be an option for you if your overall tuning system is already in place. However, I personally don't really understand how to build one (of any variation) since I've never seen a reliable and detailed schematic. I'm sure someone here may be able to help you better.

A panel trap, would be a perfect piece of absorption to have in that difficult spot of yours. Ethan Winer has a great article titled, "Build a Better Bass Trap" about panel traps (Though from what I've read from him subsequently, I've concluded that he wishes to have titled the article differently.)

Here is the link:
Build a Better Bass Trap

There is also a link at the end of the article to both a great diagram and a parts list.
Though the diagrams show how to build them onto walls, we can substitute your door for a wall. This assumes that your door is very solid.

After reading the article, I'll show you some numbers (dimensions and material weights) I've used that would target that 90-150Hz dip nicely!
Of course, that isn't to say that one bass trap would correct that null or even if it's in the best spot. Still it's an available area and should be used to your advantage.

[Max Dread]

John, that's really kind of you and I await any further info with baited breath!

I'm sure I've read that article by Ethan before, but I've read so much recently that I think any info gleamed from it will be lost in a dark dank corner of my wilting mind! IOWs, best I read it again......Now (or in a minute)!

Just a couple of points....

For a variety of reasons I would much rather build the trap onto the wall rather than onto the back of the door.

In case it would help to see a room plan, here's a couple of overhead diagrams:

Analysing Measurements for Speakers & LP in My Room, What they tell me & what next?

Part of the reason I was thinking about trying to target the 40Hz room mode is that - as the second overhead shows - I have that large alcove (well, it's not really an alcove as such) and intend to use most of it for deep "fluffy" porous absorption. I need to run a shelf through the middle of the "alcove" to house some vinyl, but other than that it can be all mineral wool! At it's thinnest, the MW will be 40cm deep, whilst at its thickest (near the back wall/in that back corner), it can be up to 60cm deep. Other dimensions: floor to bottom of the vinyl shelf, and top of the vinyl shelf to the ceiling will both be around 70cm high EACH. The length of the alcove is 175cm.

If none of that makes sense, let me know and I'll try to draw a diagram.!

But what I'm getting at is...... with that amount of porous absorber (fluffy), and with the two front wall superchunks (84 x 60 x 60cm), along with the side wall reflectors and ceiling clouds, I'm hoping that I can make some good improvements to the 90 - 150Hz problems. I've used the porous absorber calculator and found that a GSR of around 4000 rayls will give me .80 ish in the 90-150Hz region and around .35 at 40Hz.

So, I'm wondering whether I should try to hit the 40Hz ringing with the space behind the door. Or more precisely, I was thinking that I should make the "alcove" shelving and bass traps first, re-measure with REW, and then see how well 90-150Hz has been treated. Depending on the results , I can then decide what I want to target behind the door.

I was pleasantly surprised to hear from Jens that the amount of space behind the door could be used to target such low frequencies.

Yet I must say that I'm a little concerned about the warnings that these traps can be very difficult to make. Still up for giving it a go though! And for the reasons stated (and because porous won't help 40Hz much), tuning to 40Hz seems to make a lot of sense. OTOH, tuning to 120Hz will mean that if I get things wrong (but not too wrong) I'll still hopefully have an effect on "the dip".

Decisions decisions!

Cheers

Max

[John White]

Jens is definitely the resource for this. I couldn't get the Whealy calculator to which he linked to work on my Mac. Even so, I can never figure out if the space behind needs to be sealed or if it's just a percentage of the perforation over the absorption.

Here is another article on an array of absorption from Sound on Sound:
Practical Acoustic Treatment, Part 2

Adding to my confusion is that the referenced figure 3 as in, "Figure 3 shows the construction of the Helmholtz trap." is not available on that page. In addition I am further dissuaded by the next sentence, "By varying the percentage of perforation, the design can be applied to both the bass and mid range. However, predicting the performance of these traps is difficult because the Q or bandwidth varies depending on the amount of internal damping."

If it were just a matter of plugging in the numbers correlating to the material it sure would be easier.

Still, with "only" 14 centimeters with which to work, this seems to me to be your best option.
In theory a panel trap could target any frequency though I'm inclined to think there may be point at which the depth and or the mass of the panel would decrease the linearity of the absorption.