Bass traps and diffusers

[Spencerleehorton]

I will move some more panels to the back wall and retest, I’ll also build a few more and put them left and right to mix position and retest.
I’ll also share the delay timings.

 

[kags]

Here’s a drawing of a cylindrical bass absorber made from concrete form cardboard tube with 1” laminated wood caps on each end. One end has a hole cut with a pvc pipe inserted to tune it. The tube would be semi-filled, as described by OZ. I found an online calculator and did a rough calc for a 120Hz resonator from a 24” diameter tube that is 48” long. I guessed the inside dimensions and made a circular cut in one end cap with no pvc tube.

You should see what types of tubes are avail and take some real world measurements to input into calculator. This tube would need to be placed in an area of high pressure for the problem frequency, ie a corner or against a wall or floor. Using a cardboard tube, the cost for one tube is not much more than your velocity absorbers. A 24” diameter PVC pipe would be 5 times the cost.

Calculator link Helmholtz Resonator Calculator

Images attached

 

[Spencerleehorton]

Perhaps I could use the cardboard tube and wrap it in few layers of rock wool and cover with hessian?

 

[kags]

Perhaps I could use the cardboard tube and wrap it in few layers of rock wool and cover with

You mean rock wool inside the tube or outside?

 

[RoadrunnerOZ]

Perhaps I could use the cardboard tube and wrap it in few layers of rock wool and cover with hessian?

It won’t do anything - the resonance occurs inside a tube, which is why you use a tuned, damped internally, resonant tube.

Here’s a drawing of a cylindrical bass absorber made from concrete form cardboard tube with 1” laminated wood caps on each end. One end has a hole cut with a pvc pipe inserted to tune it. The tube would be semi-filled, as described by OZ. I found an online calculator and did a rough calc for a 120Hz resonator from a 24” diameter tube that is 48” long. I guessed the inside dimensions and made a circular cut in one end cap with no pvc tube.

You should see what types of tubes are avail and take some real world measurements to input into calculator. This tube would need to be placed in an area of high pressure for the problem frequency, ie a corner or against a wall or floor. Using a cardboard tube, the cost for one tube is not much more than your velocity absorbers. A 24” diameter PVC pipe would be 5 times the cost.

Calculator link Helmholtz Resonator Calculator

Images attached

This above diagram and calculator is for a Helmholtz absorber which is narrow band, a single port large diameter cylindrical Helmholtz is not as space efficient as a multi-port perforated face rectangular Helmholtz absorber which can sit flat against a wall.
The tube trap uses a different principle - based on a closed at one end, open at the other cylinder creating a 1/4 wavelength trap.
24” diameter is pretty much too big for practical purposes in a 3 x 4M room for a couple of absorbers.
As I was more talking about using 8” pipe (200mm) trap and not a Helmholtz which is tuned by the port diameter, port depth, tuned cavity depths and overall dimensions, the idea being to suspend a trap across the rear of the room out of the way against the ceiling. There’s a door on one side and a desk on the other of the rear wall which makes vertical corner traps non-viable in that room.

 

[kags]

It won’t do anything - the resonance occurs inside a tube, which is why you use a tuned, damped internally, resonant tube.

This above diagram and calculator is for a Helmholtz absorber which is narrow band, a single port large diameter cylindrical Helmholtz is not as space efficient as a multi-port perforated face rectangular Helmholtz absorber which can sit flat against a wall.
The tube trap uses a different principle - based on a closed at one end, open at the other cylinder creating a 1/4 wavelength trap.
24” diameter is pretty much too big for practical purposes in a 3 x 4M room for a couple of absorbers.
As I was more talking about using 8” pipe (200mm) trap and not a Helmholtz which is tuned by the port diameter, port depth, tuned cavity depths and overall dimensions, the idea being to suspend a trap across the rear of the room out of the way against the ceiling. There’s a door on one side and a desk on the other of the rear wall which makes vertical corner traps non-viable in that room.

I agree that 24” is too large- just an example. Your trap could also be modeled using the calculator I linked to. Set the size of the opening equal to the size of the pipe and length of the opening equal to the length of the pipe.

In your experience, would a single 8” pipe trap make a significant effect on the modal response of the room? How wide is the absorption of this trap compared to any panel or perf panel resonator with some internal fill?

120 Hz should be easily controlled with velocity absorber panels. My next remedy would be membrane absorbers with 1/4” plywood faces. I would start with one panel on each sidewall and each one measuring 2’ x 8’ (half a sheet of plywood for each resonator). I especially like to use these on the ceiling of 8’ rooms, where 70Hz becomes a problem at seated head height. I would build an array of panels that target 60, 70, and 80Hz to make sure any errors in freq center due to build anomalies become moot.

I’m interested in your pipe trap- I’m trying to wrap my head around how it differs in practice from a Helmholtz cylinder with internal damping.

 

[Spencerleehorton]

You mean rock wool inside the tube or outside?

i mean rockwool outside the tube making the diameter up to 24" or so?

ok i'll forget the cylinder trap idea and concentrate on getting more panels on either side and back wall to see if i can control that 120hz null

 

[Spencerleehorton]

i will lower my L/R panels as they are a bit high and retest, the bottom of the panel is actually in line with the tweeter!!
i'll also remove the small panels at wall/ceiling either side and place the bigger panels there.
will try just on the wall but high up first of all and then i will try angle the wall/ceiling ones.

 

[RoadrunnerOZ]

I agree that 24” is too large- just an example. Your trap could also be modeled using the calculator I linked to. Set the size of the opening equal to the size of the pipe and length of the opening equal to the length of the pipe.

In your experience, would a single 8” pipe trap make a significant effect on the modal response of the room? How wide is the absorption of this trap compared to any panel or perf panel resonator with some internal fill?

120 Hz should be easily controlled with velocity absorber panels. My next remedy would be membrane absorbers with 1/4” plywood faces. I would start with one panel on each sidewall and each one measuring 2’ x 8’ (half a sheet of plywood for each resonator). I especially like to use these on the ceiling of 8’ rooms, where 70Hz becomes a problem at seated head height. I would build an array of panels that target 60, 70, and 80Hz to make sure any errors in freq center due to build anomalies become moot.

I’m interested in your pipe trap- I’m trying to wrap my head around how it differs in practice from a Helmholtz cylinder with internal damping.

The tube trap is based on resonance of a closed (at one end) or open tube - it’s basically like a big Helmholtz absorber with a large mouth, not a reduced sized tuned port. The air inside the tube resonates at whatever frequency the length of the tube and the diameter of the tube dictate, the absorptive material provides the impedance to free resonance within the tube. The idea is to get the resonant frequency of the side to side here so a full side to side 3M open tube will be 1/2 wavelength ie. wavelength of 6M and a fundamental of 57.28Hz, The resonant frequency across the 3M wide room is 114.56Hz so the idea is to have a tube that absorbs the pressure from the corners and damps it within the tube. A floor tube trap closed one end 1.2M x 225mm will absorb down to roll off at 110Hz (absorption coefficient less than 1), a 3M 200mm pipe open both ends will roll off at around 100Hz. A 1.5M 200mm pipe closed one end will do the same so one open to each corner works best. The larger the diameter, the greater the efficiency, the longer the trap, the lower the frequency of absorption. Also as these absorb above and below resonance they cover the harmonics.
But the idea is to stay within practical bounds when limited by what space is available.
It’s sort of equivalent to packing the corners with heavy absorbing material. Seems to work. 1/2 room height (1.2M or 4ft) closed end vertical bass traps have been around a long time which seem aimed at ceiling reflections, other lengths target width and length. I just found a cheap way to make them and not pay silly money by using second hand water pipe, cardboard concrete form tube which is cheapest or newsprint roll cores.

 

[RoadrunnerOZ]

Perforated Helmholtz panel absorber calculator
http://www.mh-audio.nl/Acoustics/HelmholzPanelResonator.html

 

[Spencerleehorton]

here is how ive changed the panels and here is the graphs






seems worse now?

 

[Spencerleehorton]

ive measured the mix position and mic is 150cm away from front wall, 117cm from left, 135cm from right and 120cm high.
i have to have it slightly offset due to my preamp rack.

 

[RoadrunnerOZ]

ive measured the mix position and mic is 150cm away from front wall, 117cm from left, 135cm from right and 120cm high.
i have to have it slightly offset due to my preamp rack.

What angle are your monitors pointing at? They should be at 60° off lateral axis forming an equilateral triangle with the listening position. Monitor to monitor distance equal to monitor to ears. You’re getting cross room and lengthwise cancellation. What are the backs of the absorber panels - absorbent or hard?

 

[Spencerleehorton]

The only thing I have really moved is the speakers from the first measurements, I’ve moved them closer. Now they are 145cm apart.
The backs of my panels only have hessian.

 

[Electrobumps]

here are some test results, mic is positioned in the middle and i have done where my ears are and a few inches forward of that listening position and a few inches behind, thats why there is 3 plots.View attachment 119807
View attachment 119808
the bulging panel will be swapped with one of the floor stand ones, dont worry!!!

Good work on the traps. If you are replacing those traps straddling wall and ceiling I’d consider making those full depth. Although four inch panels help. When I started putting in 18” deep light insulation traps that really made the difference. I did an 18” deep 4x6 ceiling trap and was pretty impressed with what it did. You need more space though when you go that’s large. So ideal to do it on the cloud and soffits where it’s not imposing on the size of the room as much.

If you want more absorption you can leave the traps open sided. They are also cheaper to build and easier to hang. Although you need more fabric to wrap them.

Deep traps would really help with those LF problems you have. I used a calculator that gets you in the ball park for material, thickness and absorption. Will find the link later and post.

 

[RoadrunnerOZ]

The only thing I have really moved is the speakers from the first measurements, I’ve moved them closer. Now they are 145cm apart.
The backs of my panels only have hessian.

What’s underneath the hessian?

 

[RoadrunnerOZ]

Can you itemise the layers from front to rear of the panels?

 

[RoadrunnerOZ]

Looking at your readings again there are two frequencies the dip occur at - 114Hz relates to from left to right (3M) and 92Hz relates to from diagonal top to bottom across your front wall flat face (3.72M). If your panels are timber backed with only hessian cover then your angled traps at the front L/R face have created chamber resonators behind the panels and this will add to/create the problem.
Edit - this problem runs all the way down the room for the diagonals and the lateral wall to wall dimensions but hollow, hard faced cavities at the front is not good.

 

[kags]

The tube trap is based on resonance of a closed (at one end) or open tube - it’s basically like a big Helmholtz absorber with a large mouth, not a reduced sized tuned port. The air inside the tube resonates at whatever frequency the length of the tube and the diameter of the tube dictate, the absorptive material provides the impedance to free resonance within the tube. The idea is to get the resonant frequency of the side to side here so a full side to side 3M open tube will be 1/2 wavelength ie. wavelength of 6M and a fundamental of 57.28Hz, The resonant frequency across the 3M wide room is 114.56Hz so the idea is to have a tube that absorbs the pressure from the corners and damps it within the tube. A floor tube trap closed one end 1.2M x 225mm will absorb down to roll off at 110Hz (absorption coefficient less than 1), a 3M 200mm pipe open both ends will roll off at around 100Hz. A 1.5M 200mm pipe closed one end will do the same so one open to each corner works best. The larger the diameter, the greater the efficiency, the longer the trap, the lower the frequency of absorption. Also as these absorb above and below resonance they cover the harmonics.
But the idea is to stay within practical bounds when limited by what space is available.
It’s sort of equivalent to packing the corners with heavy absorbing material. Seems to work. 1/2 room height (1.2M or 4ft) closed end vertical bass traps have been around a long time which seem aimed at ceiling reflections, other lengths target width and length. I just found a cheap way to make them and not pay silly money by using second hand water pipe, cardboard concrete form tube which is cheapest or newsprint roll cores.

Thanks - I’m familiar with the math and physics of the resonator, I have just assumed that a room like that size would need a handful of these to make a dent in the modal pressure. I like the idea of some “pedestals” in corners that can double as a resonator and lamp table. Hanging them is a good way to save floor space, too. Great info!

 

[kags]

Looking at your readings again there are two frequencies the dip occur at - 114Hz relates to from left to right (3M) and 92Hz relates to from diagonal top to bottom across your front wal flat face (3.72M). If your panels are timber backed with only hessian cover then your angled traps at the front L/R face have created chamber resonators behind the panels and this will add to/create the problem.
Edit - this problem runs all the way down the room for the diagonals and the lateral wall to wall dimensions but hollow, hard faced cavities at the front is not good.

Yes- hopefully these panels have no hard backing, and they should easily reach below 90Hz.