Avoiding traps - AVS Forum
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post #1 of 23 Old 10-17-2013, 06:15 PM - Thread Starter
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Curious to know if a person could angle drywall across corners and at the ceiling and floor instead of spending big money on bass traps.

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post #2 of 23 Old 10-17-2013, 07:27 PM
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If you want to trap bass, you need bass traps. smile.gif The corner isn't causing bass problems, so getting rid of isn't helping.

You don't need to spend big bucks on bass traps - you can build them yourself, and they are among the most economical improvements you can make to your audio.
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post #3 of 23 Old 10-17-2013, 10:57 PM - Thread Starter
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I've miss understood the idea then.
If corners aren't the problem how does adding a trap help?

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post #4 of 23 Old 10-18-2013, 02:20 AM
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Sound pressure is greatest at room boundries (walls). Where walls meet (corners), sound pressure levels are even higher, since you are combining more than one boundry. Hence why people put bass traps in corners or tri-corners or where ever two boundries (wall & ceiling) meet.

It's not the corner itself that creates bass. It's your subwoofer that creates bass. The corners is where it is strongest, making it a more effective location for trapping (absorption).

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post #5 of 23 Old 10-18-2013, 06:46 AM
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I would say you absorb bass in corners because you absorb every modal resonance at the same time (with the same trap) - but the details of the reason don't change the practicality of the recommendation.

The bass response improves as a result of absorption because "ringing" (like a bell) has distorted the response. The ringing occurs as a result of standing waves (resonances) established between room boundaries. Absorbing the resonant energy allows the response to change back (closer) to what it was designed to be.
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post #6 of 23 Old 10-18-2013, 08:27 AM
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Multiple subwoofers = best method for even response across multiple seats
Broadband bass traps = reduction in modal ringing to 0.4 sec or so ; give that clear bass impression
...........modal ringing issue best viewed in waterfall chart

Link in my sig shows how to easily build DIY corner bass trap with cheap pink fluffy
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post #7 of 23 Old 10-19-2013, 03:33 PM - Thread Starter
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Can you have too many bass traps?

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post #8 of 23 Old 10-19-2013, 04:46 PM
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Theoretically maybe - practically, almost never.
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post #9 of 23 Old 10-20-2013, 08:39 AM
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Using room dimensions that do not cause room modes to double up, such as prime or "golden" ratios of dimensions, can help. Larger rooms move the modes lower, which may help (or not). Adding angles creates more complex reflections and additional modes but does not in and of itself improve modes, particularly in the low bass since wavelengths are so long (a quarter wavelength at 100 Hz is about three feet, about 6' (2 m) at 50 Hz).

Bass traps that are broadband absorbers can kill the HF sound by eliminating all reflections in the room, making it seem lifeless to some. That just means any ambiance is from the recording and not added by the room. Opinions vary. Bass traps designed to trap only bass often include a membrane (often a piece of plastic) on the front to provide some HF reflection.
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post #10 of 23 Old 10-20-2013, 11:50 AM
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Quote:
Originally Posted by brwsaw View Post

Curious to know if a person could angle drywall across corners and at the ceiling and floor instead of spending big money on bass traps.

Yes.

Rooms with complex boundaries will have a tendency to even out room modes and boundary reflections, but there will still be low-order modes.
This is rather complex to predict with accuracy, as the en result depends not only on room shape but also wall construction.

In domestic rooms the idea of acoustic treatment using bass traps and absorption panels is rather new, in the old days one had to rely on furniture and room construction to get good sound.
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post #11 of 23 Old 10-20-2013, 08:09 PM - Thread Starter
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Can you use the space above a T bar ceiling as a trap?

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post #12 of 23 Old 10-21-2013, 03:20 AM
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Quote:
Originally Posted by brwsaw View Post

Can you use the space above a T bar ceiling as a trap?

You mean like this?http://ethanwiner.com/acoustics.html
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For a typical unfinished basement ceiling you can take advantage of the gap between the support beams and the floor above by placing rigid fiberglass between the beams. Short nails or screws can support the fiberglass, making it easy to slide each piece of fiberglass into place. Then cover the fiberglass with fabric as shown below in Figure 3b. You can optionally pack the entire cavity with fluffy fiberglass one foot thick and you'll probably get similar results. Top
Treating a basement ceiling
Figure 3b: 705 between support beams, covered with fabric.

Treating a "dropped" grid ceiling is even easier: Simply lay fluffy fiberglass batts on top of the grid, above the ceiling tiles. The thicker the fiberglass, the better. One foot thick R38 is perfect for this if you have the space. If you don't want to bother covering the entire ceiling that way, at least put fiberglass batts around the perimeter to treat the important wall-ceiling corners. And since the fiberglass is not exposed to the room and doesn't show, you don't need to cover it with fabric.
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post #13 of 23 Old 10-21-2013, 10:55 AM - Thread Starter
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Yep. Thanks.

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post #14 of 23 Old 10-21-2013, 11:06 AM
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Note the cloth "ceiling"; the traps are far less effective if you drywall over them. smile.gif

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post #15 of 23 Old 12-05-2013, 10:55 AM - Thread Starter
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Quote:
Originally Posted by DonH50 View Post

Note the cloth "ceiling"; the traps are far less effective if you drywall over them. smile.gif

Back to traps in ceiling, can a person have both, sound isolation and traps in the same cavity? Maybe a heavy layer of carpet underlay (www.soundown.com/carpet%20under.htm) on the bottom of the floor wrapped down the sides the joists (all 1 piece per opening) then 2 layers of insulation, then screen/ mesh/ velvet, etc?
I'm thinking I'll try, it can't be worse than no insulation between the joists.
It wouldn't be over the entire ceiling, likely just around the edges where the wall meets the ceiling.
Thoughts?

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post #16 of 23 Old 12-05-2013, 11:17 AM
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if you want sound isolation and hidden bass traps, then consider a full drywall ceiling suspended on risc clips and green glue double drywall, then add soffets to the inside of your sound managed room and fill those with pink fluffy and lined with GOM fabric. You can then have a light tray ceiling also.
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post #17 of 23 Old 12-05-2013, 03:31 PM - Thread Starter
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I'll need to consider another method.

I have thought about raising my towers and using the 18" x 18"x 9' box as a trap. Any chance this would work or be of benefit to the SQ of the room?

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post #18 of 23 Old 12-31-2013, 09:23 AM
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Quote:
Originally Posted by mtbdudex View Post

if you want sound isolation and hidden bass traps, then consider a full drywall ceiling suspended on risc clips and green glue double drywall, then add soffets to the inside of your sound managed room and fill those with pink fluffy and lined with GOM fabric. You can then have a light tray ceiling also.
how much would the DD+GG on clips and channel act as a tuned bass trap? I have a drop ceiling with r19 above it in between the joists. I want to move to a DD+GG on clips ceiling for aesthetic and sound isolation reasons. I don't have much room for sofits and I am curious how the overall bass acoustics inside the room are likely to be affected by the ceiling change I described?

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post #19 of 23 Old 01-28-2014, 03:55 PM - Thread Starter
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Quote:
Originally Posted by brwsaw View Post

I'll need to consider another method.

I have thought about raising my towers and using the 18" x 18"x 9' box as a trap. Any chance this would work or be of benefit to the SQ of the room?

I've lined up 6 of these, NIB, www.questhifi.com/av4b.html for $50 a piece.
I'll use parts from all 6 units, 3 high for equipment with the PJ on top (in the equipment room) and a two shelf unit below each tower to bring the center line between the tweeter and mid in line with the center of the image.
I wonder if it would be of any benefit to add a layer of Roxul between the shelves.
Is there a potential bass trap in the bottom of each of these?

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post #20 of 23 Old 01-30-2014, 09:26 PM - Thread Starter
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Big bump

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post #21 of 23 Old 02-03-2014, 02:51 PM
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Lots of good info, but very little explanation! Let me try to fix that...

First, do you have mesurement gear? Can you measure your room's decay characteristics (waterfall or spectrogram plot)? This is critical to the selection of acoustic treatments, because all treatment options have a limited frequency range of absorption. The goal of room treatment is uniform decay rate with frequency. If you have a bass-only problem, and use something like the rafter treatment in post 12, you'll find it's sucked the life out of the mids, too. The treatment had broadband effect, but you only needed low bass.

Next, understand that acoustic treatments are not created equal. The design and placement of an absorber determines its effectiveness as a function of frequency. There are two general kinds...
- resistive absorbers, like fiberglass, that work by resisting the flow of air in a sound wave
- pressure-based absorbers that work by absorbing some of the wave energy, reducing the amplitude of the reflected wave

Now, let me digress into a bit of wave theory, so the next part makes sense. I think of a standing wave like a jump rope, oscillating between high and low in the middle of the room, but not moving much at the ends, where the walls are.
This is like the speed of air molecules in a sound wave; lots of movement, mid-room, but none at the walls.
The correlarry is that there are pressure maxima at the walls, and a pressure minimum in the middle of the room.

Resistive absorbers work best where air flows a lot, out in the middle of the room, away from boundaries. The closer you put them to a wall, the less low frequencies they absorb. However, this spatial dependence gives you the ability to "tune" the absorber response to your needs by simply spacing it away from the wall.

Pressure-based absorbers work best where there's no air flow, so at a pressure maxima like a wall or corner. These are 2-part designs, a resonant membrane or pipe plus a resistive absorber-filled cavity. Helmholtz resonators are identical to ported speaker enclosures, are tuned by volume and port size, and have similarly narrow band absorption. Diaphragmatic absorbers are membranes over resonant cavites, are tuned by both diaphragm resonance and cavity resonance, and generally have a couple octave range. Windows and deformable walls (sheetrock) are a common form of diaphragmatic absorber.

My room is naturally devoid of bass resonances because it's above grade, with wall paneling mounted on standoffs, an open attic above, and a 15' sliding glass door. No bass treatment necessary, but also very poor sound isolation; I can hear car doors, and I'm sure the sub is audible outside.

With a sound-isolated room, you minimize sound leakage, so you have to make up for the added energy that leakage would normally take away. If you're in a basement, you'll find that bass is reflected very well. The best place to start is still measurement, because you can reasonably expect to need absorption from DC on up, but have to account for thigngs like furnishings.

I hope this helps!

Have fun,
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post #22 of 23 Old 02-03-2014, 04:01 PM - Thread Starter
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Thanks for the reply.
In the very near future I will be installing my speakers and equipment in a new dedicated space in our basement.
I've learned a lot here and on other forums, enough to know it will never be perfect.
That said I still find myself stressing over the little things and if something I can afford will make a difference than I'm all in.
Although against what you've written above any measuring equipment beyond a calibrated SPL meter will come after bass traps and perhaps a couple panels at the first reflection points.
I haven't crossed the hard core line yet, I'm exited to watch and listen as is and then see what happens.

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post #23 of 23 Old 02-04-2014, 04:25 AM
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Great info above. Here are a few articles and videos about how bass traps work and different kinds of trapping.
http://www.gikacoustics.com/how-bass-traps-work/
http://www.gikacoustics.com/understanding-different-bass-trapping/
http://www.gikacoustics.com/video_bass-traps/

Glenn Kuras
GIK Acoustics

http://www.gikacoustics.com

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