Smallest size for bass traps?

[TEMAS]

I've been building some new broadband absorbers recently. My first batch last year were pretty awful if I'm honest, but these ones look much better and I'm really starting to hear a difference to my room now.

I'm now thinking of building a few dedicated bass traps. But with my room being an awkward shape and not having much available flat space left in the areas I want to put them, I'm wondering at what width and height they have to be to effectively vibrate. And if they do work, would several small ones be as affective as one or two large ones?

Thanks in advance.

[bpape]

How much you need depends on the size of the room and what else is in it.

More smaller absorbers will likely yield more control overall - but not extend as deep into the bottom end as larger ones.

Bryan

[TEMAS]

Hi, thanks for the response.

Just to be clear, I'm planning to build plywood fronted, air-tight sealed bass traps, not material covered broadband absorbsers.

The reason is that I can still definately hear some lingering bass frequencies (sub 125hz) at the rear of the room. I want to really concerntrate on reducing these reflections.

So, if the traps I build are, say, 1&1/2 foot x 4 foot, or even smaller, are they large enough for my needs or am I wasting my time?

[lordmiguel]

too tough to say without knowing the room and the charts

sounds like you're trying to find out how much coverge you need without knowing the gritty details

what you want is measurement software or a consultant

or if you're just going to wing it, just cover as much as the wall as possible

if its a small room it would probabaly be better to err on the side of too much with all the work you're going to put into it

the dimensions of each trap aren't necessarily important, but coverage and placement are

also unless you know exactly where your buildups are you should at least go to ethan's forums to make a swag on walls and placement

[Dzoing]

Originally Posted by TEMAS

Hi, thanks for the response.

Just to be clear, I'm planning to build plywood fronted, air-tight sealed bass traps, not material covered broadband absorbsers.

The reason is that I can still definately hear some lingering bass frequencies (sub 125hz) at the rear of the room. I want to really concerntrate on reducing these reflections.

So, if the traps I build are, say, 1&1/2 foot x 4 foot, or even smaller, are they large enough for my needs or am I wasting my time?

Hi TEMAS, I'm doing the same thing as you (again!)!

I'd say 0.5mx1.3m (1.5fx4f) would be ok for your needs.

Center frequency fc is determined by plywood thickness + cavity depth. Panel size is 2nd order but tend to stiffen (i.e. increase its frequency response) more or less depending on boundary conditions applied: screwed, sealed, free...

More absorbing material inside the cavity will dampen it acoustically, i.e. will lower the Q factor (broaden the band of efficiency around fc).

You'll have to know precisely which frequency domain you want to treat: 100Hz-200Hz or 40Hz-200Hz? Make some measurement. You can DIY playing a "shirp" program into a speaker while recording the response in the room at an omni miced point. A "shirp" signal is kind of a "moving sinus" in which the frequency increases continuously. Looking at the recorded waveform will show you some high levels (modes) and some low levels. Just find out the corresponding frequency (e.g. using soundforge freq analyser tool): you will know the mode response levels at the recording point.

1/8" plywood will treat deeper bass than 1/4". Chances are that you'll need to cover many corners and surfaces. But you will need to EXPERIMENT, because real acoustics is not an exact science!

Keep us posted!

[Dzoing]

Originally Posted by TEMAS

Hi, thanks for the response.

Just to be clear, I'm planning to build plywood fronted, air-tight sealed bass traps, not material covered broadband absorbsers.

The reason is that I can still definately hear some lingering bass frequencies (sub 125hz) at the rear of the room. I want to really concerntrate on reducing these reflections.

So, if the traps I build are, say, 1&1/2 foot x 4 foot, or even smaller, are they large enough for my needs or am I wasting my time?

Hi TEMAS, I'm doing the same thing as you (again!)!

I'd say 0.5mx1.3m (1.5fx4f) would be ok for your needs.

Center frequency fc is determined by plywood thickness + cavity depth. Panel size is 2nd order but tend to stiffen (i.e. increase its frequency response) more or less depending on booundary conditions applied: screwed, sealed, free...

More absorbing material inside the cavity will dampen it acoustically, i.e. will lower the Q factor (broaden the band of efficiency around fc).

You'll have to know precisely which frequency domain you want to treat: 100Hz-200Hz or 40Hz-200Hz? Make some measurement. You can DIY playing a "shirp" program into a speaker while recording the response in the room at an omni miced point. A "shirp" signal is kind of a "moving sinus" in which the frequency increases continuously. Looking at the recorded waveform will show you some high levels (modes) and some low levels. Just find out the corresponding frequency (e.g. using soundforge freq analyser tool): you will know the mode response levels at the recording point.

[Edit:]1/4" plywood will treat deeper bass than [Edit:]1/8". Chances are that you'll need to cover many corners and surfaces. But you will need to EXPERIMENT, because real acoustics is not an exact science!

Keep us posted!

[TEMAS]

Originally Posted by Dzoing

Hi TEMAS, I'm doing the same thing as you (again!)!

Yeah, sometimes you have to just learn by your own mistakes!! Its the only way to fully understand why things only work if they are done a certain way. Really pleased with my latest panels though and my carpentry in general is getting much better - I built a shed from scratch in my back yard last week!!

Originally Posted by Dzoing

Center frequency fc is determined by plywood thickness + cavity depth. Panel size is 2nd order but tend to stiffen (i.e. increase its frequency response) more or less depending on boundary conditions applied: screwed, sealed, free...

More absorbing material inside the cavity will dampen it acoustically, i.e. will lower the Q factor (broaden the band of efficiency around fc).

1/8" plywood will treat deeper bass than 1/4". Chances are that you'll need to cover many corners and surfaces. But you will need to EXPERIMENT, because real acoustics is not an exact science!

This is what I thought. I totally get the physics here. The thinner the plywood, the more 'give' it has to 'wobble'. Though I would have thought that the screws wouldn't want to be too loose or else the air-tight seal won't be as good.

Originally Posted by Dzoing

Keep us posted!

Will do. Thanks a lot. thumbsup

[Chrisc_o]

I just had a thought while reading the explanation of panel thickness: if thinner panels will help you cover lower frequencies, then wouldn't something like the heat-shrink plastic film used to cover windows in the winter be a good material for this application? Cheap and easy to work with too.

[Dzoing]

Originally Posted by Chrisc_o

I just had a thought while reading the explanation of panel thickness: if thinner panels will help you cover lower frequencies, then wouldn't something like the heat-shrink plastic film used to cover windows in the winter be a good material for this application? Cheap and easy to work with too.

[Edit: sorry there was a typo. Thicker panels will help cover lower frequencies. Anyway the paradox applies: the panel should not be too heavy]

There is a paradox. Bass trap work like a damped spring-mass system (like car suspension...), cavity being spring, membran being mass, absording material being damping. The membran must have enough mass (+ self stiffness but no inner damping in order to vibrate) for the spring-mass to work. In the other hand, the membran must not have too much mass in order to be 'bass transparent enough' i.e. to let the low frequency waves pass through it, be trapped into the cavity and engage the spring-mass-damping process...

[TEMAS]

I'd also been thinking about windows and whether it would be possible to make a transparent bass trap. Not with heat shrink plastic, but something a little more ridgid or stiff, but still very thin - there are a range of thicknesses of plastic available. But are you saying that it wouldn't work because there would need to be something of mass inside? If so, how about water?!! (I'm kind of joking here).

Edit: I've just re-read your post. You're talking about the outer layer mass, not the inside. Please ignore my ridiculous transparent bass-trap idea above!!

[TEMAS]

Speaking of ridiculous ideas, what do you reckon of my panels on hinges? These offer me 3 positions: folded away (so that the double doors can open), fully open (for maximum coverage), and folded open (to store things behind and get inside the trunk/seat).

[Dzoing]

By 'transparent' I meant acoustically not optically! The panel must be rigid (not damped) and heavy enough, but not too much otherwise it would stop and just reflect the basses insteed of trapping them.

I'm sure you can build an optically transparent membran like glass or something. Did you know that windows themselves are of the best medium-bass traps (I mean with no wall behind it)!! Actually windows will trap basses while reflecting the highs: exactly what helps balancing the room response. Shhhhh... this is a secret weapon...

Congrats, your setup looks very nice!

[Ethan Winer]

Originally Posted by TEMAS

I'm wondering at what width and height they have to be to effectively vibrate.

If the front panel is too narrow it won't be able to vibrate enough. Further, with bass trapping you want as much surface area as possible. So small and effective are mutually exclusive.

--Ethan

[TEMAS]

Thanks Ethan. How narrow is too narrow?

[Ethan Winer]

Originally Posted by TEMAS

Thanks Ethan. How narrow is too narrow?

Less than two feet. See this:

Build a Better Bass Trap

--Ethan

[stuntbutt]

1/8" plywood will treat deeper bass than 1/4". Chances are that you'll need to cover many corners and surfaces. But you will need to EXPERIMENT, because real acoustics is not an exact science!

I'm pretty sure that a thinner front panel will make the effectice frequency higher, not lower.

[TEMAS]

Originally Posted by stuntbutt

I'm pretty sure that a thinner front panel will make the effectice frequency higher, not lower.

What makes you think that?

[Dzoing]

Originally Posted by stuntbutt

1/8" plywood will treat deeper bass than 1/4". Chances are that you'll need to cover many corners and surfaces. But you will need to EXPERIMENT, because real acoustics is not an exact science!

I'm pretty sure that a thinner front panel will make the effectice frequency higher, not lower.

Oh sorry you're right that was a typo. Should read:

1/4" plywood will treat deeper bass than 1/8".

[TEMAS]

Originally Posted by Ethan Winer

Less than two feet. See this:

Build a Better Bass Trap

--Ethan

Hmmm. Thats kind of put a halt to my plans.

Ethan, if you don't mind looking at the pictures above, where I've built the foldable panels, which is at the back of the control, would it work to build bass traps on the rear wall behind the existing hinged panels? What I mean is, would the bass trap still trap the bass if the sound waves it is receiving have already passed through a 4" broadband absorber? Its the only way I can see that it would be possible to make it less muddy at the back of the room, as at the moment the panels on hinges are not working as well at sub 125hz frequencies.

Any advice much appreciated.

[Dzoing]

Originally Posted by TEMAS

What makes you think that?

TEMAS, panel is Mass, air gap is Spring. The greater the Mass, the lower the oscillating of that system. Sorry for the confusing typo.

[Ethan Winer]

Originally Posted by TEMAS

would it work to build bass traps on the rear wall behind the existing hinged panels? What I mean is, would the bass trap still trap the bass if the sound waves it is receiving have already passed through a 4" broadband absorber?

It's hard to say how much more bass trapping you'll get. But the more you have, the better. Always.

--Ethan