I was wondering if it would be possible to reduce troublesome room modes with a simple box containing a passive radiator? The idea would be that the PR would be tuned to the room mode, and would resonate at that specific frequency in the location that it is placed.
Would it be best ported or sealed? Would it even be feasible? Seems like a low-cost alternative to a membrane resonator for low-frequency applications (and it would take up a *lot* less space than traditional absorption).
Would it be best ported or sealed? Would it even be feasible? Seems like a low-cost alternative to a membrane resonator for low-frequency applications (and it would take up a *lot* less space than traditional absorption).
Hi,
it may work ( i know some guy which use 'cheap' chinese PA speaker that way as bass trap). Ported will be the obvious choice to me but i may be wrong. If you already have the passive radiator you should experiment about it.
I would tune the port and the passive radiator/box for frequency of interest and measure the room at boundaries to find the best location. I would expect better efficiency than an helmotz resonator without the radiator for a given size.
Worth an experiment in my view.
it may work ( i know some guy which use 'cheap' chinese PA speaker that way as bass trap). Ported will be the obvious choice to me but i may be wrong. If you already have the passive radiator you should experiment about it.
I would tune the port and the passive radiator/box for frequency of interest and measure the room at boundaries to find the best location. I would expect better efficiency than an helmotz resonator without the radiator for a given size.
Worth an experiment in my view.
Not to discourage you ... but if the passive radiator is being activated by sound in the room, it would be in phase with that sound and might even accent it. Also consider that it's activation might be at a rather low level, due to small surface area, and would not have a large effect, even if it did work out of phase.
That said, it is an interesting experiment ... can you let us know the outcome?
That said, it is an interesting experiment ... can you let us know the outcome?
So a membrane resonator ( which is not widely different from what Alekk is talking about ) is not useful or doesn't work?
That said i think you made a point about surface area. Membrane resonator i've seen implemented were usually the full size of a wall and with a cavity in 20 to 80cm depth behind them and their membrane weight was high.
Of course it all depend of the frequency they should take care of.
What i've had in mind was something like a 6th order vented box. You'll have both the traditional helmotz resonator at work and within it's own space inside the box the passive radiator ( which could have some weight attached to it to increase the overall efficiency) and it's own closed box fullfiled with absorbent material.
That said it would be tuned so useful for a narrow band which is very specific.
Alekk, if you want low bulkyness and wider band of action limp membrane absorber are strong contenders.
Not really different than your idea except you use much more heavy material as membrane over a enclosed cavity filled with absorbent material. They are then located against wall where we are in pressure mode so they transduce acoustic energy into mechanical which dissipate as heat.
Working example:
Tim's Limp Mass Bass Absorbers - Gearslutz
Last edited:
I've actually been using Canvas Art (click the link) which is a form of limp membrane absorber, in several of my later installations, with pretty good results.
What this is, is a fabric canvas over a wooden frame. It places the canvas about 1 or 2 inches from the wall.
It generally works well in limiting echos and reflections. Plus it looks a lot better than the stupid foam egg crates.
The one in the thumbnail hangs in my living room listening area...
Attachments
Last edited:
I've seen them Douglas and thought it is a great idea and they look great. Wesayso had used same kind of thing too cover his midband absorber ( he has a reproduction of led zeppelin's 'house of the holy' printed over fabric, i'm jaleous i not had the idea first!**).
Do you use some kind of absorbing material behind it Douglas? You can space them from wall to increase frequency lower in frequency too ( down to 200/300hz varying with space gap. Pretty effective.)
Limp membrane are probably a bit different in the way they work.
Take a look at the previous link ( to gearslutz, there is some pdf to build some and rough guide how to locate them)they use as membrane some very heavy visco elastic material* over a closed box ( hermetic). They are wide band low frequency absorbers, smaller than typical helmotz resonnator.
* eg: Auralex 'sound barrier'
**: help to keep peace in the family.
Do you use some kind of absorbing material behind it Douglas? You can space them from wall to increase frequency lower in frequency too ( down to 200/300hz varying with space gap. Pretty effective.)
Limp membrane are probably a bit different in the way they work.
Take a look at the previous link ( to gearslutz, there is some pdf to build some and rough guide how to locate them)they use as membrane some very heavy visco elastic material* over a closed box ( hermetic). They are wide band low frequency absorbers, smaller than typical helmotz resonnator.
* eg: Auralex 'sound barrier'
**: help to keep peace in the family.
Last edited:
I've tried that experimentally and to be honest it turned out to be more trouble than it's worth. I'm guessin, of course, but it seems like 90% of the improvement comes from just the standard, out of the box, artwork itself. Adding foam or pollyfill might make a difference but nothing I could actually measure...
This is accords with what I've discovered. Sometimes we just let them hang in free space. It is a bit odd to see a painting haning 1 inch out from a wall so most people will want a secondary frame installed behind the main one.
Of course... I'll look into it furhter. I suggested the canvas art because it's reasonably cheap, readily available and it's not ugly
Empirical determination of acoustic material is a pita i agree. But we are lucky this is science driven field and so we have access to simulator which are quite effective (given you can have access to some specs of materials used).
Foam could be used yes but this is almost always pricey and not visually suited to domestic use. Polyfill may work but there is better alternative material (read more effective).
Diy'er usually use glassfiber/wool. It is easy to source, cheap, effective and you have access to parameters easily. That said this may well be a source of concerns if it is not used in a closed enclosure.
In pro field instead we use stone wool (Rockwool) for obvious reason: the fibers are way bigger so much less a concern for your lung, and material is usually heavier so more effective for a given size and last but not least it won't hitch you when handled. That said it's not cheap.
An great alternative to both is wool done by recycling fabrics. In France we have this for example:
Metisse(R), l'isolation durable a la protecion feu naturelle ! | Metisse
It is effective, is good for our grandchildren and our society ( in France it is collected, treated and done by person in social 'trouble'),it have spec availlable and won't produce any concern relative to health. Not the cheapest alternative but given all other advantage it have, it is the obvious choice to me.
For calculator you have:
Porous Absorber Calculator
Or
HiFi Loudspeaker Design
Your own case is a bit different as this is not strictly speaking a porous absorber but even membrane resonator benefit from absorber located within the enclosed space behind them ( explord this through the different option availlable in first link).
For your membrane resonator see^. For porous absorber it is important to not enclose the thing or it'll loose efficiency or won't work.
Of course... I'll look into it furhter. I suggested the canvas art because it's reasonably cheap, readily available and it's not ugly
That is why i mentioned Wesayso recipe: you can cover the front membrane ( just leave a small gap to let the membrane ability to move without being restricted) by a printed fabric whatever fabric it is ( within reasonnable limits).
Given the wavelength involved it won't change this much the behavior ( and it don't have to be 'acousticly transparent' either).
This is not always true in any acoustic treatment however, sometimes it does matter a lot to have acousticly transparent fabric.
@krivium ...
The canvas art is not a porous fabric surface. Best I can tell without destroying it is that, mostly, it's a lightweight canvas fabric that is either silk screened or printed over with an acrylic ink. The surface is sealed, which is probably why backing materials have little effect.
I've put a few of these in first reflection spots and when the owner is getting all carried away showing off his system and trying to ruin my hearing, you can actually feel them moving. The movement appears to absorb energy reducing the reflections, which is why I thought they might be germaine to this thread.
The canvas art is not a porous fabric surface. Best I can tell without destroying it is that, mostly, it's a lightweight canvas fabric that is either silk screened or printed over with an acrylic ink. The surface is sealed, which is probably why backing materials have little effect.
I've put a few of these in first reflection spots and when the owner is getting all carried away showing off his system and trying to ruin my hearing, you can actually feel them moving. The movement appears to absorb energy reducing the reflections, which is why I thought they might be germaine to this thread.
This is what i understood Douglas.
But i repeat the membrane resonators i've seen implemented used some porous material in the cavity beside the fact the membrane was made from wood or plasterboard. It increase the absorption. That said they are almost always to treat low frequency in studio, not mid/hi from whqt i've seen.
About ER this is nice and expected but usually porous absorber are used at this point as what really matter is 1khz and up to be treated ( lower could be even better but not easy to implement from what i've seen, it is often done by an RFZ ( angling of wall to redirect ER outside the listening spot)).
I don't say your membrane absorbers don't work ( i think they are probably effective in a lower range than 1khz). I just point what is used in pro field and why. Anyway here again a little experiment is easy and will tell you what to expect from simulation then make a test in situ and see what you prefer.
An easy to use material is the ceiling tiles from Rockwool ( compressed maybe 2cm ( 1") in height with a fabric cover black or white colour. They absorb almost everything from 1khz. With a bit of space gap you can lower maybe to 500/700hz. Worth an experiment.
But i repeat the membrane resonators i've seen implemented used some porous material in the cavity beside the fact the membrane was made from wood or plasterboard. It increase the absorption. That said they are almost always to treat low frequency in studio, not mid/hi from whqt i've seen.
About ER this is nice and expected but usually porous absorber are used at this point as what really matter is 1khz and up to be treated ( lower could be even better but not easy to implement from what i've seen, it is often done by an RFZ ( angling of wall to redirect ER outside the listening spot)).
I don't say your membrane absorbers don't work ( i think they are probably effective in a lower range than 1khz). I just point what is used in pro field and why. Anyway here again a little experiment is easy and will tell you what to expect from simulation then make a test in situ and see what you prefer.
An easy to use material is the ceiling tiles from Rockwool ( compressed maybe 2cm ( 1") in height with a fabric cover black or white colour. They absorb almost everything from 1khz. With a bit of space gap you can lower maybe to 500/700hz. Worth an experiment.
Last edited:
I think I understood that much. But thanks for the clarification.
These art panels appear to have the most effect in the voice range, which quite nicely corresponds to where our ears are most sensitive to detail. I'm not saying they're the be-all and end-all, but they do have a beneficial side to them that people who've used them (at my suggestion) do seem to like.
I think it's fair to say that you approach this more scientifically than I do and probably in a different arena... I'm doing consumer stuff so I do try to keep things nice for the client. There are things I've seen done in studios that would send my guys off screaming into the night ...
I'll let you get back to the passive radiator discussion...
Thanks for the chat.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.