There are plenty of bookshelf speakers, anyone tried to make bookshelves that act as sound absorbers?
My main sound room is fairly reverberant and some extra absorption would be helpful.
Sound absorbers that double as bookshelves seems like an efficient idea but I haven't seen them, has any one else?
The details are that my room is about 8m x 4m x 3m (not exact multiples) and built on a concrete slab, with concrete brick walls.
Definitely no wolf will blow my house down but the bass modes are pretty severe.
The speakers are on the short wall, 2 subs placed to cancel the first cross modes and vertical modes.
The axial modes are less controlled, the wall at the other end from the speakers is broken only by one door, couple of other doors at that end in the side walls.
Book shelves on the end wall would fit the house layout, I could make it more or less all shelves.
The plan is to make the shelves from steel sheet with a C cross section, the open bottom of the shelf would be used for a limp-mass membrane absorber.
I can fit shelves 200 mm wide, thickness is flexible, maybe 75 mm.
I also plan to install some shelves between the speakers but there's a window so this wall at least has some absorption.
2 extra subs on the far wall is a further option to reduce the first axial mode but I'd like to try the shelf solution first.
The shelves will also provide a bit of extra diffusion and that should help too.
Anyone have any experience or ideas for this?
David
My main sound room is fairly reverberant and some extra absorption would be helpful.
Sound absorbers that double as bookshelves seems like an efficient idea but I haven't seen them, has any one else?
The details are that my room is about 8m x 4m x 3m (not exact multiples) and built on a concrete slab, with concrete brick walls.
Definitely no wolf will blow my house down but the bass modes are pretty severe.
The speakers are on the short wall, 2 subs placed to cancel the first cross modes and vertical modes.
The axial modes are less controlled, the wall at the other end from the speakers is broken only by one door, couple of other doors at that end in the side walls.
Book shelves on the end wall would fit the house layout, I could make it more or less all shelves.
The plan is to make the shelves from steel sheet with a C cross section, the open bottom of the shelf would be used for a limp-mass membrane absorber.
I can fit shelves 200 mm wide, thickness is flexible, maybe 75 mm.
I also plan to install some shelves between the speakers but there's a window so this wall at least has some absorption.
2 extra subs on the far wall is a further option to reduce the first axial mode but I'd like to try the shelf solution first.
The shelves will also provide a bit of extra diffusion and that should help too.
Anyone have any experience or ideas for this?
David
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I think real room mesurements are good start point.
You can get better results than I have now. 2+2 DBA in 9x4.2x3 room with some excess
windows, no EQ, no smoothing, pale curves for quite random points at 1-2 m from MS:
definitely effective solution for cuboid room if the door in rear wall does not interfere with it. I mean double bass array.
You can get better results than I have now. 2+2 DBA in 9x4.2x3 room with some excess
windows, no EQ, no smoothing, pale curves for quite random points at 1-2 m from MS:Attachments
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Door is to one side so a centrally placed sub would actually fit better than 2.
So now I think about it - the central position would not excite the 1st transverse mode either, should work well and be less expensive.
But I still want to reduce the low frequency reverberation and need some book shelves so that remains the first project.
I am a little disappointed no one has any ideas on this.
Measurements when (if) my new measurement microphone arrives.
Best wishes
David
Don't be disapointed as your idea is not common so not many people could have experimented with it.
I thought about it since you posted your idea and here is the outcome for me: limpmass absorbers can be effective but from the one i've seen they need bigger depth than the 20cm you have. They needs to be distributed over the room too, in fact most ( passive) absorbers needs to be located at place they are the most efficient in regards to the room mode you want to treat.
For this you'll have to take measurement to be sure...
And i'm not sure limpmass absorber to be the treatment most adapted to your needs ( your concrete shell is a difficult situation regarding modes as they should be indeed very resonnant).
I see two other option: first one being to 'adapt' the Qtc of your loudspeaker/sub to your room as it could help complement the system and lower the offending modes,
the second is to use a more efficient systdm than limp mass like some VPR ( same kind as Fraunhofer institute developped): they could satisfy the depth needs and are more efficient than limpmass absorber ( and at lower frequency).
That said they'll need to be located at right spot too but if you can use a whole wall to position them they should be a solution.
Who knows about or has built a vpr (fraunhofer patent) - Gearslutz
VPR - Effective VLF absorber?
In the second link you'll see post from Audiothings (Jai) which is member here too: the guy use them extensively and experimented a lot with them ( if you look at GearSlutz you'll see many post/thread about it) so he could be of help ( he is very friendly too but he is really busy).
I thought about it since you posted your idea and here is the outcome for me: limpmass absorbers can be effective but from the one i've seen they need bigger depth than the 20cm you have. They needs to be distributed over the room too, in fact most ( passive) absorbers needs to be located at place they are the most efficient in regards to the room mode you want to treat.
For this you'll have to take measurement to be sure...
And i'm not sure limpmass absorber to be the treatment most adapted to your needs ( your concrete shell is a difficult situation regarding modes as they should be indeed very resonnant).
I see two other option: first one being to 'adapt' the Qtc of your loudspeaker/sub to your room as it could help complement the system and lower the offending modes,
the second is to use a more efficient systdm than limp mass like some VPR ( same kind as Fraunhofer institute developped): they could satisfy the depth needs and are more efficient than limpmass absorber ( and at lower frequency).
That said they'll need to be located at right spot too but if you can use a whole wall to position them they should be a solution.
Who knows about or has built a vpr (fraunhofer patent) - Gearslutz
VPR - Effective VLF absorber?
In the second link you'll see post from Audiothings (Jai) which is member here too: the guy use them extensively and experimented a lot with them ( if you look at GearSlutz you'll see many post/thread about it) so he could be of help ( he is very friendly too but he is really busy).
The worst modes in the room are axial and the far wall is the location where these modes have a pressure maximum, the limp mass absorber was chosen specifically because it works on pressure and so should work well there.
I will tune the system but I'd like to improve the room acoustics as much as possible first.
Thanks for the info on the VPR, it does seem to be essentially a particular implementation of a limp mass absorber.
So I think I am on the correct track.
Best wishes
David
Well i wrote an answer which i either didn't sent or...i don't know.
Yes it does make sense.
On which design you want to base them on?
The one i've heard were based on Tim Farrant's design. The room was smaller than yours and they were distributed all around the room and part of a global approach ( including membranne absorber and helmotz resonator).
From the guy who did the room they were efficient in the 60/70hz to 150hz range. He did a lot of variation to find the ones which worked in his room.
You'll find all the info he based them on there:
Tim's Limp Mass Bass Absorbers - Gearslutz
About location of treatments one have to be aware that acoustic treatments can be... surprising here is an example:
Remove a corner bass trap and the “suck-out” went away? | Acoustic Sciences Corporation
It can be wise to have/keep flexibility in location of treatments as the kind of situation presented into the link can happen.
I'm not sure i was clear enough: i wasn't thinking about tuning the system but to see it as a couple ( room and loudspeaker) and treat it as a whole. As your room may* be highly resonant ( high Q) it could be wise to make the sub low Q to compensate and have even response.
See Adhoc1 2 comments in this thread it is close to what i try to tell:
https://www.diyaudio.com/forums/multi-way/361482-extremely-low-sealed-enclosure.html#post6373611
I'm not an expert on how the VPR works but it have been presented to me as a membrane absorber more than a LMA. I can however tell you they are effective at what they do and less location dependent than helmotz resonator from what i've heard.
*it will depend if you have doors which are leaky, roof construction,... only measurement could tell you the situation about your own room. A room (or back wall at extreme minimum as you want to locate treatment there) 'pressure map' will help to locate the treatment at the most effective place with sub located at their definitive place.
Yes it does make sense.
On which design you want to base them on?
The one i've heard were based on Tim Farrant's design. The room was smaller than yours and they were distributed all around the room and part of a global approach ( including membranne absorber and helmotz resonator).
From the guy who did the room they were efficient in the 60/70hz to 150hz range. He did a lot of variation to find the ones which worked in his room.
You'll find all the info he based them on there:
Tim's Limp Mass Bass Absorbers - Gearslutz
About location of treatments one have to be aware that acoustic treatments can be... surprising here is an example:
Remove a corner bass trap and the “suck-out” went away? | Acoustic Sciences Corporation
It can be wise to have/keep flexibility in location of treatments as the kind of situation presented into the link can happen.
I'm not sure i was clear enough: i wasn't thinking about tuning the system but to see it as a couple ( room and loudspeaker) and treat it as a whole. As your room may* be highly resonant ( high Q) it could be wise to make the sub low Q to compensate and have even response.
See Adhoc1 2 comments in this thread it is close to what i try to tell:
https://www.diyaudio.com/forums/multi-way/361482-extremely-low-sealed-enclosure.html#post6373611
I'm not an expert on how the VPR works but it have been presented to me as a membrane absorber more than a LMA. I can however tell you they are effective at what they do and less location dependent than helmotz resonator from what i've heard.
*it will depend if you have doors which are leaky, roof construction,... only measurement could tell you the situation about your own room. A room (or back wall at extreme minimum as you want to locate treatment there) 'pressure map' will help to locate the treatment at the most effective place with sub located at their definitive place.
It will be an implementation of my own that I have finally come up with, after much indecision - partly over just this need for flexibility.
The system is modular and fairly easily re-positioned
It should be reasonably simple to fabricate so it should be not too expensive and will look clean and simple too.
Oh, it definitely is resonant, obvious as soon as one listens.
A few quick tests with the speakers and a manually tuned sine wave oscillator confirmed the modes.
I will measure in more detail once the new microphone arrives.
It will be educational to see how much the doors and windows affect the theoretical behaviour.
The room and speaker are a coupled system, for sure.
But the speaker resonance can not correct for the room.
At best the speaker resonance could compensate for one mode, and only if by chance the two frequencies happened to be identical.
I plan to tune the combined system electronically because that looks both more flexible and easier.
Limp Mass and membrane absorbers are more or less similar anyway so I wouldn't debate the exact nomenclature.
The important point is that they respond to pressure and therefore are suited to be placed on a wall, exactly where I have the space.
Best wishes
David
I wasn't thinking about room modes in particular about the suggestion to 'compensate' in adapting box qtc to room resonnance: this is a trade off of energy as i see it.
The more reflective to low end the outer shell is ( resonant) the more built up of energy you'll face. This result in 'room gain': an increase of perceived output for a given range within the low end.
This is clearly pointed in the Adhoc1's link ( and what Flaesh implemented too it seems). By using a lower Q of loudspeaker you make this more easily manageable and open for interesting things: you'll trade off a bit of output from your box/loudspeaker which will switch into more lower freq of reproduction. You'll go deeper and with lower distortion as you have the room gain at play.
By itself it won't do anything about room mode ( the location can however) but it'll have a positive effect about sound quality overall.
By limiting the energy sent within the room you too lower the amount of acoustic treatment needed. This can be very nice too.
I won't discuss the way the bass absorber works too as i have not studied them enough but there is another benefit from both Vpr and limp mass which is they are 'wide band'.
By acting over a 'large' range of freq they are easyer to 'integrate'.
My experience about some Helmoltz absorber 'tuned' to room mode is that they may induce an 'imbalance'* in the way you feel the room behave. Iow it can sound unatural to me.
That is why i find often better to have multiple kind of bass absorbers mixed to make sweet spot sound good.
Anyway i wait for your findings.
* i don't know how to put words other than that one on what i experienced.
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