Room acoustic treatment

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Room acoustic treatment

Postby nescafe2906 » Fri Feb 13, 2004 3:29 pm

I have 2 rooms converted into one measuring 10ft w x 8ft 7in h x 27 ft long. One half is where I sleep, it has a bed, built in floor to ceiling wooden wardrobes of 7ft length as well as a heavy wooden bureau of drawers 4 ft x 4ft x 2 ft forming a barrier in the middle of the room that breaks it up into two sections.

I'm planning to use the other half as my tracking cum mixing room. My monitors will be in the middle of the 10ft wall at one end of the room. I'm planning to put in a 14x10ft gypsum board ceiling lined with rockwool or rigid fibreglass across the half of the room I'm tracking in as a broadband absorber. My floor is made up of hardwood parquet. My walls are plaster and brick, except for a short length along one side which is gypsum in front of plaster and brick. On the other side of the wall are 9ft of windows about 4 ft in height. I'm going to get them changed to casement style and double-glazed.
With this arrangement is it neccessary to build floor to ceiling bass traps in the corners? Or is it sufficient to use 2 symetrical 6ft x 4ft closed back bookcases with half doors in front, set at an angle to the corners, with the back of the shelves lined with rockwool? The bookshelves will probably be filled with books and stuff. Is there anything else I should be doing?

Thanks
AT
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Postby Scott R. Foster » Sun Feb 15, 2004 3:30 am

AT:

Merely lining the front of bookcases will not provide the variable depth cavity that is generally believed to be an important part of why placing a panel across the diagonal of the corner from floor to ceiling works so well. Your are proposing to take the cavity out - in a sense - by fillimng it with a bookcase.

Will the rock wool facing on the bookcase still absorb - sure - and probably quite well for its thickness - it is still in the corner after all. But I think you will lose some low end effectiveness to the device as a broadband absorber without the cavity.

Though, this might be a reasonable trade off given we are talking about a dual use space. Why not do the ceiling and see what you got? If the bottom end is still boomy - skip the book case idea and go for the full Mont in the corners.

My $0.02

Good Luck
SRF
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Postby nescafe2906 » Mon Feb 23, 2004 12:03 pm

Hi Scott,

Thanks for the reply. I am actually planning to put the two corner bookcases at an angle across the corner, thus forming a cavity. I will line the back of the bookcases with mineral wool so there will be a triangle cavity with the right angle corner, and the last surface will be the back of the bookcase lined with mineral wool.

Regarding my ceiling, my contractor is proposing a plaster ceiling, lined with 2 inch, 80kg per cubic meter mineral wool. Is that okay? Or will the plaster ceiling in front of the mineral wool be too reflective?

Also, I was trying to use some pink noise run through a spectrum analyser to try and see the response of my speakers -- there was very little energy from 60 hz down, a big bump between 80 and 220hz and very little energy from 8khz onwards. However when I looked at an analysis of the original pink noise sample, there was a largely similar shape however there was a little more energy present in the low end and top end. So I rolled off the bass response on my speakers to make the bump more even. Did I do the correct thing?

Thanks and regards,
AT
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Postby Scott R. Foster » Mon Feb 23, 2004 1:35 pm

AT:

The mineral fiber behind the plaster will enhance the isolation from other rooms that this partition [the ceiling] provides, but will not materially effect acoustics within the room.

Any absorption desired as to the room's ceiling will need to added to the inside surface.
SRF
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Postby nescafe2906 » Mon Feb 23, 2004 2:27 pm

Hi Scott,

Thanks again for your reply. Somehow my contractor's suggestion felt wrong to me so I am glad to have your confirmation before I took the plunge.

In this instance, what material should I use in front of the mineral fibre boards for my ceiling? Gypsum? 3/8" Plywood? Cloth? My aim is to make a general broadband absorber to balance the wood floors -- following the advice I've read up in this excellent group (I got a lot of useful information out of the yahoo site before).

An idea I had, is to make a the ceiling slope up diagonally (to form a bass trap between the front wall and the ceiling) and then flat thereafter for the rest of the ceiling. I was thinking of 2 feet of gypsum boards running the length of the room, and the middle 6 feet covered with fabric, with mineral wool above the whole.

Is my contractor's recommendation of 80kg per m sq too dense? Should I ask him for 65kg per sq m material? According to the Alton Everest book, there seems to be little difference above a certain density, (though the thickness matters).

Regards,
AT
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Postby Scott R. Foster » Mon Feb 23, 2004 7:12 pm

AT:

The insulation above the ceiling may be useful.. I'm not sure I would taker it out... it may serve to increase isolation, and/or thermal insulation... but as I said it won't be useful as in room acoustic absorption.

As to how to treat the ceiling, the question should IMO be addressed as part of a holisitc treatment plan for the room. In general it is hard to beat standard commercial grid and coustic tile ceilings though when the application is in rooms meant for musical purposes generally they should be enhanced by adding absorptive material above the tiles - something like mineral fiber thermal wall batts laid on top of the tiles [the tiles are generally designed to work as an absorber on sounds in the office/speech range of the band - music requires that one implement treatments so that they will work lower in the band].

Good Luck

On your bookcases - I would use an open backed design and add cloth cover 703 [or equivilent] to the back of the piece.
SRF
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Postby nescafe2906 » Tue Feb 24, 2004 4:43 pm

Hi Scott,

I think perhaps I may have been a little unclear in expressing my aims. Pardon me for the crossed semantics. I am writing from Singapore, which is a tiny little island with many people and which is a little primitive in terms of acoustics. We have mostly bad sounding soundproof Karaoke booths and tiny little studios packed tightly with foam and carpet from floor to ceiling, or very run-down, horrible sounding tiny rehearsal rooms. Land is at a premium here and most people live in high-rise apartments (as I do) so most times you are looking at studio spaces averaging 300-800 cubic ft. Most of the contractors here are not English educated and also the materials used here are a little different or go by different names. I am not from the construction industry (neither are Singaporeans big on DIY so information, materials etc are not readily available) so I have to deal with trying to figure out what the materials talked about in books or forums such as these, actually are, and then translate that into what is available here. For example, when I mention "rigid fiberglass", I've been faced with just blank looks. Here, it's mineral wool, or rockwool -- so I then have to go back and try to find out whether we are both refering to the same thing, or something different and then work out the differences.

It's warm with temperatures averaging 27-32 deg C (roughly about 85-90 deg f) with 100% humidity so most of the places are air-conditioned. Therefore thermal insulation is not an issue though air-con noise certainly will be once I've got my double-glazed windows in. Currently I have traffic noise issues about 45db A weighted, in my room.

I've actually moved my room around as I described in my first post -- but without doing the windows or ceiling yet, just to get an idea of the layout and sound. I've got the bookcases angled (but without the mineral wool pads yet). Unfortunately I got closed back bookcases but the back is just 1/4" cardboard, that is tacked to the frame. These cupboards now form a sort of arc across the front wall as the two corner ones are set diagonally against the corners to form the variable space cavity.

My nearfield monitors are placed about 1 meter/3ft from the back wall and about 4 feet from the sides of the room. They are toed in such that the tweeters are 35 inches apart. I got everything wired up and did a test run last night. With the listening position forming an equilateral triangle, the sound is up front, clear and quite in your face. The bass is quite tight and not obviously muddy. (My speakers are the active Dynaudio BM6As) Even at low listening levels, the music comes out clear and strong. From a listening perspective, I would say that I have pretty good sound.

What I am looking for however, is accuracy so I know that I can trust what I am hearing when I am mixing. Since I will also use this room for tracking, I also want to make it "sound" good. I think it was you who recommended a reflective floor and absorptive ceiling to give the room a little more control and auditory space. So this is what I am trying to do. I am actually trying to approach this from a wholistic viewpoint as you advised. My room is not bad sounding, I just wish to improve it. From simple room mode calculations, it seems that I would be having some resonances around the 300hz-500hz zone.

As I don't have a high ceiling (8ft 7in), I would rather use the lost height to improve the acoustics than isolate the room. I am thinking of a wooden frame attached to the ceiling, across which are hung mineral wool batts (density to be ascertained). The surface facing the room would be cloth or plywood, or a combination. I am thinking of building a corner to corner bass trap across the front wall of the room using a piece of plywood/gypsum/cloth/combination + mineral wool insulation, laid diagonally between the front wall and the ceiling. As money is a consideration, I would really like to know whether all this is overkill, or whether I am even going about this right way, plus advice on what materials to use and how best to achieve my goals of accuracy and control in the most cost effective way. I don't have access to acoustic tiles unless I import them so they would be really quite expensive and I also have to indent any acoustic foam I want as what is available is very limited -- 1" grey egg-crate type. I would rather work with what I do have access to here i.e. common construction materials.

(I have another smaller room 13x10x8'7" which houses my library which I also want to treat but that is perhaps another topic -- that one has a lot of open back bookcases.)

Sorry for the long post and thanks again for your time and help. It is much appreciated. BTW, I am a hardcore hobbyist, not a professional but I do want to achieve professional sounding results as far as I can.

Regards,
AT
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Postby Bob » Tue Feb 24, 2004 5:56 pm

There's a picture of rigid fiberglass on this site
http://www.ethanwiner.com/oc-703.jpg

For more details about rigid fiberglass see
http://www.owenscorning.com/comminsul/p ... &system=79

absrobtion rates (125Hz 250Hz 500Hz 1000Hz 2000Hz 4000Hz):
2" 703 0.17 0.86 1.14 1.07 1.02 0.98
2" 705-FRK 0.60 0.50 0.63 0.82 0.45 0.34

There's a picture of rigid rockwool (mineral wool) on this site
http://199.202.236.133/canada/product_details.asp?id=11
(note the PDF is interesting, and has absrobtion rates)

Sort of correct: Roxul RXL40 is similar to 703, Roxul RXL60 is similar to 705. But you can compare absorbtion rates for a more accurate comparison. I believe that RXL is heavier than 700 series, so it requires stronger mounts.

Also this forum's area
http://forum.studiotips.com/viewtopic.php?t=48
has some chats about the material, including the above owenscorning link.
Regards
Bob Golds
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Postby Scott R. Foster » Wed Feb 25, 2004 1:56 pm

AT:

Rockwool should work fine for your purposes [see the FAQ for details on desnities].

I'd pull the back off those corner bookcases and replace it with cloth covered Rockwool [at least 50mm thick - a bit thicker would be nice].

Your ceiling treatment plan sounds like it might help a good deal to tame that mid-band hum. I'd do the corners and add some absorption to the ceiling... try and minimize early reflections [see the FAQ]... and make sure the rear wall behind the mix position was broken up [diffusive] in some way.

Try those measures and then let your ears test your work.

Good Luck

PS: What kind of tunes are doing and how are you recording - to a PC?
SRF
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Postby nescafe2906 » Wed Feb 25, 2004 4:22 pm

Hi Bob, Scott,

Thanks for your replies and the links. I've actually visited the ethanwiner site before, visited recording.org etc but the other links are very useful, thanks again.

Scott, I 'm going to try what you suggested with the back of the bookcases. I'm going to shoot for a 1/4 plywood+mineral wool bass-trap across the entire front diagonal of the room and cloth covered mineral wool for the rest of the ceiling.

I've only started recording about 2 years ago. At first I simply recorded jam sessions (5 piece band: drums, bass, guitar, keyboards, sax) to two track CD in my living room. Then i started wanting to do much more. So I started delving into hard disk recording.

So far, I've done mostly acoustic and jazz stuff. I've done simple minus one projects for friends (which is when I encountered guitar hum) My main projects actually have been recorded on site on my PC based laptop -- I recorded an entire unplugged concert that a friend of mine gave in the newly built Esplanade Theatres on the Bay last year, and I'll probably get a shot at recording the Singapore Symphony Orchestra in April.

I also want to try some pop and rock music. Some of the newer (and not so new) songs have some fantastic sound design and arrangement and I would like to find out how to get that sound for e.g. the arrangement on the Cassandra Wilson cover of "Harvest Moon" still takes my breath away every time I listen to it -- it's so evocative.

So far, I've spent quite a bit on equipment and a fairly nice microphone collection (for a project studio that is). However I don't have a good room to properly use those microphones! And then when I started mixing down the concert, (on my main PC based DAW running Cubase SX) I realised that I couldn't really hear anything properly because my "studio" was set up in a corner of the bedroom. In the end, by sheer dint of mixing and re-starting over and over again, mixing, and remixing and remixing, I learnt what to look out for -- and I counterchecked every mix across my living room stereo and on headphones.

(Now however, in my new listening position, I can hear some boominess around 200hz in the piano. Maybe I'll do the mix yet again!)

After that experience, I knew that I had do something about my mixing and recording environment. And that's when I started boning up on acoustics. That was about 6 months ago.

Everything I know so far about engineering/acoustics, I've actually read up from books or from the internet. I love the internet for this, it's great!! I'm actually a classically trained pianist and I've played various genres in various bands when I was younger. I haven't played much in years (until two years ago, I was so busy at work I did no music at all) -- however I have been starting to practise the piano a little the last few months. But sound engineering and acoustics has so far been really fascinating!

I've been lurking around for two years at various web sites, and this topic is actually the first time I've posted anything on the net. I'm very happy that you've shared your knowledge direct with me.

I'll be meeting my contractor again tomorrow. Will keep you posted on my progress and maybe post some pictures once I figure how to do that.

Regards,
AT
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Postby Scott R. Foster » Thu Feb 26, 2004 12:49 pm

AT:

I'd skip the plywood on the bookcases and just line the back with cloth covered panels of thick mineral fiber.
SRF
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