Acoustic Treatment Planning
May 30th, 2008 by Andrew CockburnPosted in A Studio is Born
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This is where the rubber meets the road in terms of acoustics - previously we spent some time measuring the room and figuring out what is wrong with it, now we have to decide how to fix it!
As I said, this is going to revolve around bass traps and acoustic tiles, so lets take them in order.
Bass Traps - this is going to be the most important part of the treatment. I really need to get rid of those nasty resonances that will mess up both tracking and mixing, Bass traps will help! I figure that since the dip is so severe that I really need to go heavy duty here.
As I mentioned before, a Bass trap is usually a lump of flat material that is affected by bass frequencies and reluctantly moves in sympathy with them, enough to rob the waves of energy and therefore reduce reflections through absorption. Other designs are available (such as Helmhotz Resonators) but these work at very specific frequencies and I am after broadband treatment here. As a last resort, if the broadband treatment doesn’t take out that severe dip at 144hz, I may be forced to build one of these, not a task I am looking forward to, but we’ll see.
Since Bass traps aren’t particularly complicated to build, there are a number of different types out there, both manufactured, and in the form of plans for a DIY approach. I initially scanned the various websites for inspiration.
I had been looking at Auralex for acoustic tiles - they are pretty much the de-facto standard for these in the industry, and I noticed that they were selling a range of bass traps called LENRD (Stands for Low End Noise Reduction Device).
They are foam inserts that you place in the corners of your room and they act as bass traps. The idea of this was so appealing to me that I ordered several of these hoping for a quick fix. When I deployed them in the left and right corners of the room, there was little if any subjective improvement, although this was before I had made any measurements so I can’t be sure that there was no improvement at all. These devices get good reviews, but I believe they are just not serious enough to treat my particular problem easily, and I was a bit optimistic in buying them. I may still use them elsewhere in the studio, but I now need to plan some more serious bass trapping.
I liked the look of Real Traps Mini Traps they seemed like a good product, got good reviews, and they are actually based pretty close to me in Milford CT. I even briefly corresponded with one of the owners, Ethan Winer on the Sound On Sound forums without knowing it - seems like the type of company I would like to do business with but unfortunately, I have decided I need some pretty substantial treatment, probably 5 - 6 of these would be required, and at $200 each that would break the bank. So, a cheaper alternative was required.
Rod Gervais’ book included designs for a DIY bass trap, and there are also plans all over the internet for these (here is an example of a nicely laid out one). Ethan also features plans on his home pages. The DIY route is attractive because this is not a complex thing to do. Bass traps are really only a way of suspending an inert material in the corner of a room - in most cases Rockwool, or Owens Corning 703/705 insulation - these are both readily available construction materials that happen to have good absorption properties. In fact, Rod’s book gives detailed information on the absorption coefficients of various thicknesses of 703/705 and other materials - 705 seems to be marginally better than 703.
To build a bass trap you basically do the following:
- build a frame to hold the insulation
- Put the insulation in it
- Cover it with fabric to hold it in place and improve the appearance
Its pretty simple, and well within Ant’s capabilities, although it would be a little time consuming to build 5 - 6 of these. I was in the process of ordering the OC705 to build this with when I stumbled across Ready Acoustics website. They sell OC705 and 703 in various sizes and thats why I found them, but I also noticed that they make a line of Bass Trap Bags. The idea is that you buy the insulation, and then slide slabs of it into one of these bags to give a very cheap and ready built bass trap. The bag basically replaces the frame and fabric above and includes a loop and hooks for fixing. at $30 per bag, this was a great compromise. Much cheaper than buying real bass traps, much quicker than building them myself - a real aha moment!
I decided to go for a 4″ thickness of bass trap material, and ordered enough insulation and bags for 6 traps. The insulation was $130 for 6 48″x24″x2″ panels - I ordered 2 6 packs as I would be doubling up the width to 4″ per trap. So, $260 for the insulation which I would have needed anyway, and an additional $180 for the bags. I guess I could have saved maybe $100 or so on the bags by buying wood and fabric and building it myself but it would have taken a day at least and we will be on a tight schedule, so it seemed well worth it!
So now I have 6 bass traps on the way in an attractive grey color, where will I put them? At present the plan is as follows. I used a room mode calculator to check out the expected theoretical modes of my room, and it seems to show that there was a predicted mode at around 142hz associated with that back wall that the desk will be along. Given that, I am planning to concentrate treatment in this area. I am planning the following treatment:
- Bass traps in either corner floor to ceiling
- Bass traps above each monitor wall to ceiling
- A bass trap in the corner of the left hand wall and boiler room, floor to ceiling
- A bass trap in the middle of that left hand wall, wall to ceiling
I am really hoping that that amount of bass treatment will help to smooth out the response of the room.
Acoustic Tiles - a key part of any studio treatment, acoustic tiles are used to prevent higher frequency reflections which cause problems with stereo imaging, flutter echoes and comb filtering - all artifacts that will destroy a mix or tracking activity.
As previously mentioned, Auralex do a wide range of acoustic products, and their studiofoam wedges are standard for this kind of treatment in studios everywhere. I decided on the 2″ tiles, and bought 24 of their 2′ x 2′ tiles, as well as 12 1′ x 1′ tiles - this should be enough to do all the treatment I need.
A key thing to get right is absorption of the so called mirror points - when treating a mixing room, you want to prevent reflections between the speakers and your ear, and these occur wherever you can place a mirror and see the speakers in it if you are sitting in the mixing position, usually left, right and above, so this will be the first thing I will do for high frequency treatment. The mirror points occur to the left, right, above and behind. I will definitely be treating the first 3, I may leave the behind portion since that wall is 20 feet or more away from the mixing position. If I have some left over tiles I might do it anyway. Tiles will also be required in the vocal area as well, but I’ll discuss that in a later article.
