Step Five: Isolation and Mode Analysis

So it’s been a while 🙂 now we are going to look at our room from the perspective of increasing the isolation and doing the initial mode analysis of our room.

From an isolation perspective we can do several things: one – increase the mass of the room boundaries – walls, floors, ceiling and windows and doors; two – seal everything really well – doors, windows, electrical and other boundary penetrations; and three – decouple noise and sound creating elements from the surrounding boundaries to reduce the amount of transfer directly through the structure. And in our example room we really cannot do much else if we’re renting (for example and very commonly asked about) or even if we own it, it might not be within our budget.

Adding mass to the walls and ceiling is pretty straightforward – we add some more drywall layers. if we choose this, we probably need to strip out the trim and outlets etc so we can add the drywall, and maybe it’s just a single 5/8″ (16mm) layer, and replace the outlets, switches, fixtures, trim etc. if we have access under the floor because it’s joist and subflooring – we can add layers of drywall between the joists (sometimes we line this with “blueboard” foam insulation to even it out because of nails) and seal it all up.

Remember – check with a properly licensed structural engineer before adding this extra weight to your building and of course obtain any necessary building permits and inspections needed to correctly complete this work, maintain your certificate of occupancy and your insurance coverage.

Increasing the mass on the windows and doors can take several forms but in our example we simply create a window plug to cover the window (like heavy indoor shutters) and seal up the doors. we could add a layer of MDF to the doors to increase their mass, and if they are hollow doors, replace them with solid core doors first. Increasing the weight of the doors means we need to also look at the structure around the door to make sure it is sturdy enough and also we may need to replace the hinges as well. we can make our own stops and seals or buy them from companies like Zero International.

E.g. Increasing our room isolation

Once we finish sealing everything, adding mass where we can, then we need to consider the air flow – in our example room we only have supply vent and no return as it’s a common return for the house. in this case assuming we need some isolation on the ducting, we can build an in-room plenum to isolate the supply from the rest of the house, perhaps big enough to slow the air speed down, and advantageously direct the air flow in the room. for the return, we could build an in room or in hallway plenum which vents into the common space so the return operates as expected. this return plenum could include a fan to ensure positive air flow out to the common area or it could be passive.

With isolation design out of the way (and please don’t consider this a trivial step – good isolation like anything else can be difficult to achieve when you have limited options and budget) the next step is to consider what we need to do about acoustic treatment design. the purpose of the design effort here is really to identify your main options for the acoustic treatments and get them incorporated into your budget and planning. what you will do is measure the room response after you complete the isolation work so you K NOW what you’re really into. plus the testing will also help identify any remaining defects in the isolation.

From a design perspective, we know that small rooms have a smaller number of modes which have enough space between them to be problematic and the proximity of surfaces causing interference and reflections on our mixing activity which often results in less than ideal translation on other systems. on the other hand we also need to consider the lack of room volume and our need for other considerations in the use of the room – maybe we record acoustic instruments, or someone in the home uses it for painting and bookkeeping. so we want to make some educated guesses as to how the room may respond and also what type of treatments we can add to fit our room requirements and budget.

With the room laid out in SU, it’s easy enough to download my mode template (http://sketchup.google.com/3dwarehouse/details?mid=80c3a2c3b34f8cfc6d2f7f4796305f20) and scale it to fit the floor and two of the walls (long and short wall) and see not only the main axial modes but also first choice positions (like the 38% rule) so our subsequent design work can be checked.

E.g. Gullfo’s SU model template on the floor

With the main modes identified, we can also use several spreadsheet calculators (or Bob Gold’s online mode calculator) to verify our room characteristics such as tangential and oblique modes, first reflection points, etc. it can help to have some of the key equipment specifications handy when calculating reflections, lowest frequency response from your monitors, possible desk reflections, and speaker positioning etc.

E.g. Chris Whealy’s Control Room Calculator

Armed with this information, in the next post we’ll start to layout the room based on our assessment. we will start with standalone treatments which are typically commercial products such as those from GIK Acoustics or Real Traps, or they can be DIY treatments (we’ll start with POA (plain old absorbers) and think through more sophisticated units such as VPR or limp membrane absorbers). A subsequent post we’ll look at the use of integrated treatments and understand how they work together to go even further in shaping your room response.

Step four in the recording studio design process

Step four has arrived: you’ve worked out what you want, you budget and timeline, and given some thought to how you will use the studio, and how you will get it built. in this post, we’ll look at how to capture the relevant space in both a simple hand sketch, and then using a tool (like Google SketchUp – a free version is available @ http://sketchup.google.com/download/) to model your existing space. this sets the stage for then laying out your space and ideally having some folks comment on it (John Sayers’ Studio Building Forum (http://www.johnlsayers.com/phpBB2/index.php) is hands down the best resource for this).

For our discussion we’ll assume you have a spare room in your house which you want to convert into a control room where you could also record some acoustic instruments but most of your work is done electronically into a digital audio workstation via some analog-to-digital converter boxes/mixers etc.  we’ll assume that you do not plan on very loud mixing sessions (since you like your ears and only turn it up to do final tweaks), and your room is on the first floor over an unfinished basement (which for some reason you cannot use…) and is bounded by the outside wall, living room, and another room. your neighbors are about 30ft (10m) away and your ambient noise levels outdoors consist of some car traffic and little or no air traffic can be heard. since it is a project studio and not a commercial studio, and you cannot re-structure the house, you’ll make best efforts to add some isolation and keep your loud work for daytime hours when it is acceptable. otherwise you are really looking for good acoustics for mixing and the infrequent acoustic guitar or horn instrument.

You set up your monitoring equipment and a recording which you played at 100db (based on your sound meter) (which is what you think is as loud as you intend to get) and went outside to measure how much isolation you need. except for the near (1m) window where you were reading about 85db, the overall sound level by the neighbor’s house is about 50db so you know the window needs to get some attention but overall not too bad. inside the house, the living room and other room by the closet were reading about 70db so you know you need some work. in the basement is was around 70db but it is all storage and the leakage out the basement windows is lower than the room window but you might consider some isolation under the floor since once you fix the room window the basement will be the main source of leakage.

the following figure is what you drew on some graph paper.

you are not too concerned with getting it scaled to match the grid lines on the paper as much as keep it neat and making sure all the measurements are accurate and all the important room retails are included. in looking at the drawing we see a few things might be missing (some measurements and a light switch for the closet) but in general it has most of the information we need to start modeling the room. one thing is immediately apparent: the HVAC uses a supply into the room and the return is located in the hallway and the airflow is under the door. to seal the door and reduce the sound into the hallway, you may consider adding a return into the room or possible venting outside.

the next step is to create a model in SketchUp (SU) or other tool (AutoCAD, etc) so you can readily visualize the design and it makes working with people online much easier. in general, start with the biggest details and work down. this means draw a box which are the main room dimensions. then add the smaller closet box and remember to factor in the partition wall when creating its depth. now you have two boxes. using the offset tool (in SU) you can expand the room to factor in the drywall, framing depth, etc. once you trim and adjust the lines to reflect a good 2D view of the room from the top, you should save that as your first template. this way if you need to re-start from that point, you can simply open up a new file based on it and re-work it.

then you can add the framing, drywall, electric components, doors, etc. until your room looks like, er, your room! if this is too much detail for you, consider just working off the 2D floor plan. the following figure shows a good 3D view of the room including the relevant bits like outlets, light fixtures, and HVAC  registers are so you can do your design around them or know what you need to move.

next post we’ll start to put in some design ideas for isolation and ventilation so we can prepare for the acoustic treatments. meanwhile, get measuring! start playing with SketchUp, and start visiting and learning on the recording studio design forums!

Do I need a designer?

Of course you do! but that doesn’t mean the designer isn’t you! without wanting to use “it depends”, sorry, it depends. to be an effective designer of a complex studio construct where isolation, treatment, layout, aesthetics all play important roles, it will take a lot of study and practice to create a proper design. and if physics and acoustics and construction engineering are not your primary skill set, then its an uphill road. on the other hand, if you are simply looking to treat an existing room using DIY or commerically available acoustic treatment products (such as GIK Acoustics, Real Traps, etc), you can probably learn what you need to know in just a few weeks of self-study on the product sites and main acoustic forums.

You might need to learn a bit of math and some terminology but the essentials are learning to accept room modes as the primary problem area for small spaces, and getting the RT60 adjusted for your space. a room mode is simply the result of sound waves being trapped within a set of walls, floor, and ceiling. RT60 is the reverberation time. usually, once you add the treatments for room modes using broad band treatments (broad band meaning wide number of frequencies) you can often find that RT60 and miscellaneous echoes also come under control. in “dry rooms “(low RT60) you might need to add some panels (like plywood or boards) to get some life back into the room. in “wet rooms” (large RT60 or lots of echoes) you will want to add absorption to get all the energy (sound is energy) from bouncing around for too long a time. It depends…

Did that sound alien? possibly. but in a short time you can review product sites for examples of solving common room problems, some reading up on the acoustics forums, you can learn what all this means to you and prepare you to make the decisions needed to shape your space.