I will put shelves / cable racks above the sofa. These should help diffuse the sound as well.
Not very much, to be honest. Diffusers are usually carefully designed to provide smooth, even, broad diffusion between an upper and lower frequency limit, as needed by the room, either in 1D or 2D, as needed. Do some research on QRD, PRD, Schroeder, Skyline, BAD and similar diffusers, and you'll get a better idea.
That said, your room is way too small to be able to use diffusion. You are limited to absorption alone.
The speakers are pointing down the short axis because I want to be able to look to my right and look out the window of the studio to my garden.
So you prefer visuals over acoustics in your room?
In other words, you are more concerned about how your studio looks, then about how it sounds? Beauty over accuracy?
Long story short: There is a very basic concept in acoustics called the "Haas Effect". If your ears hear the direct sound from the speakers, followed by a reflection or echo of that same sound within a period of less than about 20 ms and at a similar level, your brain is incapable of figuring out that it was, in fact, an echo. Instead, it will identify that sound as having come from a different direction than it really came, and having a different frequency and phase curve from what it really was. In other words, you will be unable to determine the actual frequency response, and the actual direction. This is not something that you can learn to recognize, and learn to deal with: It is a physical limitation of the way our ears and brains are built. In order to avoid this problem, you must design your room such that the earliest reflections that reach your ears arrive later than 20ms after the original direct sound, and 20 dB quieter. You cannot do that if your speaker are firing across the short axis, as that places your head way to close to the rear wall.
In addition, with your current layout, your head will be in the geometric center of the room, which is the worst possible location for both modal issues and also SBIR. There is no worse location that the center of the room.
So, if you want your control room to be usable to actually turn out good mixes that translate well, then you should turn it around such that the speakers are firing down the longest possible path, and such that your head is NOT at 50% of the room depth, nor 25% either. About 38% is the theoretical best location.
Having a view on the outside world is important to me and I am willing to compromise on the sound quality of the room to have this
Well, that's your call, of course, but most people who go to all the huge expense in time, money and effort that is involved in building and tuning a studio, actually do want it to be as good as it possibly can be acoustically as their very first priority. Otherwise, they'd just sit in their living room and mix looking out the window, and save a lot of hassle, while still turning out the same mediocre mixes as they would in a mediocre studio.
By this I mean that the drawings show parallel walls & parallel ceiling and floor. The interior walls of the finished studio will not be parallel.
And once again I ask the same thing? Why do you NOT want to have parallel walls? You do understand that it's a myth, right? Since you are consulting with people you claim have already built a couple of studios, you should already know that there is no acoustic reason to have non-parallel walls in a studio? They have already explained to you that the only reason to do that is to avoid something called "flutter echo", but that in order to do that (get rid of flutter echo) the walls need to be angled at least 12°, which wastes a huge amount of space in a room, and that flutter echo can be eliminated in much simpler ways? They did tell you this already, right?
I just don't yet know the angles so haven't drawn them in.
As mentioned above, the aggregate angle must be at least 12° if you wanted to do that to control flutter echo. Splaying your walls by 12° will remove a LOT of space from the room, which would be a terrible thing: the room is already very, very small, and removing even more air volume would be a huge mistake. Take a look at specifications for critical listening rooms, such as ITU BS.1116-3 and EBU TECH-3276, and you'll find that the recommended minimum floor area is 20m2, and the reference volume for all acoustic calculations is 100 m3. Now compare that to your room, and you'll see that you cannot afford to lose any area or volume at all.
This is a VERY good point and one I will raise with the acoustician who designs the interior.
If you are allowing the acoustician to design ONLY the interior, then you have already made another huge mistake: The acoustician also needs control of the wall locations and angles, the isolation system, and the HVAC system. They all work together as a single whole.
Good point and since discussing a decoupled slab with a few builders I have dropped it from the design. Neither of the guys I mention above used one for their ICF studios.
Smart move.
I intend to have freestanding oil filled electric radiator for heating. I may use clay boards for the ceiling as clay has excellent hygroscopic properties. I don't intend to have any active cooling, this is not usually necessary in the UK for a well insulated building with good ventilation
Correct, it's not usually necessary for a usual building in the usual UK, but that's not your case. You are building a studio, which is very unusual building. It will be perfectly sealed, absolutely air-tight, twice over: Two complete hermetic seals. No air will be able to get in or out, except the air that comes in through your HVAC system. Since the relative humidity in the UK is pretty high most of the time, and since you need to get that down to 40% RH all the time, you need some method of controlling humidity. And since the very act of removing humidity from the air is a warming process and requires a cooling process to go with it (because the air has not only a sensible heat load but also a latent heat load), you need some type of equipment that is capable of doing that. In addition to the humidity coming in from outside, there's also the humidity emitted by your own body: In every single breath, you expel water vapor. If you don't deal with it, not only will the humidity inside be too high, it will also be rising continuously, until you end up with condensation on the walls.
The unit i've chosen is designed for properties up to 90 sqm so I'm sure it will be able to handle the extra load of a few right angles.
Nope. There is no relationship at all between the two! That's like saying "My car is able to hold 9 liters of refrigerant in the radiator, so I am sure it will be able to go 120 km/h on the freeway". Two different things. No relationship.
Your duct and silencer system resists the flow of air through it. This is known as "static pressure". The amount of resistance depends on several factors, but basically the total length of the system, and the internal surface area and cross sectional area, and changes in direction. You need to calculate this, not guess at it. The fans in the HRV unit are designed to be able to handle up to a certain static pressure, and this will be stated somewhere in the specifications. This is mostly independent to the air flow rate and the air flow velocity. In fact, the specs for the HRV should give you table with a range of air flow situations for several possible static pressures. You will notice that as the static pressure gets higher, the flow rates and flow speeds get lower, and if the static pressure of your system is higher than what the HRV can handle, the blades will suffer from airflow stall, and will generate more turbulence than actual air flow, since the will be incapable of moving air against such high resistance. The motor will be overloaded, will overheat, and will eventually fail, as will the motor bearings.
It's that simple. You have to do the calculations and check all the numbers and specs to make sure that the unit really is suitable for your situation. If not, you are inviting disaster.
I've never heard of insertion loss, I've looked it up and it seems to refer to a loss of signal power in telecommunications devices. How does it relate to ventilation?
Insertion loss in refers to how much acoustic isolation the silencer box produces. In other words, how much it attenuates or reduces sound levels of the air flowing through it. I'm surprised that you could not find anything about this, since the very first Google hit I got when searching for "HVAC silencer insertion loss", is this one:
http://www.engineeringtoolbox.com/noise ... -d_81.html and the second one is this:
http://www.enoisecontrol.com/wp-content ... itions.pdf There were many others too...
I have designed several studios for customers in the UK, similar to what you are trying to do, and I'd suggest that you should probably hit "reset" at this point, and go back to square one. I'd suggest that you start by buying and thoroughly studying two books: "Master Handbook of Acoustics" by F. Alton Everest (that's sort of the Bible for acoustics), and "Home Recording Studio: Build it Like the Pros", by Rod Gervais. The first one will give you the basics in acoustics that you are currently lacking, and the second one will give the basics of studio design and construction. Once you have worked your way through both of those, and fully understand them, then you'll be ready to start designing your place.
- Stuart -
I have squeezed these in alongside raising a 7 month old baby who has only just figured out how to fall asleep on her own 
I am just a hippy festival dj who has somehow managed to buy a house with enough space at the end of the garden to build a bass cave!
