Speaker layout
Drawing done to reflect the room dimensions given:
Room length = backwall to chimney breast + depth of alcove = 3400 + 370 = 3770
Room width = 3400
Alcoves = 2@ 950 x 370 (W x D mm)
A standard method for stereo speaker layout uses the idea of an equilateral triangle (all side lengths equal), and so the speakers are the same distance apart as the straighline distance from the listener to each speaker. In an equilateral triangle each corner is 60 degrees, so using trigonometry it's possible to calculate the distances based on the distance from the listening point to the chimney breast. Now, the sofa is hard against the back wall, and so allowing for an average backrest depth then the listeners ears will be roughly 400mm from the back wall. So the listener to chimney breast distance is 3000mm.
At 3000mm, the speaker distance apart is 3462mm. That's a bit of a problem though in a room that's only 3400mm wide... ha ha ha.
However, the room is used for movies, so using the same listening distance of 3000mm we get speaker positions just on the borderline of the DD specs at 440mm from the left and right walls.
Surround speakers should be level with the listens ears, so 400mm forward of the rear wall. This part of it may of may not be practical depending on the doorway and which direction the door opens in to the lounge.
Speaker heights - front soundstage tweeters all the same height if possible. Since the centtre speaker hight is dictated but the fireplace shelf then the front L&R should be the same height for the tweeters.
Surround speakers for 5.1 should be 30~60cm above the listener's ear level.
If the wires are already in place for corner mounting, try turning the spearker up at a 45 degree angle to face the ceiling. This will bouince the sound off the ceiling a-la Dolby ATMOS reflecting speakers and create a more diffuse surround effect from monopole speakers.
Front speaker angles - Tilt all three down so that the tweeters are firing at the sweet spot/prime listening posistion
Front L&R speakers, distance from rear wall - work from the manufacturer's recommended distances if possible. The Mercury 7.2 have a recommended distance of 500mm from the back wall. This won't be practical in your room unless you buy tall speaker stands, and even then it'll get in the way of the PC desk, so you'll be running much closer to the back wall than recommended.
Audio tuning - stereo speakers: Put on some music you know and set the amp to Pure mode; this will (or should) drive just the front stereo pair with a full range audio signal and minimal processing from the amp. Use a good quality music source of a track that you know well and that can be set to repeat play. You'll need to play at a fairly high volume to hear the speaker working effectively in the space. This process doesn't work at background listening levels, so if you're likely to get grief from a significant other for the racket being made then better to do this when the house is empty.
Start with the speakers close to the back wall. Ideally the tweeters should be at ear level when you're sat in the main listening position. If you can manage this then face the drivers forward so the backs of the speakers are parallel with the back wall.
Long story short, with them hard against the back wall they should sound bloody awful. Bloated bass, indistinct midrange. Move them forward about 5cm at a time and play the same 30 second segment of the track. You should hear improvements in the bass and midrange; bass notes starting and stopping more cleanly, vocals easier to follow. Repeat the move and listen process.
Once you have a rough idea where the best distance is, repeat this process but this time use the bass port bungs (the tubular bits of foam you left in the packing box). If the speakers are sitting on a cabinet, table or shelf, use the rubber feet (also in the box) to give some isolation from the surface. With the bungs in place there'll be less bass bloat where the speakers are closer to the wall than recommended.
After finding the best distance you can achieve in the space available, then start to toe-in (angle) the speakers gradually to get the front sound stage to image. With speakers that are closer together than the stereo optimum they shouldn't require much toe-in at all.
Acoustics of the room: for any sound system, the room is at least 50% of the final result. Great gear in a bad room will never perform its best. Mediocre gear in a great room will produce far better results than expected. But the room is the one element that most overlook, and even when it's recognised it's still the most difficult element to alter when it's a domestic living room.
There are some minor changes you can try though. Use books on shelves to make the alcoves and irregular shape. This'll help break up the soudwaves so the alcove isn't lensing the backwaves from the speakers so much.
Another good trick is to bring the sofa forward 40-50cm. With the sofa hard against the back wall your ears pick up a lot of the reflected sound. Give it some space and the effect is lessened. Acoustic panels can help too, but could be tricky to get the go-ahead from a significant other
Try the sofa trick when you have the house to yourself one day.
