As discussed in the opening page, the car and open air are good listening environments because they have either no reverberation or a controlled reverberation field. One of the key points was the flush mounting of the car speakers. By building your speaker boxes into the wall you can tighten the bottom end of the whole system because you eliminate the reflections that emanate from the back of the speaker, hit the wall and then come back to you in and out of phase.
Here the sound from each speaker comes back off the walls and creates a general mayhem of frequencies in the middle where the engineer would probably be sitting.
In the open air the sound waves are passing you and never come back.
Flush Mounted Speakers
Look what a difference the flush mounting makes to the speaker wave fronts. The difference to the ear is even more dramatic.( As with most techniques in the modern studio in the past 3 decades Tom Hidley was the acoustician who started it.) The angles give what is referred to as a 60 degree monitoring system. Some studios played with 90 degree monitoring which gave a wider image and is an option you may experiment with. I used it at Music Farm Studios and I really liked it as the image was really wide yet the centre was still tight.
As far as the construction is concerned it has to be solidly built. As with all studio construction glue and screw as you go. Usually a frame is made and then a box 2mm bigger than the speaker box is built so the speaker slips in tightly so you are in fact pushing hard against the air pressure when you place the speakers in it. Some people line it with rubber pads so that the speakers are suspended and are mechanically isolated from the whole frame. You could build them in concrete which would be ideal but expensive.
The angle of the speakers is set so that they create a 60 degree angle at the focus. If you want to mount them high you must angle them down so that they point at the engineer. I find it annoying when I go to a studio and sit at the console and the speakers are pointing over my head. Whilst that effect looks good it is expensive and complicated to build so don't attempt it unless you've got a good carpenter as the resultant angles are complicated especially if you have a window between the speakers.
The area underneath the speakers makes a good bass trap and the front face should be absorptive so that reflections from the front of the console are eliminated.
Flush Mounted Speakers Elevation
If a signal is placed equally in each speaker (panned centre) you will hear it as if it were coming from a centre speaker, (sometimes referred to as the Phantom Centre). It is good practice to establish the phantom speaker in your room. If your system is correctly setup you will hear a third (phantom) speaker coming from the centre between your speakers. There was a record label on New York during the early 60's that even promoted their recordings because they had a phantom speaker in their recordings. They called their recordings Dimension D and their main artist was Enoch Light and the Light Brigade. They were spectacular recordings for their time though!.
If you have a recording setup try this experiment. Bring the same signal up into the console twice with one of the signals sent through a delay unit. Now pan one left and one right with no delay. You should hear the signal coming from the centre. Now slowly add delay to the delayed signal. You will find that something occurs around 18 -20 milliseconds. Suddenly you will start to perceive the signals as two separate signals one from the left and one from the right yet when the delay was below 18ms the ear couldn't tell the difference between the delayed and the direct so they both created a centre image. (Good way to create a doubletracked guitar effect) In other words your ears can't distinguish delay below 18ms or so. (It is different if that delay is changing as in a flanger or phaser)
Now if you have a room with a reflective rear wall the signal from the speaker will pass you and then be reflected back. Sound travels at approx. 330mm (1 ft) per millisecond so if you are 3m (9ft) from your back wall you will hear the sound as a 18ms delay and your ears will be confused and think that there is another speaker behind you. Most home studios have a back wall closer than that so it shouldn't be a problem but I really recommend you don't try that system unless you are fully aware of the technical problems involved. I have seen lots of LEDE pulled out and rebuilt because the designers didn't fully understand the geometry involved and the room sounded weird.
You could try this test. Sit in front of your speaker system and cup your hand behind your ears to block the sound coming from behind you. If you notice the sound tighten significantly you have a rear wall problem.