Acoustat
I may have this wrong, but from what I have read there are only four reflection points on the side walls that matter in a two speaker setup. The reason is that each speaker has only one reflection point on each side wall that directs sound to the listening position. All the other reflections off the side walls pass either in front of or behind the listening position. The mirror technique for locating these reflection points works because a mirror on the side wall will reflect light coming from the speaker position to the listening position only when placed at the reflection point.
When I used a mirror to locate the side wall reflection points in my listening room, I used a mirror about 1 foot x 1 foot and the trial and error method. It was a PITA because I didnt have any readily available furniture on which I could place the mirror at eye level when sitting in the listening position, and I had to improvise by stacking things. So, yes, a larger mirror would have made the whole process easier for me, but a larger mirror wasnt necessary to determine 3rd, 4th and other reflection points. The easiest way to use the mirror to locate the reflection points is to have another person hold the mirror and move it around until the person sitting in the listening position can see the light from the speaker reflected.
Other reflections in the room can cause excessive reverberation or ringing, but this is different in nature from the problem caused by 1st and 2nd reflection points. Excessive reverberation causes smearing and overhang and muddies the sound. First and second reflection points create a time difference in a specific sound reaching the listener and makes it hard to determine the location of the sound within the soundstage. In other words, imaging of specific sound sources is impaired. The solution to both problems is absorption of the unwanted reflections. However, ringing (detected by clapping hands) is solved by distributing sufficient sound absorbing materials around the listening room. First and second reflection points are solved by having sound absorbing material at those specific points.
I may have this wrong, but from what I have read there are only four reflection points on the side walls that matter in a two speaker setup. The reason is that each speaker has only one reflection point on each side wall that directs sound to the listening position. All the other reflections off the side walls pass either in front of or behind the listening position. The mirror technique for locating these reflection points works because a mirror on the side wall will reflect light coming from the speaker position to the listening position only when placed at the reflection point.
When I used a mirror to locate the side wall reflection points in my listening room, I used a mirror about 1 foot x 1 foot and the trial and error method. It was a PITA because I didnt have any readily available furniture on which I could place the mirror at eye level when sitting in the listening position, and I had to improvise by stacking things. So, yes, a larger mirror would have made the whole process easier for me, but a larger mirror wasnt necessary to determine 3rd, 4th and other reflection points. The easiest way to use the mirror to locate the reflection points is to have another person hold the mirror and move it around until the person sitting in the listening position can see the light from the speaker reflected.
Other reflections in the room can cause excessive reverberation or ringing, but this is different in nature from the problem caused by 1st and 2nd reflection points. Excessive reverberation causes smearing and overhang and muddies the sound. First and second reflection points create a time difference in a specific sound reaching the listener and makes it hard to determine the location of the sound within the soundstage. In other words, imaging of specific sound sources is impaired. The solution to both problems is absorption of the unwanted reflections. However, ringing (detected by clapping hands) is solved by distributing sufficient sound absorbing materials around the listening room. First and second reflection points are solved by having sound absorbing material at those specific points.