How U determine first and second reflection points

Don't overlook 1st reflection points on the floor and ceiling! Depending on the type of surfaces and their shape they can be just as important as the side walls, albeit the ceiling reflections can be tough to deal with. If you treat the 1st reflection points properly you really shouldn't be too concerned by the second reflection points. IMHO.

Better yet - YOU sit at your normal listening position, have someone else slide a large mirror (One of those back-of-the-bathroom door mirrors works well) along the walls. When you see the reflection of the speaker in the mirror, that's your first reflection point, and where you need room treatments. Auralex foam, available at Guitar Center, or some other of the same type stuff is fully adequate.
Note that you should do the same on the ceiling too, as the reflections are just as strong.
Your speaker's imaging will thank for for the effort.

Go and buy yourself a cheap instrument.Stand in the middle of the room and strike it.Sound everywhere at once.So cover all your wall or put up all mirrors.LOL

Sorry not answer your looking for.sound that travels in straight lines????

You have it correct with the mirror method, you can also use it to find the reflection point on the ceiling and floor. Go to audioasylum and click on the Rives site as there in lots of great info and recently a disscusion on this topic.

Sit in your listening chair. Have a friend take a flat mirror (e.g., the kind you use for shaving with or in the bathroom - hopefully at least 8-9" x 8-9") and place it flat on the right side wall just behind or beside your right speaker and at about the height of the tweeter in your speakers. While you are sitting in your listening position, have the friend stand behind the mirror and start moving the mirror towards you until you see the tweeter of the right speaker in the middle of the mirror. THAT is the FIRST reflection point. Then have that person keep going until you see the tweeter of the speaker nearest the left wall in the middle of the mirror (still on the right wall) and THAT is your SECOND reflection point. Do the same on the opposite wall and you have your first and second reflection points.

Does this help?