How U determine first and second reflection points

That's a pretty good explanation of how to find the first reflection point. You, as the listener, sit in the listening position. Have another person with a large mirror (3'x4' would be nice...or you can use reflective Mylar sold at Party stores), place the mirror on the side wall next to the right speaker. Have your helper slowly slide the mirror along the wall moving toward the listening position. As you look in the mirror, there will be a point at which you see the right and left speakers reflected in the mirror. Have your partner mark the wall with some blue painters tape at the spot where you see the reflections of the speakers. Your partner will put two pieces of tape on the wall...one for the reflection of the right speaker, and one for the reflection of the left speaker. You will have to "talk" your helper to the correct postion, because obviously only you can see the speaker's reflection. If you use reflective Mylar, you can tape the Mylar to the wall and put the blue tape right on top of the reflection. The blue tape now marks the point of first reflection of each speaker on the right wall. Repeat for the left wall.

For further info, read Robert Harley's book, The Complete Guide to High End Audio.

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.