How U determine first and second reflection points

So, each of the left and right walls will have 2 relection poinsts - one from right speaker and one from left speaker?

Or, each wall will have first reflection point from the speaker on the same side and the reflection of the speaker on the other side is considered second refection point?

seems like slight difference in definiton of first and second reflection point by Tvad and Fmpnd.

Eandylee - Correct, however there are actually more if you take the floor and ceiling into account, and don't discount them as they are quite significant. It's easy to put a rug on the floor - a bit more difficult on the ceiling.

Don't forget the reflection points from the rear wall. In a wide room, these can be more intrusive at the listening position than the reflections off the side walls.

As a rule of thumb, if the distance the sound travels from the speaker to the reflecting wall and on to the listener, is more than 5 ft. longer than the distance traveled directly from the speaker to the listener, the ear will be able to distinguish this reflected sound in time and not confuse it with the image. That is why it is important to get speakers away from side and rear walls if possible.

You can see that if a speaker is more than 2 1/2 ft. off any wall, this criteria is always met. Still, the clarity of the image will always benefit with some absorption in the room, and putting it at the reflection points is a good place to start. Experiment.

So, each of the left and right walls will have 2 relection poinsts - one from right speaker and one from left speaker?

Yes.

In my view, the right speaker's reflection on the right wall is the point of first reflection of the right speaker, and the left speaker's reflection on the right wall is the first reflection of the left speaker. Think of it as the "first bounce" of the sound wave off a hard surface. After the sound wave is reflected off a surface the first time it then bounces off a second hard surface, which is the second order reflection. Also, think of a billiard ball bouncing off a first bumper (point of first reflection), and being re-directed to bounce off a second bumper (point of second reflection). The floor, the ceiling and each wall will have points of first reflection.

According to Robert Harley's book, "reflections reaching the listener from just one room boundary (i.e. side wall, floor, ceiling) are called first order reflections".

Hope this helps.