How U determine first and second reflection points

Someone told me following a while ago in room teak thread, but I don't think I understand it well. Any comments?
Have someone sit in the primary listening location, take a mirror to the side walls opposite each speaker and move it until the seated person can see the speaker reflected in the mirror. These are your first reflection points. Start from there.
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?
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 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.
I found it easier to use a small mirror if I put a lamp (no shade) at the center of the speaker, then, as above, move the mirror until you "see the light".

Other choice is to sketch your room (to scale) and find the reflection point on the drawing (where the "angle in" equals the "angle out"--in other words, the angle the line from the center of the speaker to the wall makes is equal to the line from that same point to the listening position). Lamp is much easier...

I have not worried about 2nd reflections
Between my post, Snofun3's post and Fmpnd's post, you have the same method nicely described by three different people. Pick one and have fun.
So, each of the left and right walls will have 2 relection poinsts - one from right speaker and one from left speaker?
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.
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.
So, each of the left and right walls will have 2 relection poinsts - one from right speaker and one from left speaker?


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.
WOW!!! To show you how fast we all responded, I thought I was the FIRST to respond because there WERE NO responses when I typed mine!! Good luck with your system.
Thanks guys.
Fmpnd - Ya snooze you lose guy. Sorta germain to this thread - mirro, mirror on the wall, who's the smartest....
No excuses man.
What were you supposed to be doing anyway? I'm "Working from Home".
Ah, C5150 - yes, in fact sound travels in straight lines - it's not real good at going around corners. Try having someone else stand in front of your speakers - see what I mean. Sound = straight line.
Reflections of straight lines = standing waves - not a good thing. Mirrors show you where they occur.
Nevermind, just go enjoy your instrument.
Oh - Zargon has a good point. The rear reflections are probably worse than from the sides. I've deadened both walls enough to tame that form of standing wave.
Snofun3, how about a mirror on the ceiling instead of a rug?
And with Cdc's post, the thread turned...

Tvad, I think we'd exhausted the subject anyway.

Well Cdc, If you want to see that first reflecting point on a permanent basis I suppose that's useful - why else would you want a mirror on the ceiling?

Caught me!!
Tvad and snofun3 - LOL!!!!!
so if you where to hit my instrument and stand in front of it .Lets say you line all 4 walls with humans ,line all the ceiling and floor as well.when you hit the triangle(example)no other person will hear it but you becaue you block it is 3d friends.......
I'm not sure what you are trying to say, but if you are saying that you need all of the reflections from the walls, ceiling, and floors to get 3d effect, you are overlooking the fact that on well recorded software that information is already there. To not treat the critical reflection points will distort the original signals and you will lose the value inherrent in making recordings using minimal mic'ing with realistic spatial information. In other words, there goes your depth of field everyone seems to think is so important.
Not saying that at all - sound reflects in staight lines (many, many, many straight lines - it does not zoom around corners, honest), and when the waves bounce back they cancel each other out at a certain place (standing waves). The way to stop that is to not allow them to reflect back.
Oh never mind, everyone else seems to understand just fine...Yes the earth is flat, and intelligent design is a science - happy now?
First of all to treat, means kill in hi-fi.You or we (hi-fi)claim or describe sound as a staight line.We give examples of the beam of flash light or a cue ball off of a pool table deflection.

The earth is round...sphere. when we throw a pebble in a still lake.The ripples are round.This is a 2d picture of what happens.If you can imagine 3d of this then you will get sphere.When you take a bath and fart under water the bubbles are ......spheres and smelly(bonus).When you watch under water explosions in movies example,the sound is the shape of a sphere.When the radio tower of a tv station emits a signal it is in a radius(miles traveled) sphere...

No lines....if we change the way we think and get back to what really happens instead of what we think or we read . Hi-fi would be even better than just swapping brands etc...

I want you guys to just think about this.I'm not interested in now if the speheres collide and bounce off the walls ,and off of each other.We get a problem with standing waves or /nodes.By killing some of the frequencies or musical notes.....All you have accomplished is the death of your music.The spheres will continue to collide.Only now you chopped away most of the vital info in your room by killing it.When you move your speakers around all you do is move the pressure zones about.Big spheres ,small spheres,all your rooms are individual as your are.So you cannot tell what a pressure zone is doing to your friends system. Controlling the waves and nodes.When you learn not to kill ,but to control and use the full energy of your room and system.Live music is never killed....You are trying to remove the walls out of the whole equation.Your little nearfield triangles do a very good job of this ,but then your stuck on hearing the character of your gear.At low volumes.....
The ceiling wall junctions all around the room should be treated as reflection areas. To me these are the critical areas to treat not to kill but to redirect the pressure back at the listening position. You want the energy that usually flows along the surface of these large planes to become a part of the overall experience..Tom
Uh, how the hell did a simple question that was answered very well by the first several posters turn to a 30 reply thread?
Man, some audiophiles are weird.

One needs to careful not to over damp a room. Reflected sound may not be such a big issue because sound levels follow an inverse square law, even untreated walls absorb some portion of the sound that hits them and the Haas Effect allows the brain to suppress a large part of the reflected sound. Most of the domestically acceptable room treatments are not broad band and introduce a change in the tonal balance between the direct and reflected sound which the ear/brain tags as a hallmark of artificially produced sound. If soundstage info is your paramount listening concern, then it could make some sense to suppress first and second reflection points, but if proper tonal balance is more important, then heavily damping a typical home listening room is heading in the wrong direction.

Please don't construe my comments to be anti room treatments. Properly designed and installed broad band absorption systems can work wonders for the proper reproduction of sound. It's just that the best rooms I've been in for long term listening all have a combination of reflective and absorptive surfaces.
David I suppose you could follow as so many others do the same old pitch of foam and fiberglass that so many companys build..These products often do as much harm as they do good..they reduce energy in the room and often suck out the life..Tom