How can I calculate the point of first reflection?

[Snoochers]

I've heard I can just have someone stand at a wall with a mirror and if I can see the tweeter then this is a point of first reflection. Is this generally correct?

Unfortunately I need to make some placement decisions and my room isn't yet done and I don't really want to be messing around with setting up speakers and mirrors etc. I have 3D drawing software and I can sort things out easily in there if I know what to look for.

How can I calculate the point of first reflection? Are there angles I should be looking for? Does toe-in affect this? The speakers I am wondering about are the KEF R7s, not sure if that matters. Presumably different models have different dispersion?

Any help would be great!

 

[Snoochers]

Mirror, sit in seat put mirror on side wall when you can see the speakers in the mirror thats it .

you.... didn't read my post

 

[Thomas savage]

you.... didn't read my post

No , I read the title lol

Add virtually to that title and no one else will make the same error .

 

[dfuller]

The mirror thing is the easiest and quickest approach, but tricky to do with 1 person and harder still if your room isn't put together. So, I'd advise placing the speakers (or a similar sized object) roughly where you plan on placing them and still doing it.

 

[NTK]

You can try this ray tracing sketchpad.
https://amcoustics.com/tools/amray

Toe-in shouldn't matter too much, as the R7 radiates at all directions, but the strengths of the reflections will differ due to the speaker directivity.

 

[tuga]

Try this calculator:

https://www.acoustic.ua/forms/calculator4.en.html

 

[MZKM]

 

[Snoochers]

The mirror thing is the easiest and quickest approach, but tricky to do with 1 person and harder still if your room isn't put together. So, I'd advise placing the speakers roughly where you plan on placing them.

I have a newborn and wouldn't have help and the room is still being built so it really isn't going to work that well, though I appreciate the sentiment. The mirror trick is also a bit confusing. Is the point of first reflection when I see any part of the speaker, or only the center of the driver? Also, doesn't this assume that sound travels only in a direct line like you would see in a mirror? Obviously sound expands in a multitude of directions so I don't quite get how the mirror trick works exactly.

 

[Snoochers]

View attachment 107974

Thanks for this! A quick look at this formula suggests I would get an exact distance, is that right? Presumably the area of reflection is wider than a single point. How wide is it, generally? Obviously it wouldn't be 15', but I know it is wider than 1'.

Also, how does this take into account toe-in? This presumably makes a big difference

 

[Snoochers]

You can try this ray tracing sketchpad.
https://amcoustics.com/tools/amray

Toe-in shouldn't matter too much, as the R7 radiates at all directions, but the strengths of the reflections will differ due to the speaker directivity.

Thanks for this. Perhaps I'm not understanding you correctly. Toe-in must matter somewhat, no? For example, if I was to point the speaker to the right instead of towards me presumably the point of reflection would change drastically. I imagine the point of reflection is best determined using the angle that the front faces, though I don't know.

 

[Thomas savage]

Thanks for this! A quick look at this formula suggests I would get an exact distance, is that right? Presumably the area of reflection is wider than a single point. How wide is it, generally? Obviously it wouldn't be 15', but I know it is wider than 1'.

Also, how does this take into account toe-in? This presumably makes a big difference

You can all but take away side wall first reflection by toe in but the ceiling will still remain .

Dealing with the floor to ceiling interactions are more important than the side walls in many instances imo/ime

 

[Snoochers]

You can all but take away side wall first reflection by toe in but the ceiling will still remain .

Dealing with the floor to ceiling interactions are more important than the side walls in many instances imo/ime

I don't want to take away the first reflection. I want to do the opposite in fact and make sure I leave it untouched!

 

[MZKM]

Thanks for this! A quick look at this formula suggests I would get an exact distance, is that right? Presumably the area of reflection is wider than a single point. How wide is it, generally? Obviously it wouldn't be 15', but I know it is wider than 1'.

Also, how does this take into account toe-in? This presumably makes a big difference

For simplicity’s sake, let’s assume the tweeter stays in the same point in 3D space when you toe-in. The first reflection point will still be the same, but the energy on the closest side-wall will simply be lower in SPL.

The angle off-axis that was originally hitting the first reflection point will be pushed more into the room along the closest side-wall, thus the reflection off the wall will thus go behind your seated position.

As for how wide the reflection point is, not sure.

Also, if the above was confusing, I can sketch it.

 

[Thomas savage]

I don't want to take away the first reflection. I want to do the opposite in fact and make sure I leave it untouched!

I never suggested you did lol

You don't want ceiling and floor bounce , there's no advantages to that . Sidewalls are a different story.

Wrt side walls it will depend on the material the sound waves are interacting with , density, surface angle etc and how far away they are as to whether anything would be beneficial in terms of treatment considerations.

Broadband absorption on the ceiling at the first reflection point is worth considering / experimenting with ime.

 

[NTK]

Thanks for this. Perhaps I'm not understanding you correctly. Toe-in must matter somewhat, no? For example, if I was to point the speaker to the right instead of towards me presumably the point of reflection would change drastically. I imagine the point of reflection is best determined using the angle that the front faces, though I don't know.

You are welcome . The reflection points (for specular reflections) only depend on the geometry of the room and the locations of the speakers and listener. They also determine the path lengths of the reflections, and therefore the time delay when compared to the direct sound. Toe-in will affect the strengths of the reflections (and direct sound too), since the speakers radiate sound at different strengths as different angles.

[Edit] If you have dipole speakers, then orientation will matter. It is because the back waves from the speakers are out-of-phase with the front waves.

 

[Propheticus]

Depending on the material that is reflecting the sound really. Harder and non-porous materials acting more like a mirror than softer and uneven textures which absorb or diffract.
Also the radiation pattern around the speaker is different for high and low frequencies, the first having more directivity than the latter. Bass can be Considered omnidirectional, so no toe-in would help. Yet, the effect on the sound 'imaging' or instrument placement -or whatever you call this- is not caused by lower frequencies, so that means reflections are less cause for concern (other than standing waves causing a big boomy mess).
The tweeter being more directive would decrease in power that's being reflected the more you toe in.
If the speaker's directivity consistency is bad, you might want to dampen first reflections. If the behaviour is good -some speakers are designed on purpose to combine direct and indirect sound correctly- you don't.
If your room is asymmetrical, with an open space on one side, you might want to remove reflections on the other side for symmetry...

All in all no one easy answer.

 

[Snoochers]

Thank you! The R7s have good off-axis response so this is why I want to not dull the effects off the first reflection and to do that I need to sort out where the reflections are! We're building columns and such potentially and my decisions are based partly on the acoustics.

 

[Snoochers]

I never suggested you did lol

You don't want ceiling and floor bounce , there's no advantages to that . Sidewalls are a different story.

Wrt side walls it will depend on the material the sound waves are interacting with , density, surface angle etc and how far away they are as to whether anything would be beneficial in terms of treatment considerations.

Broadband absorption on the ceiling at the first reflection point is worth considering / experimenting with ime.

Thanks this makes sense to me. It would make sense to get some panels on the ceiling for sure, floor might be a bit awkward to do since it will be a walking space but I'll think on it. The issue with ceiling is that I will have speakers on the ceiling as well so I would need to have panels that have transparent material in at least some parts of it.

 

[Thomas savage]

Thanks this makes sense to me. It would make sense to get some panels on the ceiling for sure, floor might be a bit awkward to do since it will be a walking space but I'll think on it. The issue with ceiling is that I will have speakers on the ceiling as well so I would need to have panels that have transparent material in at least some parts of it.

If you do the ceiling broadly speaking you can forget about the floor . Nothing or a simple rug would be fine , its a try and and see situation.

If you want to have a idea of what broadband absorption would do if you put it on the ceiling just buy the panels ( don't have to be huge ) once the rooms together and sling them on the floor around the first reflection point . Take some measurements and or just listen to some music , replace those panels with a rug and try nothing at all too. If you feel its worth the hassle and are happy stick the panels on the ceiling.

If your putting pillars in the room making them round should help with things .

 

[Snoochers]

If you do the ceiling broadly speaking you can forget about the floor . Nothing or a simple rug would be fine , its a try and and see situation.

If you want to have a idea of what broadband absorption would do if you put it on the ceiling just buy the panels ( don't have to be huge ) once the rooms together and sling them on the floor around the first reflection point . Take some measurements and or just listen to some music , replace those panels with a rug and try nothing at all too. If you feel its worth the hassle and are happy stick the panels on the ceiling.

If your putting pillars in the room making them round should help with things .

Thanks for this! I am going to go with a setup as suggested by Dr. Toole in his book and just hope it works well for me. I don't trust my ears enough to experiment!