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In Reply to: RE: First reflection question... posted by pkats on March 18, 2008 at 20:30:47
Any difference will be a matter of an inch or two only, much smaller than the width of any treatment placed at the first reflection point, but some qualifications to that statement do apply:
1- A lot depends on where you pivot the speaker from when you change toe-in angle. If you're pivoting on the driver axis, ie around the centre of the speaker, the r location of the first reflection point basically won't change since the tweeter is still in the same physical location, just rotated slightly. It's changes in the actual location of the tweeter that change the first reflection point, actually changes in the physical location of all drivers but they're most commonly vertically aligned so they all basically remain in the same place when you rotate on the tweeter axis. Changes in the angle of the tweeter change the part of the off axis radiation that becomes the first reflection and that will affect the frequency balance of the reflection since most speakers tend to beam towards the high frequencies. Change tweeter location and you move the first reflection point, change tweeter angle and you change the frequency balance of the first reflection, change both location and angle of the tweeter and you change both things.
2If you're pivoting the speaker on one of it's corners, then the tweeter is moving in a circle with a radius equal to the distance between it and the corner the speaker is pivoting from, or more probably the spike which will be a little closer than the corner. That changes tweeter location and you then do get a change in first reflection point location. The amount of change in the tweeter location, and therefore first reflection point location, increases as the width of the speaker and the radius from pivot point to tweeter axis increases. You're still only moving the tweeter a little bit forward/back and towards/away from the wall depending on whether you're increasing/decreasing toe in and small changes in location don't have a big impact on first reflection location.
3- if you move the speaker backwards or forwards and/or closer/further from the side wall at the same time as changing toe in, the change in first reflection point will be greater as well because now you're into larger movements of the tweeter.
racerguy suggested using a mirror and said that the first reflection point is at the point where you can see the tweeter in the mirror. He's right in most cases but not all. If the angle of toe in is large enough you will never see the tweeter in the mirror. That doesn't mean there isn't a first reflection point. Some sound will diffract around the edge of the speaker closest to the wall and that sound will create the first reflection. If you can't see the tweeter in the mirror because of a high degree of toe in, use the reflection of the outer edge of the speaker to identify the first reflection point.
Also, if your speaker drivers are not vertically aligned, the first reflection points for them won't be vertically aligned. If you can see the drivers in the mirror, then you can identify the separate first reflection points for each driver from their individual reflections. You then have to treat a wider area. If you can't see the reflections of the drivers, then you're back to using the reflection of the outer edge of the speaker cabinet as the location of the first reflection point and the first reflection points will then be vertically aligned even if the speaker drivers aren't in that case.
There's also a mathematical way to calculate the position of the reflection points.
Draw a line from the driver axis to meet the wall at right angles. Use the tweeter axis only for vertically aligned drivers otherwise you need to calculate separately for all drivers. Measure the distance from tweeter axis to wall and call it A. Mark the point on the wall where the line ends.
Draw a line from the listening position to meet the same wall at right angles. Measure the distance from listening position to wall and call it B. Mark the point on the wall where the line ends.
Measure the distance between the 2 marks on the wall and call it C.
To locate the first reflection point, calculate A divided by A +B and multiply the result by C. Measure that distance forward from the point on the wall opposite the tweeter and the first reflection point will be on a vertical line passing through that point. It's height will fall somewhere between the height of the driver and the height of your ears.
Final point. If you're making free standing panels that are going to be placed in front of the wall, the first reflection point will fall on the face of the panel rather than on the wall. Depending on the width of the panel and how far out from the wall the front of the panel is, there may or may not be a difference in location for the first reflection point. If you want to make sure a freestanding panel is in the right place, place the mirror on the vertical midline of the panel and slide the panel parrallel to the wall till you can see the appropriate reflection (tweeter or outer edge of speaker) in the centre of the mirror. The point at which that occurs is basically the location you want for the panel. You can experiment with moving the panel a few inches forward or backward parallel to the wall and 'tune' the location by ear but the location provided by the mirror will be basically perfect if the panel is close to the wall. If the panel is a fair distance from the wall and is not very absorbent, some sound will pass through it and be reflected from the wall. Moving the panel forward along the wall so that the first reflection point is closer to the edge of the panel closest to the speaker can put the front edge of the panel between you and the first reflection point on the wall for sound passing through the speaker, giving you a second 'bite' at absorption to make up for the low absorption of the panel. If the panel is close to the wall this shouldn't be necessary because the wall reflection should strike the back of the panel anyway.
You can also calculate the reflection point on the front of the panel in the same way as outlined for walls above. Just measure to the plane of the front of the panel rather than to the wall.
The location of the reflection point on the wall will not be on a straight line along the path from tweeter to the reflection point on the front of the panel since the sound passing through the panel will be refracted slightly and emerge on a slightly different angle. That's why you have to tune the panel placement by ear if it's a fair way away from the wall, I have no data for calculating the angle that the panel will refract the sound at. It will be slight which is why panels close to the wall won't be significantly affected but if the panel is, say, a foot or more away from the wall and the panel is only 2' wide, you may need to move the panel slightly for best results. As I said, this is only an issue if the panel doesn't provide enough absorption to reduce the reflection level sufficiently for your needs.
David Aiken
Follow Ups:
in point 1 .....you appear to be saying that if you rotate the dirrection of the tweeter the first reflection point stays in the same place .....if you use the narrow beam torch analogy [most tweeters have a horizontal dispersion angle thats about 5 degrees before attenuation starts ] simply rotating the flash light will change the point at which it make its first reflection. I suspect you need to define tweeter axis are you talking about the axis of the voice coil ?
the simple way of finding a first reflection in the real world is to use a mirror and a helper ....the helper moves the mirror so that in your listening position you can see the reflection of the tweeter ...you then consider the angle off the voice coil axis your viewing the tweeter if it less than 30 degree the surface of the first reflection if hard and reflective deserves treating as this will be beaming out of phase sound onto your listening position....this will cancel [destructive interference] a proportion of the direct sound. areas to consider are floor wall and ceiling .....treat wall and floors with rugs and listen for changes ....its as simple as that....you don't need to know the maths
The first reflection comes from off axis, unless you toe the speaker out instead of in and point it directly at the first reflection point. As long as the position of the tweeter remains unchanged relative to the walls, ie the physical location of the centre of the tweeter dome, the first reflection point will remain unchanged. What toe in will affect is the number of degrees off axis the first reflection is coming from.
Since tweeters are normally located in the centre of the baffle, my statement that if you adjust toe in by "pivoting on the driver axis, ie around the centre of the speaker" the first reflection point will remain unchanged is correct.
The first reflection is created by an off axis 'ray" that strikes the wall, reflects at an angle equal to the angle of incidence, and then reaches your ear. Changing the orientation of the tweeter by adjusting toe in WHILE NOT CHANGING ITS PHYSICAL LOCATION can't change the location of the first reflection point. The source position, the tweeter, is still in the same location since, as I've said, we're only changing the tweeter's orientation in this case. The only thing that will change is the angle of the off axis radiation forming the first reflection.
If you change the physical location of the tweeter by not pivoting the speaker in a way that leaves the tweeter position unchanged, for example if you adjust toe in by lifting an angling the speaker so that it's resting on one spike and rotate the speaker around that spike before putting it back flat on all 4 spikes, then you will shift the first rotation point. If you do this the tweeter will move in 2 directions relative to the wall, forward/back and closer/further away from the wall, depending on whether you increase or decrease toe in angle. With narrow baffle speakers, the radius between vertical axis of the spike you pivot on and the vertical axis on which the tweeter is located will be small and the movements will be small. Increasing toe in will move the first reflection point forward along the wall towards you but it's not going to move very much for small movements of the tweeter. It will move a smaller distance along the wall than the distance the tweeter moves.
I agree that the mirror approach is the simplest approach to locating the first reflection point, but it's really only simple if you have someone to assist you by moving the mirror along the wall. You can do it on your own if you can support a mirror on the wall in some way and keep getting up to adjust it, sit to check, then adjust and check again and again until you find the location. If you're on your own, however, doing the simple calculation I described will give you a location to place the mirror and have a damn good chance of being spot on when you check which certainly makes the process easier if you're on your own. It certainly makes it easier also if you have someone moving the mirror because if they place the mirror at the calculated position you will definitely be able to see the mirror in it and give quick instructions about moving it backwards or forwards along the wall in order to bring the tweeter or speaker edge into view.
David Aiken
by definition on axis is pointed straight at you unless the speakers are pointed at a wall which of course is not impossible [rear wall] .....I'd assumed that this was a given. imo the simple solution is always best anything complicated is prone to lead to misunderstanding or failure to impliment. I think on reading through were saying the same thing
on axis is the path directly to the front of the speaker. It relates to the speaker, not the listener. The listener may or may not be on axis and, in my experience, the listener is off axis in most setups and many manufacturers recommend slightly off axis listening.
My setup is one of the few I've come across where the listening position is on axis but I tend to prefer a slightly off axis sound from my speakers. My room, however, is asymmetrical and that creates some reflection problems. Listening on axis results in a more off axis first reflection from the right speaker which helps minimise one of my room problems so in my current room I've opted for on axis listening. In a symmetrical rectangular room I would not have.
David Aiken
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