Does speaker angle matter with reflection point calculation?

[IDQ]

Hi all - I'll pose this in a general sense first, then with a bit more specifics.

When picking primary reflection points for sidewall absorbers, should one take into account if the speakers are angled toward the listening position rather than front-facing? It seems logical that you would move the primary reflection points a little further out from the monitors towards the listening positions. I recognize that sound comes from the cones for a full 180 degrees, but certainly the energy must be less off-axis than on? Anyone know if there's a logical way to assess this?

To be more specific: I'm working on setting up a home studio in a room that is roughly 22 x 14.5 feet with a 7.75 ft ceiling. My desk is at one of the short ends, which is effectively shortened to 11.5 feet because there's a short wall at this end of the room to separate a door with a couple of stairs (it's a split level). The speakers are about 6' apart, and my listening position is about 30" forward of them. They're angled maybe 30-35 degrees towards me. If I do the "mirror" thing, (have my wife move a mirror along the wall while I'm at listening position) I see side of the monitor but never the cone. Using the front edge, this would put my primary reflection spots nearly even with the front edges of the speakers - only about 7-8" forward towards the listening position. Is this right? It just seems a bit counterintuitive - but maybe not...

[JWL]

It doesn't change the location of the reflection points, however speakers toed in will tend to have fewer reflections. It really depends on what the off-axis response of the speaker in question is.

[jhbrandt]

Shawn,

Are you saying that the speakers are 72 inches from each other and only 30 inches forward of your position? You must move BACK another 25 1/2" to be in the best position. THEN you'll need those side reflection panels.

You should have the apex of the equilateral triangle about 7" behind the center of your head. The speakers should be 60 degrees relative to each other. - Go to my publications page and download the Reflections Boundaries Mass Calculator.. 1st tab & have a look at the First reflection points calculator and associated drawing for help.

I will always calculate reflection points using a spherical model of the sound waves, even though at reflection 'frequencies' they are much more directional. And it is a fact that most speakers manufactured today offer poor of-axis response but I prefer to design for 'worst case'.

Cheers,
John

[Nick Sevilla]

I imagine you like science.

Imagine if you will, that your speaker is the end of a rocket stage.
When sound is coming out, it will radiate out like the exhaust of a rocket engine.

Now, there will be sound that basically is like a gas, very slow, and will radiate like a circle right out of the speaker, and fill the room completely, while other frequencies will be much more directional.
The lower frequencies will be LESS directional, while the higher ones will be MORE directional. Got it?

Now, in order to know which frequencies will reflect directly from your walls and into your listening position, you will need to do some maths. Measure the distance between the center of your speaker, the walls, and your listening position, as if you were measuring a trapezoid. The trapezoid will be fat at the end closest to the nearest wall to the speaker, and longer at the further wall. Draw it out on paper, or a computer.

The length of a sound wave is easily calculated. Take the frequency of any given sine wave as the basic measurement, divided by the speed of sound, avg. of 1126 ft/s.
As a starting example, take 1000Hz/1126= 0.88 ft, which is 10.56 inches (12/0.88). And so forth.
When you find the frequency which rougly equals the distance from the wall to your listeningposition (your head), this is going to be the most troublesome frequency. So use the correct diffusion/absorption on that wall, to deflect that frequency from your listening position. Whatever material you use, will be sufficient to cover above and below this most troublesome frequency.

To further put some nice complications, this frequency is NOT your 1st modal frequency, which is related only to the dimensionsof the room, and not the position of your speakers.

Cheers

http://en.wikipedia.org/wiki/Speed_of_sound

http://www.nasa.gov/centers/glenn/image ... hi-res.jpg

[IDQ]

Thank you all for the advice. It's baffling to me how much people know around here.

Part of the reason I have angled the speakers inward is that I know I don't have an equilateral triangle. Does that make any difference? Also - to move my listening position back 25" or so, while perhaps sonically correct, would most definitely make the space ergonomically and visually weird. I might be able to get the speakers closer together, and I could even move them away from me towards the wall a bit, but won't that cause other issues with bass response? (They're Mackie HR824s, so they do have a rolloff switch for positions closer to the walls).

[jhbrandt]

Shawn,

Push those speakers AWAY from you.

Get them as close to the wall as possible. This is sometimes called pseudo flush-mounting and it will even-out the bass response.
Did you download my Reflections Boundaries Mass spreadsheet? If you go through it, you will begin to understand more. I have left all the cells unlocked so that you can see the formulas, etc

Ideally, the speakers should be 60 degrees apart relative to you. This places them in optimal stereo balance for translation to other systems. If the speakers are too wide, you will tend to pull the stereo pan in towards the center causing you to mix 'mono'. Too close together, and you tend to do the opposite. 60 degrees is the 'best' compromise. - It's all about translating mixes. This is something that is difficult to determine with headphones, so checking with headphones is not an option when it comes to stereo imaging.

Cheers,
John