Low Frequency Null Points and Listening Location

hi peter...


(would you by chance have a 106" 16x9 screen??? i used the 1.5 x screen width ratio for my girlfriends setup and have been told that this way too close for DVD material... DWIN700 and transcanner)


anyway, I believe Home Theater Mag did a 3 part article on room dimensions and how to try and incorporate everyone in the sweet spot (but i believe the jist of the article was to get everybody as close to the "rule of thirds" as possible)...

i think there was also a link to a quick Excel spreadsheet that would show your room response given your theater dimensions....

not to sure how far their archives go however... (hometheatermag.com)

maybe some else has the link offhand..


cheers....jeff

Thanks for the replies. The hieght of the room is 7'6" (basement).


I am not using a dedicated sub at this time. I have a surround system where I have 3 ways as the two fronts and 2 ways as the rears an center. I get pretty good kick from the 3ways down to about 35hz. I'll probably get a sub eventually, but I'm holding out for a good one.


Will the use of a dedicated sub improve this issue? I'd probably have more freedom on placement with a dedicated sub. I hate to move the 3ways too far from where they are, which is up by the screen. Also, I have the 3ways mounted on 30" stands, which means the sub-speaker is about 36" up from the floor. I seem to get better ambience when the fronts are up in the air as opposed to being on the floor. Any issues with this?


Dennis, I was afraid that the nulls were tied to the room size/modes. But what you're saying is that placement would flatten the response. Does this flatten the response so that the nulls are less noticeable, or the opposite, or both?


Regarding the screen size/placement. I'm using a 4x3 that is 88" wide. Not quite as big/wide as the 106" 16x9. Unfortunately, I've tried the 12' distance with my setup and although the sound is much better, it's just too close the screen...



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-Peter


[This message has been edited by pcostanz (edited 09-29-2000).]

Since you don't have much latitude to move the main speakers (your bass producing units), your best bet is to get a sub. By placing it carefully, you will be able to change where in the room the null occurs.

For example, if you put the sub in the back of the room, you would still get the null, but it would be in the front of the room (kind of a mirror image of your speaker/couch position now).

The tricky bit is integrating the sub with the mains, and not getting whole new sets of cancellation problems.


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Buzz Goddard

TAG MCLAren Audio
www.tagmclarenaudio.com

In 1896 Rayleigh showed that the air enclosed in a room has an infinite number of normal modes of vibration. These are referred to as room modes or standing waves. The frequencies at which these modes occur are given by the following:



f = c/2 (sqRt (p/L)2 + (q/W)2 + (r/H)2)


where:


c is the speed of sound

L is the length of the room

W is the width of the room

H is the height of the room

And p, q, and r are the integers 0, 1, 2, 3.


As we can see from the formula, speaker placement has no bearing on the matter. The standing wave condition is set up only when the air particle velocity at the boundary is zero. Whenever air particle velocity is zero, pressure is at the maximum level. (If the walls are not perfect reflectors, losses at the walls will affect the heights of the maxima and depths of the minima.)


All room modes are represented as a sinusoidal wave form and for any given modal frequency the peak is always located at the room boundary. Thus, the location of any null or peak will be a function of the boundary and frequency and not a function of speaker placement. Since the condition for a standing wave is that particle velocity must be zero at each opposite reflecting surface, the peak of the wave must occur at the wall surface, and, hence, standing waves can only occur where the distance between the boundaries are an exact multiple of the wave length. By extension, then nulls are a function of only the distance between the boundaries and the frequency (wave length). While speaker placement will not affect the location of peaks and nulls, it will affect the maxima and minima of the effect.


In the same manner, you cannot have an asymetrical standing wave form. For example, in an 18' room, you cannot have a null at 6' without also a null at 12'. Moving the speaker from the front to the back of the room will simply alter the location of the speaker, not the location of peaks and nulls. Moving the speaker out away from the boundaries, by various distances, will also not affect the existence, or location of, nulls and peaks ... it will affect the maxima and minma of certain (not all) modes.


However, as I have previously suggested, we ought be more concerned with the minima and maxima of a standing wave rather than on the placement of the peaks and nulls (why fix a problem you don’t have?) Modal Potency, Modal Frequency Density, and Modal Bandwidth are more important criteria in establishing the playback venue. Unfortunately, you’re not going to effectively model the maxima and minima from a free spreadsheet program.



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D. Erskine

DEsign Cinema Privee
www.DEsignCinema.com

Imagine what you could do, if you could do all you imagine.