PDA

View Full Version : Lobe effect?



Hamilton
02-03-2006, 01:39 PM
There was a thread a few months ago about sound anomalies at a church due to possible "lobe effect". Being a "card carrying bottom feeder" I had never heard of the term, but immediately jumped into some reading to get m'self up to speed.

But that reading raised a question. Why is "lobe effect" a major concern involving sound reinforcement, but not a concern in home audio where our speakers are deliberately aimed at our seating location?

Thanks for the help,
Hammer

Akira
02-03-2006, 01:50 PM
In sound reinforcement the problem becomes enhanced with multiple speakers and complex arrays.
To help circumvent this and other problems, individual cabinets are specifically designed so that they work in conjuction with multiple (usually identical) cabinets in a set configuration, be it a splay or line array etc. These boxes do not necessarily sound good on their own, but come into their own as a combined cluster.
If you were to play a single home speaker, it should sound good on it's own. Where as a P.A. speaker often has an extremely high Q factor, with a tightly controlled pattern like a horn to minimize phase interference.

p.s. This is not a field that I know a lot about--system designer's territory. Perhaps someone can elaborate.

Hamilton
02-03-2006, 02:06 PM
Ok, I think I get it. Our home speakers, even though aimed at a focal point, are far enough apart to eliminate any of these problems?

:uhmmmm:

Robh3606
02-03-2006, 02:16 PM
If you have 2 drivers or more drivers reproducing the same frequency you can get areas where they reinforce and cancel due to phase issues. This depends on a multitude of issues such as driver size, frequency and driver spacing. With SR arrays you can have multiple drivers all doing the same frequency range. Depending on the driver spacing and baffle layout lobing can occur at the lower frequency range where the individual cabinets have little control over directivity. At least horns with there defined directivity can be splayed as an example to help minimize this effect.


"Ok, I think I get it. Our home speakers, even though aimed at a focal point, are far enough apart to eliminate any of these problems?"

No not at low frequencies. The distance between the drivers could be such that they can couple so you get a increase or out of phase so you get cancellation. It's not cut and dry it really depends on a few things.

Rob:)

Zilch
02-03-2006, 02:59 PM
But that reading raised a question. Why is "lobe effect" a major concern involving sound reinforcement, but not a concern in home audio where our speakers are deliberately aimed at our seating location?
See for yourself. Play pink noise (a mono source) through both of your mains simultaneously. Put the mic some distance back, in the center. Move the mic left to right.

You'll see large "notches" appear on the RTA in the HF response with horizontal displacements as small as 1/8". Those are phase cancellations occurring due to different path lengths from the acoustic centers of the drivers. The larger the displacement, the lower the frequency at which it occurs. You can "walk" the rarefactions up and down the display with the mic.

Put your head there and do the same. You'll hear the lobing. At low frequencies where the wavelength is long, it takes a major displacement for it to happen so perceptibly.

So, what is the consequence for normal stereo listening? In those instances where the program material is identical coming from both speakers, i.e., a "centered" instrument or vocal, there's only one location (a vertical plane, actually) at which all frequencies are in phase, at the precise acoustic center between the drivers. At all other locations, one or more frequencies within an octave may be out of phase (all frequencies will be out of phase to some degree) and similarly interferring, depending upon the differential distance from the two drivers, the "comb" effect.

Thus does Mr. Widget enstation himself upon Bo's coffee table for critical listening. ;)

It's a wonder that it works at all, actually. Thankfully, all normal program material is dynamic and contains a broad frequency spectrum, so we don't perceive it as a substantial detriment. However, playing the same program material from several speakers in the same listening space is a recipe for sonic chaos, virtually by definition. Same for mono through multiples, like we used to do all of the time.

[Except for subwoofers, of course, where more IS better.... :p ]

Titanium Dome
02-03-2006, 03:39 PM
(snip)

[Except for subwoofers, of course, where more IS better.... :p ]


At least up to four subs in a normal room (according to HK).

http://www.harman.com/wp/index.jsp?articleId=1003.0

Get the full PDF for details.

pangea
02-06-2006, 01:33 PM
If you search this forum, there is quite a lot written on MTM and D'Appolito, which uses the lobing constructively.

Also take a look at X-dir a free program, that shows how to position the drivers and what happens at different distances and different x-over frequencies.
http://www.tolvan.com/xdir/

BR
Roland