Speakerplans.com Homepage
Forum Home Forum Home > General > Newbie Discussion
  New Posts New Posts RSS Feed - room effects
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

room effects

 Post Reply Post Reply Page  <123
Author
Message
MattStolton View Drop Down
Old Croc
Old Croc
Avatar

Joined: 04 September 2010
Location: Walthamstow
Status: Offline
Points: 4234
Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 14 April 2018 at 11:51am
Sorry had to go to work, before finishing my daft post....

So back to OP point, some horns, if big enough, or enough of them in a line, will be more "1/2pi" to lower frequencies than others. So they appear to be more corner loaded, wherever you stick them.

Rog/Voids paper on different bass stack configs, including "stereo" stacks and corresponding dispersion patterns is a nice bit of paper. But even that shows what works at 50Hz, gives a very different pattern at 100Hz. Then try to chuck in cardio mono centre clusters, and it gets nasty, quickly.

So back to my last point in first post, some rooms, of some fixed dimension will sound "poor" for a certain placement of speaker. Whether BR or Horn will not make that much difference, they both put energy and waves into the room, and the room dimensions and wall surface finish then take over. The only difference between BR/Horn would be how many watts you have to put into each cab to get some SPL out, a decent horn should be 6-9dB louder for same watts in, as it is, for a certain bandwidth, already in 1/2Pi space.

What gets interesting is how arrays of boxes, be they BR or Horn, when the size of the array starts getting of the order of magnitude of the wavelength of interest, you will start to get cylindrical emanation of the wave. Same, allegedly, for long enough hangs of line array, when long enough, you get cylindrical wave front, at least in the near field. The cylindrical wavefront will tend to refract, and at some distance in the far field, revert to cylindrical expansion, and decay at 6dB...

So. 20m wide array of bass boxes would give some sort of pattern control to 20Hz. As you increase the frequency, the effect becomes more pronounced, because the width of the array is very much bigger than wavelength.

Same effect is why a 12" cone starts "beaming". Thinking horizontally, when the width of the cone is of the order of magnitude of the wavelength emanating from it, the wave will tend to be more directional. Technically speaking, you are actually getting cancellation off axis, as the wave coming from cone left is out of phase with wave from cone right, at some angle, and cancels.

This is most commonly demonstrated using coherent collimated light (laser) through 2 slits, so perfectly aligned waves pass through two points, and then refract through the slits and then combine onto some projection surface. You see a central bright point. And then a ring of darkness, ring of bright, ring of darkness. We will ignore light duality, as this is a wave feature....



Same with sound wave, emanating from coherent sources, so either one cone LHS interfering with signal from RHS, beaming in BR cone drivers, or multiple identical bass bins driven with same signal in phase. You end up with a Young slit's type interference pattern.

Now, going back to why a cylindrical wavefront is interesting, perhaps desirable, is that it breaks the inverse square law. Consider this drawing:




As mentioned, sound emanates from a speaker as a surface of sphere, subject to walls forming 2pi, pi, 1/2pi. At infinitely close to the speaker, 100% of the power is spread over 0 units of surface area. At distance r this same power is spread over the "surface" of the sphere, at 2r this surface is 4 times larger, at 3r, 9 times larger, so at any distance away, power intensity (SPL to us) is a function of 1/r^2, inverse square law. In our world, for every doubling of distance from the stack, you loose 6dB of SPL.

Now, if you can get the wave to emanate from its source in a cylinder, not a sphere, then the power intensity is a function of 1/r, i.e if you double distance you double surface area, not square it. In our world for every doubling of distance, you only loose 3dB! In theory, your long (enough) array has broken the inverse square law...

So. Car Park. If you take all these complicated effects into account, bit of standing wave in the hall way/entrance lobby, bit of directionality from your wide bass array, bit of roof of venue acting like a helmhotz resonator, and attenuation of HF by the walls, all you hear outside is bass.

Also, indoors, you have effectively made a band pass box! Consider this bandpass 4th order box:



The chamber with the port, is what the audience is standing in, and the port is the entrance door! In reality the speaker may be ported, there may be some form of stage door, and you well end up with some evil 8th order monstrosity of a system response!

In short, sound is fecking complicated, and then chuck in AES white papers on acoustic centres, and it all goes to shit, fast.

Heavy, 50% or more fullness, drapes to minimise/absorb waves getting to walls, and then ragged surface of wall to scatter reflections as much as possible. Job done. Perhaps some eq notches to minimise really bad room nodes (peaks of standing waves) and that is about as good as it will get, BR Horn or open baffle!
Matt Stolton - Technical Director (!!!) - Wilding Sound Ltd
"Sparkius metiretur vestra" - "Meter Your Mains"
Back to Top
MattStolton View Drop Down
Old Croc
Old Croc
Avatar

Joined: 04 September 2010
Location: Walthamstow
Status: Offline
Points: 4234
Post Options Post Options   Thanks (0) Thanks(0)   Quote MattStolton Quote  Post ReplyReply Direct Link To This Post Posted: 14 April 2018 at 12:14pm
@Toasty - cracking link to the acoustic centre calculator, that is properly broken my head, but makes perfect sense! Interesting how effective baffle area effects it too, I presume even the type of edge of baffle, square versus rounded off in some way, may have a small but measurable effect too!

As you say, horns will be interesting, as the interaction at port mouth isn't black and white, effective mouth of horn will extend forwards, albeit without quite the smooth expansion inside the horn, from floor as much as going from 1/r expansion to 1/r^2 expansion, at some point in front of horn. Large nasty area of physics some distance in front of mouth, for some distance, with multiple effects all occuring at the same time!

It will be very interesting to see how horn loading modifies what is measured and predicted for BR acoustic centres. And then how arrays of BR or horn further move the effective acoustic centre around.

Certainly adds to the complexity of trying to get cardioid arrays to work, if that is correct most peoples calculations will be a few feet out!

I think I am going to retrain as a Lampie or Vidiot. Far easier physics....
Matt Stolton - Technical Director (!!!) - Wilding Sound Ltd
"Sparkius metiretur vestra" - "Meter Your Mains"
Back to Top
 Post Reply Post Reply Page  <123

Forum Jump Forum Permissions View Drop Down

Forum Software by Web Wiz Forums® version 12.06
Copyright ©2001-2023 Web Wiz Ltd.

This page was generated in 0.109 seconds.