Film-Tech Forum ARCHIVE: Making small speaker baffles

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Posted by John Walsh (Member # 168) on 02-14-2004, 10:31 PM:

Our company is going to refurbish one of our theater complexes. It this location, there are no baffles or bafflettes behind the screen. The area behind the screen has an exit hallway, so the speakers are on top of the exit ceiling. I don't have much of a buget, but I was thinking of making small baffeletts for each speaker. The idea is that the exit hallway facing the screen could be part of the baffle; I would just place a pre-made section around the speaker, even with the existing wall.

Would there be any advantage in placing a 2ft wing on each side, and top of the speakers we're using (JBL4675's)? I'm wondering: Is a 2ft wing is too small to do any good? I'm thinking of this size because I can use one sheet of 3/4" plywood, with not much waste, for each speaker. I'd make a frame, put ductliner on the face, and bolt it to the speaker decking.

Opinions?

Posted by Adam Martin (Member # 641) on 02-14-2004, 11:40 PM:

You mean something like this?

Posted by Ian Bailey (Member # 1752) on 02-15-2004, 05:50 AM:

Ok I haven't seen these wings before can I get a bit more information on what they do.What they are made of-"duct liner" is that like fibre-glass insulation? And when installed can you hear the difference?

Posted by Ian Price (Member # 14) on 02-15-2004, 12:36 PM:

I tried to find a paper on the use of baffles for cinema speakers but I couldn’t find anything written. So I’m going to give it a whirl off of the top of my head.

Bass waves are very large. A 30-Hertz bass note is about 30 feet long.

When Altec Lansing first came out with their cinema speakers they were all horn loaded. The size of the horn has an effect on how low that horn will play. Klipsch used the walls in the corners of the room to increase the horn size with their Corner Horns. Altec used Baffles on their A1 through A5 cinema speakers to increase the bass energy coming from what were very small excursion drivers. Also when bass energy goes around the speaker it tends to cancel the signal at the back of the speaker. If you can stop the bass energy from going around the speaker, you can subjectively increase the perceived bass energy.

So why doesn’t every home speaker come with a big old flipping baffle? Baffles tend to propagate the sound energy along the baffle and destroy imaging. The entire concept of stereo speakers is imaging. The idea is to sit in the sweet spot and close you eyes and “see” each instrument in its proper position on stage. It is very hard to get proper stereo imaging in a movie theatre because you have people sitting everywhere at once. So the first thing you can get rid of is proper imaging in favor of increased bass energy. Now how do we get proper stereo imaging for all these people? Well first we add the center channel, which has always been there to anchor the dialogue behind the screen. Then we add surround speakers to place sounds coming from discreet locations. Then the speaker manufactures discovered that you could increase bass energy by using larger excursion speaker cones. Then you can port the speaker or add some sort of sympathetic vibrating device like a passive woofer to increase bass energy. All of these approaches have the problem of introducing various types of distortion to the sound.

So the problem in picking speakers for any room is how big is the room? How many people will be sitting in the room? What is more important to you, efficient, copious bass or proper stereo imaging? If you have 1-3 people sitting in a room listening to pop, jazz or classical music then a small speaker without a baffle and with proper imaging might be good. If you have 100-1,000 people sitting everywhere in a room and you need to hear clear dialogue and thunderous explosions, then a nice horn loaded speaker with proper baffles will help.

Baffles in a theatre tend to increase bass energy and limit room interference problems to more focus the speaker for clear sound.

In-wall speakers give you that baffle but with thin flexible speaker frames and very resonant walls. A theatre baffle is usually an inch thick partial board or plywood with a layer of duct liner on the face to control slap echo and to limit high frequency smear.

The best kind of baffle is a baffle wall that is continuous from speaker to speaker and to the sidewalls. But they have discovered that you can get 80% of the effect from small baffles or wings attached to the speaker. Usually these are as tall as the speakers and 3 feet wide. Yes you can make them yourself with inch-thick plywood or MDF and black duck liner on the face.

Posted by Adam Martin (Member # 641) on 02-15-2004, 01:14 PM:

Wow. And here I thought the only purpose of the baffle was to reduce echo by absorbing some of the sound bouncing back at the speaker from the rear of the screen.

Posted by Mark Gulbrandsen (Member # 72) on 02-15-2004, 02:06 PM:

John,

Actually its better to stick to Drywall for baffles because of its deadness. With todays high reproduction levels and bass energy a simple plywood baffle might be more likely to rattle and squeek unless its reinforced by a very well made frame. You can make a lesser frame, cover it with 5/8 drywall and then duct liner of what ever you normally utilize for absorbtion. be sure to go up at least a foot above the mouth of the hf horn and figure the baffle size by eigth wavelength, quarter wavelength or half, etc. depending on your size constraints. Any decent baffle around a screen speaker will be a benefit though. Also be sure to completely cover the entire back wall behind the screen and the reear wall of the room with at least 2 to 3" of material.

Adam actually hit a much more important point of a properly designed baffle, or baffle wall. It does a big job of limiting the relfected high frequency waves bouncing back and increasing distortion and uninteligibility levels. The other important function a baffle wall does is to reinforce the low frequency end of the spectrum. Typically it will increase the bottom octave 2 to 3 db. This is actually a benefit as one can cut back the lowest 1/3 frequency adjustments a bit and lower the normally high harmonic distortion produced by woofers.

I normally dump the 30 and 40 hz adjustments completely in any installation that has a sub woofer as this is normally(but not always) the highest point of distortion produced by a woofer. Extreme LF distortion is usually measured in the tens of percent!!

We will also be building quite a few smaller baffle assemblies for several upcomming jobs. I've seen over the last 25 years that way too many techs think that placing the speaker up on a shelf is all there is to installing screen speakers in a room. This is absolutely NOT true.

Mark @ CLACO

Posted by Jeff Stricker (Member # 224) on 02-16-2004, 05:54 AM:

The way I had it explained to me a baffle keeps the sound waves produced by the rear of the speaker cone from reaching the waves produced by the front of the cone. At low frequencies, these waves combine out of phase and limit the low frequency response. The demonatration I saw was using a 2ft X 2ft piece of plywood with a 6" hole in the center. The person doing the demonstration held an operating 6" speaker in his hand, then he placed it on the baffle at the hole. What a difference in the sound!

Posted by Mark Gulbrandsen (Member # 72) on 02-16-2004, 09:00 AM:

Thats exactly right Jeff for an unbaffled or unenclosed driver. The drivers that are being installed in cinemas are already enclosed and on a baffle though so the baffle wall serves to reinforce the LF energy and helps couple it to the room in a more efficient way.

Mark

Posted by Manny Knowles (Member # 1171) on 02-16-2004, 11:06 AM:

What are the odds that all of these reasons are valid?

Posted by John Walsh (Member # 168) on 02-16-2004, 11:25 AM:

Thanks everybody! This is a drawing of what I'm thinking of (hope it comes out OK...) The idea is, well, I can't spend too much money, so I was thinking of using just one sheet of drywall(thanks, Mark!) per speaker; I'll need a total of 27.

I wasn't planning so much of increased bass, (Ian P's idea) which is a good one, but I figured the baffle was too small. It was the reduced 'screen reflection' (Adam's idea) I was going for.

We will have a subwoofer here, but I can't have too much power, because the walls at this theater are not too well isolated; sounds can be heard in the next auditourium.

Thanks for several good ideas! If you think of anything else, please post it. If we do this, I'll take pictures and try to get an idea of if it was successful.

Posted by Gary Crawford (Member # 2038) on 02-16-2004, 03:03 PM:

I was curious as to how high your speaker(s) would sit off the floor. Thanks for the drawing, I saw it would be centered at 4'8".

Keep in mind the accoustics of the room. In the older theaters (with stages that is) the rooms were designed for a speaking person's mouth, somewhere near 5' off the floor. We did a restoration some years ago and just parked a speaker on the stage floor while we worked on the sound system. It sounded flat, actually awful.

One of us remembered about the accoustics, and the normal height of a person's mouth onstage, and placed the speaker on a table and a small wooden platform so it was centered at 5'. It sounded great all of a sudden.

Posted by John Walsh (Member # 168) on 02-16-2004, 03:28 PM:

Gary; The speaker(s) are on top of an hallway, like in this picture:

On the right, you can see the screen is placed in front of the hallway wall; there is no classic stage, just a area for speakers. There is a SMPTE standard that suggests to place the HF horn at 2/3 the screen height. This means that, at least for close ups of actors, most of the sound will seem to be comming out of their mouths.

Posted by Manny Knowles (Member # 1171) on 02-16-2004, 05:12 PM:

How sturdy is the construction of that "hallway"? Any concerns about that platform vibrating? Are the speakers sitting bare on that plywood or are they decoupled?

Is wall-treatment still required (or recommended) behind the screen even when baffles/bafflettes are present?

Posted by John Walsh (Member # 168) on 02-16-2004, 05:38 PM:

Hi, Manny; Good point about the hallway construction.. I'll have to look at it when I'm there again. All speakers will have the Mason "Super W" pads, so at least there's a little isolation. Again, the wall isolation is not so good, so we're sort of stuck not being able to turn the sound up.

We have THX approved auditoriums that have no ductliner/insulation of any kind behind the baffle wall, neither on the baffle backside, or on the rear 'demising' wall. But those are full baffle walls, not 'bafflettes.' So the thinking is there's no way for the sound to get back there (to echo) in the first place. I did a theater recently (non-THX) and was afraid to do that, so I did put regular insulation on the rear wall just in case.

Posted by System Notices (Member # 2357) on 03-05-2007, 11:45 PM:

It has been 1113 days since the last post.

Posted by Tom Wienholt (Member # 1510) on 03-05-2007, 11:45 PM:

Back to the original question...Is a 2' wing on each side of a speaker sufficient for a speaker baffle? If not, what dimensions are sufficient for making speaker baffles that will be effective without having to construct an entire baffle wall? Also, it was mentioned that the baffle should extend at least 1 foot above the hf horn. Should the baffle also extend below the base of the speaker? If so, how much?

Posted by Cameron Glendinning (Member # 3516) on 03-06-2007, 02:54 AM:

Ok If, 30hz is an aprox 32 ft wave, 1/4 Wavelength would be 9 ft, so ideally, including the speakerbox with the wings the total should be 9 ft, if the box is 3 ft, then an additional 3 ft of wings on each side of the box ect. It is also good not to have a speaker exactly in the middle of the mini wall, slightly off centre in theory is ideal.

No you do not have to add wings below there is no advantage on bass response, infact it is only the sides that count. By adding wings above and around the horn complete with ductliner, you are breaking up the hi frequency sound that is bouncing off the screen before it bounces around backstage.

Wings need to be structually extremly strong, MDF or birch ply 18mm plus! crossbraced with 2x4 is a minimum, or several sheets of gyprock / drywall gued together, with no gaps inside between the sheets as these can create audible distortion.

quote: Manny Knowles
What are the odds that all of these reasons are valid?

100%

If you put a slight angle on the wings 4-6 degrees, it will help break up standing waves of the back walls of the cinema.

Posted by Brian Guckian (Member # 1678) on 03-06-2007, 01:42 PM:

Couple of questions:

Should that be 8ft rather than 9ft Cameron? (4 x 8 = 32).

Also, I'm interested in the wavelength division into 2, 4, 8 parts when calculating the speaker-plus-wings width; presumably suggesting powers of 2, i.e. 2, 2 squared, 2 cubed. What is the physics behind this?

If not adding wings above and around the horn is desirable for the reasons mentioned, then how come this is done with full baffle walls? Although I remember seeing a baffle wall where the area around the horn was open (the opening being sized to the larger dimensions of the speaker cabinet).

Thanks!

Posted by Tom Wienholt (Member # 1510) on 03-06-2007, 05:59 PM:

Brian,

If you read Cameron's post carefully, it doesn't say that a baffle around the horn isn't beneficial. I think he describes what this part of the baffle does and implies that it is beneficial.

If using sheets of drywall, what should be used to glue them together?

[ 03-06-2007, 09:07 PM: Message edited by: Tom Wienholt ]

Posted by Cameron Glendinning (Member # 3516) on 03-07-2007, 04:01 AM:

Ok Brian, umm the truth is that a 32-33hz wave is infact 32ft long, so wider is more to 30hz. It is simply a demonstration of the physics behind it, and told as simply as possible. I live in a metric country! It is a bit more complicated because the baffle will reinforce 1/2 wavelengths higher up the bass chain as well. Realistically a speaker interacts with the air around it, the better control of the air, the more accurate the sound waves that are formed. The more natural the sound created. By making the structure wide enough you are simply allowing the creation of better formed waves.

Now a total wall, THX wall, Baffle Wall is the ideal. It will totally remove any early reflections or echoes backstage by sealing them away in there own room. Early reflections are the enemy and can make dialogue hard to understand. A full wall will obviously give the very best interaction between the speaker and the room.

This thread is about making small speaker baffles, As a person who has actually built a few, I have tried 2 ft ones on each sides of standard twin 15" boxes and all I can say is that to me, bigger sounds better. Now 1/4 wave theory is easy to understand.
Draw a straight line, draw a sound wave, 1/2 will be above the line, 1/2 below. If you cut the wave into 1/4, you will find that the curve is exactly the same but mirrored 4 times.

Cinema Mid- Hi frequency horns are also designed around 1/4 wavelength theory.

I do not understand nor have heard of 1/8 wavelength theory.

quote: Tom Wienholt
If using sheets of drywall, what should be used to glue them together?

I would use the standard glue for the purpose, I would just be very careful of air gaps and voids. These can create resonences and rattles, so a very even application is ideal.

Posted by Brian Guckian (Member # 1678) on 03-08-2007, 07:01 PM:

Got that, apologies as I'd misunderstood the wording regarding the baffle in the area of the horn.

I'll read up on 1/4 wavelength theory; I know the general principles and benefits of baffle walls and just wanted to go into more detail.

1/8 wavelength came from Mark Gulbrandsen's post back up the thread:

quote: Mark Gulbrandsen
be sure to go up at least a foot above the mouth of the hf horn and figure the baffle size by eigth wavelength, quarter wavelength or half, etc. depending on your size constraints

Thanks for the answers!

EDIT: Consulting Audio by F.A. Wilson, it says that to reinforce the performance of a speaker around a particular frequency (40Hz is used as an example), a wave from the rear of the driver must be made to travel a distance of L/2 (L=lambda=wavelength) via a flat baffle, to achieve a 180 degree phase shift and thus add to (reinforce) the corresponding wave from the front. Presumably this basic principle must inform the techniques referred to above?

[ 03-08-2007, 08:18 PM: Message edited by: Brian Guckian ]

Posted by Cameron Glendinning (Member # 3516) on 03-08-2007, 11:23 PM:

Marks knowledge and experience is superior to mine! so I too will look into 1/8 wavelength.

quote: Brian Guckian
Consulting Audio by F.A. Wilson, it says that to reinforce the performance of a speaker around a particular frequency (40Hz is used as an example), a wave from the rear of the driver must be made to travel a distance of L/2 (L=lambda=wavelength) via a flat baffle, to achieve a 180 degree phase shift and thus add to (reinforce) the corresponding wave from the front. Presumably this basic principle must inform the techniques referred to above?

Not exactly, in this example the speaker is open backed which is inefficient. In our case we are enclosing the speaker in a bass reflex box that also has wings. Bass reflex speaker boxs maximise efficiency and minimises cone movement at the lowest notes and allow much higher sound levels to be produced. This efficiency is then re-enforced more by the wings. This tends to create too much bass at the bottom end which will be reduced during the eq. As Mark points out earlier there is a lot of distortion in the low end of speaker boxes, this will reduce that issue.

One variation that further reduces bass distortion that I have tried is the use of a large sealed box, which is named an infinite baffle box, which I mounted into an infinite baffle wall (also known as a thx wall) the lack of a port and the slight time delay (between 1/48 and 1/24 of a second that is a by product of bass refex) does increase realism at the expense of efficiency. The use of the box was mainly to contain the back wave as so it would not bounce around backstage and possibly leak into the cinema.

Posted by Brian Guckian (Member # 1678) on 03-10-2007, 04:37 PM:

Many thanks Cameron - I'm just wondering if 1/4 wavelength theory is applied to this so that any sound energy propagated say around the back of the wings is phase shifted to reinforce the LF - by the special sizing of the wings themselves.

I understand how dividing a wavelength into 4 segments produces "mirrored" units (equal areas under the curve?) but don't understand how that relates to reinforcement or to wave coherence. Is it to do even with the behaviour of soundwaves in single-ended tubes (resonance at a chosen frequency occurs when the length of the tube is 1/4 the wavelength at that frequency)? [Help me while I stumble around in the dark [eyes]

!]

It is of course a special circumstance since we are talking about a speaker driver mounted in a cabinet (a form of baffle), which itself is mounted in a flat baffle (attached wings). The example I gave above was very different and very general (open-backed speaker in a flat baffle), but thought it might offer a starting point into understanding the 1/4 wavelength principle.

A full baffle of course is a different case since the baffle is continuous, and therefore little or no radiated energy can travel back behind the source.

Real back to school stuff, this! [Big Grin]

Posted by Cameron Glendinning (Member # 3516) on 03-14-2007, 10:01 PM:

Sorry for taking so long to get back to this. The example of an open backed speaker in a baffle based on 1/4 wavelength theory is simply someting I have not tried yet.

I have however attempted 1/4 wavelength subwoofers, at home I use a 12" driver in a 8 ft long 12" diameter sonotube, the speakers resonence is 33 hz, and the tube provides usable bass from 22 hz.

I have also tried a 18 inch driver with a 4 inch voice coiled (hi efficiency 98db per watt) version in a fellow film techers 250 seat cinema. re 30 hz in a 10 ft long, 18" diameter sonotube, the results to me are exceptional, far more natural and pleasent sounding than the standard bass reflex designs always found in cinemas.

Many people who build there own speakers describe open baffle as a much better and more natural sound of a speaker, rather than the common sound of a box. The idea is that the wings will stop the sound from mixing from the front of the cone and the back until its 1/4 wavelength, by then the sound of the back of the cone will be in phase and re-enforce frequencies lower than that.

The speaker driver is a piston, its aim is to move air, the more controled the air is, whether placed in a wall or a partial wall, the more acurite the sound wave will be. If there are no wings then sound waves will distort as they form in the air infront but then wrap around on the side and then behind the box. So wings help create a more natural shaped sound wave.

Its fair to say that 1 bass reflex(efficent)speaker would have the same spl or efficiency as say 4 sealed boxes( not efficent). Altec did make THX aproved sealed box speaker system. Apart from that ported boxes dominate cinemas today.

I hope that helps Brian.

Posted by Brian Guckian (Member # 1678) on 03-23-2007, 12:20 PM:

It sure does! Much appreciated and that's clearer now.

BTW sorry for my own delay in replying as I was away at the excellent Widescreen Weekend in Bradford, UK [shameless plug! [Big Grin]

]

Posted by System Notices (Member # 2357) on 07-11-2008, 11:10 AM:

It has been 475 days since the last post.

Posted by Marco Giustini (Member # 4544) on 07-11-2008, 11:10 AM:

I'm going to build a small baffle around three 4675C speakers. The speakers are suspended (in a way that is approved by JBL) and the audience is not "stadium seating". So the low frequency enclosures are pointing the projection booth.

Would be any advantage to tilt slightly the entire speakers to point better to the audience? I know thta bass frequencies are omnidirectional but perhaps around 500Hz (crossover point) the directivity will start to be an issue.

I'm going to build a baffle to the subwoofers too, that are not suspended; they're on the floor, but with 21 feet of stage behind, and this is not good.
Let's take 20Hz, 1/4 lambda is 13.94 feet. Subs (2x4642A) are 3.9 feet long. So 13.94ft - 3.9ft = 10.04ft (5ft for each side).
But 4642A does not have a single cone in the middle of the box. So perhaps I should consider to have 13.94ft from around each woofer. Is this correct? This will raise my baffle at 6ft for each side. Would this increase my subwoofers' power? Are my thoughts correct?

Thanks
Marco

Posted by Jonathan Bodge (Member # 4072) on 07-11-2008, 12:14 PM:

Hi John, I have a chop saw, nail gun, table saw, drywall screwgun, and a knife to cut the sheetrock. The statement about the low budget frightens me though.

Posted by John Walsh (Member # 168) on 07-11-2008, 03:06 PM:

Hi, Jonathan! What do you mean 'low budget' scares you? You've been in this biz for decades ... I thought 'low budget' was the first requirement in the theater business! [Big Grin]

This topic was awhile ago ... and I never did actually carry the idea out. But again my orginal intention was to reduce reflections/echos behind the screen for as cheaply as possible. I wasn't looking for any kind of improved bass response.

Posted by Marco Giustini (Member # 4544) on 07-11-2008, 06:09 PM:

Yes it's my idea too, since the auditorium has too much reflective and behind the speakers there is a large stage.

I'd like to improve SW response too, since it's very poor at the moment (with only one 4642) due to, I think, the large stage behind it.