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Topic: Making small speaker baffles

Tom Wienholt
Master Film Handler

Posts: 371
From: Towson, MD, USA
Registered: Dec 2002

posted 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?

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Cameron Glendinning
Jedi Master Film Handler

Posts: 845
From: West Ryde, Sydney, NSW Australia
Registered: Dec 2005

posted 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.

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Brian Guckian
Jedi Master Film Handler

Posts: 594
From: Dublin, Ireland
Registered: Apr 2003

posted 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!

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Tom Wienholt
Master Film Handler

Posts: 371
From: Towson, MD, USA
Registered: Dec 2002

posted 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 ]

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Cameron Glendinning
Jedi Master Film Handler

Posts: 845
From: West Ryde, Sydney, NSW Australia
Registered: Dec 2005

posted 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.

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Brian Guckian
Jedi Master Film Handler

Posts: 594
From: Dublin, Ireland
Registered: Apr 2003

posted 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 ]

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Cameron Glendinning
Jedi Master Film Handler

Posts: 845
From: West Ryde, Sydney, NSW Australia
Registered: Dec 2005

posted 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.

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Brian Guckian
Jedi Master Film Handler

Posts: 594
From: Dublin, Ireland
Registered: Apr 2003

posted 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]

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Cameron Glendinning
Jedi Master Film Handler

Posts: 845
From: West Ryde, Sydney, NSW Australia
Registered: Dec 2005

posted 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.

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Brian Guckian
Jedi Master Film Handler

Posts: 594
From: Dublin, Ireland
Registered: Apr 2003

posted 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]

]

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System Notices
Forum Watchdog / Soup Nazi

Posts: 215

Registered: Apr 2004

posted 07-11-2008 11:10 AM

It has been 475 days since the last post.

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Marco Giustini
Film God

Posts: 2713
From: Reading, UK
Registered: Nov 2007

posted 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

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Jonathan Bodge
Film Handler

Posts: 83
From: East Dorset, VT
Registered: Nov 2006

posted 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.

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John Walsh
Film God

Posts: 2490
From: Connecticut, USA, Earth, Milky Way
Registered: Oct 1999

posted 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.

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Marco Giustini
Film God

Posts: 2713
From: Reading, UK
Registered: Nov 2007

posted 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.

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