[rajacat]

I'm about to start a diy speaker project. Which is more effective, a cabinet with thick mass loaded sides or one with substantially thinner sides but with lots of bracing? I was thinking that the mass loaded wall would leave a cleaner interior and have fewer interior surfaces that would cause undesirable reflections. Does it make any difference as long as the sides don't vibrate and distort?

...Roy

[bardamu]

Hello,
The French did do some test in the past. The best was 2 different thickness  panelks with sand in between. I think tjey did use 9 and 12 mm with  25 mm of sand in between. If the panels are to thick the vibrations will not go into the sand but just keep trasveling in the panel. The thicnekss of the panels is also determined by the total dimensions.
http://www.asrr.org/biblioteca/Revue%20Audiophile/FICHIERS/36/PETITE/PETITE.html
I hope you can read french. Take a look at this.
Good luck or bonne chance as they say in France, Edward

[James Lehman]

It has everything to do with what bandwidth of energy will be inside that box.

If it's a sub woofer box then you won't have any mid-range coming off the back of the cone. In that case it is very likely that the wavelengths of the energy will all be much longer than any internal dimension. So reflections and standing waves are not an issue. At very low frequencies, the box is about barometric pressure.

This is assuming we're talking about a sealed or a ported box. If this is a transmission line or an open backed baffle or some variant, then it's not at all about pressure. It's just about separating the front and rear phase of the woofer.

If there will be a lot of mid-range coming off the back of the woofer cone, then you can use internal bracing as a means of controlling the reflections inside the box; to deflect them away from bouncing off the back of the box and coming right back out through the woofer (with an undesirable delay).

If total cabinet weight is not an issue, then go for both! Just remember that internal bracing takes away from total interior volume.

James.

[JLM]

Can't say without particulars.  Note that in many designs the cabinet emits more sound energy than the drivers (so you're listening to a violin).  Need to know the thickness, unbraced dimensions, and material.

[*Scotty*]

One of the best bracing designs I've seen and my personal favorite is the shelf brace. A shelf brace is just like it sounds, basically a piece of wood an inch or two less than half the depth of the speaker cabinet in width and the same dimension in length as the cabinets internal width. A series of these braces is installed in the enclosure in an alternating pattern. This ties three sides of the cabinet together with the shelf brace and breaks up the internal volume of the cabinet into a series of smaller spaces while simultaneously increasing the rigidity of the cabinet walls. The shelves also act like a breakwater protecting a harbor in that they act to reduce the intensity of sound waves attempting to pass from the top of the loudspeaker towards the bottom and back again in a reverberant fashion. Here are couple of illustrations of shelf bracing, excuse the poor draftsmanship.
 


A speaker cabinet that is built with shelf bracing can use a relatively thinner wall and still easily pass the knuckle rap test. The enclosures wall have been sub-divided into a series of smaller panels with much higher frequency resonance points that are more difficult to excite. The speaker will also have a lower total weight than one made with much thicker walls and a poorer bracing design.
Scotty

[rajacat]

The enclosure will be for mid bass drivers Acoustic Elegance TD15Ms'. The tweeter will be a compression driver http://www.diysoundgroup.com/drivers/tweeters/ba-750.html using a SEOS waveguide www.horns.pl/seos18.html . The mid bass will operate within ~ 45hz(F3) - 850hz ported. I have considered using a sealed box. In that case the F3 would be ~85hz however since I have these high end midwoofers, might as well use more of their effective range. The waveguide will sit on top of the bass bin either in a semi enclosed box or just in hardwood brackets. Hmm...so 850hz has how long of a wavelength?  :scratch:I'll have to do some number crunching.

I want to go for maximum sound quality so I'm still playing with the parameters. I've modeled, in winisd, boxes for F3 45 and 65 ported. 85hz sealed.

I also have some subs. If I can go fairly low with the midwoofers, they'd contribute to a distributed bass scheme.
Weight isn't a major factor although I'd prefer something that wouldn't need a hand truck to move.

[JLM]

850 Hz signal is 16 inches long (speed of sound is 1130 ft/sec divided by 850 cycles per second).  But I believe you meant 85 Hz, which would be 159.6 inches long.  Bass frequencies pressurize speaker cabinets, so wavelength is irrelevant.  Keep in mind that equal energy is produced inside and outside the cabinet from the driver, so cabinet pressures are relatively high.  Also keep in mind that the driver cone/dome material is very acoustically transluscent compared to any cabinet material.

Recommend non-parallel cabinet walls, especially directly behind the driver (this is why pipe and transmission lines can produce clearer midrange).  Window braces (solid panel with 4 large/squarish cutouts) or even solid panel braces with a large circle cutout are also effective.

Still need to know the exact material to be used (to determine Young's modulus), cabinet wall thickness, and maximum unbraced length. 

[rajacat]

Actually the waveguide will maintain its constant directivity down to 650hz or so and the compression driver will go that low too and more.  Then the midwoofer will take over. The AE TD15Ms'http://aespeakers.com/shop/catalog/product_info.php?products_id=56 aren't true subs. The Fs(resonance freq.) is 34.7Hz, so they reach the point of diminishing returns at that level.

bardamu, the sandbox enclosure would probably work well(I don't read French and google wouldn't translate) but it's too much work for me.

Scotty, I like those shelf braces. It looks as though they wouldn't be too hard to implement. Maybe I'll go with moderately heavy walls and use the shelf braces for good measure. I thought of using some lead tapehttp://aespeakers.com/shop/catalog/product_info.php?products_id=56 between two sheets of plywood.

[*Scotty*]

rajcat I wouldn't worry about the wavelength, in my opinion you just need to have a relatively inert enclosure whose walls don't resonate at midrange frequencies. I think critical importance should be given to the type and amount absorptive material that is applied to the walls. You may have to do considerable experimentation in this area to arrive at what you consider the correct voicing for this portion of of your system. I would use a fastening system on the TD15M driver that allows for easy removal and tweaking of the wall treatment inside the box. Whether the walls are non-parallel or not is of considerably less importance. A large wooden bell/ can be constructed with non-parallel as well as parallel sides.
Given the TD15M's Xmax of 6mm I would be inclined to use a sealed enclosure tuned somewhere between 40Hz to 50Hz with a system Qtc between .6 and .707. This would tend to limit doppler distortion and keep the bass that is produced clean and relatively low in distortion. I would let subs do the heavy lifting below 50Hz.
The sealed boxes internal volume will set the system Fs,Qtc and F3.
The lead is a neat idea but it might be a little hazardous to work with. I would wear a good mask with filters if I were cutting a lead sandwich type of board on a table saw. It will add mass as well as a large density difference between the layers of material making up the walls. The weight will also add up quickly making the box something of trial to move around the listening room. Casters anyone?
Scotty

[FullRangeMan]

I'm about to start a diy speaker project. Which is more effective, a cabinet with thick mass loaded sides or one with substantially thinner sides but with lots of bracing? I was thinking that the mass loaded wall would leave a cleaner interior and have fewer interior surfaces that would cause undesirable reflections. Does it make any difference as long as the sides don't vibrate and distort?

...Roy

I prefer the second option: a  cleaner inside to not increase the reflections.
If I remember well a JValin Avalon Eidolon test on FI mag, this speaker use 3 thicks:
Baffle 100 mm
Sides 20 to 30 mm
Back/top/botton 20 mm

This factory tour had some pictures that may help.
Please note the 3th image, the baffle is empty and will be filled with a lot of PolyEthilene blanket.
http://www.soundstagelive.com/factorytours/avalon/
On youtube there is more Avalon factory tours;

[James Lehman]

You might want to set up your speaker for bi-amp before you build a fixed, dedicated passive crosser. Then you can use an active crossover and find the sweat spot. I know 850Hz sounds like it would be a good idea, but you won't really know until you try it. With an active crossover you can sweep it around. Just close your eyes and don't worry about what the knobs are telling you. Use your ears!

[*Scotty*]

rajcat, plugging the TS parameters into an online calculator for a sealed box, for a Qtc of .707, I get a 3cubic foot box with a system F3 of 73Hz. which is pretty much in line with your calculations. The woofers Vas is the parameter that is pushing the F3 well north of 50 or 60Hz. It would be nice to know just how accurate that Vas figure is for the TD15M. The Vas of the driver single is the most difficult parameter to achieve unit to unit consistency on. I have a feeling, based on the drivers Vas, that a vented box is going to get pretty big rather quickly as you try for a lower F3.
 The thick wall approach will certainly work and wood is relatively cheap, cutting and tailing 4ft.x8ft. pieces of MDF that have been sandwiched together could pose some real weight problems however and would probably require some assistance from a helper to avoid herniated discs and the like.
 My speakers were built with a double thickness front baffle made from solid mahogany glued to 3/4in MDF for a total thickness of 1.5in.. The top and side walls of the enclosure are a single layer of MDF. The bottom is a double layer of MDF for strength. As it is, the speakers weigh in at 185lbs a piece and are a little dangerous and unwieldy due to a 14inx17in. footprint and height of 56in.
The internal volume of my speaker is a little less than 6cu.ft. and I can see how it wouldn't be hard to quickly exceed 100lbs on a 3cu.ft. cabinet using your TD15M and some lead sandwiching in the walls of your box.
Scotty

[rajacat]

Here's the three boxes I modeled.
yellow-- sealed box: W: 18.00 in., H 19.36in., D 12.00in 2.42cu. ft. F3~75Hz
blue--vented box: W 18.00        H 33.00in,  D 17.45in  6.00 cu. ft        F3 65Hz
green--vented box: W  18.00       H 33.00       D 16.52 5.679 cu. ft.      F3 42.62


You're right. Either of the vented boxes would be around 6 cu. ft. I haven't added driver volume and bracing yet. I think I might use  plywood and omit MDF because it's heavy and toxic. I live near Edensaw http://www.edensaw.com/MainSite/Store1/Content/SiteContent/1/Home/Main.aspx. They have a huge warehouse with a large selection of plywood and hardwoods. It's a great local resource and I visit it regularly looking for special sales and quality scraps of exotic hardwoods for small projects. 

I'm tempted to make a sealed box and see if I can integrate it with my subs. I'll be using an active crossover (miniDSP)http://www.minidsp.com/products/minidsp-in-a-box/minidsp-2x4

BTW here's a new toy I just purchased that makes cutting large sheet goods a breeze.

http://www.amazon.com/Makita-SP6000K1-2-Inch-Plunge-Circular/dp/B000UYTKG6/ref=pd_ys_sf_s_228013_a1_6_p
I found a reconditioned unit and saved a hundred bucks. It looks and works like new.

[*Scotty*]

Nice toy er Makita. If you go for the shelf braces they have to be cut pretty precisely. In order for them to do the job they have to slide straight into the cabinet. They can't go in on a slant like a shelf that needs to be leveled or the books will slide off and then be twisted into a level position. Everything has to be square to start with then the shelf will just slide in until it seats against the back wall.
I envision a cabinet shaped a lot like a sub-woofer enclosure and perhaps less like a tall skinny loudspeaker. I am thinking something like the bottom half of an Altec Lansing Model 19.
Scotty

[navin]

The thick wall approach will certainly work and wood is relatively cheap,

My speakers were built with a double thickness front baffle made from solid mahogany glued to 3/4in MDF for a total thickness of 1.5in..

and I can see how it wouldn't be hard to quickly exceed 100lbs on a 3cu.ft. cabinet using your TD15M and some lead sandwiching in the walls of your box.

For a 3 cu. ft box I would use a sandwich of 12mm MDF (inside) + 3mm of lead + 12mm of Marine grade Teak-Ply (outside) for the baffle I would go for 12mm MDF (inside) + 3mm of lead + 12mm of Marine grade Teak-Ply + 3mm resin bonded fiberglass + 12mm MDF. Sides would then be 27-31mm (depending on veneer/paint thickness) and baffle would then be 42-46mm.

See picture of lead sheet sandwiched between layers of ply. If you look closely you can also see a layer of resin bonded fiberglass in there.


For a subwoofer non-parallel surfaces are not really required.

[James Lehman]

Since you are already thinking about an active crossover with a sub.... check this out:

http://www.audiocircle.com/index.php?topic=110569.0

This is bi-amped. It also uses a compression driver and horn!

This is why I suggested earlier that you try it first bi-amped to get the exact right crossover freq.

I would TOTALLY recommend using one of those very inexpensive, low power, single board class D amps that run off of 12VDC for the horn.

You can still get a no-name version of the Sonic Impact T-Amp and there are quite a few other variations of the same idea on eBay.

[planet10]

I prefer the second option: a  cleaner inside to not increase the reflections.

I concur... lots of bracing will also help randomze the dimensions that will cause reflections and standing waves.

One rule i use, counter to what Scotty posted is that a brace is more effective if you divide the panel such that the aspect ratio of the subpanels is greater than the panel being braced.

dave

[*Scotty*]

Can you elucidate your concept?

that a brace is more effective if you divide the panel such that the aspect ratio of the subpanels is greater than the panel being braced.

Scotty

[planet10]

Higher is better.

dave

[James Lehman]

If you are going to use internal bracing, stay away from repeating the same dimensions between the braces and stay away from 90 degree angles!