speaker cabinets and movement in service

[Pascal]

Here's an interesting question:

As I understand it, constructing simple speaker cabinets is most easily done using mdf and veneer due the tendancy of solid wood parts to expand or contract basically leading to failure (cracked wood or joints).

However, I know there are also strategies (joinery) for dealing with movement in service in solid wood furniture such as table tops.

What kind of joinery would allow for some movement in service in a speaker cabinet?

Could the joints still be sealed from the inside somehow (silicone or other flexible sealant) similar to what is done with the mdf speaker construction for improved response?

[cgallery]

The speaker enclosure needs to be air-tight and rock-solid. So you can't easily accomodate solid wood because rock-solid means all joints have to be glued.

[dpaton]

The impetus for using MDF in multiple layers for speakers has more to do with making dense cabinets than dimensionally stable ones. The dim. stab. is just a benefit for me. Acoustically, you really want to build your speakers out of a sandwich of MDF, lead, sand, and MDF, but that gets prohibitively expensive, hazardous, and heavy after a short while ;-)

If you want pretty cabinets, make them out of MDF and veneer them, or use corner inserts of solid wood. Trying to make them out of anything but MDF or good plywood is an exercise in disaster.

-dave
builder of all things acoustic, for waaaaay too long.

[partialresponse]

The impetus for using MDF in multiple layers for speakers has more to do with making dense cabinets than dimensionally stable ones.

...in other words acoustically inert (minimal "ringing" when cabinet is rapped with the knuckles)...unless you care more about how the *speakers* look than how they sound!

- Kofi

[JimD]

The air in the cabinet of a speaker couples with the low frequency driver (woofer) and affects it's response. This is true regardless of whether it is a sealed enclosure or a ported enclosure or even a folded horn enclosure. If you were to make the speaker enclosure like a paneled door, the air leakage at the joints around the panel would not allow the proper coupling with the driver and the low frequency response would be affected. In an extreme case, you might even damage the woofer.

You don't want the enclosure to vibrate and affect the sound coming from the speaker and you do want the enclosure to act as an air spring, sort of, for the woofer which requires it to be sealed up tight except for possibly a port of specific dimensions.

A good source for drivers and some design information is "Parts Express". I think their website is just "partsexpress.com". They sell both expensive and inexpensive drivers and they list dimensions for cabinets for most of the drivers they sell. I have bought at least 8 low frequency drivers from them so far plus some higher frequency drivers and have been happy with the results.

Jim

[BigguyZ]

Have a LOT of free time? Here's a post that got me into building my own speakers, and eventually led me to WWing. The post has a lot of information on just about every aspect of speaker building and sound theory. At the very least, there's a lot of information on different providers of kits/ drivers, and lots of pretty pics to look at too. I built the Audax set talked about frequently in that thread.

http://www.avsforum.com/avs-vb/showthread.php?t=170942

[JimD]

BigguyZ,

My degree is in mechanical engineering but I have been out of school nearly 30 years. I started messing with audio things including speakers while in college so I've been at it awhile. I wrote a few senior level papers while in college on audio component design.

My only caution to you is that this field is full of subjective opinions presented as fact. There are a high level of variables involved (room you will listen in makes a big difference, type of sound you listen to, what type of sound you prefer, source of sound, etc.). Objective measurements are kind of hard to come by in most cases. There also tends to be a big snob factor (if a subwoofer has a 20 lb magnet and a cast aluminum frame it is obviously better than a stamped steel frame with a 10 lb magnet structure (not)). If a driver costs $100 it is better than one costing $40. A lot of this is not valid IMHO. I think I recommended Parts Express in my first post and I definitely think they are worth checking out. They have test equipment so you can test your low frequency drivers (when I did this I found that they were not very close to the published values) and modify the box design appropriately. They also sell kits, and crossovers, and components for crossovers. My best advice about passive cross overs is not to have one. I have made 6 speakers so far that my son and I use regularly with a 6.5 inch Pioneer woofer that happens to have a published roll off curve that matches a high power piezio tweeter pretty well. Piezios have high impedence so they can be wired in parallel with the woofer without changing the impeedence the amp sees significantly. These little two ways do not have great bass but both of us have a sub (mine is bigger) that uses an active filter and we just let the small speaker roll off naturally on the low end. I have not used any of my test equipment to determine how flat the frequency response is in the rooms we use them in because I kind of don't want to know. They sound good to us and that is what counts, in the end.

Have fun! Contrary to what somebody posted at the site you reference I think you can easily save money. The speaker supplier may save some on components but you have at least a 100% markup on his costs in the price plus another markup by the retailer. The raw drivers in a manufactured speaker are no more than 25% of the price. You can definitely use better drivers for a given price point in what you build, the only question is whether it is engineered as well. That answer can be yes also but it isn't super simple, but then that is part of what makes it fun.

Jim

[ars_lurker]

Here is a link to a speaker design that has been pretty popular for DIY projects. It uses MDF and veneer.
http://www.speakerbuilder.net/web_fi...D3W/index.html

Being a woodworker and an audiophile are two hobbies that when combined, can be financially devastating ! Good Luck!