Woofer volume and MDF?

[That Guy]

Hi guys,

Recently I picked up a pair of old Sherwood three ways. I've left these round the speaker forums and no one seems to have any knowledge of this make (Sherwood 1060B's).

They're in a terrible cabinet atm I'm hoping to part them and use them in my first DIY speakers. I have big ideas for these speakers, though limited resources.

In my dreams they are beautious (probably not a word) :D, however they probably wont turn out anything like I hope.

I plan on separating the woofers from the tweeters and mids, pretty much cutting each tower in two. Then using passive, but external, crossovers for each of the pairs. This is in the hope of modularability (definitely not a word) so I can play around with them a bit more.

However to begin with I have 2 questions for you all.

1) How can I calculate the volume needed for my woofer if all I know is that it's 6 ohms and 10"?

2)(less important) how hard is it to work with MDF? I know it's the weapon of choice when constructing speakers however I only have access to fairly basic tools (no large electronic machinery).

Cheers guys, I'll keep you posted about this project :thmbsp:

[tomt]

I'll keep you posted about this project



don't forget the pic's!

[RayW]

1) How can I calculate the volume needed for my woofer if all I know is that it's 6 ohms and 10"?

You can't. However, you can measure the volume of the current cabinets and assume that the engineer who designed them was competent. If they're ported then you'll need to duplicate that too. Make the new cabs with the same volume and, if applicable, porting and you should be ok.


2)(less important) how hard is it to work with MDF? I know it's the weapon of choice when constructing speakers however I only have access to fairly basic tools (no large electronic machinery).

Cheers guys, I'll keep you posted about this project :thmbsp:

MDF isn't really hard to work with but it is a bit heavy and tends to eat up saw blades and router bits faster than natural wood. Corners and edges can be brittle so don't drop it. I know of guys who have built speakers with nothing more than a jigsaw and a straight edge. A massive woodshop isn't required but can be helpful.

Ray

[Ausjoe]

Just a tip a lot of folks forget. After you measure the boxes figure in plenty of bracing and add that to the internal dimensions. You'll be glad you did. Usually tightens up the bass. Front to back and side to side and corners are a good place to start. 10" woofers move enough air to flex box speakers. There are also some damping materials which are fun to play with. I think Parts Express carries some. Hope that helps.

[That Guy]

Thanks for the help guys.

I've measured up the current boxes and started drawing up some sketches, when they're done i'll post 'em. Tomorrow hopefully i'll scout out my MDF source.

One question, what is the best way to join mdf, screws/woodglue/both?

Cheers

[RayW]

I usually just use glue and clamp. It saves the effort of covering screw holes. However if you don't have clamps then screws work fine. The glue itself will be stronger than the MDF so the screws aren't necessary as far as the joint goes, just the clamping strength. If you use screws make sure you sink them deep enough that you can cover without leaving a raised area. It will most likely show up through the finish if you do.

Ray

[SaSi]

One bad thing about MDF is that it is a very resonant material. Unless you use half an inch (1.5mm) thick panels, it is very likely that the boxes will resonate badly. It can also help if you use internal braces and join oposite panels.

Otherwise, MDF is a good in that it is dead, like plastic.

I would use (and have done it already) a layer of rubber, at least quarter inch thick, and attach it to the inner side of the panels. It absorbs the waves, increases the mass of the box bringing the resonance frequency lower and the resonant level as well.

You also need to calculate or reverse engineer the volume of the box required by the midrange. Typically, in three driver speaker designs, the midrange is in it's own small box or often cylinder. If it just sits in the front panel exposed to the total volume reserved for the woofer, then the design isn't good and, I'm afraid, the drivers should be in correlation, not good.

If the midrange has it's own smallbox, make sure you calculate that out from the total volume you calculate for the woofer. Small errors in the calculation are irrelevant.

Avoid the use of screws in the construction. Apart from the fact that any decent carpenter will never use screws, screws tend to collect lots of energy from the box and radiate it. It's a bit of a geek's detail for high end constructions though, so it depends on what drivers you have and how deep you want to end into building the speakers.

[tomt]

One bad thing about MDF is





alon wolf, of magico, has a few things to say, about MDF -

scroll down halfway

http://64.233.169.104/search?q=cache:0Hn0fpGGCKgJ:www.magico.net/magico.pdf+alon+wolf+absolute+sound&hl=en&ct=clnk&cd=4&gl=us&client=safari
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
http://www.magico.net/?d=02_Technology

[SaSi]

OK, a bit of exaggeration here. . . But I can totally understand the approach towards an acoustically dead enclosure. KEF and B&W did it with concrete two decades ago.

I did it with two sets of plywood, total thickness of 28mm and an extra 10mm of rubber glued to the inside of the enclosure. Total volume of each speaker about 24 lt. Based on the classic BBC monitor design with B110/T27 drivers. Need about 150~200W per channel to come to life. But when they do...