Why do some Fostex BR designs recommend HP reflectors?

[arjscott]

I have a pair of FE127E drivers, and I'm thinking of building the double bass reflex enclosure.

I noticed in the Japanese version of the information sheet that comes with these drivers, the diagram shows HP reflectors located at various positions inside the enclosure.

I'd never heard of these and the english version doesn't say anything about them. A quick search revealed they are intended to reduce standing wave problems in horn enclosures.

The diagram says 1 HP reflector should be used in the upper chamber, and 4 more reflectors should be used in the lower chamber.

A rough translation of the note says something like "Use the HP reflectors in addition to sound absorbtion material to enhance the clarity and speed of bass".

Does anyone else think HP reflectors are necessay to get the best out of this enclosure?

I've never seen any kind of reflector used in BR enclosures, only absorbtion material. Perhaps the designer just thought "lets add them as an extra performance tweak, since Fostex sells them!"?

I'm planning on using 20-30mm of felted wool or polyester around the walls of the upper and lower enclosures and adjusting by ear (who knows if my ears will actually hear any difference, but I'll try adjusting it anyway).

For those of you who are still reading, I have another question regarding this enclosure:

All of the Fostex recommended bass reflex enclosures I've seen either specify plywood, or specify wood of a thickness that is only available as plywood (for example the FE127E DBR enclosure just says 15mm).

As far as I know plywood is more suitable for resonant enclosures. I seem to remember Mr. Brines saying it "rings like a bell".

So, is plywood truely desirable for Fostex's bass reflex enclosures, or do the designers just happen to use that because it's used for all the Fostex horns?

In New Zealand at least, marine plywood is around 5 times the price of MDF of a similar thickness. I don't want to spend that much on wood, so I'll be using MDF unless anyone advises against it.

So to summarise my questions:

Is the use of reflectors and plywood in Fostex's bass reflex enclosures just a result of the designers building too many horns? Or, are their designs superior to standard enclosures that use absorbtion material and MDF?

 

[Bob Brines]

I have a pair of FE127E drivers, and I'm thinking of building the double bass reflex enclosure.

I noticed in the Japanese version of the information sheet that comes with these drivers, the diagram shows HP reflectors located at various positions inside the enclosure.

I'd never heard of these and the english version doesn't say anything about them. A quick search revealed they are intended to reduce standing wave problems in horn enclosures.

The diagram says 1 HP reflector should be used in the upper chamber, and 4 more reflectors should be used in the lower chamber.

A rough translation of the note says something like "Use the HP reflectors in addition to sound absorbtion material to enhance the clarity and speed of bass".

Does anyone else think HP reflectors are necessay to get the best out of this enclosure?

I've never seen any kind of reflector used in BR enclosures, only absorbtion material. Perhaps the designer just thought "lets add them as an extra performance tweak, since Fostex sells them!"?

At the price Fostex wants for their reflectors, it's no wonder they recommend a lot of them. However....

A case can be made for putting a reflector behind the driver. In a rectangular cabinet, you will get reflections off of the back wall reflecting through the cone. A fostex reflector will break up the reflections and reduce any energy coming back through the cone. So will a wad of stuffing, a sheet of Deflex, fiberglass lining, etc. I personally like fiberglass lining, but I haven't tested all of the options.

I'm planning on using 20-30mm of felted wool or polyester around the walls of the upper and lower enclosures and adjusting by ear (who knows if my ears will actually hear any difference, but I'll try adjusting it anyway).

Probably as good a choice as any.

All of the Fostex recommended bass reflex enclosures I've seen either specify plywood, or specify wood of a thickness that is only available as plywood (for example the FE127E DBR enclosure just says 15mm).

As far as I know plywood is more suitable for resonant enclosures. I seem to remember Mr. Brines saying it "rings like a bell".

So, is plywood truely desirable for Fostex's bass reflex enclosures, or do the designers just happen to use that because it's used for all the Fostex horns?

Well, you already know my opinion on this. Braced and damped MDF will give a very dead box. You can get a dead box using plywood also. Or, you can use use thin, undamped plywood and let the box add a bunch of mid-range distortion that some like to hear.

In New Zealand at least, marine plywood is around 5 times the price of MDF of a similar thickness. I don't want to spend that much on wood, so I'll be using MDF unless anyone advises against it.

That's the case anywhere. MDF is cheap, plywood isn't. One thing about working with MDF, you MUST use a face mask or you will be hacking up and blowing dust out of your lungs and nose. But then, you should use a mask when machining and sanding any wood. Red oak and mahogany are much more poisonous than MFD.

Bob

 

[arjscott]

Thanks for the explanation and tips, Bob.

I'm now part way through building the enclosure with MDF. The dust is certainly a downside!

If anyone is interested, and hasn't spotted this already, this post (in a thread specifically about MDF vs plywood) has a link to some tests on various types of material.

http://www.diyaudio.com/forums/showthread.php?postid=912546#post912546

Links to:

http://www.zelfbouwaudio.nl/index.php?option=com_content&task=view&id=18&Itemid=2

Convert this from Dutch to English with babelfish.altavista.com.

This clearly shows MDF is less resonant than Birch plywood if the same thickness. I know everyone will agree on this, but it's nice to see graphs.

The last few graphs show 20mm concrete added to the enclosure. Perhaps this is overkill for my requirements but definitely makes Bob's case for 1/4" cement board a good one.

Now, back to wondering why so many people prefer to use plywood. Perhaps I'll have to build some of these just for comparison.

 

[no xo]

FWIW, I built a pair of these usin 3/4" MDF, and made the cabinets 12" taller. I filled this bottom chamber with sand to act as a integrated and very stable stand,and that puts the driver close to ear level. In the top chamber I used a sheet of deflex behind the driver and in the 2nd chamber some loose polyfil. Most people who hear them are impressed with the clarity, and when I pull off the grilles they can`t believe all that sound comes from 1 small driver. Very satisfying project for such an easy build. These replaced an old pair of Infinity bookshelves at my girlfriends house, and the difference in sound is huge. And by the way no HP Deflectors.

 

[arjscott]

I have also made the cabinets taller but didn't think of filling the bottom with sand, great idea. I might try some deflex and stuffing the lower chamber too.

I suppose you didn't notice a very slight "honky" sound when you first started listening to this speaker? I've started a new thread with a few questions about why this could be happening.

Hopefully the problem is due to my enclosure, but I'm not sure that anyone else will even know what I'm talking about. Everyone seems to use this driver with good results.

 

[Scottmoose]

Re the materials, I'm with Bob on the MDF front, at least for BR or TL applications. Plywood is stronger than MDF in tension and compression, but its break-up takes place over a wider range.

Just to stress the necessity of a mask: MDF dust isn't posionous per se, but it is a carcinogen, so you want to be very careful when cutting.

 

[no xo]

Arjscott, the only time I`ve noticed a "honky" sound is when I play some Hank Williams, and there is a "tonk" along with the honky.