Henkjan's easter project 2010 - compact horn/BR with Alpair6

[Henkjan]

every year for the past 4 or 5 I treat myself to a small project at easter. the goal is to start on good friday, and have music at the end of easter-monday.

this year I decided to make something nice for the Alpair6, despite this little gem's qualities, it appears that its larger brothers get all the attention. and continuing my quest for proper sound on the desktop, it would be nice if it fitted neatly next to a computer screen or TV

I could of course have gone the easy way and to make another variation of my "Desktop TL", but was intrigued by a variation on the BR I saw in the german magazine Hobby Hifi some time ago in which they revived an old type of enclosure the horn-reflex. this basically is a BR with a horn like port.

so I tuned a BR for the Alpair6 using LspCAD, translated it to a horn-reflex using the formulas in the HH, and finetuned it using mathcad. the result is a small speaker, 12½cm wide, 20¼cm deep, and 39½cm tall.

next was buying a small sheet of birch plywood (1 sheet of 61*122cm is enough for a pair) and then wait for the weather to clear up for I have to be outside to use my table saw so I decided that this year there is also an easter tueseday

here is work in progress, and gives a nice view on the port:
paasproject 2010 003a.jpg

and here is the semi finished product:
paasproject 2010 006a.jpg paasproject 2010 007a.jpg

and how do they sound: pretty good I am glad to say. high and mid excelent as I knew it would be (it's an Alpair6 after all), and a very impressive low end, made me check that the sub was really off... but only when it is not put on a table as in the picture, for then the bass is much too heavy, with the large port in the front there is simply too much boost, but when put on stands the bass was deep and clean extending to somewhat below 50Hz.

I still have to finish of the enclosure (I'll just put some varnish on it), solder the BSC-filter with proper components, make a proper drawing, and some decent photos. and I'll experiment with some stuffing to adjust the bass when it is placed on a table. watch this space

[italynstylion]

Very nice work. I like seeing the A6 being put to use. It's great for compact designs. Does Xmax seem to be controlled well by the enclosure?

[Henkjan]

Originally Posted by italynstylion

Very nice work. I like seeing the A6 being put to use. It's great for compact designs. Does Xmax seem to be controlled well by the enclosure?

thanks. it does it very well, I have played it as loud as a T-amp would let me, and have not seen large excursions. this horn like port has a larger output than a normal port, I will post some sims later (have them on another PC)

[gvimhoof]

Cool design. Looks like a serious step up from the regular store-bought computer speakers. What's the purpose of notching the four corners on the baffle?

[Henkjan]

Originally Posted by gvimhoof

What's the purpose of notching the four corners on the baffle?

just the looks, nothing else

[rjbond3rd]

Originally Posted by Henkjan

so I tuned a BR for the Alpair6 using LspCAD, translated it to a horn-reflex using the formulas in the HH, and finetuned it using mathcad.

Hi Henkjan, can you give a description of the formulas to translate a BR to a horn-reflex? Thank you in advance.

[parb]

that is a fun and interesting little project!
any chance you'd put your drawings online? i've been looking for a fun mid-may long weekend project...

[_henry_]

it looks beautiful

[Henkjan]

thanks for the compliments.

Originally Posted by rjbond3rd

Hi Henkjan, can you give a description of the formulas to translate a BR to a horn-reflex? Thank you in advance.

here is the formula:
l = (15000(A1+A2))/(f²V)-k√(A1+A2)

where:
l is length of the port in cm
A1 is cross section of the start of the port in cm²
A2 is cross section of the mouth of the port in cm²
f is the tuning frequency in Hz
V is the box volume in liter
k is the end correction factor, recoomende values 0.45 for a free standing resonator, 0.8 for in a box like this one

advised is to set A1 to 25% - 50% of the Sd, and A2 2 - 10 times A1

this is just a general formula, YMMV

[rjbond3rd]

Hi Henkjan,

Thank you for posting the formula -- interesting! Hmm, I wonder where 15,000 comes from. I will try to puzzle this out a little