Folded Horn - 4" Fostex speaker - First build for one of these

[quakecitystudio]

This is my first time building a folded horn. I had some 4" Fostex speakers (15watts) from old studio boxes and wanted to try building a folded horn. I had some scrap pieces of wood laying around and gave it a try. Since I don't have access to a pc to use Horn Resp or have an emulator software, I started building keeping in mind a particular design I saw. I made sure not to "choke the front spacing of the woofer, and made sure that the horn line was expanding from the front of the woofer to the box opening. This being an experimental box, I threw a little paint for looks. The box is screwed together and the edges and seems are lined with wood glue to seal any possible air leaks. I had a bit of an issue with the woofer, in that apparetnly I had my circular jig for my router a little smaller than it should, and started hearing some woofer scraping inside the port. So I cut another piece of wood with the appropriate 4" opening and made sure that I didn't have that problem.

Here are the pictures:

[quakecitystudio]

The dimensions of the box :

31" height x 9" depth x 7" width

Angled board inside - 27" long and .75" thick

Horn opening at largest end - 5.5" width x 8.5" height

Left and right side - .75" thick

front and back boards - 1.5" thick

[Colin - STAPUFT]

Looks very well built and worked out. I've gotta' know- how does it sound? Looks really good, short of getting the finish up.

[Bump4life]

my question is... does it work? if so, how well?

[quakecitystudio]

The folded horn worked. There's was cone movement and sound coming from the speaker. I did have a problem and I knew that it may be an issue, and that was the area of the throat of the horn was not matching the surface area of the speaker (obviously). So I felt there was a bit of choking going on causing the speaker to overwork itself as it tried to push air down the narrow gap at the throat of the horn:

So I took a side of the box out and adjusted the inner board so that it is parallel with the front and back board. It is now a t-line with the spacing from the inner panel to facing wall being about 2.25" all the way down. Because most of the box was already built, and I didn't want to start completely over with this box, it wasn't going to have 2.25 all the way throughout the line due to the overall depth of the box being insufficient. So, I made sure that the inner panel and facing wall were 2.25" apart all the way down, and the back side up to the port ended up being slightly larger but even all the way up:

It sounds alot better with that adjustment. I'd need to find a receiver with a crossover so I can see how it really sounds. I've been testing it with a small studio amp of which I blew the power to the tweeters (so it only plays mids and lows). When I play a track on computer and use an equalizer and adjust for only the low frequencies (emulated crossover), I can sense that the ideal range for this setup is between 80 and 200 hz, leaning more towards the later.

Upon listening it's definitely louder than before and is moving a great bit more air than it did before so there's greater output of sound coming from the system, with it only being 15 watts. It's no 15, but it's surely pumping alot more sound than it did in it's original bookshelf format: (the amp on the back is what I'm using to power it)

[quakecitystudio]

I blew or burnt out the power to the tweets of the bookshelf, while using them outdoors near full volume as dj preview monitors.

[quakecitystudio]

This is a screen shot of the 200-1 hz sweep at an amplitude of 1 for 30sec. The top is with the t-line and the bottom is with the folded horn.

The shape of the sweeps are important for comparison.

It was recorded low on the Zoom H4n mic at 1 foot away from horn/port opening.

Screen grab from Audacity (Mac computer).

[White Lightning]

Very interesting ... Always wanted to try a folded horn setup ...

Thanks ...

[quakecitystudio]

Number wise:

I had to work backwards to see what frequency the box was:

27"(t-line) * 2(folded) = 54"/12"= 4.5 ft * 4(quarter-wave theory)= 18. 1126 ft/sec (speed of sound) / 18= 62.5hz

But the graph shows three peaks.

So if we go with the 4.5 ft. because that's what it actually is and not do the 4x theory -

1126/4.5= 250.2hz

[quakecitystudio]

I'm going to redo the folded horn but with different wood, and with proper port throat area. But I will say that the first build of it was very loud at around 600 hz with that narrow gap in front of the speaker.