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johnr66 3rd March 2010 04:52 PM

Big bass from small power
I have a chip amp I built that produces 20 watts into 8 Ohms (before clipping). I'm requesting assistance into a very high SPL woofer design. After looking through this forum, it seems that a horn design is the way to go. I like the long box design since it is simple to fabricate.

My requirements are:
SPL @ 1w/1m over 102db
Response down to 30Hz (or below if size is not an issue)
Box length: 48" (over or under as needed, but 48" allows me best use of the plywood)
Width: ~15" (14 to 18" allowable)
Height: ~18"
Driver: 12" woofer. $100 or less desirable.
3/4" plywood, so internal space looses about 1-1/2"

Hopefully my design requirements aren't too crazy. Thank you for your assistance. I'll post pictures of the project as it unfolds.

Rademakers 3rd March 2010 10:22 PM

In such volume 100 dB/W/m from 30 Hz and up should be possible if you have a decent driver and the design can be placed upon a solid floor and against a solid wall. Placing it in a solid corner might even get you 105 dB/W/m from 30 Hz and up. Personally I would start by looking for a correct driver, say Fs 40-55 Hz, lowish Qes, Vas around 60 L (2.2 ft^2). With only 20 W, 4-5 mm of Xmax should do fine.

Best regards Johan

Glowbug 3rd March 2010 11:52 PM

How flat of a response do you need?

johnr66 4th March 2010 03:05 AM


Originally Posted by Glowbug (
How flat of a response do you need?

Good question. I'm not after super flat response, nor do I want precipitous peaks or dead zones either. I'm happy if I can meet somewhere in the middle.

Looking at my Parts Express catalog, the driver parameters given by Rademakers seems to point to what they call "Pro Audio" drivers (PA speakers). The Fs of the PA speakers are in the 40Hz range. The frequency range of these PA speakers go no lower than ~40Hz and the hifi speakers go down in the 20s, so I'm lost in selecting a good speaker. Should I ignore the driver frequency range and just consider Fs?

Moondog55 4th March 2010 05:23 AM

I looked at building a tapped horn myself but did not want to buy even more drivers

I think if you research tapped horns that they are tuned ( if that is the correct tern ) to the drivers Fs * 0.7071 o a 40 Hertz Fs driver gives 28Hertz

Look up "Horn response"

freddi 4th March 2010 05:52 AM

here's an inverse tapped pipe I did from one sheet with ~30Hz fs 15" - taper put Z-minimum around 30
YouTube - 40Hz Inverse taper transflex for 15" woofer from one sheet of plywood

bjorno 5th March 2010 04:40 PM

1 Attachment(s)

Originally Posted by johnr66 ( I'm lost in selecting a good speaker..

Hi johnr66,

FYI, Your 20W/ 8Ohm amplifier delivers at maximum about sqrt (20/8) = 1.58 A Rms, thus can survive a 4 Ohm load if not exceeding 10 W, i.e. = 4 x (1.58)^2: that is more input power than required to feed an EminenceDelta-12LFC TH. into over excursion at frequencies below about 20 Hz. See the picture. EminenceDelta-12LFC: Price: $94.99


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