Tiny cabinet sealed box minimun size help

Hi. I'm building a small stereo tube amp PCL82 based (used on old TVs audio) Just 3W per channel and no deep bass nor very high frequencies. The idea is to build a nice retro-look wood cabinet with its own speakers, kind of a "retro futuristic boombox", old school shellac finish. :)

I get two (very) cheap 4'' speakers (one per channel) and I need to know the minimum sealed box volume of each cabinet.

Of course, no online data available for the speakers so I tested them with the LIMP software, added mass method (a coin of known weight). These are the output values:

Fs = 133 Hz
Qt = 0.01
Qes = 0.01
Vas = 4.82 liters

With such a small Qt and my not-so-high expectations, which should be the minimum size? Could be smaller than Vas?

Thanks in advance.
 

GM

Member
Joined 2003
Greets!

No such thing as a driver with a 0.01 Qt, much less a cheap one as even a powerful $$$ field coil couldn't come close to doing it. A 1.0 Qt is reasonable though. Since you have well proven software, try again if not a typo since Qt is often higher with cheap, small drivers.

GM
 
with unknown drivers or drivers without trustable t/s parameters, always measure them out, including frequency response and impendance. I use rew and dats V2 for that.

Just on a few parameters and looks, i can't tell. Vas is often the right place to start simulating, but often not where you end up.
 
Yes, the 0.01 value also looks weird for me, meaningless. But I repeated the test many times with the same absurd results. Sine test better than pink noise test, reasonable Fs but absurd Qts. I am almost giving up with calculations... Any "rule of thumb" for the old days? Remember...just 3W per channel and poor specs from the amplifier.
 

GM

Member
Joined 2003
Well, the pioneers had a Vas/1.44 vent tuned to Fs alignment for best trade-off between size, bass extension, so figure out its effective diameter/area and use a vent ~1/4 in area since it's low power.

The box will have a hollow sound, so put just enough damping on the top, back, one side to make it go away, then if it audibly 'rings' [under-damped], which it probably will, do the click test or use LIMP's impulse test to damp the vent if unacceptable and 'it is what it is':

Click test: Click Test | GM210 | Flickr

GM
 
Are the proper units used for input?
I catch myself mixing metric and English units occasionally, or sometimes ounces instead of grams, inches or mm (or should it be meters?). :eek:

Nothing personal, but sometimes it's worth double checking units or settings in the software.

Otherwise, the previous suggestions will likely get you in the ballpark.
Dave
 
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These are two plots, the unloaded in grey and the added mass (coin) in black. Of couse, I agree is a nasty plot. Now I am travelling and away from home. Next week I will try again with another computer. I have also a little USB sound card. Is just a DAC and only has outputs and no inputs but maybe it could help.