I may have to be a bit more
careful when calculating the total pipe length of these things. Looks
like I might have been off by a few cm or so, based on the impedance response
and the frequency response of the un-damped system, which shows a bump at around 50 Hz, rather than
a smooth and flat pass-band down to 40 Hz. The impedance response also
seems to be shifted upwards a bit at the lowest frequencies.
Don't even consider
doing something like this unless you intend to use it with a 24dB/octave
low-pass filter within the pass-band, and/or you plan to deal with the out
-of-band response by another means. The out-of-band "hash" really colours
the sound of this subwoofer.
Stuffing should be considered a
mandatory part of this type of project. Stuffing makes a significant
difference at the frequencies above 100 Hz, smoothing out the response.
Stuffing only half of the pipe is enough to make a significant difference.
There appears to be an audible
increase in output at lower frequencies when using this type of alignment.
The response from the mouth of this tapped pipe has a tactile feel that this
little 6.5" driver could not do on its own. It's quite impressive. However,
output is limited by the driver's excursion capability.
It looks like HornResp
does a pretty good job of predicting the frequency response of a tapped
pipe, particularly within the pass-band. Out of the pass-band, the
response starts to differ, but that's likely because of the effect of the
folds on the system's response.
From some impromptu listening
tests, the tapped-pipe, after being stuffed, has a effortless and
unrestricted sound, up to the driver's limits. It's also not that easy
to tell the driver's limits.
While the box size is small
compared to a horn with a similar cutoff frequency, it's still large
compared to a simple sealed or vented alignment with the same driver.
The internal box volume of my "proof of concept" tapped horn works out at
around 2.4 cu.ft - enough to fit a 10" driver in a decent vented alignment
capable of higher output and lower cutoff. That 10" driver is likely
going to need a lot more power though, and the alignment is certainly not
likely to be capable of 92dB/1W/1M @ 50 Hz.
From the design work and the
results, it looks like the best driver for a tapped pipe may be one with the
Fs = 45-55 Hz : For a
tapped pipe or horn, Fc, which determines the length of the horn, tends to
be somewhere around 0.7*Fs. If you start with a driver with low Fs, the
length of the horn could turn out to be impractical.
Qts = 0.45~0.55 : The
driver's Q sets the bandwidth, but it seems to be difficult to get a smooth
pass-band when Q us much higher than 0.5.
Medium Vas : If Vas is too
low, the cross-section of the pipe turns out to be pretty small, leaving not
much space for mounting the driver in the pipe. Of course, if the Vas is too
high, the box ends up being very large.
The Dayton 8" Series II driver is actually a drop-in replacement for my
test driver in terms of parameters that affect the tapped pipe's frequency
response, but it is theoretically capable of much greater SPL due to its larger
Sd and Xmax.
01 September 2009