I've been doing quite a bit (well, in fact a
lot) of reading on the Internet recently about "tapped horns" and I thought
I might give the concept a try by building a "proof of concept", to (1) see
if actual output match the simulations and (2) if the results were
actually worth it. So, I downloaded a copy of HornResp (an application that
seems to be the best utility available online for designing horns), and in
trying out different alignments using a spare driver I had on hand, I came
up with a related alignment that's typically referred to as a "tapped pipe".
First, a few definitions
that may help to clear things up a bit...
A "tapped horn" is
basically a horn that physically consists of the driver mounted part of the
way up the horn, rather than at the throat as is commonly the case.
The advantages of this configuration are increased response at the low end
(a lot less dependency on mouth size) and a smaller box, compared to a true
horn with the same cutoff frequency. The disadvantage of a tapped horn is lower
sensitivity and power handling compared to a true horn with the same cutoff
frequency due to increased driver displacement within the pass-band.
A "tapped pipe" is
basically a tapped horn where the cross-section of the horn remains constant
from the throat to the mouth. An early example of a tapped pipe is the
"Jensen Transflex", information about which can be found elsewhere on the
Internet. Tapped pipes are a bit easier to build than tapped horns (because
of the constant cross-section), but the ability to "tune" the response by
adjusting the change in cross-section is lost.
As this is going to be a "proof of concept" and likely destined for the
scrap-heap, I decided to use one of the drivers I had lying around, rather
than purchase a particular driver for the project. The intent of the design
process is to come up with a tapped pipe alignment that extends the lower
cutoff frequency as much as possible, while keeping the upper cutoff
frequency of the tapped pipe as high above 80 Hz as possible. Excursion and
box size are also taken into consideration - we don't want to end up with a
12 cu.ft. box with 0.5 watts of power handling!
The spare driver that I used for this exercise is
one of the Pyramid W61 drivers that I used to use in my "El Uglito" 4th
order bandpass subwoofer. I measured the parameters with my WT3 and came up
with the following:
After playing around in HornResp for awhile, I came
up with a tapped pipe with the following specifications:
S1 = S2 = S3 = S4 = 290 cm^2 (cross section of tapped pipe)
L12 = L34 = 11 cm (driver distance from horn / throat)
L23 = 177 cm (horn length, less L12 and L34)
The total length of
the pipe works out to 177+11+11=199 cm. To convert this into an actual box,
I divided this by four, then decided on dimensions that would result in a
cross-section of 290 cm^2 for each section (here's where I made my first error - this
approach actually results in a pipe length that's shorter than the design
target, as it does not take into consideration the effect of the 180 degree
join at the end of each section!).
HornResp predicts that the
frequency response of this tapped pipe would be as follows:
The graph suggests that the
frequency response would be essentially flat between 43 Hz (the length of
the pipe works out to 1/43 the wavelength of this frequency) to just above
100 Hz. There's a minor peak at 135 Hz, followed by a huge dip, but
all of this is happening well above 80 Hz, the usual upper cutoff frequency
of a bass unit or subwoofer. The "hash" above 200 Hz can basically be
ignored - I'm sure that the folds in the tapped-pipe are going to have a
radical impact on the frequency response at that point.
Once folded up, box size works out
to around 2.4 cu.ft., which is quite large for a single 6.5" driver. On the
plus side, there should a lot of room in that box to mount the driver,
without overly restricting the cross-section of the pipe.
The following images indicate the construction phase of my "proof of
1. All of the panels cut out and ready to assemble. It
took about half of a 4x8 panel of 3/4 ply to produce these panels.
2. Putting the first two panels together. I started with the front
panel (facing downwards in the image above and one of the internal panels.
3. Attaching the third panel. This is the
one that will eventually host the driver.
4. Attaching the 4th panel (which identical to the 2nd panel). The path that
the pipe is going to take inside the enclosure should be clear from this and
the previous picture.
5. Attaching on side.
6. Attaching the back panel.
7. Attaching the other side panel and the bottom panel. Now the internal
structure of the pipe should be pretty clear.
8. Closing up the box. Instead of permanently sealing up the box, I
opted to use screw the panel in and use a thin strip of weather sealing to
ensure an airtight seal. This will allow me to take apart the box in the
future if I want to add any stuffing or otherwise change the box's
This picture above should give a good idea of the size of the finished box.
Here the tapped pipe box is located next to my ACI SV10 subwoofer.
This is a pretty large box for a 6.5" driver. When compared to direct
radiator alignments, size is definitely not one of the tapped pipe's
advantages. In these days of large drivers and cheap amplification, it's
probably easier and better to go with a director radiator alignment for home
or home-theatre use, unless there's a specific reason why you want to
attempt one of the high-efficiency alignments like a tapped pipe, and you've
got the available space (and the approval of your spouse to use it!).
Below are the measured results, with the pipe unstuffed.
Above is the measured frequency response of the raw system, without any
damping material inserted. As I'm driving the tapped pipe subwoofer using
the subwoofer channel on my amplifier, the response also includes the
effects of an active 12dB low-pass filter @ 100 Hz. Included in green is the
theoretical response as predicted by HornResp. As I got the pipe
length wrong, there is a bump at 53 Hz which is not present in the HornResp
predictions. I also later found a minor leak around the driver, which could
account for the strange dip and peak just under 20 Hz. Apart from
that, there's pretty good correlation above 100 Hz, particularly considering
the effect of the 100 Hz low-pass filter. It's clear from the measured results that HornResp has done a
pretty good job of predicting the system's frequency response, with good
correlation up to ~300 Hz.
Above is the impedance response of the system. In green is the
response as predicted by HornResp. Again, I suspect that it's a bit
different because I may have gotten the pipe length wrong, and the small
leak could have caused the lower frequency peak to be a bit lower in level
than expected. Interestingly enough, apart from the lowest minimum and
below, the other peaks that occur for the rest of the impedance response
seem to be close matches in frequency, though the amplitude seems to be off
(probably because of losses).
Below are the measured results, this time with the first two sections of the
pipe partly stuffed with polyester fiberfill.
Above is the measured frequency response of the
system, stuffed as indicated above. While the low frequency response remains
approximately the same, the upper frequency response has changed
significantly. The peak and notch between about 120 Hz and 200 Hz has
basically disappeared. Again, as I'm driving the tapped pipe subwoofer
using the subwoofer channel on my amplifier, the response also includes the
effects of an active 12dB low-pass filter @ 100 Hz. However, the response
above 100 Hz is significantly smoother in character than the response of the
unstuffed pipe. It sounds better.
Above is the
frequency response of the stuffed system compared to the raw system. The
smoothing of up the upper frequency response is readily apparent here.
And it looks like I got the driver seal right this time - the low end
response has apparently improved.
Above is the impedance response of
the stuffed system. The peaks and dips are a bit lower than the
HornResp predictions (the green line) now, and the magnitudes are
significantly different - not unexpected, as the damping would cause that.
The graph above compares the
impedance response of the stuffed system (red) to the unstuffed system
(blue). The peaks and dips have clearly shifted down a bit. This suggests
that the resonant frequencies of the system have been shifted down by the
addition of the stuffing.
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.