The Subwoofer DIY Page v1.1 - Projects
Tapped Pipe "Proof of Concept"

last updated: 12 October 2018

Conclusions

  1. 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.
     

  2. 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.
     

  3. 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.
     

  4. 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.
     

  5. 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.
     

  6. 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.
     

  7. 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.
     

  8. From the design work and the results, it looks like the best driver for a tapped pipe may be one with the following characteristics:

    • 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.

Other thoughts
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.

Brian Steele
12 October 2018