Tapped Horn Vs. Bass Reflex Case Study

[weltersys]

Just finished a tapped horn for a B&C 18SW115-4, an incredible speaker that works well in a variety of TH and BR alignments.

During the course of testing and a few accidents, I have fed up to 120 volts of sine wave tones to the speakers, they did not sound distressed at that level. At 46v at 20 Hz, the cone has 50 mm peak to peak travel, and had just a hint of “clicky” sound.

I’m calling it the “Keystone” TH, the exit is shaped like a keystone. Dimensions are 45 x 26.5 x 22.5 inch, 14.54 cubic feet (411.72 liters) external volume.

I had an old BR cabinet 36 x 24 x 17.5 inch, 8.75 (247.77 liters) external volume. Using a large port, 7.25 x7.25 x 9.75 deep with a 13.25 diameter inside circle tuned gives a 37 Hz Fb, which matches the TH’s LF corner pretty closely.
Cabinet braces prevented the speaker to be mounted with the deep magnet structure inside the cabinet, so the cone is mounted “backwards”, effectively increasing the cabinet volume by about 15 liters over a “normal” mounting.

A lot of lower tunings were tried, cutting off 1 inch of the port at a time, the port started over 12” long. The “top hat” port tunes lower than a normal “tube” port.

Maximum excursion is about the same for a given voltage for both the TH and the BR, though the TH excursion is far lower for most of it’s range.

In listening tests with both cabinets equalized to have the same frequency response and output, the primary difference was the port noise in the BR when pushed to the amp’s limit.
When the TH was pushed to the amp’s limit, it was basically noise-free, and 6 dB louder than the BR.

The top screen shots shows the Keystone TH and the BR cabinet at the same drive level, the lower screen shows the BR with 6 dB added, the level it would be if two cabinets with double the power were used.

Two of the ported cabinets would be over double the truck space of the Keystone TH.
Using twice the amp power, it would take two of the BR cabinets to equal the level of the TH above 63 Hz, the BR would put out about 2 dB more below 50 Hz.
If the pair of BR cabinet size was reduced to the size of the single TH, their output would probably be almost identical.

The TH, requiring half the speakers and power for the same output, is a clear winner over BR.

The Keystone TH design also allows for extended LF response (at the expense of reduced upper response) by covering the mouth. The response curves with the exit height reduced to 15, 10, and 8 inches are shown.

Art

[Djim]

Originally Posted by weltersys

Maximum excursion is about the same for a given voltage for both the TH and the BR, though the TH excursion is far lower for most of it’s range.

We found very similar results with our version of the Tham loaded with 18Sound 15WL1401's. We feed them 1Kw each and we haven't seen any problems. That's why I still believe Xmax is a pitty to use as limit with modern (expensive) drivers...

[epa]

design looks neat.
do you have some more pics of it?

[jbell]

Originally Posted by weltersys

The 3015LF is almost exactly that sensitivity in a ported box of similar size.

Oh well .

Originally Posted by weltersys

When the TH was pushed to the amp’s limit, it was basically noise-free, and 6 dB louder than the BR.....
The TH, requiring half the speakers and power for the same output, is a clear winner over BR.
Art

[weltersys]

Originally Posted by epa

design looks neat.
do you have some more pics of it?

Thanks,
I'm waiting to take more pictures until finishing the second cabinet and they are painted and have grills.

[Antripodean]

I'm interested in the path and folds for your TH if you have pics/diagrams, thanks.

[weltersys]

Originally Posted by Antripodean

I'm interested in the path and folds for your TH if you have pics/diagrams, thanks.

No pictures yet.
Horn path is very simple, other than the braces it only requires three parts.
First 180 degree bend is at the bottom center (approximately) of the cabinet, next 90 degree bend back top, next 90 degree bend top front, then down to the Keystone shaped exit.

[iand]

We had this discussion a long time ago.

If you can obtain optimum drivers -- which may be impossible for a reflex due to the required parameters -- and fit a noise-free port in -- ditto -- then a reflex of the same size as a TH has pretty much the same efficiency and cutoff frequency and maximum SPL for the same Xmax.

You may need 2 18" drivers in a reflex with difficult-to-build lighter cones and coils (still with enough Xmax) to get the similar results to one heavy-duty 18" driver in a tapped horn, but if you can't see what's inside the box the results will be very similar.

Either way, a the optimum driver (or drivers) for a reflex will have very different parameters to the optimum driver for a tapped horn. Put one into the other type of box and the results will be poorer -- this is what is shown in the comparison.

Even Tom's original patent shows this; a tapped horn lets you use drivers with less total volume displacement (Sd*Xmax) which may be a lot easier to obtain.

And it doesn't suffer from port compression, unlike many (most?) reflex boxes.

But in the ideal case a TH and a reflex are pretty similar seen as a black box.

(however you can probably build close to the "ideal" TH where you can't build close to the "ideal" reflex)

[weltersys]

Originally Posted by iand

We had this discussion a long time ago.

If you can obtain optimum drivers -- which may be impossible for a reflex due to the required parameters -- and fit a noise-free port in -- ditto -- then a reflex of the same size as a TH has pretty much the same efficiency and cutoff frequency and maximum SPL for the same Xmax.

You may need 2 18" drivers in a reflex with difficult-to-build lighter cones and coils (still with enough Xmax) to get the similar results to one heavy-duty 18" driver in a tapped horn, but if you can't see what's inside the box the results will be very similar.

Either way, a the optimum driver (or drivers) for a reflex will have very different parameters to the optimum driver for a tapped horn. Put one into the other type of box and the results will be poorer -- this is what is shown in the comparison.

Even Tom's original patent shows this; a tapped horn lets you use drivers with less total volume displacement (Sd*Xmax) which may be a lot easier to obtain.

And it doesn't suffer from port compression, unlike many (most?) reflex boxes.

But in the ideal case a TH and a reflex are pretty similar seen as a black box.

(however you can probably build close to the "ideal" TH where you can't build close to the "ideal" reflex)

I agree with all you said, although in the case of the BC18SW125, this is a cone that works great in both BR and TH, but is about 6 dB more sensitive in the TH, it is still around 3dB more sensitive than two would be in the same size BR cabinet. Talking about efficiency without regard to directivity gets to be a little more dicey, the Keystone is showing more directivity above 60 Hz or so than BR, as shown in a recent post.

My Lab 2 x 12" cabs have good low end response, but I wanted more upper end to allow a higher crossover point for the 8 inch speakers used above them, after much consideration I made the leap of faith and designed and built some tapped horns.

I have included another chart below, all are at the same drive level, using 25 & 125 BW filters except the TH with extender, which had a 20 and 1K BW filter. The Lab 2x12" is nominally 3 ohms, and exactly half the size of the Keystone TH.

The others are nominally 4 Ohms, the JBL SRX 728 is a bit larger than the TH. The Keystone TH with the extender disassembled for transport would be about the same size as the 728.

The BC BR is the same cabinet in the OP, with the cone put in "normally", which raised the FB by about 1 Hz, and made the response almost perfectly flat.

Art

[lovecapacitor]

Fantastic post, Art! I just picked up a pair of these drivers for my 1st TH build. Appreciate the data point!

lc