Keystone Sub Using 18, 15, & 12 Inch Speakers
The Keystone Sub™ cabinet works well with a variety of speakers with quite different parameters and sizes.
The Keystone is named for it’s exit shape, similar to a keystone. Like keystone lens correction for off axis projection, the keystone exit corrects some frequency response problems that result when a rectangular exit is used with the particular fold pattern employed in it’s construction.
Any person wishing to duplicate my Welter Systems designs for their own use is welcome to, any person or corporate entity that would like to produce them for sale please contact me to arrange a licensing deal to avoid violation of intellectual property rights as defined in section 27 of the Universal Declaration of Human Rights adopted as international law in 1976.
It is similar in size to many 2x18” cabinets, exterior dimensions are 45 inches tall, 26.5 wide, 22.5 deep.
More information about the design are available in these posts:
Corrected plans are available in post # 487, including parts layout, thanks to NEO Dan, with the exception that the bottom of part "F" is 89 degrees, the front of "G" is a 5 degree angle, the back of "G" is 3 degrees as shown in post #478. The previous plans #94 and #97 (thanks Oliver) have some errors due to my providing Oliver with some incorrect numbers, sorry for the confusion.
Photos of the cabinet interior are in Post #99 and clarification of the 9 braces used in #206 and #451.
Distortion results are in post #12, there is a typo: The * should read: third (odd order) harmonic louder than second.
Hornresp inputs are available in post #96 and 130. The link explaining how distortion was measured expired, an explanation is in post #315, and a correction in #316.
PASC built a Keystone and another 18" TH and reports his findings in posts #114 & 115.
Frequency response of the partially covered Keystone exit, ("step down" mode) in #262.
Comparative tests using the same Keystone cabinet loaded with an 18 inch B&C18SW125 (four ohm nominal), a 15 inch Eminence 4015LF (eight ohm), and two Eminence Lab 12 (6/2 =3 ohm) were conducted.
The same 16.6 inch diameter cut out was used in each case, the 4015LF used a “doughnut” adapter, the Lab 12s used an adapter that centered the two speakers on the cut out and provided a stand off so they would not slap at high excursions. The Lab 12s have a "saddle shape" frequency contour, the 96 Hz peak needs to be removed with EQ for flat response.
There are a few other peaks and dips in the response curves due to very windy conditions, often requiring waiting minutes between gusts to record responses. It was literally a dust storm, the bottom of the cabinet was filled with dirt and tumbleweeds after the tests.
All the speakers were driven with the same level. The 4015LF is actually about the same sensitivity as the 2x12 pair, since it has about double the impedance minima.
A bass reflex Lab 2x12 (36 FB), exactly half the size of the Keystone sub is shown in the blue trace, it’s upper response is slightly reduced due to a Butterworth 125 Hz filter, all the other subs used a BW 1000 Hz filter.
As can be seen the Keystone sensitivity is about 6 dB greater than a bass reflex of half or more the size, so it takes about double the power and double the cones in bass reflex cabinets using similar size to equal the response. Two BR with double the power would be capable of few dB more output below 45 Hz, the Keystone would have more headroom above 45 Hz .
Subjectively, with the same drive level the Keystone using the B&C18 seems a bit more “punchy”, more stuff falling off shelves in the shop, and more feel in the feet of the sidewalk vibrating outside. It takes full power from a bridged Crest CC2800 effortlessly, while the 15 seemed a bit “wheezy” at a lower drive level, the 2x12 in between.
The Eminence speakers are a bit more sensitive, the 4015LF would be the best “bang for the buck” output per watt and cost. Making a slightly wider cabinet would allow two 15 inch speakers to fit, which would probably allow about 3 or 4 dB more output at the low corner and 5 or 6 dB more upper level with only about 20% increase in cabinet size.The Lab 12s sound cleaner, and with the same voltage are louder than the 4015L.
For those with limited amplifier or AC power or budget, the Eminence speakers are good choices for the Keystone cabinet.
Looking at the low excursion of the Keystone vs. BR in the region from 50-90 Hz, it is evident that the cone is under a lot of stress, audible on light (low Mms) cones.
The very stiff cones of the Lab 12 and the B&C18SW125 can take that stress and sound clean, when pushed hard the 4015LF sounds distressed.
The B&C18SW115-4 though less sensitive than the Eminence speakers, will go a lot louder with a cleaner sound given more power due to far larger Xmax, more power handling with better cooling, and a super stiff cone and suspension.
Using Hornresp simulations at rated Xmax values, (and impedance minima) average level from 35-100 Hz, the Keystone has these output levels:
BC21SW152-4 131.6 dB, 92 volts, 3.4 ohms, 2489 watts
BC18SW115-4 130.1 dB, 76v, 3.43 ohms, 1683 watts
BC18TBW100-4 127.8 dB, 59v, 3.32 ohms, 1048 watts
Eminence 2xLab12 126.3 dB, 36.5v, 2.22 ohms, 600 watts (300w.p.d.) 128.4 dB at 40 Hz.
Eminence 4015LF 121.9 dB, 38v, 5.17 ohms, 279 watts
The average impedance is higher than the minima, closer to the nominal rating of the speakers.
Other than the 4015LF, which did not sound good (by comparison) in this cabinet, all the other drivers are capable of more peak level without sounding bad.
I consider "safe" limiting for the Keystone sub using a B&C18SW115-4 to be as follows:
A) Use the DCR of the loudspeaker to determine the voltage setting, the DCR of the B&C18SW115-4 is 3.3 ohms, the minimum impedance of it in the Keystone approaches that value at Fb/Fc, where cone movement (and forced air cooling) is also at minimum.
B) The manufacturer's "Continuous Program" rating of 3400 watts is OK for peak limiting, using short time constants of a few wavelengths duration.
C) Long term "RMS" limiting with a time constant of longer than 500 ms (milliseconds) should be no more than half the "Nominal" rating of 1700 watts (850 watts), as that AES rating is conducted in free air, while drivers voice coils heat up more when loaded in the Keystone "tapped horn".
If using other drivers, simply substitute their "Continuous Program" and 1/2 of the "Nominal" RMS rating for peak and average limiting settings, using the DC resistance, not the nominal speaker impedance for figuring the compressor/limiter threshold voltage settings.
Compressor/limiters will not protect a loudspeaker from excursion related damage, the Keystone sub should use a 30Hz 24 dB per octave Butterworth high pass filter to protect it's driver(s) from mechanical damage if using amplification capable of exceeding the driver's Xmech rating.
Interesting stuff Art,
I think the keystone opening has the effect of shortening the last segment making the driver closer to the horn mouth, and the keystone mouth area could be simulated as a mouth cover plate in HR. Do you have any models you can share? Or even the dimensions? We can draw up what you have and create the model.
Thanks for the extended tests.
Are there other threads about this keystone concept?
You are the OP :D
Is there any more info you will be sharing with us?
Have you found a way to simulate anything but a circular mouth in Hornresp?
I did dozens of models that had upper dips, or not enough low extension.
The Keystone exit shape allowed the low extension without a dip below 100 Hz.
Sharing models that don't match the "as built" cabinet does not seem useful to me.
Exterior dimensions are 45H x 26.5W x 22.5 D.
The compression ratio is 2.5 to one, ultimate path length is about 312 centimeters.
The Keystone exit perhaps could be thought of as a longer path for low frequencies and shorter path for the upper pass band, providing loading to a fairly low corner while reducing some upper peaks and dips.
I'll be posting distortion figures for the various configurations, with comparisons to the same drivers in BR cabinets with similar LF corners.
Preliminary review of the data has confirmed that TH add in an upper band of distortion compared to BR, one of the reasons they "sound different" than BR or conventional horns.
intresting stuf again.
normally the lab 12 isn't realy a good th candidate,for ~35 ish (simulated)
if i ever gona bild my symetric th,i deffenetly gone tryout some different mouths.
We could call it the scrambler... wha wha wha:cheeky:
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