Keystone Sub Using 18,15,&12 Inch Speakers

[weltersys]

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.

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:

Tapped Horn Vs. Bass Reflex Case Study

Horn Extender/Wave-guide for TH

Single sheet TH challenge

Plans are available in posts#94 and #97 (thanks Oliver) .
Photos of the cabinet interior are in Post #99. , distortion results are in post #12.
Hornresp inputs are available in post #96.
PASC built a Keystone and another 18" TH and reports his findings in posts #114 & 115.

Yesterday did 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).

The same 16 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.

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. An Eminence 3015LF should outperform the 4015LF in sensitivity, though I don’t have one to test.
That said, the 3015LF extra sensitivity is due to a lighter moving mass, which means the cone is less stiff, and the voice coil takes less power. 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 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 (and probably the lighter 3015LF even more) sounds a bit distressed.

The B&C18SW125-4 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’ll be doing more subjective listening tests, comparing different amplifier and speaker combinations when the weather clears, yesterday’s high winds brought in snow today.

[NEO Dan]

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.

[ßart West-VL.]

Thanks for the extended tests.
Are there other threads about this keystone concept?

[weltersys]

Originally Posted by ßart West-VL.

Thanks for the extended tests.
Are there other threads about this keystone concept?

The OP has 3 links to other threads.

[NEO Dan]

You are the OP

Is there any more info you will be sharing with us?

[weltersys]

Originally Posted by NEO Dan

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.

A circular cover plate does not have the same effect as the Keystone shape.
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.

Art

[weltersys]

Originally Posted by NEO Dan

You are the OP

Is there any more info you will be sharing with us?

Dan,

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.

Art

[epa]

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.

[NEO Dan]

Originally Posted by weltersys

A circular cover plate does not have the same effect as the Keystone shape.
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.

Art

IMO the problem for simulation is not the shape but that the mouth opening is not at the end of the line but along the path leading up to it, and that's where the sliders come into play. Your keystone could easily be an egg...

We could call it the scrambler... wha wha wha

[weltersys]

Originally Posted by epa

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.

The Lab 12 pair definitely sounded better than the 4015LF.