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drboar

Why does not serial caps work in extending bass in a La Scala type horn?

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In a high Q closed box the bass can be extended and the peak tamed with addition of a serial cap

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The cap increase the impedance above the resonanse frequency (reducing ht ehigh Q hump) and lowers impedanse below resonanse (increasing output)

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This is a KEF B200 with a Q of 1.0 to 1.1 in this box

A horn similar to the La Scala also is a closed box with a high Q. Adding caps do increase and decrease the impedance as they should using a Eminence PA driver

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And then the proof of the pudding

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What the heck? Is it the higher Q or is it something intrinsic in hornloaded boxes that precludes the use of serial caps to extend the bass?

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Horn length, mouth size, and contour are keys if you have a driver that can extend low enough in frequency based on the design to achieve low enough bass. http://www.parow.no/Lowther/ALLFUN/Size.html is a fairly straight forward explanation. Your example is with a closed box which compromised among other things, air behind the driver - infinite baffle or air-suspension as far as the woofers are concerned.

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Pete is correct. Extension in a horn configuration is more a product of the length of the horn than anything else.

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The horn for the LS bass cabinet has almost no effect below approx. 125Hz. It just isn't big enough.

Bruce

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If you compare a Bb tuba and a trombone, the tuba can play one octave lower because the tubing of the tuba is twice as long. I can put a tuba mouthpiece in the trombone but once I reach a certain point (pedal E) the trombone will not speak or amplify the sound. Same principle.

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The horn for the LS bass cabinet has almost no effect below approx. 125Hz. It just isn't big enough.

Bruce

The key word is almost. The size of the air volume of the box and the reactance of the horn play a roll in the rolloff and smoothness of the bottom end. When the horn unloads the efficiency drops off quickly and you can't get that back as seen in the charts, especially with that small compression chamber. You can pretend the box is bigger and you are only loading the mid-bass by porting the enclosure to augment the low end.

A very simplistic view but...

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That clears things up a bit. Thanks, CECAA850.

It looks like tonal balance can be adjusted, but not the limits of frequency response. It takes a change in horn and/or driver design to do that.

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A very simplistic view ...

quite... [;)]

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The series resonant capacitor works best with a lower Qts than what is available in the LaScala.

The left side of the resonant peak is inductive, the right side is capacitive.

What is trying to be done is resonate the inductance with an external capacitance to form a zero at that specific frequency, the impedance will then drop to the DC resistance of the woofer voice coil plus the DCR of the crossover inductor at that frequency and draw more current from the amplifier.

This works best with sealed systems with a Qtc around 1.1, and will get you a half-octave of bass extension at best, with the roll off increasing from 12dB/oct to 18dB/oct below this new resonant frequency. The gains are never as high as computed because of the ESR being quite high on the large electrolytic capacitors used, and the poor tollerance causing them to miss the calculated target frequency.

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Even with a sealed enclosure you would have to wonder if the capacitor approach is cost effect. Good caps that will pass frequencies that low probably cost as much as changing the woofer.

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Changing the woofer will not get you any better bass without using a lower efficiency driver.

A 2 cu ft 2nd order sealed box with an F3 of 30hz can only be 0.18% efficient (84.55dB).
A 2 cu ft 4th order vented box with an F3 of 30hz can only be 0.36% efficient (87.56dB).
A 2 cu ft 6th order vented box with an F3 of 30hz can only be 0.90% efficient (91.54dB).

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Changing the woofer will not get you any better bass without using a lower efficiency driver.

A 2 cu ft 2nd order sealed box with an F3 of 30hz can only be 0.18% efficient (84.55dB).
A 2 cu ft 4th order vented box with an F3 of 30hz can only be 0.36% efficient (87.56dB).
A 2 cu ft 6th order vented box with an F3 of 30hz can only be 0.90% efficient (91.54dB).

I would think you could get a little louder, not lower correct?

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Correct, you can give up some more bass and gain efficiency.

6th order vented may be your best choice. This requires a better woofer, a port be added, and a filter ahead of the amplifier.

A Heresy sized box:

Driver Properties
Name: Kappalite 3012LFa
Type: Standard one-way driver
Company: Eminence Speaker LLC
No. of Drivers = 1
Fs = 37 Hz
Qms = 6.94
Vas = 106.7 liters
Cms = 0.253 mm/N
Mms = 73.26 g
Rms = 2.454 kg/s
Xmax = 9.1 mm
Xmech = 13.65 mm
P-Dia = 263.5 mm
Sd = 545.4 sq.cm
P-Vd = 0.496 liters
Qes = 0.34
Re = 5.6 ohms
Le = 0.98 mH
Z = 8 ohms
BL = 16.75 Tm
Pe = 400 watts
Qts = 0.324
no = 1.532 %
1-W SPL = 94 dB
2.83-V SPL = 95.55 dB
-----------------------------------------
Box Properties
Name:
Type: Vented Box w/ Active HP Filter
Shape: Prism, square
Vb = 1.6 cu.ft
Fb = 37 Hz
QL = 5.861
F3 = 36.91 Hz
Fill = minimal
No. of Vents = 1
Vent shape = round
Vent ends = one flush
Dv = 3 in
Lv = 6.046 in
-----------------------------------------
Active 12 dB/oct. HP Filter
Fx = 37 Hz
Qx = 2

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Great information though I am confused. This thread was started about extending bass of a La Scala vs gaining efficiency. Are we now talking that efficiency gain to raise the low bass in the La Scala without providing any real extension?

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I believe we're saying that installing a capacitor parallel (I believe) to the woofer wiring will not lower the extension of the bass cab.

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"Great information though I am confused. This thread was started about
extending bass of a La Scala vs gaining efficiency. Are we now talking
that efficiency gain to raise the low bass in the La Scala without
providing any real extension?"

My bad.

My original vented LaScala mod was for the lower Qts K43 and used 6th order tuning and the filter for same.

At lower frequencies, any horn reverts to a sealed box performance.

A 2 cu ft 2nd order sealed box with an F3 of 30hz can only be 0.18% efficient (84.55dB).
A 2 cu ft 4th order vented box with an F3 of 30hz can only be 0.36% efficient (87.56dB).
A 2 cu ft 6th order vented box with an F3 of 30hz can only be 0.90% efficient (91.54dB).

Converting a horn to a 6th order vented box below the horn cut-off will gain you quite a bit of bass, the basis of my vented LaScala idea.

(the above chart is from ex-Klipsch employee DB Keele)

I use EV filter boxes, when I can find them. Interface A, Interface D, and Sentry 3 types work well I also modify equalizers that have subsonic (infrasonic) filters, Audio Control comes to mind.

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There is no rise in effiecency. Lowering the impedance below the bass resonance increase the power input to the driver and hence input. The idea was that the LS is a closed box below the working range of the horn. I will go back to the "true" closed box and try other driver to see how sensitive this the bass extension, peak flattening is to driver variations and system Qs.

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