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La Scala - What is the path of sound in the cabinet?


rjrbass

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I have admired Klipsch speaker technology for years, and in June 2006 I had to opportunity to purchase 1978 Cornwalls, which were within driving distance.

I was so impressed with the sound that soon after, I started my quest for La Scalas. In November I found 1993 La Scalas in a second hand store and jumped at the opportunity. (I got a steal of a deal as big ticket used items move slowely before Christmas.)

Does anyone have a sketch that will explain the path of sound in the La Scala cabinet? The bottom access panels of the La Scalas have never been opened; I want to keep them this way.

I believe that the woofer faces the back of the cabinet, but I am unfamiliar with the actual sound path.

Is a triangular wedge used to disperse the sound to the sides of the cabinet? Does it run the full height of the bass bin?

How large is the opening in front of the woofer?

Is the back of the woofer sealed in the centre triangular wedge of the cabinet, or is the back portion of the woofer enclosure coupled to the folded horn section?

Thank you, in advance, for your assistance.

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Yep. The woofer faces the rear. There is a splitter
centered on the slot in front of the woofer and "V" shaped expansions
above and below the splitter. The cross-sectional area expands as
the sound moves left and right along the back of the cabinet.
After the sound turn toward the front, there is a constant area path on
each side of the doghouse and finally another zone of expanding area
between the sides and the "Vee" (roof) of the doghouse.

If
you split it down the middle and unfolded it to the rear you'd have a
4' long megaphone with a straight, square section in the middle.

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Richard, as you can see from the exploded view that Fritz provided, there are 2 V-shaped (or chevron-shaped) pieces that form the start of the expansion of the horn. The woofer does face the back of the speaker enclosure and "speaks" through a slot that's about 3"x 13" (I think). The chevron pieces start the horn, and its shape then goes around the corner on the left and right and faces forward.

Then there's a 7 1/2 inch non-tapering section. After that the large Vee section begins. Although you'd usually picture a horn as having sides that angle out from each other, the La Scala is a split horn with the Vee forming the angle, making two tapered horns that come together at the front.

Having a folded horn within a rectangular housing with no wasted space seems ingenious and simple at the same time, like most great ideas.

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not only that but at 2x2x3 feet, there is very little plywood waste in building the cabinet!

Good description there of the air path. Note that on the industrial version, the top 'chevron' area is open forming the jack cup area and handle for carrying. Just more genious.

Don't know if it was mentioned, but check out the motor board- the 15" woofer is mated to a 3" x 13" slot so the sound is compressed before exiting into the expanding horn portion.

Michael

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Thank you to everyone who contributed in clearing up this mystery.

I am still confused when it comes to the chamber behind the woofer. I understand this this is the area within the large centre "V". This area seems to be sealed off from the rear opening of the sound path.

The way that I understand it, a sealed section would require more amplification to drive the woofer, as extension of the woofer must overcome the negative pressure created when the diaphram extends and creates a low pressure behind itself. However, these speakers are very efficient, not like acoustic suspension drivers, therefore there is something that I am missing or just do not comphrend.

And yes, as IndyKlipschFan stated, this is pretty cool!

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You are correct that there is the sealed box on the magnet side of the woofer. This is generally called the doghouse. It is shaped like a doghouse and is where the woofer lives (l love that).

It does indeed act like a spring.

It is there to counteract a phenomenon of the horn. That is the mass reaction.

At high frequencies the "little local mass" of air in front of the diaphragm at the throat (small end of the horn) is propelled down the length of the horn to the mouth (big end). Of course it is really a matter those molecules hit the nearby molecules, and so on, all the way. And the clumps of molecules are really moving back and forth a small distance.

And also, there is an alternating condition of (1) clumped up molecules which, being clumped, are at high pressure, then that pressure forces the molecules (2) to have velocity. This is generally the same as how sound propagates everywhere: alternating pressure and velocity.

But in the horn, we have a confined area. At low frequencies, the driver diaphragm is not moving very fast and acelleration is low. It takes longer to complete a cycle. Now the aforementioned "little local mass" of air no longer acts a little local mass. In fact, the mass starts acting as a bigger slug of air. Therefore the diaphragm is pumping a bigger and slug (mass) of air the lower the frequency. The air slug is essentially stuck to the diaphragm.

The driver would be doing more and more work just pumping the slug back and forth rather than doing real work. That is the increasing mass reaction.

For technically trained people: The classic "mass reaction" would normally only occur at high frequencies where F=MA and so higher frequencies with high A cause a higher Force reaction. This assumes a constant mass.

The problem is that in the horn, the M of the air slug is increasing as frequency goes down, and is not constant.

What to do?

Generally, if you have a mass in an oscillating system, you can counteract it with a spring. Hence the back chamber as a spring. This is everywhere in mechanical systems. Pendulums, car suspension systems, and tuning forks work that way.

Gil

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The way that I understand it, a sealed section would require more amplification to drive the woofer, as extension of the woofer must overcome the negative pressure created when the diaphram extends and creates a low pressure behind itself. However, these speakers are very efficient, not like acoustic suspension drivers, therefore there is something that I am missing or just do not comphrend.

At DC, what you're saying is absolutely correct. But with a speaker, we're dealing with AC - or rather, the cone moves back and forth fast enough that it doesn't notice the "confined space" behind the driver. Picture a bunch of positive and negative pressure waves bouncing around inside the sealed cabinet....very roughly speaking, they cancel eachother out and in turn don't affect the behavior of the driver. This behavior happens as long as the frequency is high enough (aka, the pressure is changing back and forth fast enough). It's only with lower notes (pressure changing more slowly) that the sealed rear cabinet starts to impede the output of the driver. And in fact, this behavior introduces a 6dB/octave rolloff as the frequency goes below a certain point. And since a driver in free space has its own 6dB/octave rolloff, sealed systems roll off at a combined total of 12dB/octave.

Putting a perfect horn in front of a driver in a box actually doesn't change the behavior of the driver in that box. However, real horns aren't perfect so there are other effects that come into play, which is why you see other variations in the frequency response after a horn is added. As Gil mentioned, the behavior of the driver in the sealed box interacts with the behavior of the driver behind the horn resulting in a "weird" combined system response. If you were to try to port the rear cabinet volume, then you end up with something even weirder....which basically means harder to engineer and prone to more problems. That's not to say that improvements can't be made, but there are always costs and tradeoffs to consider. The beauty of the lascala is the fabulous economy of materials and the great performance that comes with it. Or is it the other way around?

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I can't agree with Mike B. Sorry, Doctor Who.

I do agree that that the box is not acting as an echo chamber at low frequencies. It is only to the extent that the wall is so close that an echo is near instantaneous. It is a spring. Same thing.

The motor system is doing work in three ways of creating force in the mechanical system. It is either accelerating mass, or compressing a spring, or working against a resistance. The resistance is what we want it to work against because that is what comes out the horn or the system as sound. However, these things occur at above zero Hertz, or DC.

The basic problem is that the energy of the mass and spring can't be stored back though the motor to the amp. This is because the motor is lossy as a generator (back emf) because of voice coil resistance, and the amp can't store energy at it terminals (which are a dead short)

All we can do is find a way of shoshing energy back and forth between the spring and the mass at the mechanical system without getting the motor involved.

= = = =

Some of what Mike said implies that perfect horns don't have mass reactions. Infinite length exponential horns do. There is something to be said that conical horns have less mass reaction.

I think this has to do with the extent to which exponentials have equal, normalized,transfer function working both ways. It may be that conicals do not. Long story.

Gi

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"infinite length exponential" seems to be a paradox to me... at some length of horn (not that long) the exponential expansion width (doubling cross section area each constant length of horn length, the width approaches infinity as the width angle goes asymptotic (90 degrees)... or is that what you mean? Infinite width, not infinite horn length?

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I still want to get a model of the K33, the horn and other drivers, so I can insert them into my exploded LS drawing.I need to add the cross piece in the back as well. I should make a new one that models the LSII.

Bruce

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