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Percheron

How does a folded horn like the Belle work?

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Hi guys,

So, my local HiFi store has a pair of late 70's Belles for sale. Being new to hi end audio I was intrigued and then blown away by the sound. I love speakers that look like furniture instead of modern art. This started me exploring the world of Klipsch and all the heritage designs. Looking at the belle build plans online I just don't get why the speaker would have just a small vent of sorts to move air through. Why not use a smaller speaker and have its whole face moving air? Why a 15 in. monster and not a 10 inch?

Now if it's the same principle as blowing across a glass bottle or flute, then I guess it makes sense.

Thanks!

Derek

post-62171-0-48840000-1451946149_thumb.j

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The way it was explained to me is that the compression chamber is similar to a mouthpiece on a trumpet. The sound starts off high pressure low volume and ends up low pressure high volume.

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AFAIK, the 15 in. woofer is used for efficiency, speed and low distortion. It can move a lot of air with minimal excursion. 

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I just don't get why the speaker would have just a small vent of sorts to move air through. Why not use a smaller speaker and have its whole face moving air? Why a 15 in. monster and not a 10 inch?

Now if it's the same principle as blowing across a glass bottle or flute, then I guess it makes sense.

 

 

Welcome to the forum!

 

My take is, it would not be similar to blowing across a glass bottle or flute.  Rather, think of a garden hose (no nozzle on end).  You turn faucet on and water comes out.  Now, put your thumb on the end of the hose....what happens?  You get less water per second but, it's much higher pressure and can shoot much further away.

 

With the horns, you have a 15" driver trying to push air through that little slot.  That is what creates the compression to raise the output (like the thumb on the hose).  Part of the exchange for that is, as the hose volume goes down some, you will lose some depth of bass with the horn.

 

If they used the 15" driver without the slot (a direct/radiator) you would gain some depth of bass, you would require more power to do it and, because the woofer is moving back/forth to a higher degree, you would have an increase in some kind of distortion.

 

You would get a 'flabbier' sound to the bass.  Some like it, some prefer the more crisp sound of the horn.

 

Being a Klipsch site, you might deduce most here are of the horn camp (certainly not everyone for various reasons)

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Hi guys,

So, my local HiFi store has a pair of late 70's Belles for sale. Being new to hi end audio I was intrigued and then blown away by the sound. I love speakers that look like furniture instead of modern art. This started me exploring the world of Klipsch and all the heritage designs. Looking at the belle build plans online I just don't get why the speaker would have just a small vent of sorts to move air through. Why not use a smaller speaker and have its whole face moving air? Why a 15 in. monster and not a 10 inch?

Now if it's the same principle as blowing across a glass bottle or flute, then I guess it makes sense.

Thanks!

Derek

 

 

 

The Belle was designed to be a much prettier version of the LaScala somewhat and yes, a larger diameter cone driver with less excursion leads to less distortion. You will also want a competent horn loaded sub to go with those to fill in the bottom end.

 

Roger

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Being a Klipsch site, you might deduce most here are of the horn camp (certainly not everyone for various reasons)

 

And, Mr. Coytee, just who are these subversives?  We have a room loaded with Bose where we like to discuss these things with them.

 

Dave

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Thanks for the quick replies fellas, and the welcomes!

That makes sense now, the finger over the garden hose analogy. Cool!

While it would be killer to buy the Belles for sale, they are out of my budget for sure. The Heresy is a possibility, however I've not yet heard a set, and from reading here, they seem to be a love/hate type speaker.

Really I'm in transition from a 5.1 to a dedicated 2 ch system, all it takes is money.

thanks again!

-Derek

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Thanks for the quick replies fellas, and the welcomes!

That makes sense now, the finger over the garden hose analogy. Cool!

While it would be killer to buy the Belles for sale, they are out of my budget for sure. The Heresy is a possibility, however I've not yet heard a set, and from reading here, they seem to be a love/hate type speaker.

Really I'm in transition from a 5.1 to a dedicated 2 ch system, all it takes is money.

thanks again!

-Derek

 

 

Derek,

 

First, Welcome to the Forum! :)

 

Absolutely nothing wrong with Hersey's, they just need a sub. 

 

I would want LaScalas first though if you can swing a used set? :)

 

Roger

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the larger the driver the shorter your horn can be the smaller the driver the longer your horn will be. larger driver will also afford lower distortion for any given level. The sketch you posted is not a Belle by the way I have attache a cabinet drawing for a Belle. Make it longer it will play lower but it will get a lot bigger best option is to cross at 90 Hz. to a sub. You could also consider a reflex bass mod which would help the lack bass a lot and only make the speaker taller which is not a problem. Best regards Moray James.

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Edited by moray james
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Cool info, I'm not currently planning on building my own, though I do have access to a really good cabinet maker with an awesome shop. It would be fun to do....hmmmmm......

-Derek

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....hmmmmm......

 

Yer screwed, Percheron.  Welcome to the madness...

 

Dave

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Derek, this is how I think of a folded horn.  If the horn were not folded, it would be too long to be practical, like this Herald trumpet.

463588000232000-00-500x500.jpg

But if you fold it like this pocket trumpet, it still has the same sound and same amount of tubing, just in a more compact package.

 

Gold_Bb_Pocket_Trumpet_PT-280.jpg

Some of the other guys can tell you more exactly, but I think if you un-folded the bass horn (bass bin) in a Klipsch Belle or La Scala it would be 9 feet long, something along those lines.

Edited by wvu80

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If I may comment.

 

We want high acoustic efficiency so that the diaphragm of the woofer does not move much, but still pumps air.  Movement itself will cause new frequencies to be created which is called Doppler distortion.  This is like the whistle on a train as it goes by.  You hear the change in pitch, or a new frequency.  There are other distortion products in the mix.

 

What to do?

 

PWK explains that if you have a piston moving in a lake, it doesn't pump much water, but put the piston in a cylinder, it can pump water.  The water can not run away to the sides.  Similarly, when the piston diaphragm is coupled to a tube, it pumps more air with less movement.  This is called a higher acoustic impedance on the driver diaphragm.

 

But if we are pumping air into a tube, what happens at the other end of the tube.  Actually, the air at the other end encounters what I'll call, non-trapped air, which was our problem to begin with.

 

What to do?

 

We allow the horn to gently expand in area as we go down the length.  One widely recognized expansion is called "exponential."  There is a complicated "exponential equation" to describe this.  But suffice to say, the area doubles every x distance down the horn. 

 

In the K-Horn, LS, and Belle we start with a tube (throat of the horn) about 0.5 square feet (at a high air pressure) and allow it to expand to 4 square feet.  At higher freqs this horn acts like the tube.  However now we have a big end (mouth of the horn) where the air pressure has reduced and acts like a diaphragm of 4 square feet.  Essentially we have the effect of a much bigger piston than we started with even though there no diaphragm at the mouth, only air, there.

 

The horn does not act like a tube at low freqs and this point is described as frequency cut off or fc.  There is no longer the higher acoustic resistance which helped us so much. Even worse, at low freqs the air in the horn starts acting like a slug of air and just moves back and forth.  Therefore the speaker motor has to do work moving the clumped mass of the slug of air instead of pumping sound down the horn and out the mouth of the horn. (Having to move the slug of air bogs down the speaker motor just where, in freq, we're already hurting from lack of acoustic impedance. The double whammy.)

 

What to do?

 

We can put a chamber behind the driver which acts as a spring.  There is a resonance formed by the slug of air (mass) and the spring.  This is called "reactance annulling." 

 

There is another problem at high freqs.  The moving mass of the speaker driver (diaphragm and voice coil) has to be moved by the electrical motor.  The higher the freq, the faster it has to be moved.  This requires some higher math.  I think of it as heavy sleigh on ice.  If you want to move it back and forth at a low freq, it is not much work.  But if you move it back and forth at a high rate, it is lot of work to get it started forward, stop it, move it backwards, stop it, move it forward. etc.  You are actually moving the sleigh faster in each cycle as frequency of the cycle increases. That is acceleration.  (Remember Force = Mass times Acceleration. Having to move the mass bogs down the speaker motor.)

 

What to do?   

 

Mass is the problem again and we have to find another spring. With our sleigh, it would be nice if we had a spring attached to it (forward and back) so that that the mass will oscillate at the high freq.

 

In the horn, we put a small chamber in front of the diaphragm.  But wait, the diaphragm has an area of about 1 square foot and the small end of the horn attached to the other end of the front chamber is 0.5 square feet.  That is not much of a chamber.

 

What to do?

 

This is not explained well but I believe that PWK put in the restriction of a 3 x 13 inch slot to partially seal off the horn side of the front chamber.

 

WMcD

 

See also, https://community.klipsch.com/index.php?/topic/153248-building-a-belle/page-3

Edited by WMcD
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WMcD…your knowledge is amazing, and the way you describe it is very refreshing, especially to someone that isn't as knowledgeable….thank you!

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Thanks for the kind comments.  If anyone cares, we can go a bit further.

 

I kinda glossed over why the 4 square foot mouth is good.  It is pretty much like why a big woofer (area of diaphragm) is better than a small one.  Everyone knows this intuitively.  But why?

 

The reason, again, is the acoustic resistance "seen" by the diaphragm.  We're talking about what the air is doing.  When the woofer diaphragm moves forward it pushes air, and when it moves backwards, it sucks in air.  Musical instruments do this as they vibrate and we want to recreate this with our speaker system.

 

Let us go back to grade school times.  Did you, like me, create a circle of six pennies with a seventh in the middle?  They arrange nicely. Usually we did this while the good teacher was explaining some golden knowledge, and we're not paying attention.  Right?

 

Let's imagine that we have seven woofers (say, six-inch diameter each) arranged like the pennies on a big board. An infinite baffle.

 

Let's apply bass drive to only the center woofer.  During movement forward, the air in front of it goes a little bit positive atmospheric pressure as the molecules are forced together.  But being under pressure, some go forward, but most move to the sides.  The air pressure runs sideways over the six surrounding (non-driven) pennies, which are not moving.

 

The little groups of molecules don't push back against the diaphragm which is causing the higher pressure very much.  They are not "resisting" the motion of the diaphragm very much by bouncing back against it because  they are flowing to the normal atmospheric pressure in front of the six surrounding, non-moving pennies or diaphragms. This is low acoustic resistance "seen" by the diaphragm of our center speaker.  It is not a matter of any quality of the motor behind the disk of the diaphragm.  It is just the air in front of it and whether a molecule pushes back to resist its action.  (Though we'll get back to the issue because the motor has to move the diaphragm and the air in front of it when it is forcing air into a tube or horn.)

 

But now, let us drive the six surrounding speakers or pennies. This is really creating something like a bigger diaphragm.

 

Now the air which is compressed by the center diaphragm still will run away as much as it can, but it encounters the high pressure caused by the surrounding pennies on all sides, so it can't flow there.  All. it can do is bounce off the diaphragm of the center penny and the diaphragm meets resistance of air pressure. So this is a bit like our cylinder is a piston. The surrounding pennies are like the walls of the piston, but to a lesser extent.

 

Further.  Look at the surrounding pennies.  Each creates a localize pressure and wants to flow to a low pressure situation.  But this airflow "sees" the pressure of the center penny and  the two others which it touches. Therefore, these molecules tend to push back against their own diaphragms and resist its movement. 

 

We have to consider how to describe all these events.  The bottom line is that the group of diaphragms experience resistance to moving air. But if the motor can keep the diaphragms moving, this also means that there is less running away and it can pump air better. More molecules are resisting movement and it can move them.

 

It is a bit like pumping iron or rubber bands or Bow flex.  If the resistance is high, more work is being done. A good thing for our speakers, if we can get it. This is an important point.  We started talking about air pressure running away and looped back into why, if it can't so much, the diaphragm can do more work without moving as much. 

 

This situation is potentially worse at low freqs. Above we were not considering how fast the diaphragms or pennies are moving, just that air runs way at a constant rate (speed of sound)..  At low freqs the air has more time to run away before the cycle of pressure and suck-out occurs as caused by the movement of the diaphragm. Therefore even big diaphragms work worse as freq goes down. 

 

Please note that the issue of sound pressure running away does not happen if we are feeding a tube or throat of a horn. Still, I think the above penny situation explains a lot a vector arithmetic used to describe the acoustic resistance of various size diaphragms.

 

WMcD

Edited by WMcD
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