How does a folded horn like the Belle work?

[Percheron]

Great description, good stuff. Is the design of the Belle scale able? So, if a guy had an 8 inch woofer and built a scale version of the horn, would the same apply? Or is the overall schematics of the design just apply to a 15 in speaker with specific qualities?

-Derek

[moray james]

no you cannot scale a horn. you can build bass horn a 1/4 of the full size and they will work but not as well as a full size horn but they are much more manageable in size most low frequency horns are a 1/4 size and they are still very big.

Edited January 8, 2016 by moray james

[Khornukopia]

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

 

 

 I care, and I enjoyed reading your explanations.

[Coytee]

Great description, good stuff. Is the design of the Belle scale able? So, if a guy had an 8 inch woofer and built a scale version of the horn, would the same apply? Or is the overall schematics of the design just apply to a 15 in speaker with specific qualities?

 

Though he's answered your question, I always think of it like this...  Can you scale down a Tuba?  Sure.  When you do that, will it still sound like a Tuba?  Nope, it will be a Piccolo in the shape of a Tuba.

 

[R. Swapp]

It just works.

Love my klipshorns.