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topspeed
04-13-2004, 12:58 PM
I've always wondered this and with the influx of more and more coax designs hitting the market (i.e. TAD Model 1) my curiousity is getting the best of me. When a driver moves, is the majority of the sound coming from the center, the edge, or the whole surface of the cone? For example, my B&W woofers have an enormous dust cover (for lack of a better term) thereby leaving very little of the driver cone exposed yet the mid's are your typical driver design.

I've also seen lots of soft dome tweeters that actually have a protective cap over the center of the dome thereby only exposing the outer edges of the dome. I'd always thought that would in some way muffle the sound but obviously I'm wrong. Fostex full range drivers have a bizarre wave guide(?) attached to the dust cover so what does that do? Questions, Questons, Questions!

Chances are I'm overanalyzing this. Just curious I suppose.

Any takers?

This Guy
04-13-2004, 04:06 PM
I believe it comes from the entire speaker cone, this is why the speaker sizes vary. The dustcap being used on your B&W's is likely just for adding mass to the driver to lower it's resonant frequency. The sound is still coming from the dustcap on the speaker cone though. The midrange speaker wouldn't need this obviously because it's not playing the low frequecies. Somebody correct me if I'm wrong.

-Joey

skeptic
04-13-2004, 05:39 PM
Sound it the sensory perception of the vibration of air molecules when they strike our eardurms. These molecules are set in motion by the vibrating elements, mostly the cones in loudspeakers colliding with them. Each time the cone moves towards you its material collides with air molecules which in turn collide with other air molecules. You can immagine the vibrating cones initiating waves moving outward from them just as a rock thrown in a pond sends out a wave. eventually some of these vibrating molecules reach us.

Obstruction to the propagation of waves like small objects near the source will alter the direction of propagation sometimes deliberately analogously to a lens focusing or diverging the propagation of light. Depending on their nature, size, shape, and location, they might be significant or they might not.

topspeed
04-13-2004, 09:01 PM
OK, that's pretty much what I visualized, especially the rock in the pond scenario. So in your approximation is the cap over the tweeter there to minimize "beaming" and give a more diffuse or natural sound?

Swerd
04-14-2004, 08:03 AM
I've always wondered this and with the influx of more and more coax designs hitting the market (i.e. TAD Model 1) my curiousity is getting the best of me. When a driver moves, is the majority of the sound coming from the center, the edge, or the whole surface of the cone? For example, my B&W woofers have an enormous dust cover (for lack of a better term) thereby leaving very little of the driver cone exposed yet the mid's are your typical driver design.

I've also seen lots of soft dome tweeters that actually have a protective cap over the center of the dome thereby only exposing the outer edges of the dome. I'd always thought that would in some way muffle the sound but obviously I'm wrong. Fostex full range drivers have a bizarre wave guide(?) attached to the dust cover so what does that do? Questions, Questons, Questions!

Chances are I'm overanalyzing this. Just curious I suppose.

Any takers?
Yeah, I'm in the mood to overanalyze things.
WARNING: this contains high school geometry

Imagine a 6" speaker cone. Let's simplify things and make it a flat disc, instead of a cone, that has a 6" diameter or a 3" radius. The amount of air moved by such a speaker is directly proportional to its surface area, pi × r² = 28.27 in².

Let's divide this speaker into three concentric circles like a target, with each circle an additional inch larger in radius. The inner circle (Circle 1) has a 1" radius, the next circle (Circle 2) has a 2" radius, and the outer circle (Circle 3) has a 3" radius. Their areas are:

Area 1 = pi × 1 = 3.14 in²
Area 2 = pi × 4 = 12.57 in²
Area 3 = pi × 9 = 28.27 in²

What are the additional areas are contributed by each additional ring, where Ring 1 is the same as Area 1, Ring 2 is Area 2 - Area 1, and Ring 3 is Area 3 - Area 2?

Ring 1 = 3.14 in² = 11.1% of the total area
Ring 2 = 9.42 in² = 33.3% of the total area
Ring 3 = 15.71 in² = 55.6% of the total area

The outer 1" of radius contains over half the total surface area. So the outer part of this speaker should move most of the air and produce most of the sound. With bigger speakers the outer rings have an even greater proportion of the total surface area.

Audie Oghaisle
04-14-2004, 11:38 AM
no further text

Sir Terrence the Terrible
04-14-2004, 01:39 PM
From what I understand, the bass and deep bass use the entire cone. As things move up in frequency, the radiating area moves more and more toward the center of the cone. Thats why a driver can beam at the upper reaches of its operating frequency. I may be wrong, but that is how it was explain to me.

topspeed
04-14-2004, 10:03 PM
Thankfully, I made it thru geometry (my favorite subject: shapes and figures ;)) so Swerds example actually made sense. Now Sir TT says that on smaller drivers like dome tweeters the sound comes from the center??

Hmmm, you know the saying, "Ignorance is bliss?"

They're right.

Sir Terrence the Terrible
04-15-2004, 10:56 AM
Thankfully, I made it thru geometry (my favorite subject: shapes and figures ;)) so Swerds example actually made sense. Now Sir TT says that on smaller drivers like dome tweeters the sound comes from the center??

Hmmm, you know the saying, "Ignorance is bliss?"

They're right.

Topspeed,

Think of this, and this was the example given to me. A driver cannot beam if the whole driver is being used. Its dispersion would remain constant thoughout the entire operating range. We know that is not true. Larger wavelengths require larger surface areas. So it would stand to reason that as the wavelengths got shorter, it would require less surface to propagate. That is how you get your beaming at higher frequencies.

John Dunlavy explained using this analogy. You have a flashlight with a wide beam, but with a lense that controls the width of the beam. At low frequencies the beam is extremely wide, but as the frequency increases the lense closes over the light till it is beaming forward at high frequencies. Then as it transitions to the next driver(that is smaller) the beam opens up full wide again, and so on and so on.

skeptic
04-16-2004, 04:11 AM
It might just be there to keep people from poking their fingers in it and damaging it, especially when they are laid out as demo models in stores. Speaker cones, especially light fragile ones are easily damaged by uncareless gawkers in stores who for some unexplainable reason like to poke them.

Tweeters have a notoriously poor dispersion pattern compared to other drivers. This is due to the inescapable physical fact that as the wavelength of the sound to be reproduced gets increasingly smaller (as frequency gets higher) and close to the size of the moving membrane creating it, it tends to be focused more and more narrowly which creates real problems for loudspeaker designers. This is one reason tweeter cones are small. There are some strategies to deal with this including an "acoustic lens" in front of the tweeter to help diverge the sound. Others include various direct firing tweeter arrays, indirect firing tweeters but most designers just install one front firing dome tweeter and ignore the problem. IMO, that's one reason why most loudspeaker don't sound anything like live music.

Thomas_A
04-16-2004, 06:32 AM
Regarding tweeters, there is a difference between cone and dome tweeters. The Peerless CT62 cone tweeter has more energi at wide angles than most domes, despite its greater diameter.

It's a faboluous little tweeter, the Peerless CT62.

T

WmAx
04-16-2004, 09:26 AM
Think of this, and this was the example given to me. A driver cannot beam if the whole driver is being used. Its dispersion would remain constant thoughout the entire operating range. We know that is not true. Larger wavelengths require larger surface areas. So it would stand to reason that as the wavelengths got shorter, it would require less surface to propagate. That is how you get your beaming at higher frequencies.
A larger area of the diaphgrapm being used for a given frequency, at which point it's 1/2 wavlength in air is equal to or smaller then the effective radiation area of the driver, is what causes the beaming effect. As a simplificatino, imagine a wavelength that is 2" in air......imagine a 4" driver with the entire surface area effectively radiating this frequency. The wavefront would originate approximately in a symmetrical time point from all points on the driver(let's assume the driver actually reamins pistonic at all frequencies). Now, imagine the different multiple origination points across the driver(visualize discrete points for sake of simplicity). As you move off axis, the phase difference between the signals originating from different points on the driver is evident as you meet at higher contrasting vectors. Thus causing increased combing/cancelling effects realtive to the direct axis. The farther off axis, the greater the effects. Dispersion can be increased if the radiating area can be controlled(reduced at higher frequencies), such as damping the outer areas of the diaphgragm or using a controlled bending mode diaphragm that linearly decreases radiating areas as a rise in frequency occurs. Some examples of succesful executions of bending mode diaphgragms that linearly reduce effective area of emission as frequency rises: Manger and Linaeum transducers. It should be realilzed that most drivers do not remian pistonic throughout their operating bandwidth. However, they usually remain more pistonic than is optimal, and have poor charecteristics as far as resonant behaviour is concerned once they begin to operate out of their pistonic bandwidth. The bending mode driers i gave example of, have a high degree of dampening that reduces resonant problems to neglible levels. They were purposely engineered to operate in this manner throughout most of their bandwidth. Some non-bendingmode drivers attempt to at least control the resonant behaviour and effective radiation area as frequency rises and they operate into a non-pistonic region, even if not to the great extent of the bending mode devices. One such example is Focal-Jm Labs has produced midrange drivers that used a 2 layers of stiff material and placed a highly lossy foam inbetween these layers in order to dampen resonances, especially those that occur as the driver begins to operate in a non-pistonic range.

-Chris