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View Full Version : Compression drivers; how do they work?



jarrods
10-21-2004, 08:28 PM
I have read many threads on the compression drivers like the recent "4344MKII and 275ND problem" and ones on how you should not test there ploarity with a battery etc. I really don't get how these things work and no way am I game enough to pull mine apart to have a peek.

I understand they have a diaphram; like a mini speaker cone right??? and that makes the sound ??? but what is the 'plug' I read about and how does it fit into the picture? Why is there not a correlation of polarity to forward movement of the diaphram (like mentioned in normal speaker datasheets)?

Any exploded views of a driver around or someone post a pic of the innards???

Finally the 2405H seems to be a 'normal' speaker as they mention in the datasheet that positive to black gives forward montion. So why is this, which works at even higher frequencies, seem to revert to 'normal' speaker technologies that I do understand.

thanks, Jarrod

Robh3606
10-21-2004, 08:55 PM
Hello Jarrod

Great question we need someone like Steve Schell to really answer your guestion but I will give it a go.

You can find a crosssection here

http://www.lansingheritage.org/images/jbl/specs/home-speakers/2001-k2-s9800/page08.jpg

That is for the 9800 drivers. The alnico and ferrite are basically the same except for the throat on the drivers is longer. The basic concept has not changed all the way back to the 30's if I remember my history right. A diaphram look like this, see attached, It is basically a dome driver that is inverted and fires into the phase plug. The compresion is the ratio of the not sure and were's is Steve to help on this. Basically you have a larger diaphram that goes through a phase plug to keep the path lengths the same to a smaller apeture/throat. The large formats are typically a 4 " diaphram 2" throat and the 1" are 1.75 to a 1" throat.


Rob:)

Robh3606
10-21-2004, 09:04 PM
The 2405 is a ring radiator that is horn loaded. With a compresion driver the dome is inverted and phase is in relation to the phase plug. For a compression driver to have a positive phase the diaphram has to move towards the phase plug. When you speak about the 2405 a positive on the Black diaphram moves outward then it is opposite a woofer with a pos on the red moves outward. Phase/Polarity is relative. A 2405 looks like this without the outer horn shell.

Rob:)

Mr. Widget
10-21-2004, 10:01 PM
Here is a section view of an LE85 (2420) driver with a H91 (2307) mounted on it. You can see the inverted dome diaphragm (7) next to the phase plug (5). Unlike your 2425s this model has an alnico magnet (3) that is inside the cast housing/magnet return casting.

Widget

jarrods
10-22-2004, 12:23 AM
so (7) presses against (5) the plug and makes it compress. the spikey bits of the plug move up and make a sound wave out the horn. is this plug made of a sponge type material or something harder?

this is fascinating. i wonder why this strange arrangement makes better highs sound that a small diameter cone 'tweeter' or is it just that it makes a lot MORE VOLUME highs sound than you could get out of a tweeter??? which of course suits us... no substitute for LOUD... hahaha.. well not true!

jarrod

btw Rob, those snaps of insides look great. wow that 2405 with the 'crystal' tongue looks hot. better than black.

jarrods
10-22-2004, 12:28 AM
also until seeing inside with this cut-away shot i had no idea that the horn taper is actually one long extension right down to the plug. smart stuff. so these things started in the 30's... did Lancing himself come up with this driver concept?

Mr. Widget
10-22-2004, 12:57 AM
Originally posted by jarrods
so (7) presses against (5) the plug and makes it compress. the spikey bits of the plug move up and make a sound wave out the horn. is this plug made of a sponge type material or something harder?



I hope you are joking. (7) should NEVER touch (5) the phase plug. The diaphragm vibrates air just like an exposed dome tweeter. The difference is that the area of vibrating air is compressed into the area of the slits (the dark areas between the spiky bits which need to be very rigid due to the pressures that build in this area) so the final vibrating column of air is smaller than that originally vibrated by the dome itself. The throat and horn work together to make an acoustic transformer that couples the diaphragm to the listening room.

Is a horn superior to a direct radiator? It is certainly electrically more efficient, but significantly more expensive to produce. Is the very best compression driver better than the very best direct radiator? That is a question to be debated.

Widget

johnaec
10-22-2004, 01:01 AM
Originally posted by jarrods
so (7) presses against (5) the plug and makes it compress. the spikey bits of the plug move up and make a sound wave out the horn. is this plug made of a sponge type material or something harder?No no no - the diaphragm/voice coil is the only moving part. 'See that thin air space between (7) and (5)? As the diaphragm moves in and out it compresses the air in this thin space, which is then pushed through those thin, expanding slots in the plug. It then exits via the horn throat. The plug is a hard, immovable material.

Edit: I see Widget beat me to it.

My question is on the ring radiator series - I've never had one open, but from pics I've seen of the diaphragm it looks like there's a hole in the center, hence "ring" radiator? Could someone explain the details behind these?

John

Mr. Widget
10-22-2004, 01:10 AM
Originally posted by johnaec


My question is on the ring radiator series - I've never had one open, but from pics I've seen of the diaphragm it looks like there's a hole in the center, hence "ring" radiator? Could someone explain the details behind these?



It has been discussed on some thread or other. I also believe I've read about it in a JBL Tech bulletin, but I may be making that up. It has to do with cone (dome) break up modes. The ring has an inherently higher mass break point than a complete dome which allows it to run out to a higher frequency. I have no idea why. Perhaps due to less mass? Only guessing here.

The actual vibrating diaphragm is a very thin, very narrow annular band of aluminum.

Widget

Robh3606
10-22-2004, 07:33 AM
Here is an unmounted 2402 ring radiator diaphram. The nose clamps the inner ring. The horn clamps the outer. The silver is the anular diaphram Widget was talking about.

Rob:)

paragon
10-22-2004, 08:49 AM
A compression driver is more acoustically !! efifcient, not electrically.
Try to explain (?). What`s going on in a compreesion driver compared to a cone driver ?
The room between dia and plug is called compression chamber.
If you have a cone driver which moves back and for(th ?) the air can move to all sides (Left, right, up, down...). So efficiency is very low.
In a compression driver the air can only move to the exponential slits of the phase plug (not to any other side) and it`s compressed. The following exponential horn couples the wave to the room. You have much pressure at beginning of the horn and low pressure at the horn mouth. The "speedtransformation" raises the efficiency.
If the horn is to short you get comb filter effects (look at my sim pics at off topic). The lost of pressure at the horn mouth is like a
"negative wall" which reflects waves back in to the horn.
A big "soundwall" around the hornmouth smoothes (?) this effect.

Regards Eckhard

Mr. Widget
10-22-2004, 09:42 AM
Originally posted by paragon
A compression driver is more acoustically !! efifcient, not electrically.
Try to explain (?).

By electrically efficient, I meant it was efficient in turning electrical energy into sound energy. Perhaps not the best term to use. I was simply trying to get across the idea that it is a very complex way to make a tweeter (or mid) but that the result is always a driver of greater sensitivity. I was trying not to confuse the concept of conversion efficiency with greater quality.

Widget

johnaec
10-25-2004, 08:31 AM
Originally posted by Robh3606
Here is an unmounted 2402 ring radiator diaphram. The nose clamps the inner ring. The horn clamps the outer. The silver is the anular diaphram Widget was talking about.Thanks, Rob. I'd always wondered how they dealt with the hole in the middle - now it all makes sense.

John

Guenter
10-27-2004, 05:38 PM
Widget mused: "Is the very best compression driver better than the very best direct radiator? That is a question to be debated.

surely, much like the comment of the phase plug. There is really no debate. For horns it's volume ueber alles - caveat emptor re the ultimate sound quality of horns vis a vis a direct radiator. Nevertheless, one wonderful attribute of a horn is it's 'realistic' dynamics - that's what sold me.

guenter

Steve Gonzales
10-29-2004, 07:25 AM
outstanding discussion!!!!!, I had some of the pieces of how a compression driver worked, but now it is crystal clear, awesome!!! As the question of which is better, cone or horn?; I think it could and has been debated ad nausium but just look at Project May and the mission statement and the driver choice. Also look at just about most every desirable JBL model and you will find a compression driver of some sort in the mid and/or high positions, that is no accident.

Steve Schell
10-29-2004, 01:32 PM
Since Rob asked me to chime in, I'll add a couple of thoughts to the discussion.

There is a terrible impedance mismatch between a normal direct radiator cone speaker and the air it is trying to move... sort of like swinging your fist at the air. The weight of the cone is much heavier than the air. It is very difficult to transfer energy (accomplish work) this way, and very low efficiency results. If we couple the speaker to a horn with a mouth much larger than the throat and an adequate length, now it has the trapped volume of air in the horn to push against, and it can create sound energy much more efficiently. Many benefits ensue- driver excursions are greatly reduced for a given sound output, and those distortions proportional in some way to excursion (most of them) are reduced greatly as well. For a given input power, sound output power is increased, often by an order of magnitude or more. This both increases the maximum output capability and decreases the power input for a given volume level.

A compression driver is a specialized type of speaker designed to exploit the benefits of horn loading to the maximum. Imagine removing the cone driver from the horn in the previous example, and lengthening the horn on the throat end down to a very small throat area. Now we mount the compression driver, whose internal sound channels continue the reduction in cross section right to the tiny slits in the surface of the phasing plug. Now instead of a relatively soft, heavy paper cone, we can use a formed metal or composite diaphragm, super light, thin, and shaped for maximum rigidity. The working surface is likely ten times greater than the area of the slits. Since the diaphragm must push air in and out of these slits and is loaded by the entire mass of air in the horn, it is easy to see that the pressure created by a given movement of the diaphragm will be enormous compared to a cone speaker driving the air directly. This increase in loading, or radiation resistance, allows for an efficiency (sound power output compared to electrical power input) of around 35 percent. Most direct radiator speakers operate at one percent efficiency or less (some high efficiency designs like JBLs can manage two or three percent). That's right- one percent sound, 99 percent heat. It takes quite a motor to drive a compression driver diaphragm, and everything is done to make it as strong as possible. Tolerences are tight, as a narrow gap is necessary to maintain very high flux density, often more than 20 kilogauss. The voice coil is most often edgewound, to maximize conductor length in the available space.

We usually think of compression drivers as midrange or high frequency devices, but they can be used at bass frequencies as well, with the same benefits. The Fletcher System built by Bell Labs in 1933 used a low frequency compression driver with an 18" spun aluminum diaphragm, 8" edgewound aluminum voice coil, coupled to a large reentrant bass horn. Usually, paper cone drivers are used on bass horns with large throat areas, as it allows considerable shortening of the horn.

Steve Gonzales
10-29-2004, 01:46 PM
OUTSTANDING STEVE !!

Guenter
10-29-2004, 04:58 PM
Originally posted by Steve Gonzales
look at just about most every desirable JBL model and you will find a compression driver of some sort in the mid and/or high positions, that is no accident.

Hi Steve, for JBL, I agree. I was the happy owner of a home brew - but as per spec - Olympus S8R system in the 60/70's. then in the late 70's I believe, I heard the Yamaha NS1000's monitor speakers. They changed my mind about direct radiatos pretty quick.... sold my JBL's. Kind of sorry I did though and have now rebuild the JBL's and use the 1000's with the TV. Possible my perceived problem, when I compare, is that I use the little horn/slant plate with the 375 rather than the bigger horns that match impedence to the air better (reflections and all). I do love the sound of Jazz on the JBL's (three blind mice recordings in particular) a lot better.

-cheers, guenter

Steve Gonzales
10-29-2004, 06:51 PM
Yeah, I thought that about a few exotic speakers like the Ohm Walsh and some of the Wilson offerings too until I really LISTENED, I am thankful that I never got rid of a SR8 system, that's gotta blow,but I've still got a pretty 'special' pair of 'high-end' speakers for my TV too, they are from England. I would just love to own a pair of 2441's hooked to the 2309/2310 combination, sitting on top of a custom enclosure, that has always done it for me personally, or the awesome look of the 36" slant plate lenses on those Paragon looking horns.

Robh3606
10-31-2004, 07:38 PM
Thanks Steve

Rob:)