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Why are all tweeters 1 inch in diameter ?

Frank Dernie

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I lifted that bit of information from the 1979 July/August edition of Hi-Fi Stereo Buyers Guide. Which has a review of these speakers.

"What kind of structure does it take to create this sound? Pull off the front and top grilles and you see the answer. Front bottom is a 12 -inch subwoofer with a massive 72 -ounce magnet that has its own resonating cavity vented through a 5 -inch diameter duct, ample enough to prevent the audible "breathing" sometimes heard from smaller bass vents. We poked into the
hole and found that the duct extends nearly all the way to the rear wall before opening into the resonating space. A configuration that creates the effective equivalent of an 18 -inch woofer."
Disappointing that somebody who writes for a HiFi publication is so ignorant about how reflex speakers work :facepalm::(
 

MetalDaze

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Disappointing that somebody who writes for a HiFi publication is so ignorant about how reflex speakers work :facepalm::(

To be fair, at the time bass reflex speakers were a relatively new thing. The design had been implemented and around for maybe 10 years? Ohm along with Acoustic Research and a few other companies were the first to include Thiele parameters into their design and incorporate a bass reflex system to keep the box size smaller, while still retaining decent efficiency and low frequency response.
Maybe the reviewer didn't fully understand the design when writing. Although I recall speaking with the owner and president of the company at one point (who designed these speakers) and he sort of said the same thing. If my understanding is passable.

I have a BA in science, so some of the EE parts go over my head.

As opposed to volume output, maybe they were referring to the geometry of the bass cabinet. Housing the 12'' sub alone would yield X as low frequency response cut off at say -3db. But with the port and sub, the low frequency response is now X + Z and said response is equivalent to that of an 18'' woofer if there were no port. Which that size woofer in this size cabinet would not be feasible.
 

Mnyb

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Bliesma is selling a range of 34mm tweeters roughly 1,33” is it a different set of compromises or do they have some novel way to keep the dispersion wide enough anyway ?
 

Frank Dernie

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To be fair, at the time bass reflex speakers were a relatively new thing. The design had been implemented and around for maybe 10 years? Ohm along with Acoustic Research and a few other companies were the first to include Thiele parameters into their design and incorporate a bass reflex system to keep the box size smaller, while still retaining decent efficiency and low frequency response.
Maybe the reviewer didn't fully understand the design when writing. Although I recall speaking with the owner and president of the company at one point (who designed these speakers) and he sort of said the same thing. If my understanding is passable.

I have a BA in science, so some of the EE parts go over my head.

As opposed to volume output, maybe they were referring to the geometry of the bass cabinet. Housing the 12'' sub alone would yield X as low frequency response cut off at say -3db. But with the port and sub, the low frequency response is now X + Z and said response is equivalent to that of an 18'' woofer if there were no port. Which that size woofer in this size cabinet would not be feasible.
Possibly, though I think you are maybe being generous to them. The quotation makes it look like the journalist did not understand the physics, and this isn't that unusual we have had enthusiasts here making the same mistake recently.
I haven't found historic data on the very first reflex speakers but certainly UK speaker makers were using reflex bass loading in the 1960s and understood how and why they worked.
I built my first reflex speakers in 1970 using KEF drivers and their port science.
 

briskly

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When we say that a membrane moves rigidly, this does not refer to the lack of wave modes in the operating bandwidth. A rigid body radiator is a surface that moves with a uniform axial velocity.
Only the rigid disc has an unbounded high-frequency response. All portions of the flat membrane would add constructively within the radiator width, but the phase shift from a cone or a dome's profile ensures a loss of response. You would need to equalize the large dome no matter your position. So, some deviation from uniform axial velocity is beneficial no matter the desired directivity function.
 

MetalDaze

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Possibly, though I think you are maybe being generous to them. The quotation makes it look like the journalist did not understand the physics, and this isn't that unusual we have had enthusiasts here making the same mistake recently.
I haven't found historic data on the very first reflex speakers but certainly UK speaker makers were using reflex bass loading in the 1960s and understood how and why they worked.
I built my first reflex speakers in 1970 using KEF drivers and their port science.

The way it's written does leave a lot open to interpretation. The article being as old as the speakers I wasn't too worried about the journalists technical understanding. Ohm Acoustics know what they're doing, even if the people listening do not. Being able to find published reviews for vintage speakers like these is difficult as it is. So I was happy to find some actual text.

I'm not sure about the historic data either. But AR and Bose always come up as one of the first speakers available in the US utilizing a bass reflex design.
 

sergeauckland

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Albert Thuras of Bell Labs patented the Bass-reflex enclosure in 1930 (granted in 1932). I don't think it was well understood until the 1960s, though.

I built myself a pair of bass reflex loudspeakers in 1968/9 using a Goodmans driver, and the design was published by then. Goodmans also had Aperiodic enclosures by then, using what they called an Acoustic Resistance Unit, so it was sufficiently common by the mid 1960s to be mainstream. Tannoy had also published designs for enclosures for their Dual Concentrics using bass reflex long before 1968.

S.
 
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howard416

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I'm obviously late to the game, but they're not. However, for most listening environments and applications, it's essentially the Goldilocks size.
 

Guerilla

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I’ve had a large dynaudio contour with dome midrange and 8” inch bass driver and 8 inch pr , they where nice gave them to a friend , but sadly the foam surround has rotted currently.
Change foam Not so hard.
Cheers!
 

Duke

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"Front bottom is a 12 -inch subwoofer with a massive 72 -ounce magnet that has its own resonating cavity vented through a 5 -inch diameter duct... A configuration that creates the effective equivalent of an 18-inch woofer."

A ported box inverts the phase of the woofer's backwave, such that the backwave is now in-phase with the frontwave. So AT the tuning frequency we have, in effect, TWICE the air-moving capability as with a single 12" woofer in a sealed box. (Below the tuning frequency the port's phase rotates rapidly such that it goes out-of-phase with the front of the cone, and the resulting cancellation gives us that steep 4th-order rolloff.)

My guess is that the marketing department took a bit of poetic license and focused on what's happening AT the port tuning frequency, and an 18" woofer has a similar cone area to two 12" woofers.
 

MetalDaze

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A ported box inverts the phase of the woofer's backwave, such that the backwave is now in-phase with the frontwave. So AT the tuning frequency we have, in effect, TWICE the air-moving capability as with a single 12" woofer in a sealed box. (Below the tuning frequency the port's phase rotates rapidly such that it goes out-of-phase with the front of the cone, and the resulting cancellation gives us that steep 4th-order rolloff.)

My guess is that the marketing department took a bit of poetic license and focused on what's happening AT the port tuning frequency, and an 18" woofer has a similar cone area to two 12" woofers.

Now that is something I absolutely did not know. Very cool!

And it makes a lot of sense being that the review appeared in a magazine which doubles as advertising, as well as subjective impressions.
 

andreasmaaan

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Albert Thuras of Bell Labs patented the Bass-reflex enclosure in 1930 (granted in 1932). I don't think it was well understood until the 1960s, though.

I built myself a pair of bass reflex loudspeakers in 1968/9 using a Goodmans driver, and the design was published by then. Goodmans also had Aperiodic enclosures by then, using what they called an Acoustic Resistance Unit, so it was sufficiently common by the mid 1960s to be mainstream. Tannoy had also published designs for enclosures for their Dual Concentrics using bass reflex long before 1968.

S.

Even as early as 1930, the relationship between enclosure volume, port length, port cross-sectional area, and tuning frequency was well-understood. What Thiele and Small did was to determine how to optimise a woofer's parameters for an enclosure (or vice-versa) to produce a system with the desired Q (at least at small signal levels). This reduced the trial and error involved in enclosure design, which IIUC paved the way for the big uptake in BR enclosures in the 60s.
 

Duke

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Now that is something I absolutely did not know. Very cool!

Here's a bit more coolness/weirdness:

At the tuning frequency, the air pressure inside the box is resonating exactly OUT OF PHASE with the back of the woofer's cone (which faces the inside of the box). And the STRENGTH of that resonance has all come from the woofer's motor. So...

At the port tuning frequency, the cone's motion (whether inward or outward) is being opposed by an EQUAL AND OPPOSITE force, from the reverse-phase resonance of the air in the box! So at the port tuning frequency, the woofer cone actually STOPS moving!! (This is the theoretical result - in practice there are some losses and therefore some cone motion, but not very much).

One last thing: Because of this phase inversion, at the tuning frequency, the air inside the port is effectively getting a DOUBLE DOSE of pressurization/rarefaction from the woofer's motor, and that translates directly into air movement through the port. So while the woofer cone is holding still (or nearly so) at the port tuning frequency, the air movement through the port is TWICE as much as the woofer would have moved all by itself!

And in this case, twice the air movement of a 12" woofer = approximately the air movement of an 18" woofer.

(In speakers with passive radiators instead of ports, we see this manifest in the passive radiators typically having either twice the surface area of the woofer, or twice the excursion capability, or some combination thereof.)
 

MetalDaze

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Here's a bit more coolness/weirdness:

At the tuning frequency, the air pressure inside the box is resonating exactly OUT OF PHASE with the back of the woofer's cone (which faces the inside of the box). And the STRENGTH of that resonance has all come from the woofer's motor. So...

At the port tuning frequency, the cone's motion (whether inward or outward) is being opposed by an EQUAL AND OPPOSITE force, from the reverse-phase resonance of the air in the box! So at the port tuning frequency, the woofer cone actually STOPS moving!! (This is the theoretical result - in practice there are some losses and therefore some cone motion, but not very much).

One last thing: Because of this phase inversion, at the tuning frequency, the air inside the port is effectively getting a DOUBLE DOSE of pressurization/rarefaction from the woofer's motor, and that translates directly into air movement through the port. So while the woofer cone is holding still (or nearly so) at the port tuning frequency, the air movement through the port is TWICE as much as the woofer would have moved all by itself!

And in this case, twice the air movement of a 12" woofer = approximately the air movement of an 18" woofer.

(In speakers with passive radiators instead of ports, we see this manifest in the passive radiators typically having either twice the surface area of the woofer, or twice the excursion capability, or some combination thereof.)

I had to re-read this post like 3 times LOL!

Seriously though, conversations and info like this is why I joined this forum. I love understanding why things are the way they are.

I have a pair of speakers. Well three actually (Ohm H for FR-C-FL) which use a driven 8'' woofer which has a large amount of excursion for it's size. And it's air coupled to a 12'' passive radiator. After I got them I made sure all the pore space around the binding posts, and the tweeter attenuators were sealed tight from the inside. I liked to test them by gently pushing in on the woofer. And watch the PR 'come out' roughly equal distance. And vice versa. Nice and clean and no rattles even when pushing them hard.

The bass the PR produces from the smaller box is so tight and punchy. It sounds phenomenal but it does reach it's limit before my ears say uncle. That's when I found the floor standers and the cascading wave of bass began. But only after much treatment and bass trapping.

Thanks to uncle physics, & after my listening room got renovated. I re installed my system first with minimal furniture and treatments just to have it back after a few weeks. Grabbed my beer, sat down and turned up the volume. No bass whatsoever at my listening position. I mean NONE. :facepalm: Stupid room modes and destructive cancellation.
 

wje

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With the Focal Electra "BE" series of speakers, the 007BE models had 1" Beryllium tweeters. When they upgraded their line to the 008BE series, they moved to 1.25" tweeters.
 

wje

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"Now! With 25% more tweeters!"

Sorry, I simply couldn't resist... :facepalm::rolleyes:

:p - I've had speakers from both series of Focal. The tweeter size revision, didn't make a major change in the sound. However, it did allow Focal to increase their prices by 25%. :cool:
 
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