- Dec 8, 2016
- Seattle Area, USA
Disappointing that somebody who writes for a HiFi publication is so ignorant about how reflex speakers workI 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
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.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.
"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.
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.
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.)