“For every action, there is an equal and opposite reaction.” For engineers, Newton’s Third Law of Motion is both blessing and curse. In audio, the law is primarily of concern in transducers— elements in which there is physical motion. Of transducers, that “equal and opposite reaction” is most critical in loudspeakers.
Aside from ionic units or the occasional bending-wave driver, all loudspeaker drivers move back and forth to produce the pressure wave we hear as sound. The bigger the driver, the more evident this will be, with large woofers exhibiting the greatest driver excursion.The forward motion— the speaker’s direct radiation— is the primary source of what we hear in reproduced sound. How we handle the out-of-phase rear wave is the subject of this piece.
The title is intended as a double entendre (and a clean one, at that): meticulous design of loudspeakers is indeed exhausting for the designer, and the way in which is the rear radiation is dealt with, is akin to the exhaust system of a car’s engine. —I know, I know: if you have to explain a joke, it’s not a very good joke.
I’ve previously written at length about Edgar Villchur’s devopment of the acoustic suspension speaker enclosure. Such enclosures are sealed boxes, and thus do not vent the rear radiation. Today we’ll look at some enclosures that do vent the rear radiation: bass-reflex (or ported) enclosures, acoustic transmission lines, and tuned (or tapered) quarter-wave pipes. We’ve also covered rear-loaded horns previously, and may return to that subject again. Open-baffle loudspeakers are a whole ‘nother kettle of fish which we’ll look into in the future.
So what’s a port? Think of a porthole: a ported loudspeaker enclosure has a hole, generally but not always round like a porthole. The idea is to create a Helmholtz resonator: the acoustical capacitance of the enclosure volume resonates with the air-mass contained in the port volume. If the volumes are properly chosen, the low-frequency cut-off is extended downward, and the phase of the rear radiation will be shifted so that it eminates from the port in phase with the loudspeakers’ front radiation.
Back in the day—the early experiments of Western Electric, Harry Olsen at RCA, and others–design of vented enclosures was pretty much try-and-see. In the ’60s and ’70s, Neville Thiele, an audio engineer at the Australian Broadcating Commission, and Richard Small, an American doctoral candidate at the University of Sidney, were able to reduce the design of vented enclosures to a series of mathematical “alignments” which simplified the designs, and made it much more likely to achieve a good result. (A variant of ported enclosures utilizes a passive radiator—basically a cone driver with surround/suspension but no drive mechanism—to couple the rear radiation to the outside air.)
This AES oral history interview with Thiele provides some insights into his work:
Another type of vented enclosure, rather more complex to design and build compared to a standard bass-reflex enclosure, is generally called an acoustic transmission line (or simply “transmission line”) due to its similarity to the characteristics of an electrical transmission line. The first example I can find of speaker loading similar to what we call an acoustic transmission line was used by mainstream radio manufacturer Stromberg-Carlson, starting in the late 1930s. The method is described in this patent by Roy S. Anderson, which was assigned to Stromberg-Carlson. The language is the usual can’t-see-the-forest-for-the-trees verbiage of a formal patent, but the gist is that the rear radiation of the cone loudspeaker is “for the most part, absorbed in a sound damping, circuitous passageway.”
In the postwar era, the acoustic transmission line was first named by the English engineer A.R. Bailey, who defined the form and described its design in a 1965 paper in Wireless World. (Use the link, then scroll down to page 483). As the length of the line is determined by a quarter-wavelength of the woofer’s resonant frequency, transmission lines are generally not small. Production of transmission line speakers was largely limited to British companies (including Radford and the Cambridge model shown in the header pic) and the UK market until American audio entrepreneur Irving M. “Bud” Fried cofounded IMF Electronics in England, whose transmission-line monitors and quasi-monitors Fried sold worldwide. Given the complexity and precision required to build transmission lines—including precisely measuring and placing lambswool damping material—the type has generally been limited to higher price categories.
In the late ’70’s, Fried, with his new US-based company Fried Loudspeakers, addressed both the size requirements of transmission line enclosures and the increasing popularity and imaging capabilities of mini-monitors such as the LS3/5A with his Model H system, consisting of a coffin-sized dual-channel woofer enclosure and two small satellite speakers, each containing mid-range and tweeter units. Fried used the term “subwoofer” to describe the bass enclosure, crossing over to the satellites between 50-100 Hz.; it’s the first use of the term I’m aware of.
These days a number of speaker manufacturers are still devoted to transmission lines, most notably the British pro-monitor company PMC and the small American manufacturer Alta Audio, which utilizes transmission line variants in everything from a small speaker using a single, full-range driver to massive multi-way floorstanders.
Similar to transmission lines but simpler in design and construction are tuned (or tapered) quarter-wave pipes. Like the transmission line, the TQWP feeds the back radiation of the driver into a quarter-wavelength line—but unlike the transmission line, the TQWP doesn’t have the driver at the very end of the line, but generally affixes it somewhere along the middle. First described by pioneering English audio designer P.G.A.H. Voigt (also known as founder of Lowther drivers and designer of the tractrix horn geometry), TQWP have largely been the domain of DIYers, aside from several models made by UK speaker company Castle—perhaps explained by Castle’s status as one of very few loudspeaker companies which still builds its own cabinets. TQWP are easier to build than a transmission line, but still more complex than an acoustic suspension enclosure or most bass-reflex enclosures.
In the century or so since its birth, the moving coil loudspeaker has been packed into an incredible variety of frameworks, enclosures, boxes, panels—you name it. For the most part, though, 90% of the speakers we see are simple acoustic suspension or bass-reflex designs. I’m glad to see a few diehard manufacturers still making use of the more exotic transmission lines. It’ll be interesting to see what variant and hybrid enclosures appear in the future.