rwnano
Well-Known Member
From stereo phile magazine on transmission line speakers.
www.stereophile.com
Transmission Lines
In the October 1965 issue of Wireless World, Dr. Arthur Bailey made quite a technical splash with an article entitled "A Nonresonant Loudspeaker Enclosure Design." The article was not concerned with panel resonances, but rather with suppressing the delayed output and cabinet resonances produced by the woofer's back wave. Dr. Bailey concluded that the back wave must be absorbed to avoid these problems, and that the only safe way of doing this was to transmit the rearward energy down an infinite transmission line (TL), absorbing it so totally that it never reaches the listening room. Not having the resources for a line of such magnitude, he instead approximated the line with a damped and folded port about eight feet in total length. The optimum damping material he discovered experimentally to be long-fiber wool, packed at a density of 0.5 to 1.0 pounds per cubic foot. The use of damping material is crucial to the success of the TL enclosure, and represents the major departure from the previously known acoustic labyrinth, which used very little damping. The sound velocity through the wool is about 20% lower than it is in free air, so the stuffed line effectively increases the actual length of the line.
The wool also damps the inevitable pipe resonances. Yes, Virginia, every pipeline has a series of standing-wave resonances, so that even the TL enclosure is not truly "nonresonant." The TL length is generally chosen to correspond to the quarter-wavelength of the woofer's free-air resonant frequency. The old acoustic labyrinth was normally tuned to the woofer's half-wavelength at resonance—but then woofers in the '40s and early '50s had resonant frequencies around 50 to 60Hz, where a half-wavelength is only about 10 feet.
The port output for either a quarter- or half-wave line is in phase with the forward radiation of the woofer, so there's reinforcement of the lower octaves. However, with a quarter-wave line there's the added benefit of having the standing wave out of phase with the back wave of the cone, providing a kind of "braking" action that reduces cone excursion at the woofer's resonant frequency. The TL enclosure generally succeeds in its objective of suppressing back-wave energy and, with the right woofer, is capable of providing very clean and extended bass response. Internal pressures are low: compared with a closed box, there's not as much flexing or bending of the cabinet panels. But the fly in the ointment is that the long lines needed are expensive and complicated to build.
Incidentally, there's another way of achieving back wave suppression. With the infinite baffle enclosure, there is no delayed output because the back wave never encounters the back panel. Of course, no one has yet succeeded in building a truly infinite baffle, but a good approximation is to mount the woofer in the floor when there's a basement below, or to use the wall of the house as a baffle, radiating the back wave to the great outdoors. Why, none other than our own LA was recently contemplating the idea of using an empty water well (yes, he has one in his old house) as a baffle for a 12" Dynaudio woofer.
Fried Model G/3 loudspeaker | Stereophile.com
Irving M. "Bud" Fried, an early contributor to Stereophile, hails from the city of brotherly love, and I must confess to finding it difficult avoiding a few brotherly jabs at Mr. Fried's name: something like "this Bud's for you" would surely not escape deletion by our conscientious Editor. And...
Transmission Lines
In the October 1965 issue of Wireless World, Dr. Arthur Bailey made quite a technical splash with an article entitled "A Nonresonant Loudspeaker Enclosure Design." The article was not concerned with panel resonances, but rather with suppressing the delayed output and cabinet resonances produced by the woofer's back wave. Dr. Bailey concluded that the back wave must be absorbed to avoid these problems, and that the only safe way of doing this was to transmit the rearward energy down an infinite transmission line (TL), absorbing it so totally that it never reaches the listening room. Not having the resources for a line of such magnitude, he instead approximated the line with a damped and folded port about eight feet in total length. The optimum damping material he discovered experimentally to be long-fiber wool, packed at a density of 0.5 to 1.0 pounds per cubic foot. The use of damping material is crucial to the success of the TL enclosure, and represents the major departure from the previously known acoustic labyrinth, which used very little damping. The sound velocity through the wool is about 20% lower than it is in free air, so the stuffed line effectively increases the actual length of the line.
The wool also damps the inevitable pipe resonances. Yes, Virginia, every pipeline has a series of standing-wave resonances, so that even the TL enclosure is not truly "nonresonant." The TL length is generally chosen to correspond to the quarter-wavelength of the woofer's free-air resonant frequency. The old acoustic labyrinth was normally tuned to the woofer's half-wavelength at resonance—but then woofers in the '40s and early '50s had resonant frequencies around 50 to 60Hz, where a half-wavelength is only about 10 feet.
The port output for either a quarter- or half-wave line is in phase with the forward radiation of the woofer, so there's reinforcement of the lower octaves. However, with a quarter-wave line there's the added benefit of having the standing wave out of phase with the back wave of the cone, providing a kind of "braking" action that reduces cone excursion at the woofer's resonant frequency. The TL enclosure generally succeeds in its objective of suppressing back-wave energy and, with the right woofer, is capable of providing very clean and extended bass response. Internal pressures are low: compared with a closed box, there's not as much flexing or bending of the cabinet panels. But the fly in the ointment is that the long lines needed are expensive and complicated to build.
Incidentally, there's another way of achieving back wave suppression. With the infinite baffle enclosure, there is no delayed output because the back wave never encounters the back panel. Of course, no one has yet succeeded in building a truly infinite baffle, but a good approximation is to mount the woofer in the floor when there's a basement below, or to use the wall of the house as a baffle, radiating the back wave to the great outdoors. Why, none other than our own LA was recently contemplating the idea of using an empty water well (yes, he has one in his old house) as a baffle for a 12" Dynaudio woofer.
