Many car audio enthusiasts start out by taking their parents' home audio speakers and placing them in the rear of their cars to blast out tunes. If you had parents that were hip then maybe you were slightly more sophisticated. Back in the day "alternative enclosures", like passive radiator and transmission line, were cutting edge. Not everyone knows about these enclosures; after all, they are a bit obscure. But it's worth covering.
Passive Radiator
Harry Olson first described passive radiators (PRs) in his patent Loudspeaker and Method of Propagating Sound that was issued back in 1935. Except for an article by Olson in 1954, very little was published about PRs until Nomura and Kitamura in their IEEE paper in October 1973, and Small's JAES paper in October 1974. Since then, passive radiator systems have had a relatively mild impact in the home audio market, while the car audio market failed to embrace it. However, two car audio manufacturers, Boston Acoustics and Earthquake, have recently initiated a movement towards the use of PRs in the automotive industry, taking from their experience in home audio.
So what are passive radiators, what do they do, how do they work, and what are the positive and negatives?
Passive radiators are often deceiving as they commonly look like a conventional subwoofer. However, these certainly are not subwoofers. While they look like and even appear to move like a normal subwoofer from the outside of a speaker system, what is behind them tells a different story. Passive radiators lack a key ingredient that assures they are not a woofer: the motor structure. In other words, they have no voice coil, magnet, top plate, t-yoke, tinsel leads or terminals. PR's are essentially an unpowered driver, therefore they must be mated with a powered woofer in an otherwise sealed enclosure.
Passive radiators systems are referred as a variation of a ported enclosure. While they are mathematically identical, PR's use a diaphragm to take the place of the port.
A PR is essentially made of two parts: a "weighted diaphragm" and a "spring". The weight of the diaphragm is a critical element the design and must be correct for the part to function properly. By changing the weight of the diaphragm, the resonance frequency will change, thus effecting the tuning of the enclosure. The spring is a combination of the stiffness of the suspension materials, and the air trapped within the cabinet. This too can change the tuning of the enclosure, much the same as the enclosure volume of a vented box changes its response.
PRs are tuned, by the mass loading, to resonate at a frequency below the active woofer's linear response range. A passive radiator has a useful range about a 1/4 octave above and below its resonance. However, the typical roll off is a fairly steep 18 dB/octave. The combined response of the woofer and passive radiator should produce about a half an octave bass extension at low frequencies that add up to the level produced by the woofer on its own at higher frequencies, if the PR is tuned properly. In other words, a small amount of low frequency bass that the system would normally have difficulty reproducing now exists.
In a passive radiator system, both the cone of the active woofer and PR could move in phase with each other, or any combination of opposite motions, up to 180 degrees out of phase. Keeping both cones exactly in phase would be ideal in order to reinforce the output of the woofer, but as physics would have it, this sort of resonant system is not exactly possible.
One of the advantages of this type of system is that as the frequency being reproduced nears the resonance frequency of the PR, the excursion of the woofer decreases, thus taking the burden away from the woofer when producing the lower octaves. This is why it is common to see passive radiators larger in diameter than the active woofer in the system, as the woofer does not need to have the low frequency extension it would need in other enclosure designs. Also, this occurrence can allow the smaller diameter woofer to better its response at the upper and mid bass regions.
As with the good, always comes the bad, for passive radiators are not perfect. As mentioned prior, passive radiators can reproduce tones up to 180 degrees out of phase of the woofer. Depending on the frequency generated and the positioning of the passive to that of the active woofer, the frequency response could have small amounts of cancellation. So long as the overall phase response does not contain any sudden changes or discontinuities, the human ear/brain should not detect this. However, when the phase response changes rapidly over a small range, it can be noticeable, as some people are more sensitive to this than others.
Another inherent problem is the fairly sharp roll off of the PR. The frequencies below the passive radiator's tuning will roll off very rapidly. In addition, the air in the speaker box no longer acts like a spring to control and restore the motion of the PR and especially the woofer below the resonance of the PR. Much like a woofer in a vented enclosure, power handling of the woofer can be limited below the tuning frequency. This, in effect, could damage both the active woofer and the passive radiator.
Passive radiators can be challenging to design. Most radiators now days have adjustable weight setups on the cone that make for easier tuning to the enclosure. However, picking the correct woofer, one with a low QTS (0.2 to 0.4), and designing a suitable enclosure are equally important.
Transmission Line
The origin of a transmission line (TL) has roots tracing back to the Stromberg-Carlson acoustic labyrinth, circa 1930. This labyrinth consisted of a long pipe, with the driver mounted at one end while the other end remained open, with a cross-sectional area about the same as that of the driver. In the early 1960's, Dr. AR Bailey experimented with different damping materials and techniques using the same basic concept in these folded labyrinth lines. His work has since become the standard for TL designs. In 1976, AT Bradbury used Dr. AR Bailey's density criteria of 0.5 lb/ft and published his paper describing changes in the speed of sound for different types of damping material such as fiberglass and long fiber wool.
So, what is a transmission line? As the name implies, a transmission line is a long chamber that expends from the back of the loudspeaker. At the opposite end of this line is a vent or opening (generally the size of the driver diaphragm) to the outside of the cabinet. Properly built TL's eliminate the phase cancellation of the driver in any form and make for a nearly perfect sub system. However, TL's are seldom found in car audio because of their size and complexity.
The design of a TL enclosure consists of a taper in the line, making it is possible to eliminate standing waves and resonance common to other speaker enclosures. Eliminating standing waves also protects the driver from harmful back waves that cause distortion and cone breakup. The length of the line does not allow time for air to travel through the chamber and cancel the front-wave. Because of its length, a tuned chamber, much like an open-ended pipe from a pipe organ, is created. This causes a phase shift. It is this phase shift to the rear sound wave (of the woofer) that reinforces the front-wave at the frequencies where the front-wave begins to decrease due to increased air resistance at lower frequencies, very much like a vented enclosure.
The damping of a TL is unlike the "air" spring of a sealed enclosure where the cone has to fight for motion. As a result, the efficiency is better than bass-reflex enclosures, the accuracy is better than acoustic suspension, and the frequency response and linearity is better than all systems.
Designing a transmission line enclosures requires thorough specifications and careful tuning. Suitable drivers for TL's usually have low QTS (0.2 to 0.4), low QES (0.3 to 0.4) and low Fs values. The distance the rear sound wave (of a subwoofer) travels in the enclosure is very specific. Determining the length is based a fraction of the wavelength of the woofers resonant frequency. For example, if the resonate frequency of the woofer used in a TL is 40 Hz, the wavelength of the frequency would be approx. 339 inches (see March 2002 issue, Box Basics II, for formula to determine wavelength). The channel inside the transmission line enclosure must be 1/4, 1/2 or 3/4 of this wavelength, resulting in a channel length of 84.75, 113 and 169.5 inches respectively. Because of the length of the channel, it is often common for a TL to be folded into a labyrinth, making it more compact. If stuffed properly with damping material such as wool, the actual length can decrease due to the resistive effect of the material on the air.
Both passive radiator and transmission line enclosures may not be completely suitable for the automotive industry, but they are great alternatives to the common enclosure. Give them a try- you may be pleasantly surprised.