A bookshelf speaker enclosure designed to support small drivers, without resonance spikes.
This transmission line speaker enclosure was designed around Peerlees TC55FD00-08 drivers, selected because of their compact 2-inch footprint and remarkably flat frequency response. The length of the transmission line enclosure was specifically tuned to extend the driver’s low-frequency performance, enhancing bass output and extending the flat response falloff point from 120 Hz down to approximately 80 Hz. This project is currently being manufactured, to test performance and refine the design.
Transmission line enclosures are not as common as the ubiquitous sealed or ported box cabinet. TLs are often larger and more complex to manufacture out of wood sheets (though not with 3d printing or injection molding), but they stand out for their natural, full-bodied sound. Sealed cabinets restrict bass and ported enclosure tend to suffer from unwanted resonance spikes, akin to the droning note produced by blowing across the top of a bottle. Air moves freely through a transmission line, like blowing into a straw. TL designs avoid resonance peaks and support bass by routing the air from the back of the driver through a long, damped pathway which turns back in parallel with the speaker driver. High frequencies are muted but low frequencies are able pass freely, boosting the low end performance.
Above: The Peerlees TC55FD00-08 driver frequency reproduction starts to fall off around 120hz. This transmission line was designed around a 100hz wave, which corresponds to a wavelength of 3.43 meters. 1/4 of the target wavelength is the typical transmission line length, accordingly this enclosure is 0.8575 meters long (about 2.8 feet). By wrapping the transmission line back and forth a tight package was achieved, which became the dominant feature of the design language. The enclosure is about 7 inches deep and 10 inches tall.
Above: This design would ideally be injection molded, but a prototype was 3d printed for analysis. The performance was tested by measuring frequency response curves from 50 Hz to 10k Hz. When compared against the driver without an enclosure (baffle-less) the transmission line increased the response at 120 Hz by 4dB, from -20 dB to -16 dB. and by 3db at 100hz. Next iterations will explore the impact of various internal acoustic damping materials to reduce added treble, and variations in transmission line length to attempt further low end boost.