Loudspeakers

Speakers with ports, ducts or other apertures in their cabinets are typically reflex designs, also known as 'vented boxes', and exploit the Helmholtz resonance to boost their low frequency output. This resonance, such as you might hear when blowing across the top of a champagne flute, offers the promise of better bass extension and sensitivity than might be achieved with a sealed box, but is traded against a steeper bass roll-off and poorer transient response. Transmission line bass loading is a variant of this technique first proposed by Arthur Bailey in Wireless World, 1965. Bailey's ingenious solution utilises the Helmholtz resonance while also damping any unwanted reflections or resonances within the enclosed air space by absorbing the rear radiation of the bass driver in a long (folded), damped and tapered duct. However, such a pipe has to be at least a quarter-wavelength in span to be effective, ie, 4.3m for 20Hz. This is rarely practical so in 'real world' designs the transmission line is cut short and vented, allowing some delayed, vestigal bass output to escape. PMC's ATL (Advanced Transmission Line) uses this technique, its claimed 3.3m effective length tuned to a notional 26Hz.