Loudspeakers

Speakers with ports or other apertures in their cabinets typically exploit the Helmholtz resonance to boost low frequency output. The green trace [inset Graph] shows the impedance magnitude trend for a typical reflex-loaded speaker (a B&W 803 D4) where the dip at 34Hz, between two peaks, shows the port tuning frequency, coinciding with a null in the nearfield output of the bass driver. Almost every ported loudspeaker will show a similar impedance trend unless other resonances, intended or otherwise, are 'sounding off' within the cabinet.

Transmission line bass loading also utilises the Helmholtz resonance but typical implementations will damp unwanted reflections or resonances within the enclosed air space by absorbing the rear radiation of the bass driver in a long (folded) and tapered duct. Alta's Alec uses an undamped line and so the quarterwave and other unattenuated modes listed in my Lab Report are also clearly revealed here on its impedance [black] and phase [red] spectra. PM