I first heard of Backes und Müller (usually abbreviated as B&M) way back in the mid-1970s. The original B&M speaker was indeed very interesting and technically radical, using an electrostatic sensor system to provide the feedback that the actively driven unit diaphragms required always to follow the music signal accurately. Although B&M’s active feedback technology broke fresh ground, and while the speaker ranges themselves have undergone steady development, the company itself remained a fairly modest operation in size.
Things started to change when Johannes Siegler of ProAudio brand KSdigital took over the reins at B&M in 2000. Alongside work as a recording engineer, Siegler was a freelance DSP (digital signal processing) consultant during the early ‘90s, and this led to the formal beginning of KSdigital in 1996. The two companies appear to be a good fit, as at least one KSdigital model closely resembles a B&M model in overall configuration and features.
Despite its considerable – some would say looming – height of 1.6m, the 25 is actually one of the smaller models in the BMLine range, though it’s also unquestionably costly at £35,990/pair. Like its larger 35 and 50 siblings, the driver line-up is carefully considered and organised to operate substantially as a line-source, an arrangement that has several implications. A line source tends to create cylindrical rather than spherical wavefront radiation: this can help avoid floor and ceiling reflections, and also reduces the rate at which the sound energy falls off when one moves away from the speaker to around half that of a spherically radiating design.
The way sound is radiated from a source depends on the relationship between the wavelength of the sound and the size of the source. A wavelength that’s larger than its source will be propagated spherically (ie in every direction); a wavelength smaller than the source will be ‘beamed’ along a band that’s perpendicular to the source.