Professional audio
Coaxial loudspeakers in professional audio enable sound from two drivers to come from one source. This characteristic allows a wider field of listening to a synchronized summation of speaker drivers than loudspeaker enclosures containing physically separated drivers. As well, the pattern of response is symmetric around the axis of the loudspeaker.[2]
Since the 1943 introduction of the Altec Lansing Duplex 601 coaxial driver, recording studio monitors have often been coaxial loudspeaker designs. In 1945 the improved Altec 604[3] was introduced, and it soon became the recording industry standard studio monitor in the U.S.[4] First shown in 1947 in England, the Tannoy Dual Concentric design assumed the same role across Europe from the 1950s onward. The Altec 604 combined a 15-inch (380 mm) woofer with a compression driver attached to a horn to carry the high frequencies. The Tannoy also used a 15-inch woofer and a compression driver for high frequencies, but differed in that the woofer itself served as the final horn flare for the high frequency driver.[5] Thus, its output pattern was radially symmetric, not just mirror-image symmetric as in the Altec. Both designs placed the high frequency driver behind the low frequency driver, and both were not initially time aligned. The high frequencies arrived at the listener's ear slightly later than the low frequencies.
One drawback of the design is the production line difficulty in mating the two drivers, and in replacing or reconing the woofer. Another drawback is that the low frequencies tend to modulate the high frequencies, causing greater intermodulation distortion.[6] The Tannoy style of coaxial, with the woofer forming part of the high frequency horn, had greater intermodulation distortion. Designs similar to the Altec 604 have further problems with diffraction of the low frequencies around the central horn, and with rearward emanations from the horn body reflected forward by the woofer out of time with direct sound. All of the problems with sound waves tend to increase with sound pressure level, causing significant shifts in tone as the loudspeaker changes volume.[7]
In the late 1970s, Bill Putnam of Universal Audio worked with Ed Long and his patented Time Alignment crossover design to fix the long-standing problem of the two bandpasses not being aligned in time at their crossover point. The Altec 604 was given this elaborate new crossover feature and incorporated into the UREI 813 studio monitor, which also had a second woofer physically separate from the coaxial pair. This design dominated recording studios of the 1980s, and time alignment became a feature of competing manufacturers.[5] In 1977, Bob Cavin of McCune Sound in San Francisco modified the Altec 604 for the McCune SM-4, a large proprietary stage wedge, using John Meyer's patented high-compliance compression driver diaphragm, a higher-power woofer, and processor-controlled bi-amplification with line-level all-pass time alignment.[8][9] The McCune SM-4 was the first multi-angle stage monitor, and was used by many McCune clients such as the Playboy Jazz Festival and the Monterey Jazz Festival. In the 2000s, digital signal processing (DSP) was used by Fulcrum Acoustic to reduce some of the coaxial drawbacks such the diffraction of the woofer's upper range around the central horn, by filling in this "shadow" with low frequency sound from the compression driver, and by countering the out-of-time reflections bouncing off of the woofer.[7]
Coaxial loudspeakers have been used in stage monitors, giving musicians a more even sound field because of the single-source characteristic. As well, the enclosure may be made more compact. In the 1980s, Professional Audio Systems (PAS), using Time Alignment technology from Ed Long, sold the popular SW series of compact stage wedges, offered with a 12- or 15-inch woofer, and having a projecting high-frequency horn as in the 604. Other stage monitors using coaxial designs are made by Clair Brothers, L-Acoustics, Radian Audio Engineering, RCF, Beyma, dB Technologies, Fulcrum Acoustic, and Rat Sound in partnership with Eastern Acoustic Works.[10]
David Gunness and Fulcrum Acoustic have designed coaxial loudspeakers beginning in 2009, notably collaborating with PreSonus in 2013 to create the self-powered line of Sceptre S6 and S8 coaxial studio monitors,[11] and releasing Fulcrum's own FH15 full-range horn-loaded loudspeaker series in 2018.[12] All of the coaxial Fulcrum designs use digital signal processing to reduce distortion modes. Fulcrum's proprietary process is called Temporal Equalization (TQ).[13]
