Pre- Star-Wars-ish looks for a surpising loudspeaker topped with an HPM (High-Polymer-Mylar film) super-tweeter for a 270° dispersion of the frequencies above 8,5Khz.
HPM, however, as noted by Wualta (thanks Wualta !) has no "mylar" : "High Polymer Mylar sounds a lot less dumb than "High Polymer Molecular", but the latter is the way Pioneer marketed the HPM speakers and headphones in the US. Aside from which, the HPM piezo film is a fluoropolymer, ie, a relative of Teflon, not Mylar.".
I guess neither Pioneer US when it published its dedicated july 1977 catalog nor the various european Pioneers had been told much about it because both either name HPM as high-polymer-mylar or high-polymer-film...
The convenient blur in the description was replaced by 1980 as High Polymer Molecular in US ads but not in european catalogs or ads...
The HPM-150 was designed by Bart Locanthi, formerly of JBL.
A suprising move from a brand which made its reputation on loudspeakers but it proved a wise one.
For from the HPM series evolved a new Pioneer division, still active and successful today : TAD (Technical Audio Devices), a specifically pro-oriented sub-brand chosen by most serious studios for monitoring needs.
40cm housed in aluminium diecast basket, wooden pulp & carbon fibers cone, long throw voice coil.
Rather small in size not to "upset the precisely calculated directionality characteristics", powered by a large magnet, edgewise voicecoil winding, "newly developed" light & rigid cone material, diecast aluminium frame and a "special design to assure substantially improved acoustic power handling, sufficient sound pressure and wide dynamic margin".
an acoustically compatible epoxy resin bonds the coil and cone to help increase rigidity ; aluminium diecast frame.
Horn-loaded High-Polymer omnidirectional super-tweeter ; 3" cylindrical diaphragm loaded on a "five vertical-sectoral horns so that efficiency is improved by as much as 6dB in horizontal plane without loss of vertical directivity".
3cm chipboard baffle (top, front, bottom), 2cm chipboard sides and top for "optimum acoustical properties".
Computer-assisted resonance analysis and "hard-headed practical experience" for the design of the bass-reflex port.
The top of the HPM-150 is made of four wooden poles which support an unbreakable smoked glass and acoustically transparent cloth.
Despite its somewhat "dated" looks, the HPM-150 is one of the milestones in dynamic loudspeakers, the kind one always comes back to - like a Technics SB-7000.
The HPM-150 (and the HPM series at large) is however part of those which made far more mousse outside Japan -like an RT-707 or SX-1250- and remaining available in the US or Germany beyond 1980.
The Stereo Sound Pioneer "bible" remains absolutely blank about the HPM-150.