HD77 is a model name of Marantz 4-way high-fidelity loudspeakers which were produced during the mid-1970s. They were bass reflex speakers, but came with a cylindrical piece of foam which fit into the bass-reflex port of the enclosure if the listener preferred the more accurate bass response provided by airtight speaker boxes. However, they were not fully airtight this way, but rather what is known as aperiodic. They were designed by former JBL Engineer Edmond May. The foam plug in port was in fact a "closed-cell" type which did in fact seal the cabinet completely. How it worked was it was cut slightly larger than opening and when inserted, you turned the retainer screw and it would tighten and compress the plug, thereby sealing the cabinet to make the loudspeaker a "suspension" type. The controls on baffle are resistive potentiometers, which changed the response curve of corresponding driver from soft to bright, giving great versatility to adapt within almost any listening environment.
