ACR MDT ALT EXCURSION. SIMILAR A/N.

Narrative

TWO COMPANY ACFT ON SAME FREQ; BOTH XA'S. A LOT OF BACKGROUND NOISE ON RADIO; THOUGH I HEARD CENTER CLR US FROM 8000 TO 4000'. ASKED IF THAT CALL WAS FOR US. THOUGHT CENTER SAID 'AFFIRMATIVE;' SO I SAID; 'OUT OF 8000 FOR 4000; DSNDING THROUGH 7500.' CENTER ASKED IF WE WERE LEVEL AT 8000'. I SAID; 'YOU CLRED US TO 4.' HE SAID HE CLRED THE OTHER TFC AND NOT US. THEN HE RECLRED US TO 6000'. NO OTHER TFC WAS IN THE AREA. SUPPLEMENTAL INFO FROM ACN 92907: WHEN THE F/A CALLED UP TO DISCUSS SOMETHING FROM THE CABIN; I ADVISED THE F/O I WOULD BE OFF THE RADIOS; YET I CONTINUED TO MONITOR. A FEW MINS LATER I NOTICED THE F/O RESET HIS ALT ALERTER AND COMMENCED A DSCNT FROM 8000 TO 4000'. AT 7500' HE STOPPED THE DSCNT; TALKED TO CTLR; AND RESET THE ALT ALERTER FOR 6000'. NONE OF HIS RADIO XMISSIONS DID I HEAR. HE SEEMED ALARMED; SO I ASKED IF WE HAD HAD AN ALT DEVIATION. HE SAID HE HAD HEARD A CLRNC TO 4000'; ASKED IF IT WAS FOR MDT X; THEN GOT SOME RESPONSE; SO HE HEADED OUT OF 8000 FOR 4000' AND RESPONDED TO ATC. I WAS OUT OF THE COM LOOP WITH ATC AND THE F/O BECAUSE I HAD JUST ADJUSTED THE ILLUMINATION INTENSITY OF THE FLT INSTRUMENTS AND COCKPIT LIGHTS. THE MASTER VOL CTL KNOB FOR MY RADIO PANEL IS LOCATED NEXT TO THE ILLUMINATION KNOB. MY FINGER BUMPED THE VOL CTL TO AN INAUDIBLY LOW LEVEL WHEN I ADJUSTED THE LIGHTS. THE F/A INTERCOM VOL WILL OVERRIDE (SOME DESIGN FLAW) COCKPIT INTERCOM AND ATC RADIOS; MAKING HEARING ANYTHING BUT THE F/A NEXT TO IMPOSSIBLE (ON SOME AIRPLANES; THIS AIRPLANE BEING ONE OF THESE). NO ATC RESPONSE WAS HEARD TO KEEP US FROM LEAVING 8000' WHEN WE ROGERED OUT FO 8000 FOR 4000'.