The part number for Mallory’s 22 mm panel alarms begin with the letters ZA; this is followed by three numbers which indicate the voltage range. The next letter indicates the sound loudness category followed by the letter D or A, which indicates the voltage type. The last two or three characters indicate the sound type. All sound types are available at all sound levels and voltage ranges, except for the multi-tone alarms, which are only available for DC voltage applications.