| 381 / | HD | ELECTRICAL AUDIO SIGNAL PROCESSING SYSTEMS AND DEVICES |
| FOR 127 | DF | HEARING AIDS, ELECTRICAL (381/68) {15} |
| FOR 152 | DD | .~ Electromagnetic (e.g., dynamic) (381/192) {5} |
| FOR 159 | | .~.~ Magnetic circuit or core structure (381/199) {2} |
| FOR 160 | DD | .~.~.~> Armature (381/200) |
| FOR 161 | DD | .~.~.~> Magnetic configuration (e.g., tubular or U-shaped) (381/201) |