CPC Subgroup
H04R 21/023 Full Title
Variable-resistance transducers (gaseous resistance transducers H04R23/00; magneto-resistive transducers H04R23/00) > Microphones
H04R 21/023 Variable-resistance transducers (gaseous resistance transducers H04R23/00; magneto-resistive transducers H04R23/00) > Microphones