Abstract: An audio data transmission device for transmitting binary coded audio tones over a telephone system. The audio data transmission device is adapted to be utilized with a hand-held, data collection terminal provided with a speaker emitting binary coded audio tones. The transmission device consists of a muff for receiving the microphone portion of a conventional telephone handpiece and a solid transmission element. The transmission element is adapted to be mechanically connected to the housing for the terminal for channeling the data tones to the telephone handset without dampening. The transmission element is defined to be mechanically tuned to the characteristics of the telephone microphone to minimize the generation of second harmonics of the desired audio tones.