The instrument
"The Models 315 and 315S are compact and rugged ribbon mics combining wide range response and a bidirectional pickup pattern. The bidirectional pattern reduces noise and reverberation providing the same control as an equivalent cardioid microphone. The Models 315 and 315S are ideal for high-quality public address, theater stage sound systems, and recording applications. Due to the bidirectional polar pattern, they may be used for either single performer, a dialogue or group, solo or orchestra, and other individual or chorus applications."
Details
The Models 315 and 315S are multi-impedance for connecting into mic inputs rated at 25 to 50 ohms, 150 to 250 ohms, or into high-impedance mic inputs.
Frequency response: 50 to 12,000 Hz;
Polar Pattern: Bidirectional, equally sensitive at front and back. Response at sides down 15 to 20 dB from front and rear responses,
Switch: built-in “ON-OFF” switch to control microphone circuit. The switch is an integral part of the receptacle assembly and is a slide-to-talk locking type switch
(source: courant.org)