Speech Reinforcement: Assistive Listening

By: Owen T. Heisler
Updated: 2016-10-31
Published: 2016-08-05

This article is part of a series; see Speech Reinforcement. This is a stub; more content may be added later.

Requirements

Consider the following requirements:

  • easy-to-use battery-powered receivers with headphones for use in the listening area
  • same receivers as above for use outside the listening area
  • AC-powered or line-powered (passive) permanent speakers for small rooms outside the listening area

Options

  • 70 V audio line installed to all required locations for permanent speakers
  • Wireless 72 MHz FM transmitter and receivers
  • Wireless 216 MHz FM transmitter and receivers
  • Wireless 900 MHz FM transmitter and receivers

Notes

  • 70 V is common, and requires a physical line to each speaker. Power is provided by an amplifier in the main sound system.
  • One or more of 72, 216, and 900 MHz for assistive listening, may be able to use a single antenna (75×12=900) by isolating grounds of transmitters (both power and input signal). More research is necessary. Don’t destroy any transmitters.
  • The 72, 216, and 900 MHz wireless options require antennas to be mounted. Use good coax, match the impedance, measure your antenna to the correct length, and watch for standing waves.
  • 72 MHz provides 6 channels; 216 MHz provides 3 channels.
  • Because 216 MHz is a smaller wavelength, it requires a smaller transmit antenna than 72 MHz.
  • All other things being equal, 216 MHz will provide approximately double the range that 72 MHz provides. But 216 MHz will lose more power due to cable attenuation than 72 MHz (again, use good coax).