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Speech Intelligibility and Design of Public Address Systems at Metro Stations


Speech intelligibility is an important aspect to be considered in a Public Address (PA) system. A PA system in a public transit system announces train arrival/ departure, general information, and emergency information and instructions to passengers. Poor speaker characteristics, inappropriate sound levels, reverberations, and delays can degrade sound quality and make even an expensive sound system unintelligible and useless. Addressing these issues at the design stage through proper planning and acoustic simulation studies would improve a PA system's overall performance and customer satisfaction, and minimize the need for expensive post-installation modifications.

GL's Solution, Technical Guidelines and Test Plans Development

GL conducted an audio survey of the PA system at two metro stations. The customer considered these two stations to be representative of other above-ground stations. We conducted initial site surveys of the two stations that:

  • Detailed physical measurements of the station architecture.
  • Took note of the materials used in the station architecture.

After the initial site surveys, we conducted comprehensive tests and simulation studies to:

  • Assess audio quality and speech intelligibility of the existing PA systems.
  • Measure ambient noise levels.
  • Measure STI-PA.
  • Perform extensive acoustic modeling for deriving optimal configurations.
  • Recommend steps to expand the coverage and improve speech intelligibility.
  • Recommend specifications and vendors for speakers.

GL's engineers created three dimensional computer models using the above inputs. With the 3-D models and ambient noise measurements, our engineers simulated the acoustic environment of the areas requiring PA coverage. Results of simulations studies were analyzed to determine optimal speaker type, speaker locations, and speaker power levels for the PA system.

Our studies also simulated sound pressure levels and Rapid Speech Transmission Index (RaSTI) scores. Using the simulation results, GL also recommended speaker tap settings, orientation, and volume adjustment in order to improve intelligibility in existing coverage areas as well as to provide coverage in additional areas. Based on our studies, GL recommended a number of steps to improve audio quality within the stations to meet or exceed NFPA 72/2006 requirements for a CIS of 0.7. We also developed a detailed test plan that our customer can use to assess new PA systems that will be installed at other metro stations.

Customer Benefits

GL's study and recommendations would greatly improve the speech intelligibility and increase coverage area within the public transit's metro stations. The quality improvement would benefit the patrons leading to increased satisfaction.