Institute for Telecommunication Sciences
the research laboratory of the National Telecommunications and Information Administration

Institute for Telecommunication Sciences / Resources / Audio Quality Research / Public Safety Audio Quality

Audio Quality Research in Support of Public Safety

ITS has contributed significant research in support of clear and effective communications for fire, police, medical, and other public safety officials. In the audio quality domain, communication system goals include

  • Listener can easily understand what is being said
  • Listener can recognize the voice of a known talker
  • Listener can detect stress in a talker’s voice
  • Background audio is clear

Our Start

Some first-generation digital radio systems fell short of these goals under certain conditions, most often conditions involving moderate to loud levels of background noise. But these noise environments (e.g. sirens, engines, rescue equipment, pumps and nozzles) are inherent in public safety operations and clear communications in their presence is a must.

The late Mr. DJ Atkinson heard reports of these communication breakdowns, took them seriously, and investigated carefully. He established a formal process of study at ITS based on the standardized speech intelligibly test called the Modified Rhyme Test (MRT). DJ’s tireless efforts, passionate engagement of the work, and commitment to the best interests of the stakeholders are widely recognized and contribute to his remarkable and enduring legacy. DJ’s work established ITS as a key source for rigorous, impartial, speech intelligibility test results, and the contributions continue to this day.


Speech intelligibility examples can be hard here. Speech Intelligibility Demo

MRT resources are available here. Modified Rhyme Test Audio Library

Our Work

We design, conduct, analyze, and document objective and subjective tests to discover the conditions in the above goals are not met, the severity of those failures, and potential mitigation strategies. Most often we measure speech intelligibility, report conditions that reduce speech intelligibly and investigate technology alternatives that can lessen those reductions. We disseminate our results so that manufacturers’ implementations will meet the operational needs of public safety officials. We have:

  • Designed numerous tests involving radio systems and other communication system components in many different noise environments (e.g. alarms, sirens, rescue equipment, fire fighting equipment)
  • Performed preliminary speech intelligibility testing using software tools
  • Formally tested components and systems using the Modified Rhyme Test (MRT)
  • Partnered with various agencies to allow their practitioners to participate in these MRTs at our labs. Listeners attempt to distinguish words spoken under various test conditions
  • Worked to improve speech intelligibility under difficult conditions
  • Analyzed test results and prepared detailed technical reports for distribution to all stakeholders

Our Results

We have focused on Goal 1—speech intelligibility. We continue to contribute evaluations of new technologies and environments as they develop:

  • Andrew A. Catellier; Stephen D. Voran, "Intelligibility of Selected Speech Codecs in Frame-Erasure Conditions," NTIA Technical Report TR-17-522, November 2016.
  • Stephen Voran and Andrew Catellier, “Speech Codec Intelligibility Testing in Support of Mission-Critical Voice Applications for LTE,” NTIA Technical Report TR-15-520, September 2015.
  • David J. Atkinson, Stephen Voran, and Andrew Catellier,“Intelligibility of the Adaptive Multi-Rate Speech Coder in Emergency-Response Environments,” NTIA Technical Report TR-13-493, December 2010.
  • David J. Atkinson and A. Catellier, “Intelligibility of Analog FM and Updated P25 Radio Systems in the Presence of Fireground Noise: Test Plan and Results,” NTIA Technical Report TR-13-495, May 2013.
  • David J. Atkinson and Andrew Catellier, “Intelligibility of selected radio systems in the presence of fireground noise: Test plan and results,” NTIA Technical Report TR-08-453, June 2008.

We have also worked to support talker recognition and talker stress detection (Goals 2 and 3):