Tech Talk: The Quest for Intelligent Speech

How many times have you been in a public place and heard an announcement that was impossible to recognize? What if the message was an evacuation order for assistance in exiting a burning building? Learn the critical facets of speech intelligibility and how to test for it.

You’ve heard the fire evacuation message over the loudspeaker, “Cheese seed to the ear exit. His is not a test.” As a layperson, am I taking this message seriously? Probably not.

What can we as security professionals do to make this message more understandable? Possibly just speak more clearly, that should do the trick. No, don’t confuse speech quality with speech intelligibility. Speech quality is related to the amount of audible distortion, while intelligibility is the amount of speech that can be recognized correctly.

The serious study of the intelligibility of the spoken word has now reached the security industry and, if not already, it will soon be at a project of yours. This month we will look at some of the terms, methods and equipment used to measure and test speech intelligibility.

Get Acquainted With Audible Terms

Let’s look at some facts concerning human hearing. Most people can hear frequencies within the 20Hz-20kHz range, and in 10 octaves. Speech only covers approximately seven of the 10 octaves (the doubling of any frequency). The bandwidth of human speech is around 125Hz-8kHz; anything else is considered background noise. Most of the acoustical power of human speech is between 250Hz and 4kHz. Listening to the higher frequencies will sound louder than the lower frequencies.

Following are some terms to help better understand speech intelligibility:

Pink noise — Contains all the frequencies within human hearing with a power decrease of 3dB per octave. When we hear pink noise we perceive all of the octaves as sounding equal in loudness. Pink noise is commonly used for room equalization.

White noise — A static sound that has equal energy on every frequency. Every frequency from 20Hz-20kHz is equally represented at the same velocity. Soft, static sound and is useful for sound masking in busy rooms and privacy compliance.

Speech transmission index for public address systems (STIPA) — This is a special audio test signal with speech-like characteristics. Hear a STIPA tone sample at

Speech transmission index (STI) — The numeric measurement of communication channel characteristics. This is a component of speech intelligibility as it indicates if syllables, words and sentences are comprehensible. An STI of at least 0.5 is desirable in most applications.

Common intelligibility scale (CIS) — A testing reference scale with a mathematical formula using STI. A CIS reading of 0.7 or higher is usually passing.

NFPA, DoD Sources of Standards

Speech intelligibility standards are referenced in two organizations: the National Fire Protection Association (NFPA) and the Department of Defense (DoD). Security professionals can now freely review NFPA 72-2010 at; DoD mass notification systems (MNS) documentation can be referenced at

NFPA 72–2010 makes considerable reference to MNS and emergency communication systems. I particularly recommend checking chapters 18 (Notification Appliances) and 24 (Emergency Communication Systems). While this document suggests speech intelligibility compliance and testing, it leaves the particulars up to the AHJ and designing engineers. However, it provides more guidance to the AHJ about how to evaluate your compliance.

Did you know that any government building with more than 12 people is supposed to have a MNS? DoD UFC 4-021-01, Design of Mass Notification Systems, is a good, free reference source for speech/voice intelligibility design.

Nice Devices to Test Effectiveness

Now that you have an idea of what to look for in speech intelligibility, how do you go about testing to see if a system complies to design specifications? Fortunately, there are some outstanding products out there to help.

Gold Line ( of West Redding, Conn., has pioneered speech intelligence testing for years. I appreciate how the company has carefully selected and designed equipment that will give the security professional reliable documented readings. For example, the Talkbox injects STIPA tones directly into fire evacuation systems via push-to-talk microphones, thereby testing all components of a MNS. Then there’s the DSP2B intelligibility meter with OPTCVOW Platinum software (see Tool Tip) for recording STI and CIS readings for MNS performance confirmation. The device is standalone and testing can be done with or without the assistance of a computer.

Improvements in the performance of iOS mobile platforms have opened up new opportunities for using these devices to do STIPA-type testing. Companies such as Studio Six Digital ( now offer iOS apps like STIPA Basic and STIPA Pro for speech intelligibility testing. The firm has just released its iTestMic device (see photo).

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About the Author


Bob is currently a Security Sales & Integration "Tech Talk" columnist and a contributing technical writer. Bob installed his first DIY home intercom system at the age of 13, and formally started his technology career as a Navy communication electronics technician during the Vietnam War. He then attended the Milwaukee School of Engineering and went on to complete a Security Management program at Milwaukee Area Technical College. Since 1976, Bob has served in a variety of technical, training and project management positions with organizations such ADT, Rollins, National Guardian, Lockheed Martin, American Alarm Supply, Sonitrol and Ingersoll Rand. Early in his career, Bob started and operated his own alarm dealership. He has also served as treasurer of the Wisconsin Burglar and Fire Alarm Association and on Security Industry Association (SIA) standards committees. Bob also provides media and training consulting to the security industry.

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