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© 2018 by Karl M. Slavik36 Picture 1: Sound reinforcement with high intelligibility in a gallery despite adverse acoustical conditions

SPEECH INTELLIGIBILITYIN PA SYSTEMSText & Translation Karl M. Slavik and Adrian Slavik“Sorry, I didn’t catch that.” If a thought like that is going through a listener’s head, chances are that the sound reinforcement is not working as intended – regardless if it´s in a concert hall or at a railway station. The measurement of voice
intelligibility does not only provide distinct information about the current state of a PA system, it also gives important
clues for optimizing it.
On December 24th, 1915, the world’s first

WHAT IS SPEECH INTELLIGIBILITY?

large-scale PA system was used to am-

The term Speech Intelligibility describes

plify speech and music at a public event

how much of the clearness and distinc-in San Francisco. Approximately 100,000

tiveness of spoken information is pre-listeners were provided with clear, intel-

served when transmitted over a PA sys-

ligible speech information. One shouldtem. Early in the history of audio manual-

think that the last 100 years led to the ul-ly conducted tests were used to rate thetimate perfection of sound reinforcementspeech intelligibility of sound systems:

systems and that poor voice intelligibility

A speaker at a lectern read meaninglessis a relic of days long gone, but daily PA

words and syllables, which were thenpractice shows different. Hard to under-

written down by the audience as correct

stand speech and a generally nasty soundas possible. The result was given as a per-are still part of the repertoire of some

centage, where 100 percent would haveinstalled or touring sound systems. It´sbeen a perfect score. In the 1940ies Bellannoying when you paid full price to see

Laboratories were developing first elec-

your favourite band or play and the vocals

tronic measurement methods for speech

are barely intelligible, but it’s clearly dangerous and even illegal if spoken warnings are not comprehensible.

intelligibility. The ALCons (Articulation
Loss of Consonants) refers to the lossof pronunciation of consonants, where
the percentage stands for the incorrectly understood words or consonants.
An ALCons value of 0.00% would thusmean error-free transmission (no lostconsonants).
Modern measurement methods, such as
STI (Speech Transmission Index) or STIPA(Speech Transmission Index of Public Address Systems) try to represent the resultwith a single number, with as many interference factors as possible (ergo the reality) to be included. The result is expressed
as a numerical value (STI) between 0.00
(no intelligibility) and 1.00 (perfect speechintelligibility). Good sound reinforcement

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achieves results between 0.45 and 0.65,
even in acoustically unfavourable rooms
and 0.70 to 0.90 in acoustically good

Undersized public-address systems thatrooms. For comparison: A good studiotransmission chain accomplishes typi-are unable to top the noise of a loud au-

cal STI values between 0.90 and 0.97 (asdience or drown in the noise of an in-measured by the author) between micro-

coming train are another problem. Again,

phone in the voice booth and the studiothe signal-to-noise ratio is worsened andmonitor in the control room.

speech intelligibility decreased. Sufficiently sized sound systems should be able

FRIENDS AND ENEMIES OFSPEECH INTELLIGIBILITY

to provide at least 10 to 15 dB more level
than the loudest background noise, withTechnically speaking, good speech intel-

natural limits (hearing protection) andligibility is always given if the modula-

disturbing masking effects to consider.

tion depth of the speech or test signal is

Psychoacoustic masking effects also con-maintained without a change. The signal
is transmitted without aberrations andtribute to the loss of speech intelligibil-

without masking of the amplitude, in theity. Not only external noise sources canspectrum and on the time axis.

cover the spoken word, but also speech
itself, especially at very high voice levels.

Good speech intelligibility has many en-Low-frequency components in speech canemies, but also powerful friends. Here ismask quieter, higher-frequency sounds

an overview of the most common pitfalls,from which implicitly the solution can bederived.

Room acoustics is one of the most important influencing factors. Long rever-THE AUTHOR
Karl M. Slavik is engaged in the fieldof professional sound and video technology since the 1980ies and is ownerof ARTECAST Audio Video Broadcast
since 2005. He works for customers domestically and abroad, including theARD-ZDF Media Academy, Dolby Labs,ORF, Puls 4, Junger Audio, Special
Olympics, Projekt Spielberg (Red BullRing), Radio Klassik Stephansdom, SRG
SSR and many more.

STI-Value

Quality1)

Intelligibility ofsyllables in %Intelligibility ofwords in %Intelligibility ofsentences in %0 – 0.3

Bad

0 – 34

0 – 67

0 – 890.3 – 0.45

Poor

34 – 48

67 – 78

89 – 92

0.45 – 0.6

Fair

48 – 67

78 – 87

92 – 95

0.6 – 0.75Good

67 – 90

87 – 94

95 – 960.75 – 1

Excellent

90 – 96

94 – 96

96 – 100

� Table 1
Meaning ofSTI-values inpractice Picture 2: Measurement of speech intelligibility with NTi Audio XL2beration times (RT60) as well as late reflections (around 80 to 150 ms) act like a„filler“ that clogs the useful modulation
depth of the speech signal by covering it,thus making the recognition of information more difficult.
Badly positioned loudspeakers with inappropriate directional characteristics
might provide sound for the room, suchas to opposite walls and glass fronts,
but not directly to the listener. As a result, the directly radiated sound from thespeakers at the position of the audiencemight be proportionally lower than thenoise level (reflections, interferences),speech intelligibility decreases.

Grafic: NTI-Audio© 2018 by Karl M. Slavik1) A more detailed classification of speech intelligibility (STI) into 12 categories between A+(better than 0.76) and U (worse than 0.36) can be found in EN 60268-16 Annex F and Annex G.

 Picture 3: Reverberation and other disturbances reduce the depth of modulationand thus prevent their perception.

So beware! Musically accompanied read-

non-linear distortions of the transmis-

sational theatre in emptied swimming

Another disturbance are linear andsion path. These disturbances are not only caused by clipping amplifiers and badspeakers, but often by well-intentioned,
but badly implemented signal processing. Too much compression of dynamics,
overdriven limiters, unnecessary boosts orcuts in the frequency range and also a too
stingy data reduction can worsen speech
intelligibility.

ings in former aircraft hangars, improvipools, platform announcements during
the arrival of a train, in all these cases
very precise planning and a greatly increased technical effort are needed toachieve reasonable or at least tolerable
speech intelligibility.

LEGAL REQUIREMENTS
Whether it‘s concert sound, airport announcements or speech at an auditorium: Speech intelligibility is one of themost important and decisive parameters
for the evaluation of all types of public
address systems. If regular PA systems
are also used as voice alarm systems,

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particularly strict guidelines apply. In Ger-References

many, the requirements for voice alarmsystems are regulated in DIN VDE 0833-4,
in Austria the fire protection guidelines

At large open-air concerts and sporting

TRVB 123 and 158 regulate that matter.

events, but also in smaller venues, pub-In the UK standard BS5839-8 applies for“VA” (Voice Alarm) or “VACIE” (Voice Alarm
Control and Indicating Equipment). For all
EU countries EN 50849 applies for „electroacoustic emergency warning systems“.
The details for speech intelligibility measurement are specified in EN 60268-16.

emergency voice alarm system. The authority that approves the event can therefore impose similar operational safety
and speech intelligibility requirements aswith fixed installations. Even without special regulatory requirements, it is urgent-

But when does a public-address system� Picture 4: Acoustics analyser set
with STIPA functionality: NTi Audio XL2 analyser
with measurement microphoneand generator
MR-PROlic address systems can also serve as an

qualify as a „voice alarm system“ that issubject to the mentioned standards, especially regarding its speech intelligibility(and other criteria)?
Many permanent installations, for example in theatres, hotels and conference

ly recommended to plan systems according to the current state of the art and toensure maximum speech intelligibility.
For example, in the event of injuries following a mass panic or due to bad weather, it is generally checked by the authorities whether announcements by the or-centres, are a combination of high-per-ganizers were well audible and effectively

formance sound reinforcement (for lec-used for traffic guidance (general diligence and operational safety obligations).

gibility. Herman J.M. Steeneken, TNO
Human Factors, Soesterberg, theNetherlands.
• Manual XL2, NTi Audio, 2018 https://www.nti-audio.com/Portals/0/data/
en/XL2-Manual.pdf• Beschallungsanlagen in Räumen Teil
3 (Messtechnik und STI). Seminarunterlagen (PDF) von Anselm Goertz
und Alfred Schmitz. DEGA Akademie
Raumakustik und Beschallung, 2017
(German only)• EN 60268-16, Sound system equipment - Part 16: Objective rating ofspeech intelligibility by speech transmission index (IEC 60268-16:2011)• NTi Audio Application Note „Speechintelligibility”, 2017

MEASUREMENT OF SPEECHINTELLIGIBILITY

noise signal. Spectrum and modulation

Although there are several methodsof a male speaker.

of measuring speech intelligibility to© 2018 by Karl M. Slavik• The Measurement of Speech Intelli-day, STIPA has become a fast and accurate method for many applications. The
measurement of the „Speech Transmission Index for Public Address Systems“has been further developed by NTi Audio

correspond to the time-averaged signal

The measurement of speech intelligibility can be realized in two ways: either bymeans of a signal generator (for example
NTi Audio MR-PRO), which feeds the STIPA
signal directly into the sound system un-� Table 2: Composition of theSTIPA test signaltures, audio-visual presentations, concerts, parties) and voice alarm systems.
In these cases, the specification for „electroacoustic emergency warning systems“
according to EN 50849 applies to the entire (!) system. This standard defines theperformance requirements for such systems and thus guarantees that availability and operational safety are ensured forthe highest demands. In the case of analarm the running performance must beinterrupted automatically and a usuallyautomated announcement with sufficientspeech intelligibility should be transmitted. For most systems, a speech intelligibility of STI = 0.5 or 0.55 is mandatory

and is now standardized according to EN
60268-16. Several T&M manufacturers areoffering this method. STIPA is based on
the measurement of the MTF (Modulation Transfer Function), which measuresthe loss of modulation depth in different
octave bands. As a measurement signal,
specially modulated noise in the octave
bands of 125, 250, 500, 1,000, 2,000, 4,000
and 8,000 Hz is used, which assesses thetransmission quality in the baseband
as well as the harmonics of the speech(formants) which is particularly importantfor intelligibility. In each octave band,the noise is modulated with two frequencies, resulting in an amplitude modulated

der test, or by means of a „TalkBox“ withan already built-in signal generator. This
special loudspeaker serves as a „speakerreplacement“ and is positioned in front of
the microphone of the sound system. Asan analyser the NTi Audio XL2 with STIPA
software and measuring microphone isused. The measurement itself takes about
15 seconds, a correction of the determined speech intelligibility with the spectrum of ambient noise is possible. During
the measurement no impulsive noises orother short or speech-like sounds shouldoccur. An automatic averaging process
calculates the average and the statistical deviation of several measurements.

during normal operation, and if system

The result is displayed as a speech intel-components fail, at least STI = 0.45 mustligibility index in STI (Speech Transmis-

be achieved (see, for example, DIN VDE

sion Index) or CIS (Common Intelligibility

0833-4 or TRVB 158).

Scale, CIS = 1 + log (STI)).