APPLICATION NOTE

Speech Intelligibility
MEASUREMENT WITH THE XL2 ANALYZER

Public Address (PA) systems in buildings like airports, railway stations, shopping centers and concert
halls have to clearly inform persons in danger about escape information and directions in case ofan emergency. Using the XL2 Analyzer, it is possible to determine the intelligibility of the messages
spoken through the PA by measuring at all public points within the building.

Speech Intelligibility is the measure of how accurately people understand a spoken message. The
XL2 measures the speech intelligibility according to the standard IEC 60268-16:2020 (edition 5).
STI and STIPA are the most established methods for measuring speech intelligibility. STIPA is an
optimized version of STI dedicated to portable measurement instruments. This application note
explains the principles behind these methods.

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Speech Intelligibility Measurement with XL2

APPLICATION NOTE

IndexIntroduction

3

Recommended Ambient Noise14

Can I buy STIPA for my XL2 Analyzer?

4

STI Report

15

Standards

4

Measurement Positions and Number of Measurements

17— International Standards

4— Australia

17— National Standards

4— German VDE0833-4 Standard

18

STIPA Test Signal

5

Continuous Level-dependent Auditory Masking

18

Signal Source

5

Measurement Hints

19— NTi Audio TalkBox

5— Ambient Noise19— Minirator MR-PRO

5— STI Measurement

19

NTi Audio TalkBox

6— What to do if there is much impulsive noise

20— How to use the NTi Audio TalkBox

6— Measurement configuration

20— CD Player

20

Minirator MR-PRO

7— Speech Intelligibility in reverberant spaces

21— How to set the Minirator output level7

Who can and should conduct STI measurements?

21

STI Measurements - Getting Started

7— Prepare for the Test

7

Appendix

22— Start STIPA Test Signal

8

Subjective analysis methods

22— Start the Measurement

8— Stop the Measurement and Save the Results

8

Technical methods

22— Qualification scale “A+” to “U”

9— Speech Model

23— Time Modulation

23

Averaging of STI Results

10— Frequency Spectrum

23— Start Averaging

10— Add Cycles		10

Modulation Transfer Function (MTF)

24— Finish10

STI Speech Transmission Index

25— Display of STIPA Numeric Result Page

11

RASTI - Room Acoustics Speech Transmission Index

25

Ambient Noise Correction

11— Enable Ambient Noise Correction

12

STIPA-Speech Transmission Index for Public Address

26— STIPA Modulation Ration mr1, mr2

27Commissioning New PA Systems

12— Measuring Ambient Noise13

How does STIPA compare to STI?

27

— STI Measurement

13

Calculation of % alcons from STIPA measurement28— Announcement Level

13

STIPA now

28

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APPLICATION NOTE

Introduction
Public Address (PA) systems in building complexes haveto inform members of the public about escape directions
in case of emergency. Such public buildings include airports, railway stations, shopping centers and concert halls.
However, if such announcements are misunderstood due
to poor system quality, tragic consequences may result.
Therefore, it is essential to design, install and properly verify
the speech intelligibility of such sound reinforcement systems. In addition, common public announcement systems
may have legal or contractual requirements regarding thelevel of speech intelligibility, such as in medical, industrial orcommercial environments.
The XL2 Acoustic Analyzer offers reliable measurement of
the Speech Transmission Index (STI) within 15 seconds. Besides the single value STI or CIS (= Common Intelligibility
Scale) test result, a detailed view of the modulation indicesand individual band level results is provided.
The measurement meets the standard IEC 60268-16 standard (edition 5.0 released in 2020). The XL2 also supports
noise corrections, automated averaging of measurements
and the older standard editions 2, 3 and 4.

XL2 Audio and Acoustic Analyzer

The intelligibility of speech depends on:• Signal-to-noise ratio• Psychoacoustic masking effects• Sound pressure level• Ambient noise level• Reverberation time RT60
• Reflections• Frequency response• Distortion

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Speech Intelligibility Measurement with XL2

APPLICATION NOTECAN I BUY STIPA FOR MY XL2 ANALYZER?
Yes, STIPA is an optional function for the XL2 Audio and Acoustic Analyzer. Any XL2 user may obtaina STIPA license. With the license key you may download the activation key for your XL2 Audioand Acoustic Analyzer. Ask your local representative for purchasing details. Enter your license key
at https://my.nti-audio.com .

Standards
The ISO 7240-16/-19 standard requires the verification of electroacoustic sound systems for emergency purposes. Realistic circumstances shall ascertain a measurable minimum level of speech
intelligibility in case of an emergency. Therefore, speech intelligibility from a regulatory view is not a
subjective measurement, but can be verified with several, more or less complex methods that havebeen standardized in IEC 60268-16.
International Standards:
CEN/TS 54-32

Fire detection and fire alarm systems

ISO 7240

Fire detection and alarm systems, section 16 & 19

IEC 60268-16

Objective rating of speech intelligibility by speech transmission index

EN 50849

Sound systems for Emergency purposes

EN 60849

First international standard based on BS 60849; replaced
by DIN EN 50849

National regulatory bodies recommend or require maintaining a minimum speech intelligibility.
National Standards:
AS 1670.4

Australia: Fire detection, warning, control and intercom
systems

NFPA 72

US: National Fire Alarm Code (2010 edition)

BS 5839-8

Great Britain: Fire detection and alarm systems for buildings; code of practice for the design, installation and servicing of voice alarm systems

VDE 0828-1

Germany: Electroacoustic Emergency Systems with application regulation described in standard DIN VDE 0833-4
and VDE V 0833-4-32

UFC 4-021-01

Design and O&M: Mass Notification Systems

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Speech Intelligibility Measurement with XL2

APPLICATION NOTESTIPA Test Signal
The standard STIPA signal is based on random noise limited to the bands contained in a malespeech spectrum.
Use only the original NTi Audio test signal for speech intelligibility measurementswith the XL2.
Other signals may not seamlessly loop, thus causing wrong measurement results!
Compressed formats, like MP3 or similar, must not be used.

Figure 1: Spectra of STIPA Test Signal Ed. 4 and 5

Signal Source
Choose the applicable STIPA test signal source.

NTi Audio TalkBox
The NTi Audio TalkBox simulates a person talking at a precise acoustic level, enabling the measurement of the complete signal chain including the microphone.
— Place the NTi Audio TalkBox in front of the microphone at the typical position of the talking person’s head.
— Select Track #1 for the STIPA test signal.
— Toggle the Output Mode to loudspeaker symbol; you should hear the STIPA test signal.

Minirator MR-PRO
The Minirator MR-PRO is used for electrical signal injection into Public Address systems that commonly use alarm messages from a hard drive (systems without a microphone).

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APPLICATION NOTE

NTi Audio TalkBox
The NTi Audio TalkBox is required for PA systems with voice
microphones.
It simulates a person talking at a precise acoustic level, enabling the measurement of the complete signal chain including the microphone. It offers a calibrated acoustic soundsource, emulating the speech levels of a person speaking ina normal or in an emergency situation.

TalkBox

The use of the NTi Audio TalkBox is advisable if— Regulations require a complete end-to-end system check including the microphone; this is, in any
event, the most realistic system check.
— No input is available for an electrical test signal.
— The level of the test signal is not clearly defined.
— The characteristics of the speaker’s and listeners’ acoustical environments are not negligible.
— The characteristic, sensitivity and frequency response of the speaker’s microphone is not known,
but needs to be considered.
— For any other reason it is desirable to test the entire signal chain under real conditions.
The standard STIPA signal is based on a band-limited random noise of a male speech spectrum. The
TalkBox is also capable of delivering White and Pink Noise and other special signals, and is thus avery useful tool for overall system tuning and testing.

How to use the NTi Audio TalkBox— Place the NTi Audio TalkBox in front of the microphone at the typical position of the talking person’s head.
— Select Track #1 for the STIPA test signal.
— Toggle the Output Mode switch to the loudspeaker symbol; you should now hear the STIPA signal.
In case the typical microphone-speaker distance D is less than 10 cm, then use
the STIPA test signal A with 70 dB @ 1 m, and set the TalkBox to a measurement
distance of approx. “D * 3” away from the microphone (e.g. distance D = 5 cm ->measurement distance = 15 cm). This simulates the actual speaker level at close
distances and prevents unwanted near-field effects.

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APPLICATION NOTE

Minirator MR-PRO
The Minirator MR-PRO is used for electrical signal injection
into Public Address systems that commonly use alarm messages from a hard drive (PA systems without a microphone).

How to set the Minirator output level— Determine the LAeq of the voice signal for at least 40 sec
unless the signal is a recorded announcement of shorter
duration.
— The measured level is corrected by an empirically-derived
factor of 3 dB in order to obtain an estimate of the realspeech level. This compensates for the silent parts of the
speech signal, e.g. the gaps between words (in accordance with IEC 60268-16:2020, Annex J4).
— Power ON the STIPA test signal on the Minirator.
— Increase the volume, until the announcement level at thesame measurement position = LAeq + 3 dB.
— Execute the STI measurements with the XL2 Analyzer.
Minirator MR-PRO / MR2

STI Measurements – Getting Started
PREPARE FOR THE TESTThe XL2 reads the electronic data sheet of the connected NTi Audio measurement microphone andswitches the 48 V phantom power automatically ON.
— Connect the measurement microphone to the XL2.
— Switch ON the XL2.
The 48 V phantom power indication in the upper menu bar changes to ASD; the XL2 is now readyfor acoustic measurements.
— Position the XL2 at the measurement location by using a microphone stand or tripod.
— Select the STIPA measurement function in the XL2 measurement menu.
— Prepare the environment for the measurement; for instance, mute all sound sources to establish
silence.
No impulsive noise (traffic, doors slamming, etc.) shall occur during the speech
intelligibility measurement, including speech and other noises in the vicinity of themeasuring microphone.

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APPLICATION NOTE

Start STIPA Test Signal
Select the STIPA signal source according to your application requirements.
— Switch ON the STIPA test signal at the signal source.
— Set the acoustic sound pressure level of the PA system to simulate the typical announcement level; e.g.
LAS = 85 dB.

Start the Measurement— Press Start.
The progress bar switches to RUNNING. The test result
tendency is shown at the bottom of the screen, as Bad,
Poor, Fair, Good and Exlnt (Excellent).
The following minimum intelligibility values, measured acrossall applicable areas, apply in accordance with the ISO 724019:2007 standard:— mean: 0.50 STI— minimum: 0.45 STI

Stop the Measurement and Save the Results
After 15 seconds the speech intelligibility measurement finishes automatically. The progress bar indication switches to
FINISHED and the final test result is displayed. The measurement result is stored automatically.
— Switch OFF the STIPA test signal.
— Press Enter to confirm. The measurement data is stored
on the SD-card in ASCII format.
The measurement is completed.

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APPLICATION NOTEQUALIFICATION SCALE “A+” TO “U”
The STI value is shown as a letter of the qualification scale below, which informs about the typical
STI requirements for dedicated applications.
Band

STI Range

Examples of typical usesA+

> 0.76

recording studios

A

0.72 - 0.76

theatres, speech auditoria, parliaments, courts

B

0.68 - 0.72

theatres, speech auditoria, parliaments, courtsC

0.64 - 0.68

teleconference, theatresD

0.60 - 0.64

class rooms, concert halls

E0.56 - 0.60

concert halls, modern churchesF

0.52 - 0.56

PA in shopping malls, public offices, cathedrals

G

0.48 - 0.52

PA in shopping malls, public offices

H

0.44 - 0.48

PA in difficult acoustic environments

I

0.40 - 0.44

PA in very difficult spaces

J

0.36 - 0.40

not suitable for PA systems

U< 0.36not suitable for PA systems

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APPLICATION NOTE

Averaging of STI Results
The IEC 60268-16 standard recommends the averaging of
two or three sequential results that were taken at the samemeasurement location.
The German VDE 0833-4 Standard requires a minimum of
three sequential measurements at a single position in case
STI < 0.63.
The XL2 Analyzer offers automated averaging of two toeight speech intelligibility results, based on these standard
requirements.

Start Averaging— Select the averaging page Avr.
— Turn the rotary wheel to select the parameter START
NEW and press enter.
The first measurement is labeled Cyc. 1 and starts
automatically.

Add Cycles— Press enter to confirm Add Cycle.
— Repeat the measurement at the same position as required.
The XL2 executes further measurements and adds them tothe list.

Finish— Choose Finish to end the averaging.
The speech intelligibility average and the deviation is displayed for documentation.

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APPLICATION NOTE

Display of STIPA Numeric Result Page
The symbol
played.

indicates that the averaged STI value is dis-

Ambient Noise Correction
Measuring the STI under realistic environmental conditions
is often not feasible. For instance, playing the test signal
in a railway station at emergency levels during peak hours
would irritate passengers. Additionally, at rush-hour thecharacteristics of ambient noise might be highly impulsive,
while a pre-requisite for accurate STIPA measurements is anegligible impulsivity in the ambient noise. Under such circumstances the STIPA measurement should be shifted to amore suitable time of the day, e.g. during the night.
Ambient noise affects the speech intelligibility. The ambient
noise correction has to be applied when the signal-to-noise
ratio is lower than 15 dB in any octave band.
Measurement sequence:— First, measure the realistic ambient noise, e.g. during daytime.
— Second, measure the speech intelligibility STIPA, e.g. atnight.
This sequence simplifies the STIPA measurement as follows: The XL2 immediately displays the STIPA result with ambient noise correction. The result provides
a guideline if further measurements at the same location are required. For moredetails see the chapter “STIPA Measurement Hints” in this application note.

If no ambient noise correction has been carried out on-site with the XL2 Analyzer, you can still postprocess your measurement data on the PC. The NTi Audio STI Report combines the STIPA measure-www.nti-audio.com

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APPLICATION NOTE

ment taken under quiet conditions and the actual ambient noise caused by the public, e.g. during
day-time. This emulates the expected STIPA value under real-life conditions.

Enable Ambient Noise Correction— Select the correction page Cor.
— Turn the rotary wheel to select the parameter Ambient
Noise Correction and press Enter.
The correction is enabled and the XL2 displays this screen.

You may adjust the ambient noise band
levels and the LAeq.

Commissioning New PA Systems
New Public Address systems are often commissioned prior to the grand opening. Thus, the actual
ambient noise caused by the public is not yet available. In this case, you may simulate the real-life
condition using ambient noise data from one of the following methods:— Utilize a suitable reference noise file
• Measure the ambient noise at another similar project and store this as the reference noise file.
• Select the parameter Load with the rotary wheel and press Enter.
• Select the reference noise file, as ambient noise correction for your speech intelligibility measurements.
— Edit actual noise data
• Select the parameter Edit or the LAeq level with the rotary wheel and press Enter .
• Turn the rotary wheel to adjust the noise level.
• Press Enter to continue the setup.
— Measure the actual ambient noise after the opening of the public area, and verify the STI readings.

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APPLICATION NOTE

Measuring Ambient Noise— Position the microphone at the STIPA measurement
point.
— Select Measure (without any test signal present).
The XL2 measures the ambient noise and displays the LEQ
octave band result.

You may edit the noise data.

STI Measurement— Select the STIPA result page 123.
— Execute the speech intelligibility measurement.
The XL2 displays the corrected speech intelligibility result
in large font. The actual measured result is listed below insmaller font.

ANNOUNCEMENT LEVEL
The speech intelligibility shall be measured at the announcement level.
The announcement level shall exceed the ambient sound pressure level by a minimum of 10 dB. This
shall apply at all places within the listener zone, where warning signals are conveyed to building occupants. The measured announcement level shall not be less than 65 dBA nor more than 105 dBA
at the listener position. A louder announcement is not always better; the speech intelligibility reduces
gradually at levels exceeding 80 dBA.
The announcement level measurement itself takes 60 seconds at each position in accordance with
ISO 7240. Here the NTi Audio TalkBox serves as an excellent reference speaker. It generates prerecorded announcements or your own messages at the specified sound pressure level into the voice
microphone for any time you require and until you have completed the verification at all measurement
positions. Your own announcement signals may be loaded onto the memory card and are seamlessly looped.

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APPLICATION NOTE

Recommended Ambient NoiseThe following table lists the typically applicable ambient noise levels in accordance with the Austrian
TRVB S 158 - 2006 standard.
Area Type

Sound
Level
LAeq (dB)

AirportArea Type

Sound
Level
LAeq (dB)Libraries- check in, departure / arrival hall

59-72

- reading area quiet

35-45- gates54-64- reading area noisy

50-60- custom area

63-71

- reception

50-60- passages

59-70- waiting area departure

49-64

Aisles- with carpet

28-32- quiet without carpet

45-55- noise without carpet

66-76

Bus station- quiet58-68- noisy

63-73

Manufacturing facilities- monitoring stations

70-75- common manufacturing

80-85- heavy industry

95->105

Markets- quiet47-63- noisy

63-80

Offices- Single person office

40-50

Conference room

40-45

- Open-plan office quiet

50-70

Concert halls, cinemas, theaters

60-75- Open-plan office noisy

70-85

Courtrooms

40-50

Exhibition hall

63-73

Public Areas

50-64- service support areas

55-65

Restaurant- sleeping room, TV off

28-35- quiet55-65- sleeping room, TV on

60-70- noisy

68-78

Hotels / Motels

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Speech Intelligibility Measurement with XL2Area Type

Sound
Level
LAeq (dB)

Railway

APPLICATION NOTE

Area Type

Sound
Level
LAeq (dB)Shops- waiting area

54-65

- quiet50-60- service area

60-66

- noisy

65-75- platform electric train

60-72- shopping centre

70-75- platform diesel train

75-85
Sport facilities

Restaurants- quiet60-72- customer area

72-75

- noisy

72-82- kitchen65-75- squash

60-80- ice-skating hall

69-80

- swimming hall

72-79

Schools- classroom, quiet

56-68- swimming area kids

81-87

- classroom, noisy

64-72- bowling

78-85

STI Report
The STI Reporting Tool creates measurement reports according to these standards:— AS 1670.4— CEN/TS 54-32:2015— DIN EN 50849:2017— IEC 60268-16— ISO 7240-19:2007— VDE V 0833-4-32:2016 (VDE 0828-1:2017-11)

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APPLICATION NOTESTIPA Summary Report
Report according to IEC 60268‐16{ed4}, chapter 7.6.4 and DIN VDE 0833‐4{2007‐09}, appendix F.6
Project Waiting Room
Comments Measured in empty room with TalkBox

All

Standard IEC 60268‐16 ed4.0 20111

Arithmetic mean lav
Standard deviation σ

STI
STI

lav ‐ σSTI

Position
STIPA File AltbFlur_STIPA_000 (Altb_Flur, 1)

0.58
0.06

0.52 G

STI

0.58

E

STI

0.57

E

STI

0.60

D

STI

0.54

F

STI

0.52

F

STI

0.52

F

Noise File XL2_V2x_RTA_APPEND_SLM_008 (MyLocation, 1)

2

Position
STIPA File AltbFlur_STIPA_000 (Altb_Flur, 2)
Noise File XL2_V2x_RTA_APPEND_SLM_008 (MyLocation, 2)

3

Position
STIPA File AltbFlur_STIPA_000 (Altb_Flur, 3)
Noise File XL2_V2x_RTA_APPEND_SLM_008 (MyLocation, 3)

4

Position
STIPA File AltbFlur_STIPA_000 (Altb_Flur, 4)Noise File XL2_V2x_RTA_APPEND_SLM_008 (MyLocation, 1)

5

Position
STIPA File AltbFlur_STIPA_000 (Altb_Flur, 5)Noise File XL2_V2x_RTA_APPEND_SLM_008 (MyLocation, 2)

6

Position
STIPA File AltbFlur_STIPA_000 (Altb_Flur, 6)
Noise File1 / 3 XL2 support website https://my.nti-audio.com
Project: Waiting Room
TheSTIPA
STIReport
Report is free to download on thefor all registered users (enable all macros when opening the document).

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APPLICATION NOTE

Measurement Positions and Number of Measurements
The ISO 7240-19:2007 standard specifies to measure the speech intelligibility at the number of measurement points equal or greater than listed in the following table:
Minimum number of measurement points

Area [m2]< 25

125-1003

100-500

6500-1500

10

1500-2500

15> 250015 per / 2500 m2— The distance between measurement points shall not exceed 12 m.
— The measurement points shall be equaly distributed throughout the area.
— No more than one-third of the points shall be located on the axis of a loudspeaker.
— The measurement microphone should be maintained approximately 1.6 m (5 ft) from the floor torepresent standing individuals and 1.2 m (4 ft) for seated individual as appropriate.

AUSTRALIA— One measurement is sufficient in case STI > 0.56.
— Take a minimum of two measurements if the reading is < 0.56 STI.
— Take a third measurement in case the two measurements differ by more than 0.03 STI.
— Take the two measurements that are closest in score and calculate the average to determine thefinal speech intelligibility at the individual measurement point.
— The measurements shall be made at a sufficient number of representative points in each area ofcoverage.
— Calculate the arithmetical average ‘lav’ of the intelligibility values on the STI and the standard
deviation s2.
— The relation between the arithmetical average and the standard deviation shall be: Iav – s2 > 0.50 STI.
If the result is within ±s2 of the limit, then the measurements should be repeated at a larger number of measurement points.
— The mean value of intelligibility and its 95% confidence interval, over the whole area of coverage
shall be calculated.

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APPLICATION NOTEGERMAN VDE0833-4 STANDARD
STI > 0.63

One single measurement is sufficient.

STI < 0.63

Execute three sequential measurements for this measurement position.
— If the maximum result deviation of these three measurements is > 0.03 thenfurther three measurements shall be executed.
— If the maximum result deviation of these measurements is > 0.05 then thecause of this instability shall be evaluated and removed.
— The arithmetic average of the executed three or six measurements has tobe reported.

An STI > 0.63 ensures that the speech intelligibility is higher than 0.5 with a confidence level of 95%.

Continuous Level-dependent Auditory Masking
The masking function emerged through the various editions of the IEC60268-16 standard as follows:
ed2.0

old edition released in 1998, with fixed masking function

ed3.0

old edition released in 2003 with stepped level dependent auditory masking function

ed4.0

old edition released in 2011, with continuous level dependent auditory masking function

ed5.0

actual edition released in 2020, with continuous level dependent auditory masking
function (identical to edition 4.0)

Figure 2: Comparison of STIPA Masking Curves

Best intelligibility is achieved at message levels in the range of 70..80 dBSPL. At higher sound pressure levels the ear triggers a self-protection mechanism, which is reflected in a reduced intelligibility
index. For example, an STI of 1 at 70 dBSPL may be down to 0.7 STI at a higher sound pressure level.

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APPLICATION NOTE

Measurement Hints
AMBIENT NOISE
The ambient noise has to be sufficiently constant during the measurement. A signal-to-noise ratio of
15 dB or higher is recommended to achieve best speech intelligibility. Impulsive ambient noise during the measurement, such as speech, causes severe measurement errors. The STIPA result is usually too high.

Fluctuating noise is detected by measuring the direct STI in the absence of the test signal. Carryout these measurements at least at a representative set of locations. If the STI is too high (e.g. STI> 0.2), the measurement results are likely to be erroneous. Then the STIPA measurement should be
carried out without this noise being present. Utilize the ambient noise correction for such instances.
At locations with varying conditions (e.g. public areas with few people and other areas with crowds)
the worst-case STIPA results should be measured. Consult the local regulations (e.g. the NFPA codein the U.S.) for directives concerning measurement locations and number of required STIPA measurements under which circumstances.

STI MEASUREMENT— The intelligibility index is measured in the range from 0 to 1, whereby 1 is perfect. The minimum
intelligibility requirement including measurement uncertainty is typically 0.5. In areas where the occupants are familiar with the alarm messages, a minimum intelligibility of 0.45 STI can be assigned.
— The variation of STIPA test results should be not more than 0.03 STI at one test position. In caseyou find the deviations higher than 0.03 STI, then verify and eliminate the causes for these discrepancies and repeat the measurement (e.g. measure STIPA during night time).

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APPLICATION NOTE

— Low STIPA readings can be caused by• Excessive sound reverberation, echoes or reflections• Poor speaker directivity or speaker coverage• Speaker level setting incorrect; e.g. low signal-to-noise ratio.

WHAT TO DO IF THERE IS MUCH IMPULSIVE NOISE
In a 24/7 production environment or near a highway, impulsive noise may be permanently present
and STIPA measurements should not be carried out. In such instances the onsite conditions have to
be simulated in a laboratory for STIPA testing:— The real noise spectrum should be measured e.g. with the XL2 Audio and Acoustic Analyzer SLM
function, averaging over a sufficient period of time.
— A diffuse sound field of non-impulsive noise with same frequency shape and octave band levels
as measured has to be generated in the laboratory.
— The real speaker-listener environment has to be acoustically reproduced as close as possible in
the laboratory.
— Then the actual STIPA measurement can be carried out; no post processing is required.
This approach is also mandatory for systems that include Automated Gain Control (AGC), if such
systems cannot be tested in the original environment without annoying members of the public.

MEASUREMENT CONFIGURATION
Simulate the emergency situation as close as possible:— Position the microphone at 1.0 to 1.2 meters above ground in sitting areas or 1.5 to 1.8 metersin standing areas (typical positions are not directly in front of the speakers).
— The person taking the measurements should be out of the acoustic field, so as not to affect themeasurement results. For this purpose the measurement microphone can be mounted on a microphone stand and connected with the ASD Cable to the XL2.
— An earlier draft of the VDE 0833 standard defines measurements to be carried out in a grid from6 x 6 meter up to 12 x 12 meter. For example, large areas like exhibition halls may use a 12 x 12
meter grid. The measurement points have to represent the area under test!

CD PLAYER
Only high-quality CD Players should be used to reproduce the STIPA test signal as only limited timeshifts (±20 ppm) ensure reliable STIPA test results. Pitch control and shock protection should bedisabled. We recommend that only professional CD Players be used. Verify the time shift of the CD
Player with a 1 kHz test signal:— Insert the NTi Audio Test CD into the CD Player and start track #1, which is the 1 kHz test signal.

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APPLICATION NOTE

— Connect the XL2 directly to the audio output and measure the signal frequency in RMS/THD+N
mode. The displayed frequency should be in the range from 0.99998 kHz to 1.00002 kHz.
STIPA test signals from other test system manufacturers may sound similar but are not compatible.
Only the NTi Audio STIPA test signal CD V1.1 or higher should be used in combination with the XL2Analyzer.

SPEECH INTELLIGIBILITY IN REVERBERANT SPACES
Send the announcements via directed speakers to the public area. Any non-directed sound returns
as reflections and reduces the speech intelligibility.

Who can and should conduct STI measurements?
Even though the STIPA method is complex, the measurement of STIPA using the XL2 Audio andAcoustic Analyzer is very simple. Operators with a basic acoustic knowledge can easily conduct
these measurements. The instrument’s internal storage ability also simplifies the measurements inlarger buildings, where many measurements must be taken at many locations. The detailed accessto the measured MTF (Modulation Transfer Function) matrix enables experts to post-process allmeasurement data.

Further Information:
For further information please visit www.nti-audio.com .
Detailed information on speech intelligibility measurements are contained in the IEC60268-16 (2020.
edition 5.0) standard, which also describes the test procedures and the requirements in practice.

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Appendix
SUBJECTIVE ANALYSIS METHODS
Although frequency response, reverberation, distortion, signal-to-noise ratio or loudness are related
to intelligibility, the conventional measurements of these parameters together only marginally relate
to intelligibility. When added issues, such as directionality of drivers and the environment conditions
are taken into consideration, the question is: How well a spoken message can be understood at
different locations?
The fundamental approach to measuring intelligibility is to let a trained human speaker read a number of existing or synthetic words, whereas a representative number of listeners individually write
down what they believe they understood. The statistical analysis of these notes results in a value
representing the percentage of words being understood correctly. Standardized procedures according to this method are PB-words, CVC or SRT (Speech Reception Threshold). However, conducting
such tests is rather time-consuming and costly, and, in some hazardous locations, even impossible.
Therefore, these methods are mainly used to verify alternate measurement methods.

TECHNICAL METHODSAs early as 1940, Bell Laboratories started to develop measurement technologies to determine theintelligibility of speech. Nowadays, highly-developed algorithms such as SII (Speech Intelligibility
Index) and various forms of the STI (Speech Transmission Index) objectively quantify speech intelligibility measurement.
These techniques consider many parameters which are important for intelligibility such as— Signal-to-noise ratio— Psychoacoustic masking effects— Sound pressure level— Ambient noise level— Reverberation time RT60
— Reflections— Frequency response— Distortion

Figure 3: Speech Intelligibility may be expressed by a single number value. Twoscales are most commonly used: STI (Speech Transmission Index) and CIS (Common
Intelligibility Scale)www.nti-audio.com

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The basic principle of STI measurement consists of emitting synthesized test signals instead of ahuman speaker’s voice. The speech intelligibility measurement acquires this signal and evaluates it
as it would be perceived by the listener’s ear. Extensive investigations have evolved the relationshipbetween the alteration of speech characteristics and the resulting speech intelligibility. These findings are incorporated into the speech intelligibility meter that is able to display the intelligibility result
as a single number between 0 (unintelligible) and 1 (excellent intelligibility).

Speech ModelFirst of all, measuring the speech intelligibility requires amodel for speech signals. For instance, speech may be described as how a frequency spectrum evolves over time.
Superimposition of spectra defines the long-term speechfrequency spectrum. The intensity of each frequency modulates over time.
Figure 4: Average octave band spectrum of amale speaker

Time Modulation
Level of frequency components varies, i.e. is “modulated”
by the speaker. Figure 3 shows the envelope of a speech
signal in the 250 Hz octave band. The shape of the envelope is given by averaging the time evolution of the speech
content.
Figure 5: Envelope of a speech signal (250 Hzband).

Frequency Spectrum
The spectral analysis of a male voice averaged over a longer
time results in a typical characteristic as shown in Figure 2.

Figure 6: Frequency spectrum of the envelope(250 Hz band).

www.nti-audio.com

Analyzing the spectra of time modulation intensity showsthat a speaker modulates the speech spectra with frequencies in the range from 0.1 Hz to 24 Hz. A set of modulation
frequencies from 0.63 Hz to 12.5 Hz sufficiently represents
these modulations.

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APPLICATION NOTEMODULATION TRANSFER FUNCTION (MTF)High speech intelligibility requires that the spectral intensity modulation and the overall spectrum bepreserved at the listeners’ ears. Therefore, the three core intelligibility measurement methods STI,RASTI and STIPA are based on measuring the MTFs (Modulation Transfer Functions) in 7 octave
bands. For each octave band, one MTF quantifies the degree of preservation of the intensity modulations in this band. These functions quantify how much the intensity modulations are preserved in 7
octave bands, thereby covering the long-term speech spectrum.

ReverbReflections
Modulated Octave Band Noise
ReverbModulation Index = 1

Reflections

Resulting signal
Modulation Index = 0.5

Figure 7: Reverberation,Modulation
ambient noiseand reflection are responsible for degrading
Transfer Function for one octave bandthe modulation index.

Resulting signal
ulated Octave Band Noise
Modulation
Index
0.5
Figure
8
shows
the
MTF
of
one
octaveband.
This
is derived
from= measuring
the 1/3rd octave moduModulation Index = 1
lation frequencies, thus resulting in 14 frequencies between 0.63 Hz and 12.5 Hz. Each modulationtransfer function determines how well the modulations are preserved in the associated octave band.
Modulation Transfer Function for one octave band

Figure 8: Modulation Transfer Function for one octave band.

Based on the MTF results as well as sound pressure level, the hearing threshold of the octave band,
frequency response and psycho-acoustic effects (masking effects), it becomes possible to reliably
determine the preservation of speech intelligibility from speaker to listener. The calculations are basedon extensive and profound evaluations and comparisons with subjective methods.
Measuring the complete MTF – as required for STI – is rather time consuming. 14 * 7 = 98 individual
measurements must be executed. Therefore, different approaches have been developed to reduce
test duration and to enable speech intelligibility measurements with portable instruments.
TNO in the Netherlands has verified the correlation of the STIPA method offered by the XL2 Analyzer
to the full STI to within ±0.03 STI.

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APPLICATION NOTESTI SPEECH TRANSMISSION INDEXThe STI result is based on the full set of 98 measurements. Since this approach requires a rather
long test period, it is less frequently applied in practice. However, STI represents the most detailed
method to measure the preservation of speech intelligibility during transmission and is mostly used
if alternative approaches don’t provide reliable results due to unfavorable environmental conditions.

Figure 9: STI considers all 14 modulation frequencies and all 7 octave bands resulting in 98 modulation index results.

In practice, the STI result is mostly calculated from the impulse response (MLSA) that has been acquired e.g. with a PC-based system. This approach is much quicker, but requires post-processing
with spectral frequency weighting and lot of experience. The measurement assumes a linear behavior of the setup, i.e. there must be no non-linear processing or conditions, including compressors or
limiters and close-to-zero wind speeds, which is a rather rare situation. Microphone and speakers
are not allowed to be moved during measurement. As handheld instruments aren’t fixed during measurement, it doesn’t make sense to support MLS testing in handheld instruments.

RASTI - ROOM ACOUSTICS SPEECH TRANSMISSION INDEX
Effective with the standard IEC60268-16, edition 4.0, 2011, is RASTI no longer a permitted method
for speech intelligibility measurements.
RASTI has been developed for special cases, such as a human lecturer speaking into a small room
without echos, but not for electro-acoustic systems. In order to reduce the test time required foreach STI measurement, a faster method called RASTI was developed. However, both the ability tocomprehensively test and the resistance to outside interference are compromised. This leads topoor correlation between subjectively evaluated STI and RASTI. For a long period RASTI was the
only method available for measuring the quality of speech transmission with a portable instrument,
and has been extensively utilized to measure public announcement systems in the aviation industry,
despite the compromises.
RASTI acquires only a few segments of a complete MTF table, and represents an extreme simplification of STI. Therefore, tight restrictions must be met to acquire reliable speech intelligibility resultswith RASTI. Furthermore, the RASTI result does not consider significant parameters such as the frequency response, echoes or frequency-dependent reverberation times. For a RASTI measurement,
only two simultaneously-generated frequency bands are considered, i.e. the 500 Hz and the 2 kHzband which are then modulated with four and five frequencies respectively.

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Figure 10: RASTI uses 9 different modulation frequencies in 2 octave bands. The yellow marked octave bands and modulation
frequency.

RASTI is normally used only to quantify the intelligibility index of the channel between two persons.

STIPA - SPEECH TRANSMISSION INDEX FOR PUBLIC ADDRESS
A rising awareness of security issues, new technological developments and the shortcomings of
RASTI triggered the speaker manufacturer Bose to develop a new method for speech intelligibility
measurements of PA installations. The result of these efforts is STIPA, which allows quick and accurate tests with portable instruments.
STIPA applies a simplified procedure to calculate the MTF. But STIPA determines one MTF by analyzing all seven frequency bands, whereby each band is modulated with two frequencies.
Supposing that no severe impulsive ambient noise is present and that no massive non-linear distortions occur, the STIPA method provides results as accurate as the full STI method. If however
impulsive ambient noise is present during the normal system operation hours, it is usually possible
to mitigate the effects by also acquiring a measurement at a more favorable time e.g. under slightly
different conditions in the area, or during the night time - and to calculate an unbiased overall measurement by using the results of both test cycles.
The old and already replaced revision IEC60268-16:2003 described a STIPA method where the125 Hz band and 250 Hz band are combined and the yellow marked modulation frequencies are notconsidered.

Figure 11: STIPA specified in old IEC60268-16:2003 standard (replaced by 2011 edition).

The handheld NTi Audio analyzers offer sufficient processing power for more precise speech intelligibility measurements than standardized. The NTi Audio STIPA method (verified by TNO) considers
all 7 octave bands and all 14 modulation frequencies resulting in more accurate speech intelligibility
results. This method has been standardized in IEC 60268-16, edition 4 (2011) and edition 5 (2020).

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Figure 12: NTi Audio STIPA Method (standardized in IEC 60268-16, edition 4 (2011) and edition 5 (2020).

STIPA Modulation Ration mr1, mr2
For good speech intelligibility it is mandatory that the integrity of the transmitted voice signal modulations are preserved. Therefore STIPA is based on measuring the MTF(Modulation Transfer Function). This function quantifies thedegree to which the voice modulations are preserved in individual octave bands. The STIPA method determines the
MTF by analyzing the seven frequency bands. Each band ismodulated with two frequencies, resulting in the modulation
ratio mr1 and mr2:
— mr1: modulation frequency 0.63 Hz to 2.5 Hz
Figure 13: Modulation Transfer Function Results presented by XL2 Analyzer— mr2: modulation frequency 3.15 Hz to 12.5 Hz

The mr levels should be as close as possible to the value of 1.00 for perfect intelligibility - as lower
the value is as less modulation has been transmitted.
All indexes together combined with psycho-acoustic models provide the single-value speech intelligibility result.

HOW DOES STIPA COMPARE TO STI?
STI measured in public address systems can be very time consuming. A complete set of 98 measurements of modulation transfer functions (MTF) has to be obtained and summed. Due to the complex nature and the time required, almost no really useful STI measurement systems were available
for years. With the appearance of MLS based systems, STI was more often obtained, as it can becalculated out of the transfer function, as long as the entire system is strictly linear and synchronous. Microphone and speaker are not allowed to move during the measurements, which prohibits
employment of handheld instruments. Thus it doesn’t make sense to support MLS measurements
in handheld instruments. Alternatively, by using the dedicated STIPA test signal, measurements canbe accomplished with handheld instruments.
STIPA, a derivative of FULL STI, has been developed specifically to cope with the non-linear processing environment common to advanced sound systems, and to reduce the measurement timewww.nti-audio.com

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required to a practical level. The results of the STIPA measurement method and the Full STI method
correlate by approx. 99% at common evacuation installations. Typically, with FULL STI, the maximum deviation is about 0.02 STI. With STIPA the maximum deviation is approximately 0.03 STI for
repeated measurements.

CALCULATION OF % ALCONS FROM STIPA MEASUREMENT
Alcons (%) = 10^((1-STI)/0.45)
The calculation of STIPA based on Alcon measurements is not reasonable due to the difference inthe measurement principles.

STIPA NOWSTIPA (Speech Transmission Index for Public Address systems) supports fast and accurate tests withportable instruments. Portable STIPA analyzers, e.g. NTi Audio’s XL2 Audio and Acoustic Analyzer,
are able to evaluate speech intelligibility within 15 seconds per room position and are thus well suited
for wide-area measurements and high productivity.

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