STIPA Pro User Manual

STIPA Operation Guide

This is the help guide for STIPA, the speech intelligibility measurement tool. STIPA works by analyzing how much of the amplitude oscillations embedded in a special audio test signal can be extracted, to determine how easy it will be to understand someone speaking in the room, and by extension how well a sound system will be understood.

STIPA has been adopted by many local and national fire codes and other public standards as the tool of choice for measuring speech intelligibility.

Our implementation of STIPA is based on v4 of the standard and complies with all sections of the standard.

STIPA Pro and STIPA Basic

This guide applies to both STIPA Pro and STIPA Basic. The same calculation algorithms are used for both modules. STIPA Pro includes the option to turn on an off level-dependent auditory masking, STIPA Basic does not.

STIPA Basic does not include Save / Recall, Save to Photo Roll, Test Averaging, LEQ Display, and the Detail View screen. It is suitable for field STIPA measurements, and uses the same test signals as STIPA Pro.

Basic Operation


STIPA Compliance Study


Basic Operation

This section describes how to use STIPA to measure speech intelligibility.


The STIPA signal will be played in the room, and STIPA module will sample the signal using the built-in iOS device microphone, or an external mic. Results will be shown as the test is running, and at the end of the test.

You can play the STIPA signal from the App itself, or you can download it from our website and burn it to a CD. Do not alter the signal in any way and avoid compressing it (keep it as a WAV file, do not change to AAC or MP3 in iTunes). Also, if the sample rate of the playback device varies from the sample rate that the iOS device is running at by more than a few percent, it will skew the results.

Running a STIPA Test

Once you have the signal playback set up, you are ready to run a test. Note which of the 6 test result slots is selected (the highlighted segment), as this is the test number that the results will be stored in. (See Averaging, below).

Start the signal playing, and tap the RUN button. The test signal analysis will begin, and you will see a progress bar begin to move across the screen. While the test is running, the only other sound should be normal room background noise.  At the end of the test (20 seconds), the test will automatically stop. You can stop the test before 20 seconds have completed, and the results of the tests up to that point will be shown, but they may not be as precise or accurate as a full 20 second test.

At the end of the test you can see the STIPA overall value (the large number on the screen), or you can inspect the individual m1 and m2 values by tapping the Details button.

While the test is running, the SPL levels for each of the octave bands, and the overall SPL level, are shown on the screen. At the completion of the test, the Leq (average SPL) for the entire test are shown, and stored with the test results, so that they can be recalled at a later time.

Averaging - Pro Feature

The STIPA standard suggests averaging six 15-second tests, and discarding or repeating any tests that vary more than 0.02 from the mean. To provide for this, we have set up our App with 6 test slots, and an average button. Select any of the 6 test slots and run a test. When complete, the test slot number will be green. This only indicates that a test has been completed for this test slot number, not the test pass/fail status. Unused test slots will appear in red. The number of completed tests will be shown below the test slot selector.

You can select test slots at any time by tapping the number of the test. To view the average of all of the completed tests, tap the ҁvgӦ#10;button.

Adding Occupational Noise to a Test Run – Pro Feature

There may be situations where you wish to run a test during time that is not representative of the normal use of the space. For example, it may be more convenient to run the test at night, when the space is not occupied, but then you would be missing the inclusion of the normal occupational noise of the space during the day. Note that this feature requires running STIPA as a part of the AudioTools app. Contact support of you do not have this, or just download AudioTools from the app store.

In this case you can save recording of the noise during the day and add it to the test that you are running at night. To do this, follow these steps:

-Record the Noise Navigate to the Recorder module, on the Utilities menu. Go to the settings page, and select Mono, 48kHz sample rate, and WAV compression (none). Turn off Record Monitor to prevent feedback. On the recording page, make sure the gain slider is set to 0dB. Now record a sample of the occupational noise in the space. You should make this recording longer than a typical STIPA test, but if it is shorter the recording will be looped during the test run. Save this recording using the Save/Recall page, and make note of the name.

- Select the Recorded Noise When you are ready to do the test, from the STIPA module, select the settings page, and tap the button under the Added Occupancy Noise label. This may say “Select Stored Audio File”, or it may be the name of a previously selected file. Pick the desired noise file from the list and tap recall. If the file sample rate is not correct you will see an error message. Tap Done to return to the main screen.

- Run the Test Run the test as usual. The recorded noise file will be silently mixed in with the live incoming audio prior to the STIPA calculations. You will see “Using External Noise File” on the screen beneath the test selector.

- Clearing the Noise File If you no longer wish to use the recorded noise, you can either go to the settings page and tap “Clear Noise File” or simply exit back to the menus and re-open STIPA.


There are several items shown on the screen for each test.

Overall STIPA value This value is shown in large digits on the screen. Larger numbers are better, i.e. 1.00 is “perfect”.

Tests Status Completed tests are shown in green on the test selector bar, and unused test slots are shown in red. Tap Avg to see the average of all completed tests.

Test Qualification Band One of the letter slots will be highlighted, indicating the relative speech intelligibility level. The highlight is also color-coded, with green = good or excellent, yellow = fair, and red = poor.

SPL Leq This graph shows the Leq (average SPL) of each octave band of the test. Use it as a guide to see the signal SPL level for each octave band of the test. The yellow band on the right indicates the current SPL level at all times and is not related to the completed test results.  - Pro Feature

Sample Rate Variations

If you play the test signal from your iOS device, the sample rate of the signal will match what the STIPA module is expecting. However, if you play the test signal from an external source, it is possible that the sample rate of that external device will not be exactly 44100Hz, and you may not get reliable STIPA results. To test the sample rate of the external player, find or create a sine wave signal that is at a known frequency, preferably above 4000 Hz. Play that signal while you are running the FFT module in AudioTools, and turn on Peak Tracking. Set the FFT Size to 16384 or greater.

Note the frequency display for the test signal. It needs to be within 0.05%, or 2 Hz in 4000 Hz., or 4 Hz in 8000 Hz.

Save / Recall - Pro Feature

When you save a STIPA test, all completed tests (up to 6) will be stored under the test name, and will be recalled anytime that you recall the test. Also, results from all completed tests will be included in the tab-delimited XLS file that is created when you save results. Use the normal Export function on the Save / Recall screen to export this data to a computer.

Examples of STI qualification bands and typical applications


Nominal STI Value

Type of message

Examples of typical uses




Recording studios

Excellent intelligibility but rarely achievable in most environments



Complex messages, unfamiliar words

Theatres, speech auditoria, teleconferencing, parliaments, courts

High speech intelligibility



Complex messages, unfamiliar words

Theatres, speech auditoria, teleconferencing, parliaments, courts

High speech intelligibility



Complex messages, unfamiliar words

Theatres, speech auditoria, teleconferencing, parliaments, courts

High speech intelligibility



Complex messages, familiar words

Lecture theatres, classrooms, concert halls

Good speech intelligibility



Complex messages, familiar context

Concert halls, modern churches

High quality PA systems



Complex messages, familiar context

PA systems in shopping malls, public buildings offices, VA systems, cathedrals

Good quality PA systems



Complex messages, familiar context

Shopping malls, public buildings offices, VA systems

Target value for VA systems



Simple messages, familiar words

VA and PA systems in difficult acoustic environments

Normal lower limit for VA systems



Simple messages, familiar context

VA and PA systems in very difficult spaces



Not suitable for PA systems



Not suitable for PA systems

NOTE 1 These values should be regarded as minimum target values.
NOTE 2 Perceived intelligibility relating to each category will also depend on the frequency response at each listening position.
NOTE 3 The STI values refer to measured values in sample listening positions or as required by specific application standards.


STIPA v4 Compliance Study

1. Abstract

This sections describes the method for processing a STIPA v4 signal through the STIPA module of AudioTools, which maintains compliance with the 4th edition of the STI standard (IEC 60268-16:2011).

The purpose of this section of the document is to show that once a STIPA signal is captured in AudioTools and processed in the STIPA application, the determination of the STI follows precisely with the guidelines set forth in IEC 60268-16.

Since it is important for the method by which STI is determined to be accurate, careful examination of the signal path and processing of the test signal is provided in order to determine how the STIPA application of AudioTools complies with the standard.

This section is presented in the form of logical signal flow, where each step of the determination of STI is documented. The processing in AudioTools is directly compared to the specific signal-processing step in the v4 STI standard.

2. Introduction to the Speech Transmission Index (STI)

The determination of the Speech Transmission Index (STI) was developed in order to quantify the reduction of the modulation envelope of speech by means of calculating the modulation transfer function (MTF) (Steeneken and Houtgast, 1980). The STI measure has been defined in IEC 60268-16:2011, and provides an efficient method of evaluating speech intelligibility in both unamplified direct conversations as well as when the speaker is amplified through a public address (PA) system. This is due to the fact that the STI takes into account distortions due to noise, reverberation and echoes, if present.
The calculation of the STI through the direct measurement, the technique that AudioTools takes for calculating STI, involves replacing actual speech with an analytical signal, which contains the spectral and temporal characteristics of speech. By injecting the test signal at some source location Si into a device-under-test (DUT, in most cases, an acoustical enclosure or room), and recording the resultant signal at an arbitrary receiver location Rj, the resultant modulation reduction factor, mr can be determined. The MTF is therefore given by:

where the MTF is determined at a given modulation frequency F by the ratio of the modulation of the response signal mr and the modulation depth of the probe signal mt. The calculation of the MTFs are determined for speech frequencies; octave-bands from 125-8000 Hz, fi, and for one-third octave band modulation frequencies, Fj, between 0.63-16 Hz. More details about the test signal are given in the next sections.

3. Test measurement setup

The following equipment is used for this validation study:

  • Apple iPadTM 2 running iOS 5.0.1
  • Studio Six Digital iAudioInterface2.
  • Sencore Type 1 Omnidirectional Microphone.
  • Genelec 8030a powered loudspeaker.

4. STIPA test signal spectrum and modulation depth

Studio Six Digital provides a STIPA v4-compliant signal. The signal is also available to be played back from within the STIPA module of AudioTools.

Looking at the way that the signal is generated shows its compliance with the STIPA v4 standard. The generator that creates the test signal is designed to the specifications of the standard by being comprised of the following:

  • Seven bands of noise, one-half octave wide centered on the following frequencies: 125, 250, 500, 1000, 2000, 4000, and 8000 Hz. The noise is pink in shape within a band.
  • Noises are generated by adding together closely spaced sine waves with a step size of 1/length of the sample. In the case of the sample that is designed to be looped continuously, that length is 40 seconds. Therefore, the frequency step size is 1/40 Hz per sine wave.
  • The rolloff of the noise bands outside the passband are extremely steep by virtue of the signal construction.
  • The seven bands are then individually intensity modulated with the frequencies shown in Table 1. These frequencies correspond to those presented in Table B.1 in IEC 60268-16:2011.
  • The depth of the intensity modulations are 0.55 for each of the frequencies and a 180 degree phase shift is applied between component one and two as described in Annex B of IEC 60268-16:2011.
  • The STIPA signal of AudioTools has a male speech spectrum, which fulfills the requirement of the Standard presented in Table A.4 of IEC 60268-16:2011.

The signal used for the STIPA module in AudioTools fully complies with the signal guidelines set forth in IEC 60268-16:2011 with respect to frequency spectrum and intensity modulation frequency and depth.

Table 1: Modulation frequencies used in the STIPA test signal

5. STIPA signal acquisition time

The recommend signal acquisition time for determining STI from the STIPA signal is between 15-20 seconds. The duration of the signal acquisition time in the STIPA module of AudioTools is 20 seconds, which complies with this measurement window.

6. Integrity of the test signal

A measurement of the sample rate integrity while using the measurement system was carried out. This is to determine if the sampling rate error falls in the window for error. The maximum allowable error rate is 0.05%. An 8000 Hz sine wave generated in the FFT module of AudioTools was looped back through the analog input of the iAudioInterface2. The signal was measured with the FFT module using a 32k point FFT. The peak of the sine wave signal was measured at 8000 Hz, which is within the 0.05% frequency discrepancy window.

7. Determination of the STI using AudioTools STIPA

The following section describes the decomposition and analysis of the STIPA signal in the STIPA module of AudioTools for the determination of the STI. This section deals with the signal after a portion of the signal has been recorded into the input buffer of the software module. The input starts as a full-range STIPA signal, which has been passed trough the system under test, and has been degraded by noise, distortion (if present) and reverberation/reflections. A block diagram of the signal flow in AudioTools STIPA is shown in Figure 1. The probe signal is the reference unprocessed STIPA signal and the response is the STIPA signal measured in the environment in which STI is to be determined.

The STIPA signal is first bandpass filtered into seven bands from 125 Hz to 8 kHz. The signal is then rectified by squaring to look at the intensity envelope within a band. In order to extract just the low-frequency intensity modulations, the signal is then decimated by 150x. The native sampling rate of the STIPA module of AudioTools is 22050 Hz. The signal is first decimated by 15x, and then 10x. The sample rate after decimation is 147 Hz.  The transmission index (TI) is then determined by comparing the intensity envelope of the probe to the response.  This entire process is repeated for each of the seven octave bands of noise and then the STI is determined by taking a weighted sum of each of the bandճ TIs.

FIGURE 1: Block diagram for STI calculation.

a.    Filter STIPA into Octave Bands

The first stage of signal processing involves filtering the STIPA signal into seven octave bands. This is accomplished with 8th order IIR ANSI Type 0 filters centered at 125, 250, 500, 1000, 2000, 4000 and 8000 Hz.

b.    Signal Rectification

In order to extract the intensity envelope of the test signal, it must be rectified. This is accomplished by squaring the input signal. The rectification comes directly after band pass filtering.

c.    Low-pass filter through decimation

The signal must be low-pass filtered next to extract only the low frequency modulations that are analyzed in the calculation of the STI. This is accomplished through a two-stage decimation filter. The audio coming into the STIPA module of AudioTools is sampled at 22050 Hz. The decimation filters are 15x and 10x 8th order IIR ANSI Type 0 filters. They are applied to each of the bands of filtered and rectified STIPA signal. They are applied in series. After both decimation filters, the new sampling rate becomes 147 Hz for the remainder of the STI calculation.

d.    Determination of the Modulation Reduction Factor (MRF)

After the band-passed STIPA signal has been decimated, the next step is to determine the ratio between the response and probe signals of the intensity modulation at each of the two frequencies in Table 1 for each octave band. The ratio of each is the modulation transfer function (MTF). The MTF is then converted to a signal to noise ratio (SNR) by dividing the MTF by 1-MTF. That quantity is multiplied by 10log10. Per the STIPA standard, the SNR is restricted to a range of +15 dB SNR and -15 dB SNR. If the SNR is above 15 dB SNR, the value is transformed to 15 dB, and if the value is below -15 dB SNR, the value is transformed to -15 dB SNR. Then the SNR is shifted by 15 dB and divided by 30 in order to generate the TI, which is a value between 0 and 1.  This is calculated for each of the modulation frequencies in each of the seven octave bands. The TI for each octave band is then averaged across the two modulation frequencies in the band.

e.    Correction of MRF based on Level Dependent Auditory Masking

An option in the STIPA module of AudioTools that is turned on by default, in order to comply the with STI v4 standard, is the correction of the TI for each octave band based on the sound pressure level (Leq) of the octave band directly below the band being analyzed. The STI standard uses this to account for level dependent auditory masking. AudioTools STIPA calculates the Leq for each octave band after filtering during the recording process. The correction takes places for the 250 Hz Р8000 Hz octave bands. AudioTools STIPA calculates the auditory masking factor for each of the octave bands using the formulas in section A.3.2 of IEC 60268-16:2011 and multiples the calculated TI by this corrected factor.

f.    Calculating the mean modulation transfer function

The final determination of the STI is completed by taking a weighted average of each octave bandճ adjusted TI. In order to comply with the v4 STIPA standard, the  frequency-weighting factors: αk, and redundancy-correction factors: βk are employed from Table A.3 in IEC 60268-16. These values are reproduced in Table 2 in this document. The male weighting factors are used, since the STIPA signal has a male spectrum. The weighted sum is determined and the resulting display of the STI is displayed on the STIPA output screen in AudioTools. In addition to the STI number, a nominal qualification band is given, which has been derived from the Annex F of IEC 60268-16 standard. These nominal grades are from U to A+.

TABLE 2:  Frequency-weighting factors: αk and redundancy-correction factors: βk. Factors assume male speech and the sum of αk Рsum of βk = 1.

8. Conclusion

The STIPA module of AudioTools has been designed following the careful guidance of IEC 60268-16, v4. The standard provides step-by-step instruction for both the generation of a v4-complient STIPA signal as well as the decomposition of that signal in order to extract the necessary information to determine the STI. The STIPA module of AudioTools performs each of the necessary steps to the recorded STIPA signal that have been described in the current STI standard. In addition, the quality of the filters in AudioTools are designed to exceed ANSI Type 0 standards.


IEC 60268-16, ғound system equipmentАart 16: Objective rating of speech intelligibility by speech transmission index,ӠIEC, Switzerland (2011).

Steeneken, H. J. M. and Houtgast, T., ҁ physical method for measuring speech transmission qualityӬ J Acoust Soc Am 67, 1980, 318-326.