Am I making myself clear?

Voice announcements in public address systems must be understood or the result is just noise pollution and lack of intelligibility can have legal implications. Philipp Schwizer, CEO of NTi Audio argues the case for intelligible advertising, information and emergency announcements.

Intelligibility of messages is very important as most PA systems also serve as evacuation information systems as regulated by ISO 7240-16 and ISO 7240-19. These standards require that STI (Speech Transmission Index) is measured. The technical requirements of STI measurements are standardized in IEC 60268-16.

There are a few important driving parameters that either help to enhance or compromise good intelligibility. Signal to Noise (S/N) ratio is very important. The Sound Pressure Level (SPL) of the announcement should ideally be 10-15dB higher than the background noise. As background noise is always stochastic in nature it is important to measure the LAeq. The S/N in the worst-case scenario must also be considered. If the announcement signal has the same level as the background noise, your STI will be degraded to 0.5 by this fact alone, ignoring all other influences.

Getting to know the environmental response to reverberation (RT60) is also very important when trying to achieve good STI readings. High reverberation time is the enemy of intelligibility. If your room exhibits an average reverberation time of two seconds, the STI is limited to a maximum of 0.5, regardless how well you set all the other parameters.

For these reasons it is important that the installer/contractor is using appropriate test equipment throughout the installation process. Only with all components working optimally together can the required performance result be achieved.

How to get the most out of the system
1. Make sure that all the installed speakers have the same polarity, guaranteeing the highest SPL output for the given wattage.

2. Test the presence of every speaker. This can be achieved without any disruption to the public by using a 19kHz high-pass filter.

3. Verify that the number of speakers wired together matches the specifications of the driving distribution amplifier. For this you need an impedance meter. A decent meter, such as the MR-PRO, also provides you with a direct reading of the connected speaker power.

4. Verify the reverberation time of your environment and if necessary make the architect or construction company aware of necessary room treatments. Be aware that the reverberation time is frequency dependent and should be measured over the full voice band.

5. Verify the electrical systems distortion levels to identify damaged signal processors or amplifiers. For this you need a pure sine wave tone and a distortion analyzer such as the XL2.

6. Get an accurate picture of what your background noise level is. You will need to tune your PA system to run 10-15dB above this level if possible. For this you need an SPL meter with averaging functionality. Bear in mind that at an absolute level above 90dB the intelligibility starts to degrade again as our ears become less sensitive at such high levels.

7. If the PA System operator uses a microphone for evacuation announcements, STI-PA tests should be conducted with a calibrated audio source such as a TalkBox. A remote control facility to switch the test signal off and on while you move between measurement points may save your sanity, particularly in large environments.

8. After you have optimized all these points you must finally measure the speech intelligibility using a STI-PA meter and, if done right, you will pass the minimum requirement of 0.5 for evacuation systems at all measurement points.

Most of the above measurements can also be conducted using a PC-based measurement system. However, as the standard requires that you measure at various positions and particularly at worst–case positions, it is advisable to conduct these measurements with handheld tools.

ISO 7240: Defines functional and technical requirements and outlines the operation of sound systems for emergency purposes.

IEC 60268-16: The standard for measuring sound system equipment in general. Part 16 defines the objective rating of speech intelligibility by speech transmission index.

S/N ratio: Defines the ratio between the electrical or acoustic signal level with the signal switched on and the ambient noise level with no signal present. It is expressed in dB.

A-weighting: A filter characteristic that describes the hearing sensitivity of the human ear at lower levels. It shows the highest sensitivity in the range of 3 kHz and rolls off at high and at low levels.

LAeq: This is the averaged sound pressure level over a period of time with the A-weighting filter applied. The time period can be defined from seconds to hours and should be indicated with the measurement result.

RT60: A measure for the reverberation time, expressed in seconds. This is the time required for a reverberant sound field to decrease by 60dB after switching it off. The higher the RT60 value, the longer the sound components will bounce around in the room affecting other sound components.

Polarity: A test procedure ensuring that all loudspeakers are working synchronously. e.g., when applying a positive DC voltage, all speaker cones move outwards. For convenient polarity testing a saw-tooth-like signal is applied to the sound system and the near field of every speaker is tested with a polarity tester.

Impedance: The AC resistance of a speaker or a speaker system, expressed in ohms. With 100V lines, the connected power can be calculated with the formula U*U/R.

Distortion: Describes unwanted signal components (harmonics & noise) which appear mostly in frequencies other than the source frequency. Distortion is often expressed as Total Harmonic Distortion plus Noise (THD+N). It is the ratio of all unwanted signal components over the total signal energy.

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