Using iSTI in practice
Anybody who has previously worked with STI-capable measuring devices will probably find iSTI simple and intuitive to work with. If you are new to Speech Transmission Index measurements, there are a couple of things to keep in mind.
Getting started with your app
Users of iPhones and iPods are used to their apps "doing something" immediately when you press a button. However, working with iSTI takes at least a few minutes of preparation before you can get your first meaningful measurement.
First of all: you need to generate a test signal before you can do any kind of measurement. ALL measurements of the STI, whether with iSTI or any other device or piece of software, require use of a test signal. You need to generate this signal at the source (talker) position of the channel you wish to investigate
Test signal generator
The iSTI app features its own test signal generator; you can turn it on at any time using the button marked "signal." A so-called STIPA test signal will then be generated continuously, until you press the button again. The line-out signal is available through your dock connector and headset jack. If nothing is connected to the headset jack or dock connector, then the output signal will be played through the iOS device's internal speaker. This is not a good setup for doing measurements, but it will enable you to hear whether the generator is running while you are making your connections.
As soon as you hear the distinctive modulated noise of the STIPA test signal coming from the sound source of your choice, then you are ready for your first measurement.
Instead of using the test signal generated by your iPhone or iPod, you may choose to play back a STIPA test signal from CD or from a PC.
You iPhone app features a STI analyzer as well as a generator. The analyzer part of the iSTI app does exactly what you would think: it analyzes the test signal at the receiver (listener) end of the tested channel. You can either use the internal microphone of your iPhone, or an external microphone connected through the dock connector or headset jack. If you hear the test signal playing, simply press "Start" and wait for results to appear on your screen. Once the measurement finishes, press "Store" to confirm the measurement and save the result.
You may choose to generate and analyze using a single device, or run the iSTI app on two separate devices (one functioning as a generator, the other as an analyzer) - or play back a test signal from CD and use you iPhone only as an analyzer. A separate iSTI signal generator app may be downloaded for free.
For the most accurate results, you should calibrate iSTI professional. There are two parameters that may need calibration: the sample rate of your test signal and the sensitivity (sound level) of your microphone.
If you are using the same iPhone both as generator and analyzer, then you need not worry about sample rate calibration. However, if you are using a separate CD player or iPod, these may suffer from small inaccuracies of their internal clock frequency governing their sample rate. In other words, if you are analyzing signals from an external source, always carry out a sample rate calibration. Just follow the steps in the calibration menu.
You can also calibrate iSTI in terms of absolute A-weighted sound pressure level. The internal microphone of the iPhone varies slightly in terms of sensitivity; without calibration, you may over- or underestimate the sound level by several dB. If you own a Sound Pressure Level meter, or a sound source of know sound pressure level, you can calibrate your internal microphone as well as external microphones connected through the dock connector or headset jack.
Ensure stable measuring conditions
All STIPA measuring devices may report erroneous STI results if measuring conditions are unstable. Check if the following conditions are met before trusting your measurement results:
- Check if the STIPA test signal is actually being played back at an audible level, and received by the analyzer. If no signal is recorded at all, then the STI modulation depth measurements featured in the STI model will yield falsely optimistic results. Check if the sound levels you are measuring are realistic.
- Always monitor if the STIPA signal is being played back without interruptions, at a constant signal level
- Monitor that no sources of noise or speech are present, other than those you wish to include in your measurement (as interfering noises) on purpose.
- Discard any measurement during which impulsive noises were observed (slamming doors, pops and clicks of audio playback devices, etc.)
Measuring to professional standards
For professional applications (such as PA system certification) you should always follow the following guidelines:
- Use iSTI Professional or Standard. iSTI Lite lacks several features necessary for truly professsional measurements, such a calibration support, and does not comply with IEC-60268-16 3rd and 4th edition.
- Use an external, professional measurement microphone. A convenient option is to connect the line-out on a Sound Pressure Level meter to the line-in on your iPhone/iPod.
- Calibrate your external microphone; make sure that the A-weighted level is measured accurately by your iPhone/iPod. Levels on your SPL meter and iPhone should differ by at most 0.5 dB.
- If you are using an external test signal source (CD player, PC or separate iPod/iPhone): calibrate your sample rate
- Start each measurement session with a "back-to-back" measurement: connect your line-out to your line-in, generate test signal and analyse. Since your tested channel is near-perfect (just a short wire between input and output), your STI should be at least 0.98. This is only true if the signal level of your input line is properly matched to your output; the signal may require attenuation in order to measure an STI close to 1.
- If you are having problems getting near-perfect STI scores on back-to-back measurements, check the A-weighted sound pressure level that iSTI is reporting. The Speech Transmission Index justifiably penalizes high sound pressure levels, from the 3rd edition of the IEC standard onwards. Try measuring back-to-back with the "level depency" option in the settings menu set to "disabled." Do not forget to enable "level depency" before carrying out acoustics measurements. Alternatively, you can choose to attenuate the electrical signal so that iSTI reports A-weighted sound pressure levels of 75 dBA or lower.