IMPORTANT NOTICE. As of December 2016, iSTI is no longer supported. We decided to focus completely on our Bedrock product line. While we understand that this is a disappointment to many loyal iSTI users, we have come to the conclusion that through our hardware-based product line we are capable of providing a professional, reliable and certifiable solution that simply cannot be achieved with mobile apps - at least not efficiently and cost-effectively. We apologize for any inconvenience.

STIPA measurements on iPhone OS: iSTI

Welcome to the information and support pages for iSTI! Getting started with the STI method was never easier. Simply download the iSTI app from the App Store, install it on your iPhone or iPod Touch and you are ready to start measuring.

iSTI comes in three different editions: Lite, Standard and Professional. Of all STIPA measuring products on the market, iSTI Professional is the most full-featured and versatile solution, turning your iPhone, iPod or iPad into a measuring device that complies with the latest standards. If your application is less demanding, you may find that iSTI Standard or iSTI Lite also fits the bill. The table below highlights the differences between editions.

** This is an indication of the app store price in USD. This is subject to change without notice.
Prices in other currencies are subject to iTunes general conditions and may vary. Check your local iTunes app store.

To see how iSTI Professional appears in the App Store, follow this link. You can download the app through iTunes, or by accessing the App Store directly on your device (search for "Embedded Acoustics").

What is iSTI, exactly?

iSTI is an application that measures the so-called Speech Transmission Index, or STI. The STI is used for predicting speech intelligibility. This is done by means of simple acoustic measurements. An artificial test signal is played back at a source position; the signal is recorded at a listener position. The STI method then produces an index between 0 and 1, that quantifies the speech transmission quality of the transmission path. The higher the STI value, the better speech intelligibility will be.

Theoretically, speech intelligiblity can only be measured using panels of human subjects. This is, of course, very time-consuming, difficult and expensive. In the 1970s, Tammo Houtgast and Herman Steeneken, both scientists at the Dutch research institute TNO, invented a physical measuring method that accurately predicts speech intelligibility: the STI. The STI has been gradually evolving since. Today, it is the most widely used and accepted method for assessing speech intelligibility. There are 10,000 engineers, scientists and consultants who use the STI regularly. STI measurements are required by many regulations and standards.

Professional and fully compliant to the IEC standard

iSTI Professional and iSTI Standard are fully compliant, professional tools for measuring the Speech Transmission Index according to IEC-60268-16 3rd edition (2003) and 4th edition (2011).  iSTI lites complies with IEC-60268-16 2nd edition (1998).

Provided that you use suitable cables, adapters and (external) microphones, iSTI is just as accurate and reliable as any STI measuring device on the market, at much lower cost.  In fact, every line of code in iSTI Professional was written by members of the team that have been developing the STI core technology at TNO for the last 15 years. iSTI was thoroughly validated both on the source code level and through the tests - using the same test bed and test conditions applied for the original design of the STI method.

The internal microphone of the iPhone is of a sufficiently high quality to be used for STI measurements for casual screening purposes. These can be carried out with no other hardware than just an iPhone. If the highest level of accuracy is required, such as during certification procedures and formal evaluations, then a calibrated external microphone is required. Many professionals will already own a Sound Pressure Level meter. Simple connect the line-out of the SPL meter to the line input of the iPhone, calibrate your microphone, and you are ready to go. Various standard cables are for sale for connecting external devices to your iPhone dock connector or headset jack connector.


Since iSTI imposes a fairly heavy load on your iOS device's processor, we recommend against running it on older models (iPhone 3G or older, and iPod Touch 1st and 2nd generation) The app also runs on iPad and iPad 2,  although no additional features are currently offered that take advantage of iPad's capabilities. You may be able to run iSTI on older devices, but this is not guaranteed. In order to free up processor capacity for iSTI, we recommended that you stop all non-essential apps from running in the background when measuring with iSTI. When running iSTI Professional on your iPhone, we also recommend that you put the device in airplane mode. This slightly increases the quality of the audio signal and prevents unintended interruption of your measurements by phone calls.

What else do I need to know?

You should be able to find everything you need to know on this website. You can navigate through the information using the menu on the left. Should you run into problems we have not foreseen, please contact us by email: At times things do get pretty busy around here, but our staff is committed to helping you out as quickly as we can.


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