Measuring the Range of a Megaphone
Some time ago the Australian Navy asked us to build a sound projection system for ship-to-ship communications. The acceptance criteria included the requirement that certain spoken commands could be heard with >90% accuracy when the speaker and the listener were separated by 1 km of open water.
This month we released our new “Teraphone” for civilian use. For safety reasons the output power is less than that of the Navy version. We’ve also made it portable by reducing the weight and, of course, it costs less than the Navy version because we no longer have to use Mil-Spec components and materials.
Importantly, though, the Teraphone is still all about the long-distance transmission of speech and not just broadcasting simple sounds such as sirens and whistles. We all know how frustrating it is to hear something over a megaphone or public address system that is definitely “loud” but is so blurred & muffled that you really have no idea about what is actually being said.
The “loudness” or relative intensity of sound is conventionally measured in decibels (dB) and there are plenty of instruments capable of measuring that. But how do you know how well spoken messages have been understood? And over what distance is the same level of understanding maintained?
Believe it or not, there is actually an international standard for this kind of thing. It’s IEC 60268-16 “Sound system equipment – Part 16: Objective rating of speech intelligibility by speech transmission index”. Edition 4.0 was issued in 2011 (http://infostore.saiglobal.com/store/details.aspx?ProductID=1477162).
STI is a numeric representation measure of communication channel characteristics whose value varies from 0 = bad to 1 = excellent. On this scale, an STI of at least 0.5 is desirable for most applications.
The science behind this standard for Speech Transmission Index (STI) was originally developed by Tammo Houtgast and Herman Steeneken and their colleagues at the Dutch TNO. There is a good summary on Wikipedia at https://en.wikipedia.org/wiki/Speech_transmission_index and you can find more information in a paper by Steeneken that can be downloaded at http://www.gold-line.com/pdf/articles/p_measure_TNO.pdf
While I don’t think it has yet been adopted as a full international standard, an extension to Speech Transmission Index for Public Address systems (STIPA) is well advanced and becoming widely used.
STIPA does not use actual words but instead uses a standard (and therefore reproducible) test signal which does not sound like speech but which, in terms of frequency content as well as intensity fluctuations, is a signal with speech like characteristics. Quoting the Wikipedia article: “Speech can be described as noise that is modulated by low-frequency signals. The STIPA signal contains such modulations at 14 different modulation frequencies spread across 7 octave bands. At the receiving end of the communication system the depth of modulation of the received signal is measured and compared with that of the test signal in each of a number of frequency bands. Reductions in the modulation depth are associated with loss of (speech) intelligibility.”
The standard STIPA test signal can be downloaded from a number of sources and an App designed by the Dutch TNO team can be purchased for use on an iPhone or iPad.
We are going to use this STIPA signal and App to determine the performance of the “Teraphone”. Our simple set-up will not allow us to claim conformity with IEC 60268-16, but it will certainly sufficient for screening and comparative purposes.