Frequently Asked Questions
Speech Transmission Index Measurements
Q: What equipment do I need to perform Speech Transmission Index measurements?
A: First of all, you need a measuring device or analyser, such as the Bedrock SM50. This is the instrument that computes the STI. In addition, you need a source of the STI test signal. The test signal is often supplied as a digital audio file or CD; the STIPA test signal used with the SM50 is supplied on a flashdrive. This test signal needs to be played back and presented to the input of a channel under test (e.g. by presenting it to the line input of a public address system). The STI characterizes how the tested channel (everything between the test signal source and the STI analyzer) influences intelligibility.
Q: Do I need a Talkbox as well?
A: That depends on your application. If your test channel begins with a microphone, then you need a calibrated source to play back the STIPA signal to the microphone (a calibrated artificial mouth or a talkbox). The spectrum of the STIPA signal need to match the spectrum and level defined in the standards. This can also be achieved without a talkbox, using a loudspeaker and an equalizer.
Q: What is the difference between STI and STIPA?
A: STIPA is an implementation of the Speech Transmission Index (STI) originally developed for Public Address (PA) systems. So STIPA is a specific method to measure the STI; the results are called STI values, not STIPA values.
Q: What is the difference between "Full STI" and STIPA?
A: A Full STI measurement samples all 14 modulation frequencies from which the STI is calculated in every single octave band. STIPA is based on only 2 modulation frequencies per octave band, but these modulation frequencies are different between bands, so that the entire modulation spectrum is covered across all octave bands. Full STI measurements have become extremely rare, since STIPA is equally accurate for nearly all applications (and much quicker to measure).
Q: Are STIPA signals compatible between vendors?
A: Yes, as long as the test signal and the analyser both comply with the 4th edition of IEC-60268-16. This excludes some older STI measuring equipment (check for the edition/revision number of the IEC standard). The Bedrock SM50 complies with all the latest standards.
The Bedrock SM50
Q: Can I purchase additional measurement methods or modules for the SM50?
A: Embedded Acoustics keeps developing and releasing new modules for the SM50. There is no need to buy these; they are available free from charge. Simply update the SM50 firmware to the latest version to obtain the most complete package of modules currently available.
Q: I need to have module X or functionality Y to be implemented on the SM50. When can I get it?
Q: Is there a version with extended battery capacity? Or is there a way to swap out batteries myself?
A: The SM50 is fitted with a battery package with a capacity of 10.5 Wh. That is already the largest capacity that is manufactured for this type of battery. If you need to measure for a long time, especially with the display turned on all the time, the capacity of the internal battery may fall short. In that case we recommend using an external USB battery pack or "power bank."
Q: How often does the SM50 needs to be calibrated and/or serviced?
A: The SM50 needs to be calibrated at least once every 2 years, but many users prefer to have the device calibrated annually - same as most sound level meters. Contact us for service and calibration, or for information on a qualified calibration service provider that is nearest to you.
Q: Can't I calibrate the device myself?
A: You can (and probably should) check and adjust for the sensitivity microphone using the "calibration" menu of the SM50, together with a calibration tone from an external calibrator. Difference in temperature and air pressure, and a certain amount of "drift" in sensitivity over time, can lead to reduced accuracy of sound pressure level measurements. Tone calibrators are available from various vendors, in different price ranges. Make sure that the calibrator fits the 1/4" microphone of the SM50. In addition to checking and adjusting the device yourself with a tone calibrator, you should still send the device back to the factory or a qualified service partner for a more thorough checkup once every one to two years.
Q: Is there a way to update the firmware of the talkbox?
A: Not outside of our factory. The talkbox needs to recalibrated after every firmware update.
Q: Can I upload my own signals to the talkbox?
A: Unfortunately, no. You can play your own signals through the talkbox by means of the line input jack. But keep in mind that the frequency transfer function and the sensitivity of the talkbox will not be calibrated when used this way.