by Pat Brown
In this article you will learn how to use a talkbox as a smartphone calibrator.
A “talkbox” is a self-contained powered, processed, loudspeaker/signal generator. Designed to aid in speech intelligibility testing, the response extends from 100 Hz – 10 kHz, ± 1 dB. The acoustic output level is 60 dBA @ 1 m, which is designed to emulate a human talker. I purchased a my first talkbox several years ago, and have found many applications for it beyond speech intelligibility testing. I’ll be sharing some of these in a series of articles.
The topic of instrument calibration comes up pretty often these days. As the public becomes more concerned about sound level exposures, the need for accurate SPL measurements is on the rise.
Anyone can have a sound level meter on their smartphone. The problem is that without a calibration reference the reported levels often disagree and cannot be trusted. Microphone calibrators are of no use, because they are designed for 1/2-inch or 1/4-inch microphones, not smartphones. A “talkbox” loudspeaker provides a solution to this (and other) calibration problems.
From the NTI website:
The TalkBox is an acoustic signal generator for speech intelligibility measurements in evacuation and announcement systems, as well as for level alignment of teleconference or other audio systems. It allows performing the complete end-to-end evaluation of the speech intelligibility STIPA from the talker’s microphone to the listener’s ears.
The TalkBox simulates a human talker (60 dBA @ 1 meter according to IEC 60268-16) with the STIPA test signal and spoken messages. Further it provides a wide range of test signals like sine, pink noise, white noise and delay.
The TalkBox features human head-like dimensions and is based on a solid-state-generator. It replays the STIPA test signal at the precisely equalized frequency response, also ensuring best performance through the internal amplifier and the precision loudspeaker. A variety of supplied or user-defined test signals may be precisely output for different alignment applications.
While mainly designed for speech intelligibility testing, a talkbox makes an excellent calibrator. My Bedrock BTB65 has a flat grill. The SPL at the grill is 94 dB-SPL for the STIPA signal, pink noise, male speech, female speech, and sine wave tone. Photo 2 shows the SPL at the grill, measured with the NTI XL2 meter with Type 1 microphone for the 1 kHz sine wave stimulus. One kilohertz is used as the calibration frequency because all sound level meter weighting scales are normalized to 1 kHz and should therefore not affect the measurement.
Photo 2 – The SPL at the grill of the BTB65
With this information, the talkbox can be used to calibrate microphones. The grill position is convenient for establishing the distance, but don’t assume that it is 94 dB-SPL. Measure it with a calibrated sound level meter, and then use that level when calibrating smartphones and other devices. The fact that it is 94 dB for the BTB65 (Photo 2) is probably more coincidental than by design, but very handy. The same measurement performed on the NTI TalkBox returns 90 dB-SPL, so one would have to enter this level into the proper field of the calibration utility (Figure 1).
Figure 1 – The calibration routine from FIRCapture. The field where the calibration source SPL is highlighted.
So, you can put a microphone up against the grill to feed it a reference level. A smartphone can be placed against the grill for calibration (Photo 3). Your sound level meter app or PC-based measurement program should have a calibration routine (Figure 1). Just enter the reference level and hit “calibrate” and it should lock to it and report the sensitivity in mv/Pa.
Photo 3 – An iPod Touch™ running a sound level meter app, placed against the grill and calibrated to 94 dB-SPL
It is extremely useful to have a readily available absolute acoustic reference. I find the talkbox far more useful than a mic calibrator, and a practical replacement for non-critical applications. There are plenty of other uses for these handy devices which I will cover in a series of articles. pb