In Electromagnetic Articulometry (EMA or sometimes EMMA), an electromagnetic field is used to track movements of small receiver coils glued on the speech articulators. Three (in two-dimensional EMA) or six (in three-dimensional EMA) transmitter coils are fixed on a cubic plastic apparatus surrounding the speaker's head. The transmitter coils produce alternating magnetic fields at different frequencies in the range of about 10 kHz. The fields from the transmitter coils pass through the receiver coils and generate an electric signal. The voltage of this signal is related to the distance and orientation of the receiver relative to the transmitter coils. This relationship is used to calculate the position of the receivers as a function of time. The voltages in the receiver coils are captured at a sampling rate of 200 Hz. Audio data are also collected in parallel with the articulatory data.
Sources on EMA
Perkell, J. S., Cohen, M. H., Svirsky, M. A., Matthies, M. L., Garabieta, I., & M. T. Jackson. 1992. Electromagnetic midsagittal articulometer systems for transducing speech articulatory movements. Journal of the Acoustical Society of America 92, 3078-3096.
Hoole, P., Zierdt, A. & C. Geng. 2003. Beyond 2D in articulatory data acquisition and analysis. In M. Solé, D. Recasens & J. Romero (eds.), Proceedings of the 15th International Congress of Phonetic Sciences, 265-268. Barcelona, Spain.
Sample EMA data
Movies of sensors on the tongue body and tongue tip during repeated /taka/ as well as movies of the same sensors on the tongue of the same speaker who is also a trumpet player engaged in a so-called 'double-tonguing' exercise at two different rates, 150 bpm (comfortable rate) and 480 bpm (extremely fast).
Panel below shows articulatory movements in the Hungarian word zafír-ban 'sapphire-DATIVE' acquired with the Electromagnetometer (Gafos and Benus 2006). The top panel represents the acoustic waveform, with broad phonemic IPA transcription of the segments in this word. The bottom three panels represent the vertical (solid curve) and horizontal (dashed curve) position of the receivers attached on the tongue dorsum (TD), tongue body (TB), and lower lip (LL). As the tongue body smoothly moves from vowel to vowel in this word, the TB, TD receivers can be seen to trace a bell-shaped trajectory from a retracted position for the first back vowel with low horizontal values of TB and TD, shown by the first valley in the dashed curves, to an advanced position for the second, front vowel, with higher horizontal positions for TB and TD, shown by the peaks in the dashed curves, and back to a retracted position of the final back vowel, shown by the second valley in the dashed curves.
Referencing this page
Gafos, A. (2006). EMA. http://www.ling.uni-potsdam.de/~gafos/courses/Phonocog_UP_CogSys/emma