As my research focused on use of Physiological Microphone - PMIC for stress and speaker verification, I thought it would be a good idea to identify different sensors that have been used for speech signal analysis/recording.
Any sensor which has an ability to measure displacement would be thought as a good option to record speech as then it can record jaw movement, cheeck movement, tongue movement so on and so forth, recording movement of an articulator, or any vibration that is due to speech production (bone vibrations) at different locations other than the standard mic-near-the-mouth standard approach.
So, the study does focus on understanding details of displacement sensors - and the principle behind measuring displacements. - The sensors to measure displacement are strain gauges, piezoelectric crystals, and accelerometers.
Microphones - the acoustic ones usually record the pressure variations - caused due to sound travel in air - acoustic vibrations. Simplest form is the diaphragm based which measure the difference in pressure on either side - pressure gradient microphones. There can also be a pressure microphone with diaphragm open to atmospheric pressure and any pressure variation on either side can be recorded as a displacement, hence can be treated as omni-directional.
Examples of a few regular microphones.
1. Crystal Microphone - piezocrystal on one side of diaphragm, frequency response 80Hz to 6000Hz, needs amplifier as the signal is weak. p+ve points - dynamic range, sensitivity, and lack of distortion is good. good stability if used outside of humid conditions.
2. Dynamic Microphone - moving coil wound on a permanent magnet. Sensitivity is less as compared to crystal one because of the coil and magnet weight and also may not perform faithfully in presence of other electromagnetic sources.
2. Dynamic Microphone - moving coil wound on a permanent magnet. Sensitivity is less as compared to crystal one because of the coil and magnet weight and also may not perform faithfully in presence of other electromagnetic sources.
3. Condenser Microphone - diaphragm is one of the capacitor plate and back plate (another parallel) combined together act as a capacitor. Requires a power supply (capacitor polarization voltage). Flat response for speech / music range, expensive and high impedance hence requires a pre-amplifier for impedance matching and signal transfer.
4. Electret Microphone - condensor microphone with permanent charge on the plates. Similar flat response over a frequency range, can be made smaller in size, but require battery power.
5. Probe Tube Microphone - useful to measure sound pressure levels within a closed or nearly closed cavity. Consists of usual transducer, but the diaphragm is coupled to a length of tubing of very small diameter. The tubing is inserted into the cavity to be measured. The length of the tubing defines the resonance characteristics hence the probe tube microphone is calibrated to appropriately yield a flat frequency response.
