Acoustic transduction

1. Acoustic transduction • Speech sounds - rapid variations of air pressure and velocity around their normal values • sound field - variation of air density and pressure are functions of time and space and propagate as acoustic wave • let assume the air to be homonogeus in a room • speed of acoustic wave propagation depends on temperature (in K): • wave equation describes propagation of sound, if pressure is represented by a scalar field p(a,t), a=[x y z]T

2. Wave propagation (2) • one of the solutions of wave equation is the monochromatic plane wave of frequency f=w/2P • where A is the wave amplitude and k=[kx,ky,kz]T is the wavenumber vector and has a direction normal to the propagating wavefront. • Distance l=2P/|K|=c/f is called wavelength and describes spatial period of propagating wave • in spherical coordinates (r,f,q) sound pressure depends only on the distance r from the source • any sound field can be expressed as superposition of elementary plane and spherical waves

3. Formants

4. Room acoustics • Reflections from surfaces, diffusion and diffraction by objects inside the room - reverberation effect • T60 - reverberation time, defined as the time needed for the acoustic power of the signal to decay by 60 dB after sound source is abruptly stopped • T60 is nearly independent from the listening position in given enclosure, it can be approximated by Sabine formula: • where V is room volume in m3, S is total surface area of the room in m2 and a is the average absorption coefficient of the surfaces • reverberation times up to 1 s (for frequencies 500-1000 Hz) do not cause any loss in speech intelligibility • impulse response h(t): described the path between source and receiver, all reflections • early reflections - perceived if delay > 50 ms, shorter perceived as part of the direct sound

5. Room acoustics (2) • speech intelligibility: “Deutlichkeit” index, centre of gravity, modulation index

6. Room Impulse Response • Simplest method: apply impulse excitation and observe the response of the system: balloon popping, gunshots, but it may not guarantee SNR and flat frequency response, also overload possible • to overcome these difficulties: excitation using maximum length pseudo-random sequences (Schroeder, 1979) - flat spectrum, auto-correlation of the sequence of length L becomes a close approximation of delta function when L is large: • then the room impulse response can be simply obtained by reproducing the acoustic signal corresponding to the sequence and then by simply cross-correlating the excitation sequence p(n) with the signal y(n) acquired by the sensor • sound ray concept- diffracted by edges, scattered by small obstacles

7. Impulse response measurement • How can it be measured? Speecon,2001

8. Microphones • Converts the acoustic energy of sound into a corresponding electrical energy; usually realized with a diaphragm whose movements are produced by sound pressure and vary the parameters of an electrical system (resistance, capacity, etc) • characterized by • frequency response (flatness in speech sounds range) • signal-to-noise ratio (SNR) • impedance (better if low, connected to low impedance amplifier gives lower hum and electrical noise), usually specified for 94 dB SPL • sensitivity: output voltage (in milivolts) or power (in dBm) • directional pattern: cardioid (supercardioid, hyper-, shotgun, etc), bidirectional (figure of eight) or omni-directional (circle) • mountings: hand-held, head-mounted, table stand (desk-top), Lavalier • Small or big diaphragm 0 dB SPL=0.0002 mbar (threshold of hearing ; 0dBm corresponds to 0dB referenced to 1mW Microphone polar response

9. Microphones: basic transduction categories • Passive: converts directly sound to electrical energy, active: needs additional energy source (battery, phantom power) • electromagnetic and electro-dynamic microphones: • ribbon - duralumin ribbon moving in permanent magnetic field • moving-coil- inverse of loudspeaker, bigger than ribbon, thus higher voltage induced • widely used, good frequency and transient response, moderate cost • rather old • electrostatic microphones: • condenser: capacitor with dielectric inside, one of plates can move, pre-polarization needed, very high output impedance; excellent frequency and transient response, low distortion • electret: with built-in pre-polarization condenser (100 V), power supply needed, good frequency and transient response, low distortion, but lower dynamic range and sensitivity as for condenser m. • piezoresitive and piezoelectric microphones: • variation of resistance • carbon: small cylinder with granulates of carbon - by vibrations granules can separate, changing the electric resistance of cylinder;low quality • crystal and ceramic: Rochelle salt - the same principle like carbon mike; low quality • special microphones: pressure-zone (PZM, for speech reinforcement), pressure-gradient microphone (for directional acquisition), noise-canceling, micro-mechanical silicon microphones, optical wave-guide

10. Ribbon microphones • Principle of work: duralumin ribbon moving in permanent magnetic field • Could be very good and expensive: (Royer labs) • Features: • Very high overload characteristics – max SPL > 135 dB • Extremely low noise • Absence of high frequency phase distortion • Excellent phase linearity • Equal sensitivity from front/back • Consistent frequency response regardless of distance • No power supply required • Strong proximity effect • Strong wind effects

11. Moving coil • A moving-coil microphone contains a diaphragm exposed to sound waves. The diaphragm carries a coil placed in the magnetic field. The voltage induced in the coil is proportional to its amplitude of vibration, which, in turn, depends on the sound pressure. • Moving coil microphones are cheap and robust making them good for the rigors of live performance and touring. They are especially suited for the close micking of Bass and Guitar speaker cabinets and Drum kits. • They are also good for live vocals as their resonance peak of around 5kHz provides an inbuilt presence boost that improves speech/singing intelligibility • However the inertia of the coil reduces high frequency response. Hence they are NOT best suited to studio applications where quality and subtlety are important such as high quality vocal recording or acoustic instrument micking

12. Condenser microphone • A condenser microphone incorporates a stretched metal diaphragm that forms one plate of a capacitor. A metal disk placed close to the diaphragm acts as a backplate. When a sound field excites the diaphragm, the capacitance between the two plates varies according to the variation in the sound pressure. A stable DC voltage is applied to the plates through a high resistance to keep electrical charges on the plate. The change in the capacitance generates an AC output proportional to the sound pressure. In order to convert ultralow-frequency pressure variations, a high-frequency voltage (carrier) is applied across the plates. The output signal is the modulated carrier. • Are the best, need Condenser microphone. AP = acoustic pressure, C = variable capacitance, 1 = metal diaphragm, 2 = metal disk, 3 = insulator, 4 = case.

13. Electret microphone • An electret-type microphone is a condenser microphone in which the electrical charges are created by a thin layer of polarized ceramic or plastic films (electrets). The ability of the electrets to keep the charge obviates using the source for a high-voltage polarization • Output impedance is relatively high (typically about 1k to 5k) • Signal output is limited (relatively low sensitivity) • Noise is relatively high • Sound level handling ability is low (typically < 90dB SPL) • They are normally available from retail outlets very cheaply Electret-type microphone. AP = acoustic pressure, Uo = output voltage, 1 = diaphragm, 2 = electret, 3 = case.

14. Piezoresistive mics • In a carbon-button microphone, the sound field acts upon an electroconductive diaphragm that develops pressure on a packet of carbon granules. The contact resistance between the granules depends on the pressure. When a DC voltage is applied across the packet, the alternating resistance produces an AC voltage drop, which is proportional to the sound intensity. Carbon-button microphone. AP = acoustic pressure, R = variable resistance, 1 = electroconductive particles, 2 = diaphragm, 3 = electrode.

15. Microphone arrays • Selective acquisition of speech in spatial domain, detection, tracking and selective acquisition of speaker automatically • beamforming: spatial filtering: filtering and sum approach: compensate for difference in path length from source to each of the microphones delay in time domain  linear phase shift in frequency domain • dereverberation, talker location - time difference of arrival,power field scanning, MUSIC

16. Microphones in speech recognition • Training and testing condition mismatch: the same microphone preferred • microphone normalization - multichannel recording and matching of signals • noise canceling head-set preferred in ASR, but users don’t like this • room acoustic influence on recording and ASR • ASR in car: • non-homogenous acoustic environment - dependence on microphone position • Speecon project: consumer devices environment • gradient microphones in adverse condition: aircraft cockpit • feature selection: filtering • cochlear model and binaural processing: special microphones and filtering methods • use of microphone arrays • active noise cancelling: new buzzword