Acoustic transduction

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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),
  ax y zT

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Wave propagation (2)

• one of the solutions of wave equation is the
  monochromatic plane wave of frequency fw/2P
• where A is the wave amplitude and kkx,ky,kzT
  is the wavenumber vector and has a direction
  normal to the propagating wavefront.
• Distance l2P/Kc/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

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Formants
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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 gt 50 ms,
  shorter perceived as part of the direct sound

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Room acoustics (2)

• speech intelligibility Deutlichkeit index,
  centre of gravity, modulation index

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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

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Impulse response measurement

• How can it be measured?

Speecon,2001
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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 SPL0.0002 mbar (threshold of hearing 0dBm
  corresponds to 0dB referenced to 1mW

Microphone polar response
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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 cylinderlow 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

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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 gt
  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

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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

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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.
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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 lt
  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.
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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.
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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

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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 dont 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

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