Microphones and Loudspeakers

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Microphones and Loudspeakers

• Architectural Acoustics II
• April 3, 2008

2
Final Exam Reminder

• Wednesday December 10
• 300 600
• Greene 120 (this building, first floor)
• Handwritten notes on 2 sides of 8.5 x 11 paper
  are allowed, along with a calculator
• No laptops

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Transduction

• Conversion of one form of energy into another
• For microphones acoustical ? electrical
• For loudspeakers electrical ? acoustical
• Two basic categories of transducers
• Sensors
• Small
• Low power
• Dont affect the environment they are sensing
• Actuators
• Large
• High power
• Meant to change the environment they are in

4
Simple EE Review

• V IR (Ohms Law)
• V voltage (volts)
• I current (amperes)
• R resistance (ohms)
• V Blu (Electromagnetic induction)
• V voltage
• B magnetic field (Teslas)
• l length of wire (m)
• u wire or magnet

Velocity (m/s)
Rossing, The Science of Sound, Figure 18.2, p. 370
http//www.tiscali.co.uk/reference/encyclopaedia/h
utchinson/images/c01347.jpg
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Simple EE Review

• Capacitors (formerly known as condensers)
• Q CV
• Q charge (coulombs)
• C capacitance (farads)
• V voltage (volts)
• C A/d
• A area of the capacitor plate (m2)
• d plate separation distance (m)

Image from http//upload.wikimedia.org/wikipedia/e
n/b/b5/Capacitor.png
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Basic Microphone Types

• Dynamic (moving coil)
• Condenser (capacitor)
• Electret
• Ribbon
• Piezo-electric (crystal or ceramic)

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Dynamic Microphone

• Sound pressure on the diaphragm causes the voice
  coil to move in a magnetic field
• The induced voltage mimics the sound pressure
• Comments
• Diaphragm and coil must be light
• Low output impedance good with long cables
• Rugged

V Blu
Long, Fig. 4.1, p. 116, 2nd image courtesy of
Linda Gedemer
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Condenser Microphone

• Diaphragm and back plate form a capacitor
• Incident sound waves move the diaphragm, change
  the separation distance, change the capacitance,
  create current
• Comments
• Requires a DC polarizing voltage
• High sensitivity
• Flat frequency response
• Fragile
• High output impedance, nearby pre-amp is necessary

Q CV
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Electret Microphone

• Same basic operation principle as the condenser
  mic
• Polarizing voltage is built into the diaphragm
• Comments
• High sensitivity
• Flat frequency response
• Fragile
• High output impedance, nearby pre-amp is necessary

Q CV
Long, Fig. 4.1, p. 116
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Ribbon Microphone

• Conductive ribbon diaphragm moving in a magnetic
  field generates an electric signal
• Comments
• Lightweight ribbon responds to particle velocity
  rather than pressure
• Both sides are exposed resulting in a
  bidirectional response
• Sensitive to moving air
• Easily damaged by high sound-pressure levels

Long, Fig. 4.1, p. 116, 2nd image courtesy of
Linda Gedemer
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Piezo-Electric Microphone (a.k.a. Crystal or
Ceramic Microphone)

• Diaphragm mechanically coupled to a piezoelectric
  material
• Piezo (lead zirconate titanate (PZT), barium
  titanate, rochelle salt) generates electricity
  when strained
• Comments
• No polarization voltage
• Generally rugged
• See Finch, Introduction to Acoustics, Chapter 7,
  Piezoelectric Transducers for details

Long, Fig. 4.1, p. 116
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Microphone Parameters
1/2-inch diameter BK measurement microphone
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Microphone Parameters
Neumann U87 Ai Large Dual diaphragm Microphone
Slide courtesy of Linda Gedemer
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Frequency Response and Incidence Angle
Long, Fig. 4.8, p. 121
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Frequency Response and Incidence Angle
Off-axis coloration
Slide courtesy of Linda Gedemer
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Transient Response
Slide courtesy of Linda Gedemer
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Other Microphone Types
Shotgun Microphone
Rossing, The Science of Sound, Figure 20.10, p.
398
http//aes.harmony-central.com/115AES/Content/Audi
o-Technica/PR/AT897.jpg
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Other Microphone Types
Parabolic Microphone
http//homepage.ntlworld.com/christopher.owens2/Im
ages/TelingaMount.jpg
http//hyperphysics.phy-astr.gsu.edu/hbase/audio/m
ic3.html
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Other Microphone Types
Contact Microphones
www.BarcusBerry.com
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Other Microphone Types
Pressure Zone Microphone (PZM)
www.crownaudio.com
www.shure.com
Slide courtesy of Linda Gedemer
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Use of Boundary Mics
Slide courtesy of Linda Gedemer
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Effects of Floor Reflections
Slide courtesy of Linda Gedemer
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Soundfield Microphone

• 4 diaphragms in a tetrahedral pattern
• Essentially measures omni pressure (W) and X,Y,
  and Z-dimension pressure
• Used for 1st-order spherical harmonic encoding of
  a sound field (1st-order Ambisonics)

http//www.soundfield.com/soundfield/soundfield.ph
p
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Microphones and Diffraction
Blackstock, Fundamentals of Physical Acoustics,
Figure 14.12, p. 487
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Directivity Patterns

• Single-diaphragm microphones are typically
  constructed to have one of a variety of
  directivity patterns
• Omni directional
• Bidirectional
• Cardioid
• Hypercardioid
• Supercardioid
• General mathematical form A Bcos(?)

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Directivity and Ports

• In a directional (ported) microphone, sound
  reflected from surfaces behind the diaphragm is
  permitted to be incident on the rear side of the
  diaphragm.
• Sound reaching the rear of the diaphragm travels
  slightly farther than the sound at the front, and
  it is slightly out of phase. The greater this
  phase difference, the greater the pressure
  difference and the greater the diaphragm
  movement. As the sound source moves off of the
  diaphragm axis, this phase difference decreases
  due to decreasing path length difference. This is
  what gives a directional microphone its
  directivity.

Shure Pro Audio Technical Library
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Directivity Patterns
Omnidirectional
Bidirectional
Cardioid
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Directivity Patterns
Hypercardioid
Supercardioid
All Five
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Directivity in 3D
Omnidirectional
Bidirectional
Cardioid
Slide courtesy of Linda Gedemer
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Directivity in 3D
Supercardioid
Hypercardioid
Slide courtesy of Linda Gedemer
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Directivity Patterns
Omni Bi- directional Cardioid Hyper-cardioid Supercardioid
Pattern
Polar Equation 1 cos? 1 cos?/2 1 3cos?/4 0.370.63cos?
Output at 90º (dB re 0º) 0 -8 -6 -12 -8.6
Output at 180º (dB re 0º) 0 0 -8 -6 -11.7
Angle for which output is 0 NA 90º 180º 110º 126º
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Combining Patterns Dual Capsules
Neumann U87Ai Georg Neumann GmbH
Slide courtesy of Linda Gedemer
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Basic Cone Loudspeaker Principles

• Paper (or other light-weight material) cone
  attached to a coil suspended in a magnetic field
• Audio signal (voltage) is applied to the wire,
  causing it to move
• Mechanism is enclosed to prevent dipole radiation
• Typical characteristics
• Sensitivity
• Impedance
• Frequency response
• Directivity

Rossing, The Science of Sound, Figure 20.13, p.
402
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Speaker Directivity

• Directivity Factor
• I usually measured on axis
• Directivity Index

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Speaker Directivity
Slide courtesy of Linda Gedemer
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Speaker Parameters
JBL Control 29 AV-1
Slide courtesy of Linda Gedemer
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Speaker Parameters
JBL Control 29 AV-1
Slide courtesy of Linda Gedemer
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Enclosures
Direct radiator or Acoustic suspension
Bass reflex
Bass reflex with passive radiator
Bass reflex with acoustic labyrinth
Slide courtesy of Linda Gedemer
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Cabinets and Diffraction
Svensson and Wendlandt, The influence of a
loudspeaker cabinets shape on the radiated
power, Baltic Acoustic 2000.
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Cabinets and Diffraction
Svensson and Wendlandt, The influence of a
loudspeaker cabinets shape on the radiated
power, Baltic Acoustic 2000.
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Cabinets and Diffraction
Svensson and Wendlandt, The influence of a
loudspeaker cabinets shape on the radiated
power, Baltic Acoustic 2000.
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Cabinets and Diffraction
Svensson and Wendlandt, The influence of a
loudspeaker cabinets shape on the radiated
power, Baltic Acoustic 2000.
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Array Behavior

• Proper calculations
• Far-field approximations
• Change in behavior with number of elements
• Change in behavior with phasing
• Change in behavior with spacing
• Change in behavior with frequency

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Array Calculations

• p(R) pressure at position R
• A agglomeration of various constants
• ri distance from element i to position R
• e-jkr - d Greens function for a point element
• k wavenumber
• d phase

• Sweep R in an arc centered at the center of the
  array to create a polar directivity plot.
• This expression does not account for the
  directivity of individual elements in the array!
  All are assumed to be point sources or
  omnidirectional microphones.

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Far-Field Approximation

• I intensity of the array
• n number of array elements
• ß kdcos(?) d
• k wave number
• d distance between array elements
• ? angular position relative to the center of
  the array
• d constant phase difference between elements

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Intensity vs. Log Magnitude
Intensity
Log Magnitude
8 elements at 10 cm spacing, 1 kHz, R at 10 m
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Number of Elements
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Phase (between elements)
0º
60º
110º
140º
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Frequency
1 kHz
500 Hz
4 kHz
2 kHz
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Spacing
10 cm
5 cm
40 cm
20 cm
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Other Array Ideas

• Random spacing to address side lobes
• Constant beam width