Loudspeakers

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Loudspeakers
Image galaxyaudio.com

• Jared Bench
• ECE 5320
• Spring 2004

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Contents

• Sound Basics
• Types of Acoustic Actuators
• Loudspeaker Basics
• Loudspeaker Model
• Loudspeaker Characteristics
• Applications
• Conclusions

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References

• http//www.iee.org/TheIEE/Research/Archives/Exhibi
  tons/Sound/ SoundRecordingandReproduction.cfm
• http//www.signalsystemscorp.com/ancindex.htm
• http//europa.eu.int/comm/research/industrial_tech
  nologies/articles/article_503_en.html
• http//micro.magnet.fsu.edu/electromag/java/speake
  r/
• www.epanorama.net/documents/audio/speaker_impedanc
  e.html
• http//stereophile.com/features/99/index4.html
• http//www.electronixwarehouse.com/education/speak
  ers/ howtheywork.htm
• http//stereos.about.com/od/homestereotechnologies
  /a/ speaker_tech.htm
• http//users.erols.com/ruckman/ancfaq.htm

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Further Reading

• New Developments
• http//europa.eu.int/comm/research/infocentre/expo
  rt/success/ article_698_en.html
• Modeling in a Control System
• http//www.egr.msu.edu/radcliff/LabWebPages/home/
  papers/AcousActu.pdf
• Speakers
• http//stereos.about.com/od/homestereotechnologies
  /a/ speaker_tech.htm
• http//www.howstuffworks.com/speaker.htm
• http//www.epanorama.net/documents/audio/
  speaker_impedance.html
• ANC
• http//users.erols.com/ruckman/ancfaq.htm
• http//www.signalsystemscorp.com/ancindex.htm

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Definition of Sound

• What is Sound?
• Sound is a mechanical vibration transmitted by
  an elastic medium.

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Sound Basics

• Sound is generated by vibration of an object or
  surface. The vibrating surface radiates pressure
  waves into the adjoining medium.
• Examples
• Speaker cone
• Violin body
• Human vocal cords
• Turbulent airflow
• Many others!

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Acoustic Actuators

• An acoustic actuator converts electrical signals
  into sound waves

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Image pathwayoflight.org
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Types of Acoustic Actuators
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• Analog Loudspeaker
• Dynamic Loudspeaker
• Electrostatic Loudspeaker
• Magnetic Ribbon (Planar) Loudspeaker
• Digital Loudspeaker (In Development)
• Acoustic Piston Devices
• Piezoelectric Materials

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Electrostatic Loudspeakers

• Electrostatic loudspeakers use the principle that
  like charges repel and opposites attract
• A thin plastic membrane is stretched over a rigid
  frame of some sort. It is then coated with a low
  mass electrically conductive substance like
  graphite power or metal flake.
• Two stiff, flat, electrically conductive
  structures called the stators are then made. Each
  stator has the same area as the thin membrane.
• The stators are connected to a power supply to
  provide the voltage to charge them. They are
  mounted on either side of the diaphragm, at a
  point exactly equidistant between the two
  stators.

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Planar Magnetic Loudspeakers

• Planar magnetic speakers are similar to
  electrostatic loudspeakers.
• Unlike electrostatic speakers they do not need an
  external power source to charge metal plates.
• Operate by passing a current through a metal
  ribbon. As the current passes along, the ribbon
  is attracted to or repelled from the magnets
  surrounding it, generating sound waves.
• Used for high and mid frequencies.

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Electrostatic and Planar Speakers

• Advantages
• Detailed high and mid frequency performance in a
  wide arc around the speaker.
• Drivers are very relatively efficient.
• Disadvantages
• Can be expensive
• Less durability
• Wide frequency performance can be very expensive
• Very little low frequency reproduction
• Electrostatics must have an external power source
  to charge the stators
• Stators and membranes can come into contact with
  each other, causing a short-circuit (and smoke).

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Digital Loudspeakers!

• This piezoelectric array is a prototype digital
  loudspeaker made from a ceramic strip. It
  translates an electrical voltage into physical
  movement of the ceramic strip due to electrical
  field.

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Image 1 Limited
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Dynamic Loudspeakers

• A dynamic loudspeaker consists of a diaphragm
  suspended in a magnetic field

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Image electronics.howstuffworks.com
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Dynamic Loudspeaker Operation

• Current flows through the coil of the speaker,
  inducing an alternating magnetic field in the
  coil.
• As the polarity of the magnetic field alternates,
  it is alternately attracted to and repelled by
  the permanent magnet. This causes the coil to
  vibrate.
• The vibrating coil causes the attached cone
  shaped diaphragm to vibrate and reproduce the
  sounds generated by the original source.

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Dynamic Loudspeaker Model

• Can be split into three primary groups
• Voice Coil Electrical Properties
• Voice Coil DC Resistance
• Voice Coil Inductance
• Equivalents of Mechanical Components
• Suspension Compliance Inductor
• Cone Mass Capacitor
• Suspension Loss Resistor
• Radiated Sound
• Radiation Impedance

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Dynamic Loudspeaker Model
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Effect of Enclosure

• One can construct a similar branch for the
  enclosure, using the lumped parameters
• Port mass Capacitor
• Enclosure Compliance Inductor
• System Losses Resistor
• Port Radiation Impedance

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Enclosure Model
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Combined Model

• Complete Driver Enclosure Electrical Model

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Typical Driver Characteristics

• 0 Hz Impedance is completely dominated by the
  DC resistance of the voice coil
• 0 Hz to Fundamental Frequency Suspension
  compliance begins to dominate and is inductive in
  nature.
• At Fundamental Frequency Impedance is purely
  resistive (phase angle 0), determined by the
  series combination of the voice coil resistance
  and the suspension loss.

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Typical Driver Characteristics

• Above Fundamental Frequency Impedance drops
  (phase angle lt 0), and is capacitive in nature.
• Midrange Impedance approaches DC resistance of
  the voice coil. Typically about 10 to 20 higher
  than the voice coil resistance. This impedance
  is specified by manufacturers as nominal
  impedance.
• Higher Frequencies Inductance of the voice coil
  begins to influence impedance.
• Over the majority of the range of operation, the
  voice coil resistance dominates. The impedance
  NEVER becomes purely inductive, or even remotely
  close.

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Impedance Characteristics
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• Typical two-way loudspeaker, showing electrical
  impedance magnitude (solid trace) and phase
  (dashed trace) plotted against frequency in Hz.
  (Image stereophile.com)

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Dynamic Speaker Disadvantages

• Disadvantages
• Cannot reproduce entire frequency spectrum on
  their own
• Large Mass (Not smooth or uniform)
• VERY Inefficient
• Performance declines sharply for off-axis
  applications

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Active Noise Control (ANC)

• Active control is sound field modification,
  particularly sound field cancellation, by
  producing a mirror image of the offending sound.
• In theory, the disturbance is thus cancelled, and
  the net result is no sound at all.

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Passive vs. Active Noise Control

• Passive noise control includes
• Insulation
• Silencers
• Vibration mounts
• Damping and absorptive treatments
• Works best at mid to high frequencies but is
  difficult at low frequencies.
• Active noise control is more practical for low
  frequencies.

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Benefits of Active Noise Control

• Low-frequency quieting that would be too
  expensive, inconvenient, impractical, or heavy by
  passive methods alone.
• Improve performance and/or efficiency (i.e. a
  less restrictive muffler passage)
• Increased material durability and fatigue life
• Lower operating costs due to reduced facility
  down-time
• Reduced operator fatigue and/or improved
  ergonomics

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Types of Active Noise Control

• Active noise cancellation (ANC)
• Control of acoustic disturbances
• Active structural-acoustic control (ASAC)
• Control of vibration of a flexible structure
• ASAC is distinguished from ANC only in how it is
  applied

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Active Noise Control Applications

• Aircraft interior noise by use of lightweight
  vibration sources on the fuselage and acoustic
  sources inside the fuselage.
• Helicopter cabin noise by active vibration
  isolation of the rotor and gearbox.
• Noise radiated by ships and submarines by active
  vibration isolation of interior mounted machinery
  and active reduction of vibratory power
  transmission along the hull.
• Internal combustion engine exhaust noise by use
  of acoustic control sources at the exhaust outlet
  or by use of high intensity acoustic sources
  mounted on the exhaust pipe.
• Industrial noise sources such as vacuum pumps,
  forced air blowers, cooling towers and gas
  turbine exhausts.
• Lightweight machinery enclosures.
• Tonal noise radiated by turbo-machinery and
  aircraft engines.
• Low frequency noise propagating in air
  conditioning systems.
• Electrical transformer noise.
• Noise inside automobiles using acoustic sources
  inside the cabin and lightweight vibration
  actuators on the body panels.
• Active headsets and earmuffs.

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Active Noise Control Headphones

• Cancel low-frequency noise while passing mid and
  high frequency sounds such as conversation and
  warning sirens.
• Used extensively by pilots.

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NOISEGARD HMEC 300 HEADSET
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Active Exhaust Mufflers

• Several automobile manufacturers are now
  considering active mufflers for future production
  cars.

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Image Katholieke Universiteit Leuven Department
of Mechanical Engineering
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Industrial Fan Noise Reduction

• Speakers placed around the fan intake or outlet
  not only reduce low-frequency noise but they
  also improve efficiency to such an extent that
  they pay for themselves within a year or two.
• -SignalSystemsCorp.com

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Automobile Interior Noise

• Active noise reduction systems are available to
  automobile manufacturers for reducing low
  frequency noise inside car interiors.
• These systems use the car speakers to superpose
  cancellation signals over the normal music signal
  to cancel muffler and tire noise and other
  sounds.

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Conclusions

• Loudspeakers can be used in many applications,
  not just to create noise, but to reduce it!
• In addition to noise reduction, vibrations can
  also be reduced, increasing fatigue life.

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