Loudspeakers Jared Bench ECE 5320 Spring 2004 Image: galaxyaudio.com

Loudspeakers

Jared Bench

ECE 5320

Spring 2004

Image: galaxyaudio.com

22/33/33 Loudspeakers Loudspeakers Jared BenchJared Bench

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

Contents

Sound BasicsSound Basics

Acoustic Acoustic ActuatorsActuators

Loudspeaker Loudspeaker BasicsBasics

Loudspeaker Loudspeaker Model Model

Loudspeaker Loudspeaker CharacteristicsCharacteristics

ApplicationsApplications

ConclusionsConclusions


References

http://www.iee.org/TheIEE/Research/Archives/Exhibitons/Sound/ SoundRecordingandReproduction.cfm

http://www.signalsystemscorp.com/ancindex.htm http://europa.eu.int/comm/research/industrial_technologies/

articles/article_503_en.html http://micro.magnet.fsu.edu/electromag/java/speaker/ www.epanorama.net/documents/audio/speaker_impedance.html http://stereophile.com/features/99/index4.html http://www.electronixwarehouse.com/education/speakers/

howtheywork.htm http://stereos.about.com/od/homestereotechnologies/a/

speaker_tech.htm http://users.erols.com/ruckman/ancfaq.htm









Further Reading

New Developments: http://europa.eu.int/comm/research/infocentre/export/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









Definition of Sound

What is Sound?

Sound is a mechanical vibration transmitted by an elastic medium.









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!









Acoustic Actuators

An acoustic actuator converts electrical signals into sound waves








Image: pathwayoflight.org


Types of Acoustic Actuators

Analog Loudspeaker Dynamic Loudspeaker Electrostatic Loudspeaker Magnetic Ribbon (Planar) Loudspeaker

Digital Loudspeaker (In Development) Acoustic Piston Devices Piezoelectric Materials









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.









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.









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).









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.

Image: 1 Limited









Dynamic Loudspeakers A dynamic loudspeaker consists of a

diaphragm suspended in a magnetic fieldSound BasicsSound Basics







Image: electronics.howstuffworks.com


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.









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









Dynamic Loudspeaker Model

Voice Coil Resistance

Voice Coil Inductance

Suspension Compliance

Suspension Loss

Cone Mass

Radiation Impedance

Z









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









Enclosure Model

Port Mass

System Losses

Enclosure Compliance

Port Radiation Impedance

Z









Combined Model

Voice Coil Resistance

Voice Coil Inductance

Suspension Compliance

Suspension Loss

Cone Mass

Radiation Impedance

Z

Port Mass

System Losses

Enclosure Compliance

Port Radiation Impedance

Z

Complete Driver + Enclosure + Electrical Model:









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.









Typical Driver Characteristics Above Fundamental Frequency –

Impedance drops (phase angle < 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.









Impedance Characteristics

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









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









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.









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.









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









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









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.

Active Noise Control Applications









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.

NOISEGARD HMEC 300 HEADSET









Active Exhaust Mufflers

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

Image: Katholieke Universiteit Leuven Department of Mechanical Engineering









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









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.









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.








Documents

Loudspeakers Jared Bench ECE 5320 Spring 2004 Image: galaxyaudio.com