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Senior Project Electrical Engineering 2009
Conceptualizing and Constructing the Smart
Speaker Designing a power-limiting device for
the common 2-way loudspeaker. Presented by Ryan
Gwinn Project Advisor James Hedrick
Abstract
The goal of this project is to design a governor
that limits the power reaching a loudspeakers
drivers regardless of the input signal connected
to its external terminals. This system, not meant
to be an add-on, but part of a speakers design
will be created using the parameters of two
drivers physical/material limitations. To
accomplish this, a two driver enclosure complete
with a 2-way passive crossover will be designed
and constructed. Individual limiting circuits
will then be placed between the crossover and
each driver within the loudspeaker.
Motivation
Design/Performance Specifications

• Limit the power reaching the loudspeakers
  drivers regardless of the input signal.
• React to the input signal within 15 milliseconds.
  
•  Function with a changing audio file containing
  significant dynamic range.
•  When not active, the device must have minimal
  effect on the loudspeakers performance
  parameters (frequency response etc.).

Loudspeakers surround us all in our everyday
lives for some they represent a livelihood, and
for many, a passion. Their failures can be
incredibly costly and while numerous systems
exist to protect speakers, nearly all function
within the amplifiers. The protection of
loudspeakers must begin to consider the speakers
themselves. Clip-protection, gain controls, and
signal processors are not enough.
The Basic Design
It is important to remember that a 2-way
loudspeaker is already a system. The block
diagram to the left is an alteration of this
system.
A MATLAB line-level signal (sine sweep, then
audio file) is sent to a power amplifier.
Next the speaker level signal enters a 2-way
passive crossover, which acts as two band-pass
filters in order to send the proper frequency
range to each driver.

• Limiter to be implemented three ways
• As a feedback system (to the amplifier).
• With an active switch.
• As a passive element.

Each (independent) speaker-level signal enters
its respective limiting device, which attenuates
the signal if over the predetermined cut-off
voltage. The signal then reaches the drivers,
ideally with no loss in quality.
This project had several successes. First, two
loudspeakers were designed and constructed. One
(the control) modeling the BR-12 by Paul
Holsopple. It was designed following the criteria
outlined in Speaker Building 201, and tested
according to Joseph DAppolitos Testing
Loudspeakers. The second speaker was slightly
altered with the crossover removed from the
cabinet as seen on the left. This allows for easy
access when adding additional circuitry. Because
of limitations in acoustical testing equipment,
both speakers were tested for frequency response
using a program developed for this project within
MATLAB.
Results and Future Work
Currently a system using feedback is being
developed, though it is not fully operational.
Moreover, certain design issues were realized
preventing the incorporation of active switching,
and a passive approach. Developing an active
switch requires an additional power source, and
has many boundaries preventing it from not
effecting regular performance. The ideal passive
system is currently inactive as of now due to the
inability to dissipate excess variable ac signal
power as well as problems such as over-heating.
However, a test system has now been created
(including test programs) that in the future can
serve both as a learning tool and a first step of
reaching this projects final goal.