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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 loudspeaker’s drivers regardless of the
input signal connected to its external terminals. This system, not meant to be an add-on, but part of a speaker’s 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
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.
•Limit the power reaching the loudspeaker’s 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
loudspeaker’s performance parameters (frequency response etc.).
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.
Limiter to be implemented three ways:
1) As a feedback system (to the amplifier).
2) With an active switch.
3) As a passive element.
Results and Future Work
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.
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
D’Appolito’s 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.
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 project’s final
goal.