Cirrus Logic AN168 - Manual (Page 5)

AN168
AN168REV2 5
1.2.1.3 Speaker Housing Requirements
The quality of the speaker housing affects the
performance of the system because the speaker can
induce vibrations in its housing if it is not properly
mounted. These vibrations tend to create “buzzing”
artifacts which are not linear and result in poor
echo canceler performance.
Speakers that are supplied after-market in a
housing and speakers that are part of the car's radio
system generally do not present problems. It is the
speakers that are glued or otherwise rigidly affixed
to their plastics that create nonlinear buzzing
artifacts.
Speakers should be soft-mounted to their housings
by using soft pliable acoustic foam. Care should be
taken to minimize any physical means by which the
speaker can induce vibrations in the plastics.
The Test section contains a test procedure for
testing and eliminating buzzing artifacts.
1.2.2 Placing the Speaker and Micro-
phone
The placement of the speaker and the microphone
affects the gain selection portion of the electrical
design of the system which will be covered shortly.
The microphone should be placed as close as
feasible to the talker's mouth. This maximizes the
signal-to-noise ratio (SNR) of the talker's speech.
In a car, the optimal place for the microphone is
near the rear view mirror, usually attached to the
driver’s visor.
There are two considerations for the speaker
placement. The more important of the two is that
the speaker be placed such that there is a minimum
of movement in the air space between the speaker
and the microphone. This will minimize the
number of updates and corrections that the adaptive
filter makes during the call, resulting in the
transmission of very little residual echo to the
far-end listener. The second consideration is that
the speaker should be placed as far from the
microphone as possible. This minimizes the
acoustic coupling between the speaker and mic and
allows the mic preamp gain and speaker driver gain
to be maximized.
The optimum placement for the speaker in a car is
the top of the dashboard. Whereas this does not
minimize the distance between the speaker and the
mic, it does limit the changes in the acoustic path,
allowing the adaptive filter to update less often,
resulting in less residual echo transmission. Other
placement options, below the dash, driver's side
door, and passenger's side door, favorably decrease
the acoustic coupling, but result in the driver or the
passengers being positioned directly in the path
between the speaker and the microphone.
1.3 Electrical Design
The electrical design process consists of the
component selection of the speaker driver, the gain
selections of the speaker driver and the mic
preamp, and the implementation of an acoustic
sidetone. The primary design consideration of the
electrical design process is to limit the distortion in
the acoustic path to less than 2% THD.
1.3.1 Selecting the Speaker Driver
Many system designers overestimate the quality of
their speaker drivers. For example, a speaker driver
that claims to be “5 Watts” on the cover of its data
sheet is not suitable to drive 5 Watts of power into
the speaker of a full-duplex echo cancelling
system. The reasons are two-fold:
1) The “5 Watts” number is usually a Typical
number, not a Maximum or a Minimum speci-
fication
2) “5 Watts” is specified with a THD of 10%, not
the 2% number that we are designing to.