Hardware Considerations in Product Design
E-10
E.9 Hardware Considerations in Product Design
E.9.1
Speaker Selection
The physical properties of the speaker have a substantial influence on the
quality of the reproduction of speech. The diaphragm of the speaker should
have a large surface area to efficiently produce low frequencies. The mass of
the voice coil and diaphragm should be small to efficiently produce high
frequencies. Most home speaker systems, for example, use a large woofer to
produce sound below a few hundred Hertz, a tweeter to produce sound above
6–8 kHz, and a 4–6 inch diameter midrange speaker to produce sound
between these ranges.
The most valuable portion of a speech signal is contained in the frequency
range between 100 Hz and 5000 Hz. Therefore, it is possible to get reasonably
good speech quality with a small speaker. The 2.25-inch diameter speaker is
a common size used with TI speech devices.
Other speaker properties that affect how well the speaker reproduces sound
are the stiffness and the material used for the diaphragm, the impedance of
the voice coil, the strength of the magnet, and the depth of the speaker cone.
A stronger magnet and a deeper cone are normally preferred, but these
properties result in a larger speaker. The other properties depend on the
complete system.
Each speaker has unique characteristics (for example, its frequency
response) and it is important to understand how a speaker sounds in your
product. The speaker characteristics are likely to change from speaker to
speaker during the manufacturing process. We recommend being proactive
in the speaker selection for your product, and not to change the speaker
without a careful evaluation of the speech quality.
E.9.2
Speaker Enclosure Design
The speaker enclosure design is just as important as the speaker selection.
This is especially true when using a small speaker to reproduce low-frequency
sounds.
A speaker produces sound by increasing and decreasing air pressure at a rate
equal to the frequency of the sound. The air pressure increases as the
diaphragm moves toward the listener and decreases as it moves away from
the listener. This assumes that the air displaced as the diaphragm moves
forward does not fill the void behind the diaphragm. When the diaphragm is
vibrating at higher frequencies, the air does not have time to rush around
behind the diaphragm. This is the reason speaker efficiency at the higher
frequencies is not as dependent on the speaker enclosure. However, this is
not true for lower frequencies.
A speaker enclosure can be used to prevent air flowing from the front to the
back of the speaker. The size and shape of the speaker enclosure affects the
performance of the speaker. Two types of enclosures are commonly used:
closed box and bass-reflex.
Summary of Contents for MSP53C691
Page 1: ...MSP53C691 Speech Synthesizer December 2000 MSDS Speech User s Guide SPSU020 ...
Page 22: ...2 10 ...
Page 96: ...C 6 ...