SiSonic Design Guide - AN16
Confidential. Intended Customer’s Internal Use Only.
3.0 MECHANICAL DESIGN CONSIDERATIONS
The purpose of this section is to provide mechanical design information relating to the
• Choosing locations for the mic(s) and acoustic port hole(s) in the case
• Designing the acoustic path, including gasket design and assembly considerations
• Designing for a wideband frequency response
• Echo prevention and troubleshooting
• Optimizing the land pattern, solder stencil design, and soldering process
3.1 CHOOSING THE MIC AND PORT HOLE LOCATIONS
Choosing a location for the microphone in a design can be challenging. For analog mics in
particular, the traces from mic to chipset should be kept as short as possible and as far as possible
from potential noise sources. However, the layout of many mobile product designs require that
the mic and traces be near antennas, power amplifiers, motors, hard disk drives, switching power
supplies, etc. The design engineer must also consider the available board space, component
height restrictions, port-hole location(s), acoustic path dimensions, and gasket size, location, and
ease-of-assembly in mass production when choosing a mic location.
The external acoustic port hole in the product housing should be located near the mic to simplify
the gasket and associated mechanical design. The port-hole must also be far enough from
speakers and other acoustic noise sources to minimize the strength of these unwanted signals at
the microphone input. In near-field use modes like talk mode in a mobile phone, the port-hole
location is more critical than in far-field modes since small changes in distance can change the
strength of the acoustic signal arriving at the microphone. In both types of applications, the port
hole should be located where it won’t be blocked during normal use.
If there are multiple mics in a design, then the mic and port-hole locations are further constrained
by the related product use-modes and any audio algorithm requirements. Picking good locations
for the microphones and port holes early in the design process can prevent costly PCB layout or
plastics changes late in the product design cycle.
3.2 ACOUSTIC PATH DESIGN
The acoustic path guides external sound into the microphone. The overall frequency response of
the microphone in the product design is determined by the standalone microphone frequency
response and the physical dimensions of each part of the acoustic path, including the case port-
hole, gasket(s), and PCB port-hole. The acoustic path must not have leaks that can cause echo or
noise problems, and needs to be designed for manufacturability.