Vibration is bad for electronic devices such as radio transmitters and computers. Screws can work loose and cards and cables can come unplugged. As a result, equipment is sometimes inoperable after a long journey. Fortunately, that kind of damage is usually easily repaired, unlike the harm caused by resonance.
Excitation at the natural frequency
Like a guitar string, every object has a frequency it vibrates at if plucked. This is its natural or resonant frequency. These vibrations gradually die away, unless the string is plucked, or excited, a second time. In that case, they get stronger. If the excitation occurs repeatedly at a rate matching the natural frequency, the vibration amplitude increases dramatically. This is called resonance.
In the case of electronics, excitation at the natural frequency can make board-mounted components vibrate enough that they touch or their solder joints fail. The right frequencies may cause circuit boards to flex and delaminate.
Providing vibration attenuation
A well-engineered transit case protects delicate equipment from vibration damage by isolating it from external sources of vibration. Elastomeric or wire rope mounts hold the payload away from case walls. This reduces both the amplitude of the vibration and the energy transmitted.
A challenge transit case designers face is shock mounts also have a natural frequency. If the case is excited at this frequency, the mounts will actually transmit energy to the payload, rather than provide attenuation.
In some applications, the excitation frequency is predictable, e.g., a case moved by road or rail or sited close to a pump or motor. More often, as when a case will be moved by air, there is no single excitation frequency. In those situations, a power spectral density (PSD) method is used to describe the excitation spectrum.
The need for testing
Engineering data for vibration isolating mounts usually includes the natural frequency. By selecting a mount where this is several multiples of the excitation frequency, vibration transmission should be minimized. However, natural frequency numbers are often derived from static testing and real-world performance in complex dynamic situations can be significantly different. As a result, an isolator may not protect the case payload as well as expected.
The only way to verify that a case and isolation mounts will provide sufficient vibration is through instrumented testing.