When is resonance good

If it is determined that resonance is in fact the cause of excessive vibration, what can be done to stop or minimize the effect of a resonant condition? The natural frequency of a system is dependent upon two main factors; stiffness, and mass.

Where k is the stiffness and m is the mass. Therefore, in order to change the natural frequency, we need to change either k or m or both. Typically, the objective is to increase the natural frequency such that it is above any expected vibration frequencies. If the natural frequency is above or significantly far away from any expected vibration frequencies the resonance will likely no be excited.

This theory forms the basis for any structural redesigns implemented to avoid resonance. In practice, the following rules can be used to shift a natural frequency and minimize the vibration response of a system;. If changing the natural frequency is determined to be the best solution, it is important to fully characterize the system before attempting any structural redesigns. Recently we performed a startup vibration analysis on a small building adjacent to a MW natural gas power turbine.

On startup it was noted that there was a large increase in vibration in the building when the turbine went through the rpm range. A modal impact test of the building showed a natural frequency at the same cpm frequency, confirming the presence of a resonant condition. One could easily assume just adding stiffness to the support structure of the building would reduce vibration amplitudes. However, it was known that the first shaft critical of the rotor was around cpm.

If stiffness were blindly added to the structure it could easily shift the natural frequency into the cpm range thus making the vibration significantly worse. Breaking of a bridge by a gentle breeze is another. Collapsing of some buildings by earthquake is a third.

It is usually a bit tricky to understand how the ideas I have developed from pushing a friend on the swing can apply to the different examples above. Lets start with the antenna on the radio.

A radio has to receive the radio wave sent out by the radio station. By the time this wave reaches your radio, it is very, very weak. Somehow, the electronics in your radio must be able to sense this very weak signal, so it can amplify it into your news or music. This is the purpose of the antenna. The radio wave moves the electrons in the antenna, and the antenna can produce a current that is big enough to be felt by the electronics in the radio.

How does it work? A radio antenna is usually a long, thin metal rod or wire. Radio wave is an electromagnetic wave. It is made up of an electric field and a magnetic field that oscillates and can travel through air or vacuum. This means that when it reaches the antenna, an electron in the antenna would feel an electric force that keeps changing direction.

This is the effect of the oscillating electric field. It moves the electron, together with many others, and produces a current along the antenna. The electrons in the antenna has a natural frequency. When an electric field is on, the electrons would be pushed towards one end of the antenna. If the field is then switched off, the electrons would repel each other and push themselves towards the other end of the antenna.

In an acoustic guitar, the body acts as a mechanical amplifier of sound waves that are triggered by the strings. The guitar body is constructed in such a way that the resonance frequency occurs even at different pitches. The resonance frequency is not welcome in loudspeakers. The loudspeakers are constructed in such a way that the various components do not vibrate at their resonance frequency. This would mean that sounds in the same frequency range as individual components would be reproduced much louder than others.

This is where the crossover network comes into play: in multi-channel systems, it directs the signals to the speakers according to their frequency. Resonance effects should not occur in the loudspeaker cabinet either. These can occur when the membrane on its back radiates sound into the interior of the loudspeaker.

This sound can cause the cabinet to vibrate and thus have a negative effect on the sound image. In order to prevent this, the loudspeaker cabinet is fitted with a damper that absorbs the sound waves radiated to the inside. The exception to this rule are bass reflex speakers. These cabinets have a tubular opening through which the sound radiated to the inside — at certain low frequencies — can escape into the room.

This works on the principle of the Helmholtz resonator, which we explained in our text on subwoofers.