Recording Studio: Resonance

Recording Studio: A resonance occurs when a structure or material naturally oscillates at a high amplitude at a specific frequency. This frequency is known as a structural resonant frequency. Typically a structure will have many resonant frequencies.

Recording Studio: A definition of resonance

“the state of a system in which an abnormally large vibration is produced in response to an external stimulus, occurring when the frequency of the stimulus is the same, or nearly the same, as the natural vibration frequency of the system.”
When the damping in a structure is small, the resonant frequencies are approximately equal to the natural frequencies of the structure, which are the frequencies of free vibrations of the molecules of the material itself.
Furthermore, an individual recording studio resonance is the condition when a natural frequency of a structure or material and the frequency at which it is being excited are equal or very nearly equal. This results in the structure or material vibrating strongly and is the classical resonance state. This resonance state can often lead to unexpected behaviour of the structure or material.

Recording Studio: Fundamental Frequency

The lowest natural frequency, often called the fundamental frequency, is related to the material of which the structure is made. The greater the mass or density of the material the lower the fundamental frequency of vibration. The natural frequency is also related to the speed that a waveform can propagate through the structure. This is determined largely by the molecular make up of the material. Gas, for example, has many free molecules with high kinetic energy, so the waveform can move quickly through the material. A solid has far fewer free molecules and is much denser, therefore the waveform moves more slowly.

Recording Studio: Measuring resonance

In order to measure a recording studio resonance of a structure or material with a Prosig P8000 data acquisition system and DATS Professional signal processing software it is necessary to attach an accelerometer to the structure. It is then required to excite or stimulate the structure with the frequencies that it is normally exposed to in its working life. For example, an automotive car tyre would need to be subject to the frequencies it would encounter whilst in use. This would normally be accomplished by use of a shaker or a large heavy hammer. The tyre for example would need to be tested in isolation, and not connected to anything else like the vehicle suspension or wheel rim as these other parts have their own resonant frequencies and would make the capture and analysis of the tyre resonant frequency difficult.
The measured response from the accelerometer will be relative to the excitation and will only exhibit frequencies that are present in the excitation. The excitation must be an acceptable representation of the normal working frequencies applied to the structure or material. If the structure has a resonance in this frequency range there will be a large peak in the response spectrum. The frequency of this peak will correspond to one of the resonant frequencies of the structure or material. If no peak is detected then the resonant frequencies lie outside the operating range of the structure or material. In order to find the resonant frequencies of a structure or material it may be necessary to apply a wider range of frequency excitation.