Thus, whenever you are called upon to recommend a sound absorptive treatment using perforated materials, your first step must be to determine the frequency distribution of the offending noise. If it is a broad-band noise, you will take the"transparency" approach; if it is a narrow-band noise, you must design a Resonant Absorber tuned to the frequency (or frequencies) where most of the sound energy lies.

These two design approaches are given in detail in sections III and IV, along with illustrative examples.

Take care, when you measure the noise spectrum, that you account for the full range of operating conditions of the equipment and/or the work materials. A change of operating speed or work material can significantly affect the noise energy distribution.

Also, you should allow for a possible change in the dominant noise frequency as the equipment ages and wears.