It's fairly easy to control personal exposure to noise by limiting exposure time or using hearing protectors like ear plugs or ear muffs. Engineering controls are usually applied to the machinery that produces noise.
Imagine a 200 foot x 175 foot manufacturing facility with a 30 foot high ceiling. In this building are a variety of cut-off saws, grinders, millers and other metal-working machines. There is also a small foundry shake-out station. When normally operating, the overall noise level is 89 decibels (A-scale). Individual machines can produce up to 106 decibels at the point of operation. The HVAC system has long runs of rectangular duct which vibrates visibly. Management would like the overall noise level to be below 85 decibels.
One of the options management has under consideration is to hang sound-absorbing panels from the ceiling. This approach, while effective, would be prohibitively expensive. We feel a more effective approach is to identify each machine that produces noise over 85dB, and then control the noise at the source. This allows for much greater flexibility in designing controls and selecting noise absorbing materials.
Almost every sound-producing device generates a mix of frequencies and intensities (decibels). What we perceive with our ears or measure with a sound level meter is a composite of all these frequencies and intensities. Usually, an Octave Band Analysis (OBA) is needed to fully analyze the sound produced by a machine. The OBA will allow you to measure the intensity (in decibels) at 31, 62, 125, 250, 500, 1,000, 2,000, 4,000 (and sometimes 8,000 depending on the OBA model used) Hertz (Hz). This is important. Low frequency noise (up to about 125 Hz) is more difficult to control but has less potential for impairing hearing. The higher the frequency, the easier it is to control. This is fortunate, because noise intensity at higher frequency is more damaging to hearing than the same intensity at lower frequency.
The most effective noise control method is to leave the noise producing machine off; but, that defeats the purpose of having the machine. Other, similar approaches would be to put the offending machine outside the work area. In some cases, that will work. An HVAC blower fan could be mounted outside the building it serves, for example. But, in most cases, the noisy device has to be inside the workplace, usually near work stations. For the remaining situations, noise can be controlled (i.e. absorbed or deflected) at the source, usually in the form of barriers or small enclosures. The choice of material to construct these controls depends on their sound absorbing capacity. Engineering tables, showing this capability for a variety of materials, are available. Controlling noise at the source is generally more effective and more economical to install and maintain than other approaches.