Acoustic noise refers to waves of pressure travelling through air or other gases. Sound is acoustic energy in the audible range, i.e. acoustic energy capable of being heard.
- Human generated
- Rotating pumps
- Industrial machinery
- Air traffic
Once acoustic noise couples into an instrument (or any mechanical structure, for that matter), it becomes structural vibration. Therefore, the effects of environmental acoustic noise are very similar to, and often indistinguishable from, structural vibrations.
Acoustic noise affects instruments in different ways, depending on their sensing method and level of precision. For imaging equipment, it can lead to blurred images or the appearance of sawtooth patterns along feature edges. For quantitative instruments, it can reduce the accuracy of measurements. In extreme cases, acoustic noise can prevent the measurement from being taken altogether. As noise levels fluctuate throughout the day, acoustic noise can also reduce the repeatability of measurements.
Since vibrations are carried through the ground and up through the structure, it is easy to determine the path of travel and to decouple the sensitive instrument from the noise source or place an isolator in between the two. Sound waves, on the other hand, come from all directions. Wherever there is an air passage, there is the possibility of acoustic transmission. Thus the ideal acoustic barrier would be airtight. For the purpose of enclosing instruments that are in use, this presents obvious challenges in terms of access, visibility, and usability.
Another challenging characteristic of acoustic noise is its potential for reverberation. Reverberation is the persistence of sound after the original sound has been removed due to the sound waves’ reflection off of surfaces. Because of sound’s tendency to reflect, the acoustic characteristics of every location varies according to the size and shape of the room and the objects present in the room. This variability makes it difficult to anticipate how sound will behave in any given environment and thus makes it more difficult to design an acoustic solution that performs well in all environments.
When locating the source of acoustic disturbance, it is tempting to just listen for the source to determine its exact location. This approach, however, is inherently flawed as the audible range for acoustic noise only extends from twenty hertz to twenty kilohertz. There can be troublesome noise below or above this spectrum that may be negatively impacting the utility of sensitive equipment. Using research grade microphones, site survey equipment, and data analysis software to gather and analyze acoustic noise data is the best way to evaluate the noise present in a given location. As with vibrations, it can still be difficult to predict how an acoustic isolator will perform when in use with a specific instrument in any given location.
The best way to reduce the effects of acoustic noise is to eliminate the source of the noise: turn off the offending piece of machinery or institute a “Quiet Please!” policy when taking measurements. Unfortunately, some sources of acoustic noise are inherent to the environment, such as external wind or a building’s HVAC equipment. Some sources are otherwise impractical to eliminate, in which case you’ll need to isolate the testing setup itself.
The first step is to determine what is the best solution available for the equipment needing protection. There are several low-cost options, such as acoustic curtains or baffles that can deflect acoustic energy that is transmitted directly from a nearby source. These low-cost solutions may provide sufficeint isolation for applications that are not severely sensitive to acoustic noise, but please note the amount of reduction provided by these solutions is minimal. Another option is to construct your own acoustic enclosure. Homemade enclosures can be made using plywood, cardboard, or Styrofoam, with various lining materials used to absorb noise at higher frequencies.
Active noise cancellation, such as the Bose noise cancellation headphones, has yet to become a viable technology in the acoustic enclosure field due to the difficulty of isolating multi-directional sounds in such a large volume. Characterizing that level of acoustic noise in such a large area is untenable (currently).
The most surefire way to isolate acoustically sensitive instruments from offending noise is to provide an acoustic enclosure, which provides the highest level of noise reduction over a broad frequency spectrum. High performance acoustic enclosures will provide an almost airtight seal around the instrument to choke off the avenues of travel for acoustic energy. When evaluating an acoustic enclosure, you should keep in mind how you use your instrument and make sure that ergonomic requirements are sufficiently addressed.