Issue 04 / July 2010
Generator Sound Level Concepts
By Allen Bair
According to Wikipedia, Sound is a traveling wave which is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard.
The emitted sound level of a standby generator set has become an item of increasing interest, and will continue to rise in importance as our communities become increasingly dense. The way we measure sound is not easily understood, and it doesn’t help that we measure sound on a log scale. Indeed, I think we all can contemplate concepts easier when they are linear in nature.
Useful sound measurement is made on the log scale in dB(A). This scale equates to the human range of hearing, which is generally around 20 to 16,000 cycles. The typical sound data measurement is made seven meters (23 feet) from the source, four feet above ground, and usually is an average of four to eight points along a seven meter diameter around the source. This base line of data is made in a free field, so that reflective sound energy does not skew the data.
Specifications which utilize exhaust silencer data as their only reference to sound are missing the point by a large margin, since many mechanical and installation factors add to the actual dB(A). It would present a true picture of the complete generator set sound emissions to state a dB(A) level at seven meters, since measurements made there will automatically include the exhaust, as well as mechanical and installation noises in the total dB(A).
So it is that I would “borrow” two terms from economics in understanding sound better; i.e., micro & macro. Stay with me now, don’t get side tracked. MICRO: Exhaust silencer references, as one example, with dB(A) limits at 500 hertz, or some other frequency, are obtuse and meaningless in the overall dB(A) level, unless we are engineering a new silencer, or, enclosure, and are concentrating on this element of micro data as a design point around which we are concentrating in developing a new and better silencer. MACRO: The total sum of all the micro elements of sound including silencers, rotating mechanical noise, piston/combustion noise, engine cooling air fan noise, generator cooling air fan noise, combustion air inlet, etc. All together, these add up to become the dB(A) at seven meters that we can actually hear. And, because this includes ALL the micro elements emitted being added into one data point, I believe it’s the most useful to the design engineer.
Open style gen sets below 200kW are <88 dB’s or lower, whereas >200kW range between low to mid 90 dB’s. Putting these gen sets inside weather protective enclosures will further reduce sound three to seven dB’s, even without serious sound absorption attempts. Whatever enclosure is utilized must pass UL2200 tested heat runs before awarded its UL label.
Since sound is a straight line form of energy, it is generally attenuated by changing air flows to directed labyrinths constructed of sound absorbing materials; such as mineral wool or sound absorbing foams. Noise is no longer allowed to propagate straight through louvered sheet metal. By this means mechanical noise is reduced. Exhaust gas noise can be reduced by placing the silencer within an enclosure cooling air plenum. These Level 1 sound attenuated enclosures further reduce sound four to nine dB’s over standard weather protective enclosures.
Once we’ve sound attenuated a unit in a Level 1 factory enclosure, eliminating usual mechanical noises, further sound attenuation (Level 2) is mainly achieved by treating the velocity of air. Typically this is accomplished by increasing physical size of the enclosure as the openings and labyrinth passages for air movement must get physically larger, thus reducing air velocity. In total, these factory standard enclosures will reduce sound from 15 to 18 dB(A) at seven meters. Suppliers can also utilize specialty enclosure manufacturers to achieve reductions of 25 to 40 dB(A) at seven meters. It’s only money!
Once the generator set is installed at site, it may not achieve its published base line test data due to reflective surfaces nearby. Reflected sound is additive and will skew rated factory data upward.
Generator set suppliers are capable of providing a standby set that meets specified dB(A) levels at seven meters, when specified. In addition, sometimes property lines influence the level of attenuation required. Formulas, such as the Inverse Square Law, can be used to correct for distances. However, here’s a word of caution. Turns out sound can be as much an “art”, as a “science”. Now, isn’t that swell!
