“The manner in which we take air in and out of our box has done two things: it has actually allowed us to improve our cooling, and we have been able to lower the overall sound. Technologically, some of the best results come from some of the simplest things — redirect the air flow and take advantage of it.”
Generate Power, Not Noise
Sound specifications are readily available for many generators, but what exactly do these numbers mean? To put decibel ratings into perspective, Rose notes, “In a normal office, background noise is going to be around 60 dB.”
The measurement of sound in decibels is logarithmic. Each decibel is separated by a factor of ten. Therefore, 40 dB is not twice as loud as 20 dB — it is actually about nine times as loud! A 10-dB increase translates to almost triple the noise.
The industry standard for a generator or compressor is to measure the sound produced at a distance of 23 ft. (7 meters) from the unit. Some of today’s generators are capable of producing under 60 dB. “Our Ultra Silent product is as low as 56 dB,” says Rose. “You could stand next to this unit, talk in a normal tone and not feel uncomfortable.”
Lowder reports that Sunbelt Rentals specifies a decibel limit on the Terex Power and MQ Power generators it buys. “We make sure they are right around 65 dB,” he explains.
Compressors Strive To Meet Standard
Compressors are even more of a challenge when it comes to sound attenuation. In addition to engine noise, there is also noise generated by the airend.
The industry has set an internal target. “The whole industry is trying to achieve 72 dB,” says Gary Mueller, district manager for the Mobilair compressor line at Kaeser. He reports they have met the 72-dB goal. “I don’t think anybody is below 72 dB.”
Similar to generators, air flow is critical to compressors. “You have to have air coming in the proper way for cooling, as well as to feed the compressor,” says Mueller. A lot of attention has to be focused on attenuating the intake and exhaust.
But attention must also be focused on the airend. Almost all towable compressors have moved to rotary airends, and rotary compressors are gaining popularity in other applications.
Yet, reciprocating compressors are still popular in certain applications, such as service trucks. “Reciprocating compressors are much noisier,” says Mueller. This is the result of their design, with pistons moving up and down inside of a cast iron casing.
While rotary compressors cost more up front, in many applications, the long-term costs are actually less due to their longevity. Unlike pistons in a reciprocating compressor, the rotors in a rotary screw unit don’t actually touch other components.
The speed at which the compressor operates and the size of drive motor also influence the noise emissions. Kaeser accomplishes the desired flow at lower compressor speeds by using a larger airend and a direct drive. “If our engine is running at 2,500, so does the airend, reducing noise significantly,” explains Mueller.
“In order to get the same air output, other manufacturers — using smaller airends — would have to run as fast as 10,000 rpm,” claims Mueller. The downsides are increased wear and greater heat generation.
Efficiency is also important. If you can get the same air flow with a smaller engine, less noise is generated and you burn less fuel.
Emissions Levels Tighten
While noise emissions in North America continue to come under scrutiny, Europe leads the way in terms of regulations. “That’s one of the reasons we have been able to stay ahead of the curve,” says Mueller. Companies that sell products in Europe must comply with stringent noise regulations.
Although local cities have noise ordinances, and contracts often specify permissible sound levels, there is no sweeping legislation on the immediate horizon. However, that doesn’t mean the noise emission issue won’t continue to play an even greater role on your jobsites. Sound-attenuated units are in demand, and they command premium prices.
The trick is going to be in how you balance the need to keep the jobsite quiet against the added cost. It is possible to make quieter units, but are you willing to pay the premium and deal with bulkier units?