Quiet Please

Sound-attenuated packages balance benefits and cost.

Noise emissions are quickly becoming a concern, with many engineers specifying sound limits in contracts. “If it is in a residential area, sound attenuation plays a pretty big part because nobody wants a big diesel motor running right beside their house,” says Steve Lowder, rental manager, Sunbelt Rentals.

The demand for sound-attenuated equipment is on the rise. “We are even getting asked for it more and more in non-residential areas,” says Lowder. “In a lot of specifications that are written, the engineers are putting into the specs that the units have to be sound attenuated.”

Even when it is not required, sound-attenuated equipment still finds its way onto jobsites. “A lot of times in residential areas, even if the engineer doesn’t specify that it be sound attenuated, the contractor will ask for it,” Lowder notes.

Although they are more expensive, Lowder reports that customers are willing to pay higher rental rates for quieter equipment. Consequently, he places a high priority on sound attenuation when selecting equipment such as generators for his rental fleet. The quality of the generator ranks first in terms of criteria, followed by sound attenuation, with price taking a distant third place.

Sound vs. Cost

There are practical limits to what can be realistically done to attenuate sound. The trick becomes balancing the cost of the sound-attenuated packages with the benefits. Tim Gunnels, manager of product and market development, Terex Light Construction, explains the dilemma.

There is a tradeoff between noise, size and cost, he reports. Consider generators and compressors, for example. It is possible to manufacture units that are much quieter than those currently available. But there are size and cost limitations that must be taken into account. You have to find the optimum solution that meets required sound levels, yet doesn’t add unnecessary cost. “Some competitors focus too much on sound attenuation, leading to very high costs,” Gunnels adds.

Given a large enough package, it would be possible to reach very low noise emission levels. “The difficult thing for manufacturers is growing the size of the product,” says Jim Rose, director of product management, MQ Power. “If we could mount every generator in a 40-ft. box, you would never hear it. But of course that would not be user friendly. So manufacturers have to be very conscious of the overall size of the respective product, then deal with the noise accordingly.”

Those in the sound-attenuation business have been at it for a while, so the opportunity to do something unique is limited. Two basic areas of opportunity include air flow and sound-attenuating materials.

There are different grades and qualities of sound-attenuating material. Foam, wool, fiberglass insulation and more exotic materials can be used in the enclosures. “Generally, the more exotic the materials you put in that box to keep it quiet, the more your overall costs go up,” says Rose. “That is where each manufacturer has to make a decision of how quiet they really need to be vs. the cost.”

Air flow is also critical to sound attenuation, but it also has to be adequate to cool the engine. For instance, the design for Terex Power Super Quiet generators brings all the air in on one end and exhausts it on the other end.

“There are no openings on the side where the doors are and therefore no places for the sound to escape,” says Gunnels. “This one-way flow helps in noise and cooling.” It is very quiet when approached from either side, he notes. It is around 60 dB when approached from the sides and slightly louder when approached from the end. Louvers in the intake end of the enclosure, with sound-dampening material, help control noise from the engine.

“How you direct air flow in and out of the package is going to determine how much noise is emitted,” Rose agrees. That is where design really come into play.

“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?

“Anybody can do it better,” says Rose. The challenge is keeping the size down and keeping the cost down. You have to maintain that competitiveness.”

A Tip for Smaller Units

Brad Parker, Rental Service Corp. store manager, shares the following low-tech solution to redirect sound from smaller generators (3,500 to 6,500 W) away from the work area.

“Attach a 20- to 30-ft. piece of flex hose (one that can handle the heat) over the exhaust. What this does is direct some of the sound down the pipe and away from the work area,” he explains. “Of course, it only works if there is an available place to direct the sound that will not cause additional problems for another location.”

When using this technique, he cautions, “It’s important that the hose only covers the exhaust, rather than seals it. Sealing the exhaust can cause back pressure problems and force the engine to stall.”