For as long as I can remember, engine exhaust silencer manufacturers have used attenuation “grades” to describe the expected performance of their products. “Critical”, “hospital”, and “residential” are some of the common grades used today.
As an attempt to differentiate themselves from one another, silencer manufacturers have come up with new grades such as “super critical”, “super hospital” and even “extreme”!
Although silencer attenuation grades are usually accompanied by a broad estimate of their actual noise attenuation performance (i.e.: 18-21dBA), these levels are neither regulated, nor standardized† across the industry. This means that one manufacturers’ “critical grade” silencer might be superior or inferior to another’s manufacturer’s “critical” silencer. With this in mind, you might see how any specification that relies on a silencer “grade” can leave itself open to interpretation.
So, how can you avoid a subpar product from finding its way to your client’s facility? I recommend to my clients that they use something clearly measurable when specifying exhaust noise attenuation requirements.
Here are two sample specifications, both obtained from real projects, but only one requiring clear and measurable results:
Specification #1: A critical exhaust silencer shall be provided in accordance to the engine manufacturer’s recommendations for silencing. The silencer shall provide extreme noise attenuation for environments with low background noise, where slight noise emissions would be objectionable.
Specification #2: Silencer shall be sized as recommended by the engine manufacturer and selected with a minimum sound attenuation of 25dB at 500 Hz. Additionally, the exhaust sound level, measured at a distance of 10 feet, 90 degrees from the exhaust discharge, shall be 85 dBA or less.
Big difference, right? While “critical” and “extreme” could be interpreted quite loosely in Specification #1, any decent sound meter can be used to test against Specification #2.
When designing your next emergency power system, keep your client’s neighbors happy (and your client out of trouble) by adding some measurable requirements to your engine exhaust specifications.
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† Updated 3/2015: EGSA has now published a standard to address this issue. Read more.
David,
Good article. One aspect that I have noticed often forgotten in writing the specifications is the location of the engine relative to the building. Reflected noise can become a property line issue when not considered in the preparation of the muffler/enclosure specification.
John
Thanks for the feedback John. You make a good point… nearby structures, and even buildings in adjacent properties, can reflect the exhaust noise and create problems for the owner.