Noise Attenuation: A Light Touch

Ed.note: The next two letters pertain to John S. Clark’s article "Congregations In Comfort,” which appeared in the January 2008 issue of Engineered Systems (page 72).

I have been a consultant in acoustics and noise control for over 35 years, and I find that an important consideration in worship spaces (as well as or school rooms) is the hearing conditions, another comfort issue, if you will. True, the matter of economics figures heavily into both environments. But good hearing is mandatory for churches as well as our schools (especially among the young). In this modern era of forced air
HVAC, fan and circulation noise is always with us, so noise control is always an issue if these occupied public spaces are to perform their mission well.

Acoustical consultants work daily on this optimization. We devise ways of engineering cost vs. benefit and we strive to satisfy both. For instance, I have designed and recommended that an inexpensive lined “plenum,” fashioned solely by the mechanical contractor, be tucked somewhere into the air supply and the air return ducting systems in lieu of “expensive attenuators.” I also recommend the use of flexible ducting instead of some lengths of hard-walled sheet metal ducting (flexible ducts allow fan noise to escape harmlessly before the air is delivered to the occupied space). The grilles over air return duct opening are a frequent and a strong source of noise in worship spaces. Simply omitting them, or widening the spaces between their vanes, is the quietest and cheapest solution. So there are ways to beat the odds in this noise control game.

Angelo Campanella, Ph.D., P.E.

I found the “Silence May Not Be Golden” to include several misguided generalizations on acoustical design of air-handling systems for worship spaces. Based upon my 30 years of experience with the design of air space for good hearing, here are some facts readers may not be aware of:


The background sound level appropriate for a new worship space can be identified knowing such parameters such as the farthest talker-listener distance, the room shape, and whether a sound reinforcement system is planned. Just as other HVAC design parameters are engineered, the HVAC system can be engineered to meet the needed target value of background noise so that people will hear well. What else is a worship space for it not for good hearing?

Three things make noise in air-handling systems: fans, air turbulence in ducts and fittings, and air turbulence at diffusers and grilles. Lowering air speeds is the method of choice for controlling noise of the second and third; fan noise can only be controlled via used of duct silencers or duct liner if there is insufficient attenuation offered naturally by the supply and return duct system.

My point for readers of Engineered Systems is that air-handling systems for worship spaces can be engineered to meet both thermal comfort and acoustical needs. Readers may be interested to know that acoustical consultants across the country providing advice on such matters are listed on http://ncac.com/, the website of the National Council of Acoustical Consultants.

Edward M. Clark, P.E.
Principal
Ostergaard Acoustical Associates
W. Orange, NJ

Clark responds:
Being able to hear the “Spoken Word,” in a worship space, is the first priority in dealing with the acoustic solutions applied to both the building space and the HVAC system. Most church construction projects have an acoustical consultant as a member of the design team. This consultant works with the architect, the HVAC engineer, the sound system engineer and the church’s building committee to arrive at visual solution, a comfort solution and an acoustical solution that fits within the construction budget.
 
In my over 40 years of experience as both a designing mechanical engineer and a member of church building committees, this team process works very well. The mechanical engineer team member must be aware of all the noise sources from fans, duct turbulence, and diffuser/grille terminal velocity. A range of economic solutions is available to deal with all the objectionable space air noise. The range of solutions can always be rated as good, better, and best, depending on the money available.

Integration Options

Regarding Kevin Dickens’ article on water vs. air cooled chiller plants for data centers (“This Time, Water Cooled Was All Wet,” April 2008, page 34), I believe there is a significant flaw in the analysis that skews the energy analysis towards the air cooled plant: The air cooled plant has an integrated economizer (the dry coolers are in series with the chillers) while the water cooled system economizer is non-integrated (the heat exchanger is in parallel with the chillers). Clearly, the water cooled economizer could also have been integrated (in series with the chillers) allowing it to provide precooling when it is not able to provide full cooling. In fact, ASHRAE Standard 90.1 requires such an integrated design for most applications.

Steven T. Taylor, P.E., FASHRAE
Principal,
Taylor Engineering, LLC
Alameda, CA

Dickens responds:
We actually faced this question during the design, and your logic is right on target. The user was just as smart as you (at least on this point) and raised the exact same question. As I wrote in my article, I was skeptical to start with and this “discovery” by the enduser had me convinced, once again, that we were on the wrong track. When we ran the numbers, we found exactly what you would expect: The energy numbers for the water cooled equipment decreased accordingly and the head-to head on energy alone went to the water-based system. But note that on the energy side of the equation it was always close.
However, the water costs were still so significant that we still were seeing total life-cycle costs that were less in the Northeast, and almost break-even in the South. The final decision, therefore, was based on the relative simplicity of the air cooled system and the water conservation aspect ... a green issue as important to the user as energy use.

Note that I do not think, nor did I pose, that this system would work everywhere. The comparison is so dependant on climate, local utility costs, life-cycle duration, and user mindset that ultimately it simply becomes another option to consider, especially in northern climates.

I so appreciate your response because it does raise the serious issue I failed to discuss in my limited space regarding a point I should have made considering my complete analysis. And regarding the 90.1 requirement for series arrangement on water-based systems, I must, admit ignorance. But while once I was lost, now I am found, and I will explore this aspect of the Standard as will those who have the opportunity to see your comments and my response.