Mark Rogers, of acoustical consultants The Greenbusch Group in Seattle, at PNW Section May 2008 meeting on basic architectural acoustics. Photo by Ivan Tashev.
An introduction to basic architectural acoustics was the topic for the PNW Section's May 2008 meeting. It was presented by Mark Rogers, Director of the AV Department at the Greenbusch Group, a Seattle acoustical consulting firm.
Mark gave a survey of three topics; room acoustics, sound isolation (or transmission reduction), and HVAC (heating, ventilation, air-conditioning) problems (i.e. noise and vibration control). A fourth aspect, the design of the sound system, would not be addressed.
He began with an overview of acoustic fundamentals including wavelength, period, amplitude, and frequency, followed by a discussion of the Fletcher Munson equal loudness curves and the A weighting filter. A chart of the loudness of various sounds was shown, comparing the levels of conversation, jet plane, etc. and actual noise examples with specific level changes for the audience to hear a 3, 10 or 20 dB difference. Next was a review of sound propagation and the. differences between large, medium and small acoustic spaces. RT60 was discussed as was NRC (noise reduction coefficient) of products.
Diffusive surfaces were explained. Usually diffusion is good in rooms, like anti-glare for acoustics. Resonant absorbers (Helmholtz) built into the structure were mentioned. Next we went into room shapes and acoustic effects, such as flat (reflective), concave (which will focus sound for better or worse) and convex surfaces (which tends to scatter sound). Echo and reverberation were covered.
Mark spoke about the 3 room modes - axial, tangential, and oblique. Sound resonates between surfaces, like an organ pipe. All rooms do this, and are a function of the geometry of the room.
The hard part is deciding what to do on all the surfaces - using a combination of shapes, absorption, diffusion and reflection.
Also discussed were elements of sound transmission and HVAC noise. Sound isolation is often needed, and ways to do it were described. We reviewed mechanical noise from HVAC systems. This is often the dominant noise/vibration in a commercial building.
Noise Criteria (NC) curves define how quiet a room is by specifying the maximum spectral content under a certain curve. A low NC is expensive to achieve.
Active noise control was mentioned, electronic systems that try to cancel out noise. They are easiest to implement at low frequencies.
Mark recommended several texts on acoustics, and fielded many questions ranging from the Kingdome ceiling panels to the effect wind has on outdoor sound to new laws on nighttime entertainment sound levels.
Reported by Gary Louie, PNW Section Secretary and Rick Chinn, PNW Section Chair 2007-8
