Cause

In very tall spaces, it’s common to encounter a ceiling made of steel decking or concrete. These building materials are considered to have low absorption coefficients, in other words; they are highly reflective and do not absorb sound well. These two physical characteristics of a room will increase the amount of time a particular sound will remain audible, even after the original sound source has stopped.

As sound travels towards the ceilings, it will encounter these highly reflective surfaces. Only a very small amount of energy will be absorbed in the surface, allowing the sound wave to have enough energy to continue travelling within the room. In technical terms, this is known as the RT60 value of the room, and can be described as how long the residual sound will take to reduce in volume by 60dB after the original sound source has stopped. It normally requires a sound source of 100dB to accurately measure this value as a typical noise floor level is 40-45dB (office noise).

Guidelines

There are some published guidelines on the optional suggested RT60 values for various rooms depending on their purpose of use. Primarily, for smaller rooms (compared to a cathedral, church or concert hall) these are weighed to provide greater speech intelligibility. So, how can we reduce the RT60 of a room? Especially one with high ceilings. There are two main solutions; lower the ceiling by installing an acoustic ceiling (ceiling tiles) or add broadband absorptive material by way of ceiling clouds or baffles.

Absorption materials come in different forms and some are not certified (Class 1/A fire/burn and smoke) to be used in public assembly spaces. We recommend high-density glass wool (6lbs/ft3), which is dense enough to absorb lower frequencies and is not too rigid to reflect high frequencies.

Ceiling Coverage

The right amount of coverage in the ceiling is also important, we recommend a minimum 40% of overall treatment required for the room. However, covering the ceiling with acoustic treatment could be the only practical solution in some cases. In this situation 100% of the acoustic treatment will have to be installed on the ceiling. This can be challenging, primarily due to the lack of usable space, and ceiling obstructions such as lighting, ducts, air registers, speakers, sprinkler heads, etc.

A combination of horizontal ceiling clouds (Nimbus) and vertical ceiling baffles (Saturna) would be a good compromise. The Nimbus clouds will take care of reflections between the ceiling and floor; the Saturna baffles will reduce the sound travelling from side to side within the ceiling area. Staggering the height of Saturnas is recommended. Sometimes, you may have to choose one or the other, based on the situation. Lowering the Nimbus from the ceiling will provide another advantage of dual absorption, by using the back of the panel to absorb and stray reflections that reflect back from the ceiling. You could also increase the panel’s absorption performance in the lower frequencies.

Final Steps

Care should always be taken when embarking on this type of project as it’s important not to violate any code regulations. If you’re looking to reduce noise in a room with a high ceiling, be sure to contact Neel Swinnen or Steven Kemland. Our team of experienced acousticians is here to help.

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