Do Trees Block Sound? How Do We Soundproof a Room? We Answer Some Common Questions About Acoustics

27 Mar Do Trees Block Sound? How Do We Soundproof a Room? We Answer Some Common Questions About Acoustics

Do trees block sound? How do you soundproof a room? Why is there a fountain in the lobby? As acoustical consultants, we’re often asked which noise control measures are the most effective. In this blog post, we answer a few of the most common questions, and dispel a couple of recurring acoustical design myths along the way.

“The sound of passing vehicles gets annoying. If we plant some trees, will they reduce the noise?”

Unfortunately, no. Sound travels through air. Therefore, if air can directly pass through a barrier, then sound waves will too. While it might seem like vegetation reduces noise, it’s all perception. That “sound-blocking” tree or shrub merely breaks the visual connection between us and the noise source, making the noise seem quieter.

To screen noise effectively, a barrier should be solid (i.e., no gaps), have sufficient mass, and provide adequate height to create an acoustical shadow at the receiver. Earth is great at this. Hedges or trees? Not so much. However, there is research to support that biophilic design promotes wellness and reduces stress levels, so, there’s no reason that vegetation can’t be incorporated into a solid barrier.

“We bought our daughter a drum set. How do we soundproof?”

The term “soundproofing” implies total elimination of noise, which, by definition, is not possible. By enclosing a noise source with different materials, we can reduce the levels of noise—often to very quiet hearing thresholds. Effective materials usually consist of a mixture of absorption (such as foam or insulation), massing (with dense products, such as concrete or steel), and springs (such as resilient mounts or isolators).

However, when the energy of the noise interacts with these materials, they become excited, making them transfer points of noise themselves. This process is called structure-borne noise transfer. The BC Building Code has recently placed an emphasis on the control of this indirect (or flanking) noise transfer, which you can read more about here.

With effective noise control, comfortable noise conditions can be achieved. So instead of “soundproofing” we prefer terms such as sound “attenuation,” “mitigation,” and “reduction.”

“If we replace a wall’s existing insulation with acoustical insulation, that should dramatically improve the sound isolation performance of the wall, right?”

Not exactly. This is a common misconception. Absorption within a wall cavity prevents the wall from behaving like an oversized spring. How? Insulation dampens the residual sound energy that has been transmitted into the wall from one side. This happens before that energy passes through the other side of the wall cavity. However, the total sound isolation performance of a wall depends on more than the insulation alone.

Generally speaking, good acoustical walls have a combination of heavy outer linings that are either resiliently mounted to, or installed on, wide, separated studs with insulation. Simply replacing existing insulation within a wall cavity without separating studs or increasing the massing on the outside(s) of the wall is unlikely to substantially increase its acoustical performance.

If you were opening up an existing wall to improve its acoustical performance, we recommend either installing a separate, discontinuous (i.e., mechanically decoupled) set of studs with insulation or remounting the outside linings on resilient clips—with care not to short-circuit the clips. Alternatively, you can also improve the wall’s acoustical performance by adding another layer or two of dense acoustical plasterboard over the existing plasterboard.

“Okay. Then instead of acoustical insulation, can we install another internal shear layer between the studs to improve the sound isolation performance of the wall?”

We would strongly advise against this. This approach could reduce the sound isolation performance of a wall by up to 5 to 15 sound transmission class (STC) rating points, due to air spring and coupled resonance. A detailed explanation of this acoustical weakness, called mass-air-mass resonance, can be located here.

“Our office can get noisy sometimes. Is it true that lower background noise means less disturbance?”

When background (or ambient) noise levels are too low, people are more likely to be disturbed by intermittent noise sources such as vehicles moving, people talking, and alarms ringing. Optimizing the background noise levels electronically (with an active sound-masking system), mechanically (with the ventilation system), or naturally (with fountains), can successfully mask these disturbances and promote room functions.

Popular guidelines for target background noise levels include the Canadian Mortgage and Housing Corporation (CMHC) and the ASHRAE Handbook.

At BKL, we’ve been providing advice on acoustics since 1966. If you want to learn more about acoustics, or have questions about practical design solutions for your commercial, residential, or industrial project, please contact us today.

Written by David Stepanavicius

Photo by Darren Nunis on Unsplash