How acoustics impact our ability to hear
Community spaces can involve a plethora of different uses within a building. Many of the uses can become acoustically sensitive depending on the activity being performed. From an acoustic perspective, areas of interest could involve multipurpose rooms, meeting rooms, private offices, lobby or atrium spaces, open/shared work areas or group activity rooms. When designing these spaces for acoustics, there are three main categories of architectural acoustics that are typically considered. These are:
Interior acoustics or ‘Room acoustics’ refers to the behaviour of sound in an enclosed space. We can all remember the acoustics of the high school gym. You could hear the coach, but not understand what they were saying! If sound bounces around for too long in a space, it can feel too reverberant and interfere with speech intelligibility, music appreciation, or just feel uncomfortable.
When evaluating the room acoustics of a space, there are objective parameters that describe how well a space would work for things like speech, or music, or acoustic privacy within an open office. Here are some of the parameters:
- Reverberation Time (RT60) is defined as the length of time it takes for sound to decay by 60 decibels from its initial level. The longer the time (measured in seconds) the longer initial sounds can linger and interfere with the next sound that a listener needs to hear. An ideal reverberation time for a large meeting room is 0.6-0.9 seconds.
- Acoustics Strength (G) is a measure of how much the room or space amplifies sound naturally compared to outdoor sound propagation (in decibels). Ideally, you’d want a room that increases the sound level compared to outdoors, but a room that has too much acoustic strength can also end up being too loud for its volume if not designed appropriately (think of your kids screaming in the washroom!).
- Distinctness (D50) is a measure of how well speech can be intelligible in a space. The measure evaluates the arrival patterns of reflections from a speaker to a listener. Acoustic reflections that arrive shortly after the direct sound can help enhance our brain’s ability to distinguish speech. Reflections that arrive more than 50ms after the direct sound (such as late echoes) are detrimental to our brain’s ability to distinguish speech. The ratio of these quantities (D50) determines how intelligible sound is. D50 values of 50% or higher indicate ‘good’ speech intelligibility.
When evaluating a space for sound isolation, the focus is on ensuring that a noise sensitive space is adequately insulated from disruptive sounds in the vicinity – be it a hotel room from an adjacent hotel room, a music classroom from the adjacent hallway or classroom, or a mechanical room within the building. There are two main measures of sound isolation. They are:
- Sound Transmission Class (STC) is an integer rating of how well a building partition attenuates airborne sound. The rating system is designed to roughly reflect the sound level reduction specific to speech between partitions. There are standardized testing methods that will evaluate the performance both in a lab setting and in a field setting. There can be up to 10 points difference between the two based on site conditions, so it is important to also review the flanking conditions for a given partition. As an example, meeting rooms or multipurpose rooms would benefit from partitions with a rating of STC 50 or higher.
- Impact Isolation Class (IIC) is an integer rating of how well a building partition attenuates impact noise. This is different from airborne noise in that the source of noise is a structural vibration that enters the partition.
When evaluating the space or a building, it is often critical to determine the expected level of noise introduced from building services. This noise will eventually define how quiet a space becomes when there is no activity in it. It doesn’t matter if the reflection pattern in a lecture theatre is optimal in a case where background noise from a nearby fan dominates the acoustic environment. Sources of background noise that are evaluated typically include:
- Airborne noise from mechanical rooms and other mechanical equipment
- Duct-borne noise from mechanical units (e.g. Air Handling Units) that travel through ductwork and exit into a space through the terminals, or break out through the ductwork themselves.
- Airflow noise from turbulent airflow in the HVAC system
- Exterior noise from traffic, nearby industry, flight paths, etc
- Exterior structure-borne noise from nearby subway or rail systems
Any or all of these sources could potentially cause an unwanted intrusion into a space, and would need to be considered separately.
Designing a space and having it be successful is a multidisciplinary effort. Any one of the above aspects has the potential to make a space acoustically unusable. With proper planning and early considerations for these aspects, potential problems can be avoided with relatively little cost.