The effects of low-frequency sounds are often subjective—symptoms such as headaches, fatigue, and loss of concentration may be reported. However, many study subjects report a sense of relief when a low-frequency noise is removed—even if they had not noticed it was there.
Commercial noise culprits
In commercial applications, low-frequency noise is concerning because sources can be difficult to spot and correct. Common sources in commercial structures include air handling systems, refrigeration compressors, climate systems, and even vibrations within the structure itself. Low-frequency noises are also harder to mitigate, as they propagate over longer distances than high-frequency noises, and are less likely to be weakened by passing through structural components like walls and windows, if those components are not insulated for sound.
The basics of acoustics
To understand how to mitigate noise in designs, it helps to understand how sound behaves in a building.
Sound is produced when a vibrating object causes the air around it to vibrate. The moving air particles collide and transfer the vibration into a pressure wave that bounces around the available space. In a room, sound will bounce off multiple surfaces, continuing to bounce around until their energy is expended. Therefore, empty spaces with lots of hard surfaces can quickly become noisy. When sound encounters something absorptive, such as insulation, carpet, soft furniture, or sound panels, the waves lose energy from interaction with the absorbing material.
In a building assembly, walls, floors, and ceilings can act as a proxy for sound speakers. In transmission through walls, most of the sound energy travels through the stud from one side to the other. The acoustic energy vibrates the wall covering and it, in turn, causes vibration in the wall stud. The sound energy transmits efficiently at some frequencies and less so at others as it travels through to vibrate the covering on the other side, re-radiating the sound into the adjacent space. The acoustic performance of wall assemblies is dependent on the construction detail. The measure of the ability of a wall to block transmission of sound is its sound transmission class (STC) or its outside-inside transmission class (OITC). These ratings describe how well a wall system reduces airborne sound transmission. The main difference between the two is the STC rating is determined over the frequency range from 125 to 4000 Hz while OITC extends the range lower to 80 Hz to account for transportation sound sources.
Sound traveling through walls tends to be airborne. Between floors, airborne sound travels through the floor assembly to the space below much the same way it travels through walls. The performance measurement for airborne sound transmission through a floor assembly is also the STC rating. Sound transmitted to the space below can be caused by impact to the floor itself, like people walking, dropping of items, or the switching on and off of rooftop mechanical equipment. Impact isolation class (IIC) measures how well a floor or ceiling reduces structure-borne impact noise. An assembly that is good at reducing airborne noise is not necessarily good at reducing impact noise, so both measurements are important to consider.
Sound transmission class
The STC value provides an assessment of a system’s ability to block sound. A higher STC rating means a better wall design. The STC value is usually determined by testing per ASTM E90-09(2016), Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements, and calculated per ASTM E413-16, Classification for Rating Sound Insulation. ASTM E336-20, Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings, and ASTM E413-16 are used for field testing. These standards provide a relative rating of how much sound transmits from one room through a partition and into another room. As an example, an architect who must design a room with a sound source emitting 100 dB and isolate an adjoining room for office workers will need to choose between various STC values when selecting a wall construction. A wall construction with an STC of 38 would not perform as well as a wall with an STC rating of 50. The latter will provide approximately 12 dB more sound-blocking performance. It is important to bear in mind a 10 dB reduction is viewed as a 50-percent reduction in loudness.
Outside-inside transmission class
OITC value provides an assessment of a system’s ability to block sound outside the building. Like STC, a higher OITC means a better performing enclosure wall. The OITC value is usually determined by testing per ASTM E90-09(2016) and calculated per ASTM E1332-16, Standard Classification for Rating Outdoor-indoor Sound Attenuation. ASTM E336-20 and ASTM E966-18a, Standard Guide for Field Measurements of Airborne Sound Attenuation of Building Facades and Facade Elements, are used for field testing.
Impact isolation class
The IIC value provides an assessment of a system’s ability to resist transmitting impact sound from a space above to an area below. A higher IIC rating means less noise from walking or impacts to the floor will be transmitted to the room below. The IIC value is usually determined by testing per ASTM E492-09(2016)e1, Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-ceiling Assemblies Using the Tapping Machine, and calculated per ASTM E989-18, Standard Classification for Determination of Single-number Metrics for Impact Noise. ASTM E1007-19, Standard Test Method for Field Measurement of Tapping Machine Impact Sound Transmission Through Floor-ceiling Assemblies and Associated Support Structures, and ASTM E989-18 are used for field testing.