Building Science Part 5 Acoustics

Poor acoustics impacts the total occupant experience (OX) including wellbeing, happiness and productivity. Unwanted, excess noise is an ongoing problem in many commercial buildings today, and its sources go far beyond the expected noise of traffic, airplanes and construction crews. Noise can originate from within a building and can have an adverse effect on the building occupants. Noise can negatively impact people’s hearing ability and diminish productivity. Because of this, the concept of sound control is vital in ensuring the comfort and wellbeing of occupants and their positive experience of your CRE investment.

To begin with, we’ll take a look at the physics of sound to gain an understanding of how sound works. With an understanding of the properties, CRE professionals can better direct the outcomes they want for their spaces.

Understanding Acoustics and Sound Control

The Science of Sound

Acoustics is the science of sound, including its production, transmission and its effects. When we hear sound, it’s actually our eardrums vibrating due to the sound energy in the air. Sound waves can travel through many different types of material, including air, water, wood and metals. We refer to any sound that is unwanted, or in excess, as “noise.”

There are two types of sound paths: airborne sound and structureborne sound. Airborne sound is what we hear when sounds radiate from a source directly into the air. Examples emanating from the building’s exterior include passing traffic, music, aircraft, highway and industrial noise. On the interior of the building, depending on their decibel levels, voices, music, motors, machinery and office equipment are often sources of airborne noise. Another leading cause of unwanted interior airborne noise is conditioned airflow through uninsulated HVAC ductwork.

Structureborne sound, also known as “impact noise,” is sound that travels through solid building materials. Examples of this are the sound of footsteps on floors; door knocks and slams; plumbing, and mechanical equipment vibrations. Think of the disturbances created by steady rain on a metal roof over a typically quiet building, such as a library, and you’ll understand what a disruption structureborne noise can be.

Sound waves propagate, or spread, in three dimensions as expanding spheres of pressure waves. Imagine blowing a soap bubble, and then another inside of it, and so on, until they expand out infinitely through the air. These sound waves radiate directly around the source and they decrease in loudness as they get farther from the source. Sound energy is reduced by half as the distance from the source doubles.

There are three properties of sound: frequency, wavelength and amplitude. Frequency is also known as pitch. The second property, wavelength, is exactly what it looks like — the distance between the start and the end of a sound wave cycle. Amplitude, or loudness, is the third property.

Measuring Sound

When we measure sound, we're actually measuring the sound pressure of a wave, which is the relative amplitude of that sound wave. The resulting amount is measured in decibels. Most activities among people, such as conversations in offices and homes, are in the 50 to 80 decibel range. For example, in an office, restaurant or other public place, once sound rises above 80 decibels, it gets difficult to communicate effectively. A difference of up to 2 decibels is not noticeable. Changes are only obvious when you’re dealing with sound level changes in the 5- to 8-decibel range.

Acoustical Performance

Acoustical test methods fall into four categories: sound absorption, airborne sound transmission, impact sound transmission, and airborne sound transmission through suspended ceilings.

Sound Absorption

This first property is the ability of a material to absorb rather than reflect sound waves. For absorption, most building components are measured for their noise reduction coefficient (NRC). Fundamentally, sound absorption, or the lack of it, is concerned with controlling sound energy within rooms and enclosed spaces. The higher the NRC, the better the material is at absorbing sound energy. As you might expect, such materials as brick, concrete block, wood floors, gypsum board and tile do not absorb much sound, while carpet with foam padding and felt finishes are better at absorbing sound.

Airborne Sound Transmission

Sound transmission loss is the decrease in sound energy as it passes through a building. The metric used to quantify that reduction is the sound transmission classification (STC). The STC value indicates how well sound is controlled from room-to-room, including through walls or through floor/ceiling assemblies. In this class, the higher the STC rating, the better. The rule of thumb is that a 10-point increase in STC means a decrease in the perceived noise by one-half.

Impact Sound Transmission

Impact sound transmission loss is the decrease in sound energy measured after the impact noise that’s generated above transfers through the floor-ceiling assembly and is transmitted into the air below. Imagine someone hopping around upstairs, over your head. That’s impact sound transmission. It’s rated using an impact insulation class (IIC) number. The IIC number is an estimate of how much the sound energy is reduced. The higher the number, the better the system.

Airborne Sound Transmission Through Ceilings

Airborne sound transmission through ceilings will typically happen when there are adjacent spaces that are connected by a common air plenum. The ceiling attenuation class (CAC) is similar to STC, but in this case, the measurement is specific to controlling sound from one space to another over the ceiling. When it comes to CAC, the higher the number, the better.

There are many ways to control sound within building assemblies. You should always use materials that tested well in these areas.

Require Sound Absorptive Materials
One way to control sound is through absorptive materials and systems. Require your spaces to use less sound-reflecting surfaces, and install acoustical ceiling systems. Cover floor and wall surfaces with absorptive finishing materials, such as carpet, fabrics and draperies. Isolate HVAC equipment and line ductwork with sound-absorbing insulation.

With the popularity of hard surface flooring, acoustical ceiling tile systems are more important in helping to reduce reflected noise and are very effective in reducing overall room sound levels. Acoustical ceilings absorb rather than reflect airborne noise and improve conversational privacy in open spaces.

Consider hanging acoustical baffles with sound-absorptive properties. Cover both walls and ceilings in large open spaces. Fiberglass and perforated metal panels on walls are especially effective in high-ceiling areas.

Halt Airborne Sound Transmission
Steel stud partition walls, wood stud partition walls, wood joist floor-ceiling assemblies and metal buildings all demand different components for optimum acoustic control.

Steel & wood stud partition walls: Here’s a fact—Light-gauge steel studs, typically used in walls, are acoustically resilient, so they reduce more noise on their own than concrete or wood. Air sealing, though, always improves sound control, regardless of the structure involved. By adding sound-absorbing cavity insulation to a framed wall, you will greatly improve the sound performance for occupants on both sides of the wall. Drywall plus fiber glass insulation is the most cost-effective option.
Wood joist floor-ceiling systems: Remember, wood easily transmit sound, so air sealing does improve sound control to an extent. You will get a more significant performance when you use sound-absorbing cavity insulation and you break the structural tie between the ceiling and the framing system with resilient channel or by using hanger wire. Floors with the highest STC values include fiberglass insulation and a wire suspended drywall ceiling over standard construction.

Halt Impact Sound Transmission
IIC ratings apply to lightweight concrete floating floors and to wood floor joist ceiling systems. There is a significant increase in acoustical performance by mounting ceiling drywall to resilient channel and adding sound-absorbing cavity insulation. Lightweight concrete floating floors are common in commercial high-rise construction. Typically, concrete slabs are good at reducing airborne sound transmission, but they’re poor at reducing impact sound transmission. So, for a 4-inch homogeneous concrete slab, the STC is typically greater than 50, which is quite good at reducing airborne sound energy, but the impact insulation class value is less than 25. So, to improve sound control, you can use resilient underlayments under floating floors to isolate the finished floor from the concrete slab and/or you can use suspended finished ceilings to break the structural tie between the ceiling and the slab to improve the IIC. The use of both underlayments and suspended ceilings does the most to improve the IIC.

Demand Sound Control

In addition to controlling sound within building assemblies, there is also the issue of sound flanking — isolating assemblies for optimum acoustical performance. You can substantially increase acoustical performance by controlling air leakage, isolating structureborne sound paths and with compartmentalization.

Like air and moisture, sound energy “leaks” through paths of least resistance. Acoustical partitions are effective, but sound does transmit around them. Blocking above, between and under partitions limits sound leaks for maximum sound control. Let’s look at some specifics.

Partition wall height: You can extend the wall all the way
through the ceiling plenum. Caulking to air seal will
maximize results.
Blocking between floors: Another location to consider is
between floors. The addition of sound-blocking material
will minimize sound transmission. This breaks the
continuous path for sound through the plenum.
Air sealing is important: We can’t stress enough the
importance of air sealing. This is no place to skimp
especially if you are investing in good systems and
assemblies otherwise.
Underfloor barrier: If you have underfloor spaces, you
can put in a barrier material, attach it to the ground and
attach it to the joists. Be sure the blocking material here
will withstand moisture.
Electrical outlets: With electrical outlets, airtight
electrical boxes block sound, or you can air seal with
caulking. If possible, stagger the outlets on opposite
sides of a shared wall so there is no direct path
for sound.
Gypsum board attachment: If you are using two
layers of gypsum board, you want to make sure that the board seams are staggered. A tip about resilient channels: using screws that are too long and reach the studs will short circuit your investment in this method and the sound control function by creating a direct sound path.
Plumbing and conduits: Plumbing and conduits can vibrate due to water movement and electrical energy. Mount plumbing and conduits directly to stud members with gasketed resilient hangers.

Sound Flanking