What You Need to Know About Acoustics (Part 2)
Acoustical applications of fabrics
Rose Brand has partnered with our friends at Stages Consultants to create a blog series about the acoustical applications of fabrics. In this 3-part series, we’ll discuss some of the more common approaches for using fabrics in performance spaces and also the things to consider when choosing a fabric for your project.
In our last blog installment we gave an overview on use of Rose Brand fabrics in performance space acoustics. In this second installment we take a closer look at the acoustically important differences between fabrics and identify some considerations in choosing the right fabric and integrating it into your design.
The Acoustical Differences in Fabrics
Fundamentally, absorption of sound occurs when air molecules interact with the pores of a porous material. Acoustical energy, transmitted through air, is dissipated as a result of viscous effects and thermal conduction with the material. The effective absorption of a material is determined by a combination of characteristics: the porosity and thickness of the material, how deeply air penetrates it, whether it is mounted to a hard or soft backing, and whether there’s an airspace behind it.
Looking at fabric itself, porosity and thickness are mainly determined by its fiber, weave, and pile. Many theatrical fabrics are made of synthetic fiber, like polyester, which is inherently (and permanently) flame resistant. Others are made of natural fibers like cotton and silk, which can be treated for flame resistance. Those fibers are spun into thread and the thread is woven into fabric on a loom. The gauge of thread and density of the weave can change the porosity of the overall fabric structure as much as the choice of fiber. Warp or weft threads in the weave can be woven to protrude from the surface of the weave to create pile, which may be cut or left uncut depending on the desired finish appearance of the fabric.
All of these differences affect the acoustic properties of the fabric. Viscous effects, that cause sound absorption, occur at the sub-millimeter scale. Either a natural or synthetic fiber may provide great friction, depending on the weave structure, pile density, and height of the fabric. Fabrics with a denser weave have more fibers crammed into a smaller space, making the fabric more absorptive. The weave could potentially impede air flow entirely. Pile adds thickness and more fibers per square inch. It will also increase sound absorption just like a denser weave. Denser and thicker pile fabrics are easily spotted by their weight per yard. Generally, a heavier-weight material provides greater absorption as long as it remains porous, but solely depending on the weight of the fabric when selecting a fabric for acoustical treatments has its pitfalls. A lightweight synthetic velour at 13 oz. per yard is more sound absorptive than a heavyweight muslin at 10 oz. per yard. The more complex, tighter weave of the velour offers more resistance to the air than the simple weave of the muslin.
However, lightweight fabrics can be used very effectively for acoustics when combined with a porous backing. For example, a wall, upholstered with Rose Brand’s Sonic Stretch fabric or Frazzle over a porous substrate made of glass fiber or mineral wool, could yield as much absorption as a double velour stage drapery. In these cases, the fabrics are not used to absorb the sound. The fabric just needs to be porous and great looking. The substrate does the real work of absorbing sound, while the fabric is a protective and attractive covering.
Placement and Construction of Fabric
The thickness of a porous absorber plays a role in how well it absorbs sound at mid and low frequencies. Thicker fabrics will usually absorb more sound at bass and mid-range frequencies than thin fabrics. Most times there is a direct correlation to the fabric weight. A general rule of thumb is that the thickness of an absorber needs to be 1/10th of the wavelength of the sound to be a significant absorber and 1/4th of the wavelength to completely absorb sound at a particular frequency/wave length. High frequency sounds have very small wavelengths, while low frequency sounds have long wavelengths, as illustrated in the figure below.
For example: How thick does an absorber need to be in order to significantly absorb sound at 4000hz with a 3 ft wavelength. Using the Rule of thumb we multiply 3 ft by 1/10th. The material needs to be 0.3 ft or 3.6 inches.
How thick does an absorber need to be in order to significantly absorb sound at 125 Hz with a 9 ft wavelength. Using the Rule of thumb we multiply 9 ft by 1/10th. The material needs to be 0.9 ft or 10.8 inches thick.
Clearly it’s impractical to use a single material thick enough to absorb sound at mid-range or low frequencies, so we employ a few “tricks” to make materials more effective. Additional thickness is developed by sewing fabrics into layers or creating “fullness” through use of pleats or folds. A comparison guide of standard approaches to curtain fullness is available here.
A word of caution — fullness increases acoustical absorption by increasing the amount of material there is to absorb the sound. The added material means there will be more weight on mounting points and more space will be required to store fabric curtains that are tracked or drawn like blinds into storage pockets. There are many other non-acoustical considerations when selecting the right pleating and fullness for any application. The standard bolt width of a fabric will determine the spacing of seams in a curtain which will influence the spacing of the pleats. It’s important to consider how the curtain will be used and handled once it is installed. Consider the effects that folding or rolling a curtain for storage will have on the fabric over time. There are many more considerations that your Rose Brand representative can help you sort out.
One more note on fabric placement. The motion of air molecules (particle velocity) approaches zero at room boundaries. So, fabric placed directly against a hard wall is less effective as a sound absorber than a curtain hung an inch or two in front of the wall. The air trapped behind a porous fabric helps increase low-frequency absorption, while the separation from the wall maximizes viscous and thermal interaction between air molecules and the pores of the absorber. It’s not unusual for a curtain in a theatre to be sewn in box pleats or ripplefold at 100% fullness and hang twelve to eighteen inches from a wall.
Next month, in our last installment, we’ll review and compare test data for some of our favorite acoustic fabrics in the Rose Brand product line, and suggest ways of incorporating them in acoustical designs in ways that are predictable.
Fabrics with Acoustic Absorption Data Available from Rose Brand
Stages Consultants provides world class acoustics and theatre design consulting for performing arts buildings. We bring to every project the knowledge, creativity, design skills and leadership offered by only the most experienced members of our profession - with the individual client oriented service that a small firm can deliver best. You can learn more about them and their services at www.stagesconsultants.com.