What You Need to Know About Acoustics (Part 3)

Acoustical applications of fabrics

Stage Curtain at Baruch College Photo: Todd Kaplan

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 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.

Read Part 1 here. Read Part 2 here.

In this third and final installment, we look at some acoustic data for Rose Brand’s fabrics and discuss how these differences might be applied in room acoustics optimization.

What’s It All About? 

We are looking at ways to use fabric primarily to provide sound absorption in rooms. Sound absorption reduces reverberation and loudness, and can also control reflections that may create strange sound images, echoes, or resonances. The quantity, concentration and distribution of sound absorptive materials in a room will vary depending on several factors; room size, the shape of the room, and the types of activities that will take place in the space. These factors all need to be considered to determine the quantity and distribution of absorptive materials around the room is desired. Some situations will require a concentrated amount of material in a small area and others will require an even distribution around the room. 

The process needed to determine how much absorption is needed and where to locate it in different rooms is a subject beyond the space available here, but there are a number of excellent room acoustics textbooks and expert help available through the National Council of Acoustical Consultants website (http://NCAC.com).

Choosing Fabrics and Deciding How to Use Them

Once it has been determined much absorption is needed in a room, and roughly where it needs to be placed, it’s time to take a look at materials to see how much absorption they provide. The amount of sound absorbed by a material is measured and reported as an “absorption coefficient” represented by the Greek Letter alpha (α). Values for alpha range from zero to one and you can think of an alpha of zero (0% absorptive) as representing a perfectly reflective surface and an alpha of one (100% absorptive) as a perfect absorber – an open window that sound goes through and never comes back.  

Alpha is reported for standard 1/3-Octave or Octave Bands to help understand how materials perform differently at low, mid, and high frequencies. The Noise Reduction Coefficient (NRC) is a convenient one-number way to compare material performance, and is an average absorption for the four mid-frequency octave bands (250Hz, 500Hz, 1kHz, 2kHz) and rounded to the nearest 0.05.

Acoustic absorption data across six octave bands are available for eight of Rose Brands popular velour fabrics.  To describe these fabrics in a bit more detail we’ve summarized their key absorption data below.  (Note that the samples at 100% fullness were box-pleated.   All samples were mounted four inches from the test chamber wall surface.)

The data below suggests that all of these fabrics are very effective sound absorbers, so it might be most important to choose the one that suits your visual preference and then figure out how much area of that fabric is needed.  If area is limited, the highest NRC fabrics will provide the most overall absorption at mid-frequencies.  In most cases, a lightweight velour will provide good high frequency performance, but much less absorption at low frequencies.  If absorption of low frequencies is required, choose heavier fabrics and add fullness via  box, knife, or pinch pleats. 

The Effect of Fullness on Absorption

With a little number crunching, we find that the change in absorption efficacy by hanging these Rose Brand fabrics with 100% fullness (using twice the fabric of a flat panel) yields an increase in absorption at all frequencies.  The improvement for each fabric and frequency band is summarized here:

As indicated below, the most substantial improvements in absorption are seen in lower frequencies and for fabrics that are lighter weight.  On average, we find about a 40% improvement in absorption by draping fabrics at 100% fullness — a useful improvement, if space to hang additional fabrics is limited. There are visual reasons for using fabrics with more fullness as well — drapes look richer and the visual depth of field and light absorption are enhanced as well.

The Effect of Spacing and Using Multiple Layers of Fabric 

There are additional ways to enhance the performance of fabrics.  All samples were measured at 4” from the test chamber wall, because less space greatly reduces performance. Fabric directly against a wall has some absorption at high frequencies if it is porous enough because the wavelength of sound at high frequencies is very small.  As we move away from the wall, we start to see the frequency of absorbed sound decrease as wavelengths get longer.

We start to see a substantial change at low frequencies (approximately 50% or more at 125Hz for a wool serge) as a result of increasing the spacing from four inches to eighteen inches, but there is little or no effect at higher frequencies.  The precise effect of spacing from the wall is not readily predictable because it depends on a number of factors including the exact materials for the fabric and wall surface, whether the gap is uniform, and wavelength of sound.   Even the way the fabric is mounted (limp or taut) can make a difference in this particular effect.  However, spacing fabric off the wall most certainly matters and measurements of specific arrangements can predict performance in use.   

We can also consider the effect of doubling up material.  After all, providing 100% fullness uses twice as much material.  So, why not use two flat panels next to each other?  Unfortunately, it doesn’t quite work out as well as we might hope. Measured results for doubling up flat panels show almost no change in performance despite a doubling of fabric quantity.  So, fullness pays but two flat panels don’t offer much benefit if they’re mounted back-to back near a wall.  

There are times when doubling-up two flat panels (or two fabric panels with some fullness) is worthwhile. If we separate two panels by twelve inches instead of keeping them tight together we can improve absorption, even for an already very absorptive fabric like wool serge, by around 5% in mid/high frequencies and 15% at 125Hz.  While this might not seem like a big change, it can have important impact on tonal balance and quality of reverberation for some performance conditions in a theater. This does take up a lot of space, especially in a small room, or a large room where maximizing occupancy is desired. Why use this approach then?  Every situation is different, but this is a pretty common condition in performance and recording spaces for variable acoustic curtains on double (or triple) tracks or banners that travel in and out of a space vertically by rolling or stacking. 

Fabric Covered Panel Options

When space is at a higher premium, it can be preferable to keep absorptive material tight to the walls. Acoustics designers often turn to glass fiberboard or mineral-wool substrates for their very good sound absorptive properties and then hide them behind a decorative fabric surface.  Many of Rose Brand’s lightweight fabrics are opaque while remaining porous, and can serve the purpose of protecting and decorating a glass fiberboard without adversely affecting its acoustical value.  

One particularly interesting fabric is Rose Brand’s Rhapsody, which allows virtually any digital image to be printed on it at widths up to 120-inches.  The following table shows how Rhapsody performs when used to cover a two-inch thick six pound per cubic foot glass fiber panel.

Covering a plain glass fiber board to protect it is important in any location where people might come into contact with it, and choosing a fabric covering that maintains or enhances absorption is important.  Rhapsody provides a big increase in low-frequency performance with only small reductions at high frequencies. Absorption by the fiberboard across the frequency spectrum is more even when wrapped with the Rhapsody fabric and the visual possibilities of a printed panel are endless.

Fabrics with Acoustic Absorption Data Available from Rose Brand

Rose Brand offers Acoustically-rated Fabrics & Foam, including acoustic absorption data across six octave bands for some popular velour fabrics. View them on the main Rose Brand website. 

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.