Sound-Absorbing Drapery: Theory & Application
Sound-control acoustic curtains
Acoustical drapery is a good option where sound absorption is required and standard 1” or 2” thick wall panels are not possible. It is particularly useful where the material must be retracted, such as over glass, allowing for a clear view during non critical-listening functions.
Recommended applications include:
- As blackout curtains and reverberation control in a home theater with a large proportion of glass/windows
- To provide alterable acoustic conditions in churches that have both classical/traditional “choir & organ” and contemporary “praise & worship” music programs
- To attenuate chatter and din in a large hall or crowded dining room (noise reduction)
- To eliminate sound reflections off recording studio and vocal booth windows
- To provide tunable acoustical control (“live” to “dead”) for listening conditions in a music studio
Note that sound absorption material is often called “soundproofing.” I believe that the term “soundproof” is usually misapplied, which will be discussed in another post. Sound absorption curtains are not able to significantly reduce noise transmission between two adjacent spaces; they are rather designed to improve sound quality and reduce reverberation levels within the room that they are installed.
The acoustic curtain material must be thick and highly porous. The thicker the absorption material, the more effective it will be against a longer wavelength (lower frequency) of sound. In order to be effective against “bass” frequencies, sound-absorbing acoustical panels typically need to be at least one to two inches thick. Since it would not be practical to have a 2” thick drapery, you must select the thickest, heaviest fabric your budget will allow. I recommend a 32 oz. fabric weight (or greater).
The material must also be highly porous. These pores act as thousands of tiny sound traps, capturing the energy and turning it into heat. A non-porous vinyl, leather, or acrylic-faced fabric will reflect much of the sound energy, reducing the effectiveness of your acoustic drapery. Choose a fabric that could soak up and absorb water, or has a plush, velour surface.
I have had success with the following:
If cost is a concern, you could choose a less expensive fabric with 24 oz. minimum thickness. Note, however, that this thinner fabric will be less effective at bass and lower midrange frequencies. Another option would be to use multiple layers of a thinner material (e.g. two layers of 16 oz. fabric).
In order to improve the low and mid frequency sound absorption performance of your acoustical curtain, the material must be pleated. This will cause the fabric to be “gathered,” such that it loops in and out (i.e. does not lay flat). The pleating should be as deep as possible in order to expose more sound-absorbing surface, thus increasing effective thickness and improving low frequency sound attenuation.
Minimum pleating at 50% fullness is required; 100% fullness is recommended.
For a more complete explanation of drapery pleating and fullness, please visit:
Distance to Wall
The drapery will also become more effective at absorbing longer sound wavelengths (lower pitches) if it is spaced several inches from the wall or window. Up to a point, the deeper the spacing, the more improvement to low frequency absorption you will see. Consider spacing the drape 6 to 12 inches from the surface behind.
The means and mechanism of mounting will not affect acoustical performance but does determine how easy it will be to open and close the drapery. Motorized winching systems are easy to operate but very expensive. For most applications, simple manually-operated rigging is the most cost-effective option. If you purchase your drapery from a theatrical supply house, they can also guide you on the appropriate mounting hardware.
More information on the various rigging systems is available HERE:
The following is a comprehensive list of known suppliers of 32 oz. drapery products:
Mainstage Theatrical Supply
San Diego, CA
Please contact me if you know of other suppliers, and I will add them to this list.
The retractable nature of sound-absorbing drapery allows us to easily test its sound absorption performance. Sabine absorption coefficient ratings can be calculated from the average tested reverberation times with and without the material exposed. Onsite acoustical testing data for the above material specification is as follows:
Material: 32 oz. Magic Velour with motorized rigging. Drapery fully retracts into a drywall pocket
Hall: Community Church of Vero Beach – Vero Beach, FL
Acoustical Design: Eric Wolfram
Tested reverberation time, drapery exposed (T30, average of 6 positions):
125 Hz = 2.184 seconds
250 Hz = 1.996 s
500 Hz = 2.000 s
1000 Hz = 1.830 s
2000 Hz = 1.808 s
4000 Hz = 1.462 s
8000 Hz = 1.040 s
Tested reverberation time, drapery fully retracted (T30, average of 6 positions):
125 Hz = 2.350 seconds
250 Hz = 2.142 s
500 Hz = 2.150 s
1000 Hz = 2.040 s
2000 Hz = 1.986 s
4000 Hz = 1.562 s
8000 Hz = 1.106 s
Material Surface Area: 685 ft2.
Room cubic air volume: 266,086.64 ft3.
Material Absorption Coefficient (calculated from above)
125 Hz = 0.62 Sabines/ft2
250 Hz = 0.65
500 Hz = 0.66
1000 Hz = 1.07
2000 Hz = 0.94
4000 Hz = 0.83
8000 Hz = 1.09
Based on the data above and similar tests at other facilities, the sound-absorption qualities are surprisingly good. Although fabric wrapped fiberglass panels provide much better low frequency absorption, this drapery is an excellent option where material retraction is necessary. It is also very effective at altering the reverberation period and acoustic response of a space to support a wide variety of musical styles and instruments.