Soft, porous materials such as heavy cloth, felt, woven mats and wool absorb the sound that would otherwise echo, and they work best when hung clear of the wall by about a quarter of the sound’s wavelength, 17 cm for 500 Hz. To bring the 510 m³ hall to a reverberation time of about 1 s, a full hall of 100 people needs about 46 m² of heavy hangings, a half-full one about 89 m² and an empty one about 131 m², nearly all the wall area, so extend absorption to the roof as well.
Part of Community Gathering Hall
Sound-absorption materials are the porous, fibrous or resonant surfaces that convert sound energy to heat and so shorten echo in a room.
Soft Surfaces Absorb, Sealed Ones Reflect
Sound that strikes a hard, sealed surface bounces back. Sound that enters a porous material loses energy as air moves among its fibres. The gathering-hall guide recommends hanging woven textiles on the walls (tapestries, blankets, curtains), irregular walls and a lower ceiling of woven material. How well a material absorbs is its coefficient α, from 0 (reflects everything) to 1 (absorbs everything); the room’s total absorption is the sum of α × area.
Two rules follow from the physics:
- Thickness and spacing in wavelengths. Wavelength λ = c ÷ f with c ≈ 343 m/s. A porous layer absorbs a given frequency well only if it is thick, or spaced from the wall, by roughly λ ÷ 4, where the air moves fastest.
| Frequency | Wavelength λ | Quarter wavelength |
|---|---|---|
| 125 Hz | 2.74 m | 0.69 m |
| 250 Hz | 1.37 m | 0.34 m |
| 500 Hz | 0.69 m | 0.17 m |
| 1,000 Hz | 0.34 m | 0.09 m |
| 2,000 Hz | 0.17 m | 0.04 m |
- Do not seal the pores. Paint, wax or plaster on cloth, felt or matting turns it into a reflecting skin.
At 500 Hz the quarter wavelength is 17 cm. So thin cloth tight against a wall absorbs the high frequencies of consonants but little of the low frequencies. Hang curtains 10-20 cm from the wall, or fold them in generous drapes, to reach down toward 500 Hz and below.
Worked Example: How Much Hanging Does The Hall Need?
Inputs: volume V = 510 m³ and surfaces as in Acoustic Design for Speech; empty-hall absorption 20.5 m²; α of heavy draped hangings 0.5 against 0.03 for the plaster they cover (net gain 0.47 per m²); a seated person 0.4 m²; target RT60 = 1.0 s, which needs A = 0.161 × V ÷ RT60 = 82.1 ÷ 1.0 = 82.1 m².
- Absorption still needed = 82.1 − (20.5 + 0.4 × N) for N people.
- N = 100: 82.1 − 60.5 = 21.6 m²; hangings = 21.6 ÷ 0.47 = 46 m².
- N = 50: 82.1 − 40.5 = 41.6 m²; hangings = 41.6 ÷ 0.47 = 89 m².
- N = 0: 82.1 − 20.5 = 61.6 m²; hangings = 61.6 ÷ 0.47 = 131 m², against 157 m² of wall.
So covering the walls is nearly enough for an empty hall and not feasible alongside windows, doors and the speaker’s bare wall. The roof’s underside (134 m²) is the other large surface. A woven secondary ceiling at 3 m, as the guide suggests, adds absorption over 120 m². The coefficients are typical values from room-acoustics references for about 500 Hz and are not stated elsewhere in this corpus; the real values of salvaged blankets, felt or reed mats should be measured by comparison with a known case, such as how the hall sounds before and after a hanging.
What You Need
- Heavy cloth, blankets, felt or woven mats, in the thickest available
- Poles, rope or a rail to hold them off the wall
- A frame for a woven ceiling if the roof is high
- Fire clearance from every hearth, flue and lamp
Method
Step 1: Start with the far wall (to remove the late echo) and the side walls near the audience.
Step 2: Hang the fabric in folds at about one and a half times the wall width, spaced 10-20 cm off the wall.
Step 3: Add the roof layer where the hall is tall: hang panels or stretch woven matting.
Step 4: Leave the speaker’s wall hard.
Step 5: Re-test by clapping and by listening from the back rows.
Common Mistakes
- Sealing the cloth with paint or plaster.
- Hanging it flat on the wall and expecting bass absorption.
- Using too little for the half-full hall.
Safety
Hangings burn, trap dust and shelter vermin
Cloth and matting are fuel; keep them out of reach of the hearth, flue and lamps and away from exits. Avoid salvaged foam, which burns fast and gives off toxic smoke. Damp fabric grows mould and harbours mites and rodents, so keep the hall dry and shake out or wash hangings each season.
What’s Next
- Learn next: Heating a Large Space Efficiently — the other half of comfort in a big hall
- Combine with: Partition and Divider Systems — curtains doing two jobs