Speech is understood in a hall when reverberation is short, and the reverberation time follows from the room’s volume and its total sound absorption: an empty hall of 510 m³ with plastered walls, a hard floor and a boarded roof rings for about 4 seconds, which blurs speech, while 100 seated people and 40-80 m² of heavy hangings bring it down to about 1.0 to 0.8 seconds. Put the speaker against a solid wall at one end, and cover the far wall with absorbent hangings so that late echoes do not return to the front rows and the speaker.
Part of Community Gathering Hall
Acoustic design for speech is the shaping of a hall’s volume, surfaces and absorption so that a speaker’s words stay distinct across the whole audience.
Reverberation Is Volume Divided By Absorption
The gathering-hall guide gives the qualitative rules: hard parallel walls and a high ceiling create echoes that blur words; irregular surfaces, hanging textiles and a lower ceiling of woven material at about 3 m help; one wall can be angled 5-10° off parallel; and the speaker stands against a solid wall at one end of the long axis, because that wall reflects the voice toward the audience. The arithmetic behind these rules is Sabine’s formula:
RT60 = 0.161 × V ÷ A
where RT60 is the time in seconds for sound to fall by 60 dB after the source stops, V is the room volume in m³ and A the total absorption in m² (the sum of each surface’s area times its absorption coefficient α, plus people). The constant 0.161 is s/m for SI units at ordinary air temperature.
Worked Example: The Hall With And Without Its Audience
Inputs: floor 10 × 12 m (120 m²); walls 3 m high, ridge at 5.5 m, so volume V = 10 × 12 × 3 + ½ × 10 × 2.5 × 12 = 360 + 150 = 510 m³. Surfaces: long walls 2 × 12 × 3 = 72 m², end walls 2 × (10 × 3 + ½ × 10 × 2.5) = 85 m², total wall 157 m²; roof slope length √(5² + 2.5²) = 5.59 m, area 2 × 5.59 × 12 = 134 m²; total surface 120 + 134 + 157 = 411 m².
Absorption coefficients at about 500 Hz, typical values for these surfaces and assumed here: plastered wall 0.03; hard floor 0.02; bare boarded roof 0.10; a seated person 0.4 m² of absorption; heavy draped hangings 0.5.
- Empty hall: A = 157 × 0.03 + 120 × 0.02 + 134 × 0.10 = 4.71 + 2.40 + 13.4 = 20.5 m².
- RT60 = 0.161 × 510 ÷ 20.5 = 82.1 ÷ 20.5 = 4.0 s.
- With 100 people: A = 20.5 + 100 × 0.4 = 60.5 m²; RT60 = 82.1 ÷ 60.5 = 1.36 s. With 50 people: A = 40.5, RT60 = 2.0 s.
- Add 40 m² of heavy hangings on the walls, replacing plaster (gain 40 × (0.5 − 0.03) = 18.8 m²): A = 79.3 m²; RT60 = 82.1 ÷ 79.3 = 1.04 s. With 80 m² of hangings: A = 98.1 m²; RT60 = 0.84 s.
- Absorption needed for a 1.0 s target: 82.1 ÷ 1.0 = 82 m²; for 0.8 s: 82.1 ÷ 0.8 = 103 m².
So the unfurnished hall is far too live, a half-full hall still is, and the hangings matter most when the hall is not full. Because the hall is used with 30 people as often as with 100, the hangings must do the work the audience does when it is absent.
Limits of the formula: Sabine’s formula assumes a diffuse sound field and small average absorption (below about 0.2). Here the average α with 80 m² of hangings is 98.1 ÷ 411 = 0.24, where Eyring’s formula (RT60 = 0.161 × V ÷ (−S × ln(1 − ᾱ))) gives 82.1 ÷ (411 × 0.272) = 0.73 s, shorter than Sabine’s 0.84 s. Sabine therefore errs on the safe side at high absorption.
Worked Example: Which Reflections Help
Sound travels at about 343 m/s in air at 20 °C. Reflections arriving within about 50 ms of the direct sound add to speech; later ones blur it. 50 ms corresponds to an extra path of 0.050 × 343 = 17.2 m.
Inputs: hall length L = 12 m, speaker at one end (x = 0), listener at distance x down the hall, the far wall at L.
- Path via the far wall = L + (L − x) = 2L − x. Direct path = x. Extra path = 2L − 2x = 24 − 2x.
- Extra path exceeds 17.2 m when x < 12 − 8.6 = 3.4 m. A listener at x = 2 m hears the far-wall echo 20 ÷ 343 = 58 ms late; at x = 6 m it arrives 12 ÷ 343 = 35 ms late and reinforces.
- The speaker hears the far-wall echo after 2L ÷ c = 24 ÷ 343 = 70 ms, which disrupts speaking.
So the far wall helps the back half of the audience and harms the front rows and the speaker; it should be absorptive (hangings, mats), while the speaker’s end wall stays solid and reflective, and side walls carry the rest of the absorption. The 50 ms figure is the usual early/late boundary for speech.
What You Need
- A volume and surface survey of the hall, as above
- Heavy hangings or woven mats for 40-80 m² of wall (Sound Absorption Materials)
- A solid, plain speaker’s wall
- A watch and a loud clap for a rough check
Method
Step 1: Compute the empty RT60 from the survey.
Step 2: Set a target of about 1.0 s or less for speech, and compute the absorption needed (82 m² for 1.0 s here).
Step 3: Place absorption on the far wall first, then the side walls, leaving the speaker’s wall bare.
Step 4: Check by ear: clap once in the empty hall and time how long the sound takes to die. This only estimates RT60 in a quiet place, since the sound must fall 60 dB before it vanishes.
Step 5: Re-test with an audience, and adjust.
Common Mistakes
- Designing for the full hall when it is usually half full.
- Absorption on the speaker’s wall, removing the helpful reflection.
- Flat thin cloth tight to the wall, which absorbs little at low frequency (see the sound absorption article).
Safety
Absorbers are fuel
Hangings, mats and woven ceilings are combustible and sit in the building where the whole community gathers. Keep them well clear of the hearth, flue and any lamp or candle, use the fire guide’s clearances for combustible surfaces and keep every exit unobstructed by hanging fabric (Fire Prevention in Settlements).
Sources And Honest Limits
Sabine’s reverberation formula, Eyring’s variant and the speed of sound are standard acoustics results, computed here for this hall. The absorption coefficients (0.03, 0.02, 0.10, 0.5, and 0.4 m² per seated person), the 50 ms early-sound limit and the speech target of about 1 s are typical values from room-acoustics references; they are not stated elsewhere in this corpus and should be checked against an acoustics text. Coefficients vary with frequency and with how a material is mounted, and the figures are for about 500 Hz only. The hall’s volume and surfaces are an assumed geometry.
What’s Next
- Learn next: Sound Absorption Materials — what to hang and how
- Combine with: Stage and Presentation Area — the speaker’s end