- Report
- FR-010
- Published
- Written by
- Rui Pacheco
- Reading time
- 02 min / 339 words
Listening to a hall
Ostra Cultural Hall measures its own acoustics during every performance without recording a single word.
Section 01
Microphones that never record
Ostra Cultural Hall has forty-two acoustic sensors spread across the auditorium, the lobby and the backstage corridors. None of them records sound. Each one measures level, spectrum and decay on the device itself and sends only the numbers, once a minute, so nothing that is said or played in the hall can ever be reconstructed. That was the condition the resident orchestra set before agreeing to the system, and it turned out to cost us almost nothing in what we could learn from the room.
Section 02
The room with a full audience
Acoustic design is usually tested in an empty hall, with a starting pistol or a loudspeaker. A hall is never empty when it matters. Over its first season Ostra recorded the reverberation time of every performance against the number of people in the seats. With the house full the room lost almost half a second of reverberation in the low frequencies, more than the model predicted, because winter coats on the seat backs absorbed far more sound than the upholstery had been designed for.
Fig. 010.1/ Reverberation time by occupancy
Unit / s
- Empty2.3 s
- 25%2.15 s
- 50%2 s
- 75%1.86 s
- Full1.78 s
Section 03
Tuning by data
The fix was simple once it was measured. The cloakroom moved to the lobby, so coats no longer reach the auditorium, and four adjustable banners behind the stage now deploy automatically according to the ticket count for each performance. The hall now sounds within a tenth of a second of its design target whether it holds two hundred people or fourteen hundred. The sensors stay on, because the next change, new seats or a new kind of programme, will alter the room again.
Section 04
A room that listens back
With the banners responding to ticket counts, low-frequency reverberation now varies by less than 0.2 seconds between an empty hall and a full one, against almost half a second before. The musicians noticed before we told them: the double basses no longer disappear in sold-out performances. The sensors have since caught two smaller changes, a new carpet in the side aisles and a curtain left drawn behind the stage, both found within a day because the room sounded different in the data before anyone complained about it.
Fig. 010.2/ Low-frequency reverberation, before and after
Unit / s
| Period | With banners | First season |
|---|---|---|
| Empty | 2.3 s | 2.3 s |
| 25% | 2.25 s | 2.15 s |
| 50% | 2.2 s | 2 s |
| 75% | 2.16 s | 1.86 s |
| Full | 2.14 s | 1.78 s |
