Theater: Intelligibility Compliance per IEC 60268-16

The Challenge: Life Safety Certification Required

The Royal Civic Theater, an 800-seat proscenium venue built in the 1960s, was undergoing a major renovation that included a PA system upgrade. The local authority required that the new system meet a minimum STI of 0.50 at all occupied seats for life safety voice alarm certification. The existing system, installed in 1995, had never been formally measured for intelligibility.

The theater featured a raked stalls section with 550 seats, a deep balcony with 250 seats, and a volume of approximately 6,800 cubic metres. The existing PA comprised four column speakers on the proscenium arch and two delay fills at the rear of the stalls. The system had served adequately for basic announcements but was now required to meet a quantified intelligibility standard.

Measurement: STIPA at 20 Positions

Using the SonaVyx STIPA measurement tool per IEC 60268-16, the acoustic consultant measured at 20 positions distributed across the stalls, balcony, and side aisles. The STIPA test signal was played through the existing PA system at the normal announcement level.

The measurement campaign revealed severe intelligibility deficiencies:

• Average STI across all 20 positions: 0.44 (poor)
• Best position (stalls row 5 centre): 0.58 (fair)
• Worst position (under balcony, row P): 0.32 (bad)
• Balcony rear row: 0.38 (poor)
• 8 of 20 positions fell below the 0.50 minimum: 40% failure rate

The per-band MTF analysis from SonaVyx showed that the 2 kHz and 4 kHz octave bands had the lowest modulation transfer, consistent with high-frequency energy being directed onto the ceiling rather than the audience by the old column speakers.

Diagnosis: Directivity and Coverage Gaps

The AI diagnostic engine identified three compounding issues:

1. Poor speaker directivity: The column speakers had approximately 40-degree vertical dispersion, meaning significant energy hit the ceiling and side walls rather than the seating area. This increased the reverberant field relative to the direct sound.
2. Under-balcony shadow: The deep balcony overhang blocked direct sound from the proscenium speakers to 120 under-balcony seats. These seats received only reflected and diffracted sound, with STI values 0.10 to 0.15 points lower than unobstructed seats.
3. No zone processing: The existing system ran a single signal path to all speakers. The balcony, under-balcony, and stalls each required different delay, level, and EQ settings for optimal intelligibility.

RT60 measurement showed 1.6 seconds at 500 Hz — within acceptable range for a theater, but demanding tight directivity control from the PA to maintain a high direct-to-reverberant ratio at each seat.

Solution: Digitally Steered Line Arrays with Zone Processing

The new system comprised:

• Two digitally steered line arrays on each side of the proscenium, with beam steering software directing energy precisely onto the stalls seating rake and the balcony face
• Four under-balcony fill speakers with independent delay and EQ to serve the 120 shadowed seats
• Zone-specific DSP: separate EQ and delay for the main arrays, under-balcony fills, and balcony direct coverage

The delay times were set using SonaVyx impulse response measurements at each zone boundary, and the EQ was tuned using transfer function measurements at representative positions.

Results: Full Compliance with Margin

Post-installation STIPA measurement at the same 20 positions using SonaVyx before/after comparison:

Metric	Before	After
Average STI (20 positions)	0.44 (poor)	0.62 (good)
Minimum STI	0.32 (bad)	0.55 (fair)
Positions below 0.50	8 / 20 (40%)	0 / 20 (0%)
Under-balcony STI	0.32	0.58
Balcony rear STI	0.38	0.56

All 20 positions exceeded the 0.50 minimum with at least 0.05 points of margin. The life safety certification was granted. The improvement was most dramatic under the balcony, where the dedicated fill speakers raised STI by 0.26 points — from effectively unintelligible to clearly understandable.

Lessons Learned

• Directivity control is the primary tool for STI: The line arrays' tight vertical beam steering put 90% of the acoustic energy on the audience, compared to approximately 50% from the old column speakers.
• Under-balcony areas need dedicated speakers: No main system, however well-aimed, can adequately serve deeply recessed under-balcony seating. Dedicated fills are essential.
• Zone processing multiplies the benefit: Independent delay and EQ per zone allowed each area to receive optimally timed, spectrally balanced sound tailored to its specific acoustic conditions.
• 20-position measurement provides confidence: The SonaVyx STIPA campaign proved that every seat met the standard, not just the best positions. This comprehensive verification is what building certifiers require.