Glossary

αw — weighted absorption coefficient

A single number from 0 to 1 that summarises how much sound a material absorbs across the frequencies that matter for speech, calculated to EN ISO 11654. 0 means it reflects everything; 1 means it absorbs everything. Felt products vary widely with thickness, density and mounting, so always check the value for the specific product.

NRC — noise reduction coefficient

A North American single-number rating (ASTM C423) that averages absorption at 250, 500, 1000 and 2000 Hz. It is similar to αw but calculated differently, so the two numbers aren't directly interchangeable.

Absorption class A–E

EN ISO 11654 groups αw values into classes: A (0.90–1.00), B (0.80–0.85), C (0.60–0.75), D (0.30–0.55) and E (0.15–0.25). Class A is the most absorbent.

Class A	αw 0.90–1.00
Class B	αw 0.80–0.85
Class C	αw 0.60–0.75
Class D	αw 0.30–0.55
Class E	αw 0.15–0.25

Equivalent absorption area (m² Sabine)

For free-hanging objects such as pendants and baffles, absorption is measured per object rather than per square metre. The result, in square metres, tells you how much perfectly absorbing surface one luminaire is equivalent to.

Reverberation time (RT60)

The time it takes for sound in a room to fade by 60 decibels after the source stops. Long reverberation times make rooms echoey and speech hard to follow. Adding absorption shortens it.

UGR — unified glare rating

A measure of discomfort glare from luminaires, calculated for a specific room and viewing position. Lower is better. The European workplace lighting standard EN 12464-1 sets a maximum UGR for each type of task, and glare ratings can differ between sizes of the same product, so check the figure for each one.

IES / LDT photometric files

Data files that describe exactly how a luminaire distributes its light. Lighting designers load them into software such as DIALux or Relux to calculate light levels, uniformity and glare. IES is the North American format; LDT (EULUMDAT) is the European one. We provide both.