Soft materials are used in auditoriums because they absorb sound energy, preventing it from bouncing uncontrollably off hard surfaces. Without them, speech becomes muddy, music loses clarity, and echoes make the space unusable for its purpose. Every soft surface in an auditorium, from the upholstered seats to the heavy stage curtains to the fabric-wrapped wall panels, plays a deliberate role in controlling how sound behaves in the room.
How Soft Materials Absorb Sound
When a sound wave hits a hard surface like concrete or glass, most of that energy reflects back into the room. When it hits a soft, porous material instead, something different happens: the sound wave enters the tiny air pockets within the material, and the air molecules are forced through narrow, irregular channels. This creates friction between the moving air and the solid fibers of the material, converting the sound energy into a tiny amount of heat. Viscous shearing of the air, direct friction against the fibers, and flexing of the material’s structure all contribute to this energy conversion.
The result is that less sound reflects back into the room. This is absorption, and it’s the single most important reason soft materials appear throughout auditorium design. Hard, smooth surfaces reflect nearly all the sound that hits them. Soft, porous surfaces can absorb a large percentage of it, depending on the material’s thickness, density, and how it’s mounted.
Controlling Reverberation Time
The key measurement in auditorium acoustics is reverberation time, often expressed as RT60: the number of seconds it takes for a sound to decay by 60 decibels after the source stops. Too much reverberation and words blur together. Too little and music sounds thin and lifeless.
The ideal RT60 depends on what the space is used for. A classroom works best below 1 second. For clear speech in a lecture hall, about 1 second is ideal. Multipurpose auditoriums aim for 1.5 to 2.5 seconds, a compromise between speech clarity and musical richness. Concert halls designed purely for orchestral music may push to 3.5 seconds, though some speech clarity is sacrificed at that level.
Soft materials are the primary tool designers use to bring reverberation time down to the target range. A bare concrete room might reverberate for 5 or 6 seconds. By adding absorptive surfaces strategically, designers shave that number down to whatever the space requires.
Where Soft Materials Are Placed
Placement matters as much as the material itself. One of the most common acoustic problems in rectangular rooms is flutter echo, a rapid, metallic-sounding repetition caused by sound bouncing back and forth between two parallel hard walls. Adding soft absorptive panels to one or both of those walls breaks the cycle. Acoustic panels placed on walls, ceilings, and even floors can eliminate these echoes and smooth out the overall sound of the room.
In practice, designers don’t cover every surface. Auditoriums need a mix of absorption, reflection, and diffusion. Some hard surfaces are intentionally left to help project sound from the stage to the back rows. The ceiling above the stage often includes angled reflective panels that direct sound outward toward the audience. Soft materials are concentrated where reflections would cause problems: rear walls, side walls at ear level, and areas where parallel surfaces face each other.
Over-absorbing a room is a real risk. If too many surfaces soak up sound, the space feels dead and performers on stage struggle to hear themselves. Designers balance absorptive soft materials with reflective and diffusive surfaces to create a room that feels both clear and alive.
Specific Materials Used
The most common absorptive materials in auditorium walls and ceilings are mineral fiber and fiberglass panels, typically wrapped in fabric for appearance. These rigid panels are porous enough to trap sound energy effectively and can be manufactured with specific absorption ratings for different frequency ranges. Thicker panels absorb lower frequencies better, while thinner ones handle higher frequencies.
Stage curtains are another major absorptive element. Heavy velour fabrics, typically 25 to 32 ounces per yard, provide excellent sound absorption. A 32-ounce cotton or polyester velour with full pleating (sewn at 100% fullness, meaning twice as much fabric as the width it covers) absorbs significantly more sound than a flat, lightweight curtain. The pleating creates folds that trap sound waves at multiple angles, and the weight of the fabric prevents it from being acoustically transparent. The distance between the curtain and the wall behind it also affects performance, with greater spacing improving low-frequency absorption.
Why Auditorium Seats Are Upholstered
Upholstered seating serves a surprisingly specific acoustic purpose beyond comfort. A human body absorbs a significant amount of sound, particularly at mid and high frequencies. This means an auditorium sounds noticeably different when it’s empty versus full. If the seats themselves were hard wood or plastic, the acoustic difference between an empty rehearsal and a packed performance would be dramatic, making it nearly impossible to design the room to sound good in both conditions.
Soft, padded seats absorb sound in a way that approximates the absorption of a seated person. When someone sits down, their body replaces the seat’s exposed surface, and the total absorption stays relatively stable. This is why researchers have spent considerable effort characterizing how different chair designs perform when occupied versus unoccupied, and developing prediction models that estimate occupied absorption from unoccupied measurements. The goal is consistency: the auditorium should sound nearly the same whether 30% or 95% of the seats are filled.
Fire Safety Requirements
Because auditoriums are public assembly spaces, every soft material used in them must meet strict fire safety standards. The two primary benchmarks are NFPA 701, which tests flame resistance of textiles used in public spaces, and ASTM E84, which measures flame spread of building materials. Stage curtains, wall fabrics, and acoustic panels all need to pass these tests before installation.
Cotton velour curtains are typically treated with flame-retardant chemicals to meet NFPA 701, though this treatment degrades over time and requires reapplication. Many modern installations use inherently flame-retardant (IFR) polyester velours instead, which achieve their fire resistance through the fiber’s chemistry rather than a surface coating. This makes them more durable and reliably compliant over the life of the installation. Fiberglass and mineral fiber wall panels generally carry a Class A fire rating, the highest level of resistance to flame spread.