Sound Blanket

Taming the Soundscape

One of the greatest challenges for a production sound mixer is controlling the acoustic properties of a shooting location. Hard surfaces like hardwood floors, concrete walls, and large windows are acoustically reflective. Sound waves from an actor’s voice bounce off these surfaces, creating reverberation and echo. This ‘reverb’ can make dialogue sound distant, muddy, and unprofessional, and it is extremely difficult to remove in post-production. The sound blanket is the single most important tool used by the sound and grip departments to combat this problem on set.

It is crucial to understand that sound blankets are used for acoustic treatment, not soundproofing. Their purpose is to improve the quality of sound within a space by absorbing reflections. They are not designed to block sound from entering or leaving a space, which is a far more complex task requiring mass and structural isolation. Sound blankets are a portable, flexible, and effective way to quickly tame a ‘live’ room and make it suitable for recording clean dialogue.

Acoustic Principles in Practice

Sound blankets work through the principle of acoustic absorption. They are typically made from a dense, porous material—often similar to the thick blankets used by moving companies, but sometimes with specialized sound-absorbing cores. When sound waves strike the blanket, their energy forces the fibers within the material to vibrate. This friction converts the sound energy into an infinitesimal amount of heat, thus preventing the sound wave from reflecting back into the room.

Their effectiveness is a function of their density and thickness. The heavier and denser the blanket, the better it is at absorbing low-frequency sounds. Most professional sound blankets are fitted with grommets along the edges, allowing them to be easily hung from C-stands, frames, or rigged to walls and ceilings by the grip department.

On-Set Application

On set, a location that looks perfect visually may be an acoustic nightmare. The sound department will work with the grips to strategically place sound blankets to mitigate these issues without interfering with the camera’s shot. Common uses include:

• Reducing Room Reverb: In a large, empty room with parallel walls (a recipe for flutter echo), blankets are hung on C-stands just out of frame to create ‘soft’ walls that absorb sound instead of reflecting it.
• Dampening Noise Sources: A noisy appliance like a refrigerator or an air conditioning unit that cannot be turned off can be surrounded by a ‘fort’ of sound blankets to reduce its hum.
• Controlling Floor Reflections: Blankets are often laid on the floor, just outside the camera’s view, to absorb sound reflecting off hard surfaces like wood or tile. This is especially important for reducing the harshness of dialogue.
• Creating Negative Fill: Just as a black flag is used by the lighting department to absorb light (‘negative fill’), a sound blanket is used to absorb sound. They can be placed strategically to prevent an actor’s voice from bouncing off a nearby wall and back into the microphone.
• Makeshift Vocal Booths: In a pinch, a small ‘booth’ can be constructed from stands and sound blankets to record clean voice-over or ADR on location.