Ecasound Reverb Decay Time Parameters Explained

In Ecasound, reverberation decay is primarily controlled by the built-in Freeverb operator (-efv), which simulates acoustic spaces using physical modeling. This article explains the exact parameters governing reverb decay time within Ecasound, detailing how room size and damping interact to dictate the overall length, tone, and tail of the effect.

The Built-In Reverb Operator (-efv)

Ecasound's native reverb processing is powered by an integrated Freeverb implementation invoked with the -efv chain operator. The full syntax for this operator is:

-efv:roomsize,damping,wet,dry,width,mode

While parameters such as wet, dry, and width govern the mix balance and stereo image, decay time is strictly governed by two parameters: roomsize and damping.

1. Room Size (roomsize)

The roomsize parameter is the primary control for overall decay time (RT60).

• Value Range: 0.0 to 1.0 (typical values range between 0.7 and 0.95).
• Function: In Freeverb's comb filter network, roomsize scales the internal feedback coefficients. Increasing this value keeps the audio recirculating longer through the delay buffers, which directly lengthens the decay tail.
• Impact on Decay: Setting roomsize close to 0.0 produces an extremely short, metallic decay akin to a tiny resonant enclosure. Setting it closer to 1.0 dramatically extends decay time, producing a long, sustained wash characteristic of a cathedral or large hall. Values above 1.0 will result in feedback instability and infinite decay.

2. Damping (damping)

The damping parameter governs frequency-dependent decay, simulating the absorption of acoustic energy by walls, air, and soft surfaces.

• Value Range: 0.0 to 1.0.
• Function: Damping controls low-pass filters placed inside the feedback loops. Higher values cause high frequencies to decay faster than lower frequencies on each recirculation cycle.
• Impact on Decay:
  • A damping value of 0.0 allows high frequencies to sustain as long as low frequencies, resulting in a bright, prolonged reverberant tail.
  • A higher damping value (e.g., 0.5 to 0.8) rolls off the highs rapidly, causing the perceived decay time of top-end frequencies to shorten while preserving lower-mid resonance.

Practical Application

To configure a long, ambient decay with natural high-frequency loss, set a high roomsize and a moderate damping level:

ecasound -i input.wav -efv:0.85,0.4,0.3,0.7,1.0,0 -o output.wav

To configure a tight, snappy decay resembling a tiled room:

ecasound -i input.wav -efv:0.25,0.1,0.2,0.8,1.0,0 -o output.wav