Chapter 11 - Reverbs and Delays

Creating a sense of space

A Brief History of reverb

Real SPaces

Nothing is better than using the acoustics of a real space. By sending audio to a room through a speaker, we can record the characteristics of the room. The best, and probably the most famous example of this is Capitol Records in Los Angeles. To create reveb at Capitol, guitarist and inventor Les Paul was commissioned to give the studios reverb. He built eight large echo chambers 30 feet below the building. The rooms each have speakers on one side and microphones on the other. Recording engineers can pipe audio into the rooms and re-record the sound.

Listen to the segment at 6:30 of American Roots to hear recordings with and without the echo chamber but really, listen to the whole thing.

Plate

If the building you are in wasn't designed to have excellent reverb spaces, like Hall, Chamber, and Room, (the three types of acoustic spaces), electromechanical reverb simulations were created by attaching a transducer (like a speaker driver) to a large plate of sheet metal. A pickup (microphone) captures the vibrations and the result is output as an audio signal.

An affordable way to reproduce reverb is through a spring. Operating much like plate reverbs, Spring reverb has become a characteristic sound for guitars, and spring reverbs are often built into guitar amplifiers. An excellent explanation of how a spring reverb works can be found here. On the right is a video showing a spring reverb unit that I removed from a guitar amplifier.

Spring

Going Digital

Digital reverbs of every type have been created using various signal processing algorithms.

Convolution Reverb

A Convolution Reverb consists of a recorded sample of an acoustic space (called an Impulse Response or "IR"), Convolution reverbs record and process room characteristics unique to a real acoustic space. The IR can be created by using a burst of all frequencies (pink noise), or a filter sweep used by the excellent program, Altiverb. Altiverb's useful video on recording IR's is here. While convolution reverbs are extremely useful in audio post production, the are only a snapshot of an acoustic space, and not an accurate reproduction where frequencies are constently changing over time.

Hearing the Types of Reverb

Below you can hear the characteristics of some of the reverb types. On the left is a snare drum treated with electromechanical reverbs. And on the right, for audio post, is a door close sound effect processed with various convolution reverb spaces.

Impulse response available on the web

Here is a sample of websites that offer free impulse responses. Airwindows Impulses, Voxengo, EchoThief, Intelligent Machinery. While Impulse responses are very useful in audio post for reproducing spaces, be aware that they only capture a moment in time of the reverberant space. The reverb in real spaces change over time.