Reverb chambers, or “echo chambers,” are purpose built rooms, that are designed to have a long reverberation time. In modern usage, reverb chambers have a speaker at one end of the room, and a microphone (or pair of microphones) at the opposite end of the room. The sound to be reverberated is sent into the room via the speaker, and the reverberated signal is picked up by the microphones.
The use of specific chambers for generating reverberation can be viewed as dating back to the time of ancient Greece and Rome. The Roman architect and military engineer Marcus Vitruvius Pollio, in his treatise De architectura, described the use of “resonating vessels” in the design of theaters. In a recent paper, it has been argued that these vessels extended the reverberation time for performers on stage.
The modern use of reverb chambers in recorded music can be traced back to Bill Putnam’s work. The Harmonicats’ “Peg o’ My Heart,” released in 1947, used a speaker and microphone that Putnam set up in the studio’s bathroom to create a reverberant sound that was much different than the dry studio recordings of the time:
During the 1950s and 1960s, most of the high end recording studios in the United States and United Kingdom incorporated echo chambers into their designs. These custom built echo chambers came in various shapes and sizes, but had many aspects in common:
- The echo chambers ranged in size from 1000 square feet, up to several thousand square feet. Larger volumes provide a longer reverb time, given that all other factors (wall/floor/roof materials, room humidity, room temperature) are the same.
- The walls are usually covered with materials that were highly reflective of sound. Tile was common, as was several coats of plaster. As the plaster dried over the course of a few years, the reverberation time would increase.
- Efforts were made to break up the symmetry of the room, in order to avoid standing waves that would lead to unpleasant resonances in the reverberation. Some room were wedge shaped, with the speaker in the narrow part of the wedge, and the microphones in the wider part of the wedge. Rectangular rooms often had objects (such as standing sewer pipes) placed into them in order to break up standing waves.
- Once the room was constructed, the speaker and microphones were set into place. The speakers and microphones tended to be aimed towards the closest walls, and away from each other, in order to avoid picking up the direct “un-reverberated” signal. Many reverb chambers have made use of the same speaker and microphones for several decades!
The sound of a reverb chamber is very distinctive, and is difficult to simulate with artificial reverberators:
- The reverb has a quick onset.
- Not many early reflections – the initial onset is fairly diffuse
- The modal density is higher than is found in most artificial reverberators (i.e. digital, spring, plate), but lower than a concert hall. This results in a rich reverb, that is free of metallic resonances and ringing/beating sounds in the decay.
- The low frequencies will decay at a rate determined by the material of the reverb chamber. Some of the concrete reverb chambers will have a VERY long low frequency reverb decay. In order to compensate for this, it was very common to filter the low frequencies out of the sound sent to the speaker in the reverb chamber. At Abbey Road Studios, it was standard practice to use a passive filter with two frequencies filtered at the corner frequencies of 500Hz and 10,000Hz when patching a chamber on a send from the console.
- The high frequencies in a reverb chamber have their maximum high frequency decay determined by the humidity of the air. No matter what the size of the reverb chamber was, the RT60 at high frequencies wouldn’t exceed 1.25 to 1.5 seconds. This meant that higher frequency sibilants would decay away relatively quickly.
The lore of echo chambers is strong in the mythology of recording studios. Echo chambers were often used as places for musicians to meditate, or pursue less savory activities. Simon and Garfunkel recorded the backing vocals for “The Only Living Boy in New York” from within an echo chamber, adding to the ethereal sound of that song: