Acoustics 101: After the Music Stops (Reverberation)
From an occasional series about acoustical concepts, inspired by current events.
If you arrived 20 minutes late for last month’s performance of Yves Klein’s “Monotone-Silence Symphony” in New York City, you might have thought you missed it all. You would have heard only silence and witnessed musicians sitting motionless onstage. In reality, it was only half over.
The symphony’s unique structure generated media and audience interest: 20 minutes of a D-major chord sustained by 70 orchestral and choral musicians, followed by 20 minutes of “performed” silence. Although composed in the 1940s (by an artist most famous for creating monochrome paintings) this symphony is performed infrequently – and never before in New York.
A New York Times art critic described the evolution of the 20-minute chord: initially “gentle” and “fragile” but later sounding “strangely electronic” and inhuman, ending “as abruptly as it had begun.” He called the silence “as absolute (and enjoyable) as any I’ve ever experienced in a crowded place in New York City…”
Acoustical Relevance. Although admittedly an extreme example, the “Monotone-Silence Symphony” can help draw attention to important acoustical concept for successful performance spaces: reverberation.
For the second half of this 40-minute symphony, the NYC audience was forced to consider what came after the music. However, most people don’t think about this. Reverberation is the persistence of sound in an enclosed space, whether that sound is a sustained, 20-minute chord or just a single staccato note.
Reverberation time is measured in seconds from when the sound is generated to when it decays to the point of inaudibility. It’s impacted by the interior surfaces and size of the performance space. Sound waves radiate through the air until they strike an obstacle that reflects, absorbs or transmits them. The air itself also absorbs sound, effecting higher frequencies first.
The classic example of a highly reverberant space is a cathedral, where reverberation times may exceed 10 seconds. Multiple, excessive hard surfaces allow sound levels to build up and reflect many times before losing its energy. In any performance setting, excessive reverberation can make it impossible for the musicians and audience to hear definition and detail. Notes blur together; articulation and timing become muddy, and clarity is lost.
Most professional music venues today are designed to provide reverberation times from 1.3 second to 2.2 seconds. In proscenium theatres used also for drama, acoustical shells onstage surround the musicians and keep sound from being absorbed in the fly loft space. Acoustical shells couple the stage and hall together acoustically, better enabling musicians to project their focused sound into the reverberant hall.
Free Resources. For an overview of other acoustical concepts and their relevance to performance venues, free educational guides are from Wenger are below, including An Acoustics Primer and Planning Guide for Performance Spaces. These Guides provide general information; we also recommend consulting with an acoustician about your specific situation.