analysis | sound modelling ryan bessey + paul whelan sacred acoustics
caves churches garages reverberance worship
paul whelan
a primer on acoustics Before launching into a comparative analysis of the spaces, there are a few very important technical concepts that need to be established. When a sound is generated it propagates outwards at a constant speed with a characteristic directivity pattern. The eardrum will first resonate to the portion of the sound that is directed towards it, the direct sound , which often represents a tiny fraction (1%) of the total sound energy of an event. A few micro-seconds later the eardrum will respond to additional versions of the sound arriving after they reflect off different surfaces within the room. This is the indirect sound , which arrives with a time delay. Because of the way our brain processes the information provided by our eardrums, the indirect portion of the sound, arriving with a less than 50 to 80 milliseconds time delay (early reflections) gets combined with, or reinforces, the direct sound. After the early reflections, later reflections of the sound resonate at the eardrum because of a number of longer pathways, including those with multiple reflections, by which sound travels. Since the energy of sound decreases with distance travelled and with each reflection, the portion of sound pulse arriving in late reflections is of a lower magnitude. These numerous late reflections arrive from multiple directions and can continue for up to a few seconds in rooms with many hard (acoustically reflective) surfaces. The time it takes for these late reflections to cease is known as the reverberation time and is measured in seconds.
As I lock my bike in an underground parking garage, the sounds around me have a very familiar ring: the door closes behind me with a self-propelled but muffled clunk. I thought about this acoustic familiarity briefly, but then forgot about it during the workday. One day it came to me – it sounds like a church. Now each time I hear that echo of the door closing, I am instantly transported back to the catholic churches of my childhood. The clunk of the entry door sounds like the sacristy door closing as the priest enters the altar. The metal on metal between lock and frame sounds like the metal on metal latch on a closing confessional booth. Even the voices of people at the other end of the garage sound like the prayer exchange between a priest and a small congregation at an early morning mass. Why do these sacred and prosaic places sound so similar? What are their shared acoustic characteristics? My first thought was that that both spaces have a lot of hard reflective surfaces, but I was perplexed by the vast difference in their spatial characteristics — a parking garage comprises two parallel plates while most traditional churches have soaring spaces. The obvious answer was that I didn’t have a clue so I connected with Ryan Bessey, an acoustician. Ryan performed a comparative analysis based on acoustical models of my wide-open parking garage and the relatively tall, former sixth century Byzantine Church of Saints Sergius and Bacchus in Constantinople, converted during the Ottoman Empire to the Kucuk Ayasofya Mosque in, now, Istanbul. Despite the difference in spatial dimensions and histories, it turns out that both the garage and the church share similar reverberation characteristics.
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