The sound of intentional imperfection
The Divine Bell of King Seongdeok, housed in the Gyeongju National Museum, does something most bells are designed to avoid: it produces a distinctive "beat". This acoustic phenomenon is not a flaw. It is the result of intentional design by Silla Dynasty metalsmiths in 771 CE. Modern analysis reveals the bell is physically asymmetrical. The wall thickness varies, ranging from 11 to 25 centimeters. This asymmetry causes the bell to vibrate at two slightly different principal frequencies when struck. These frequencies interfere with each other, creating a periodic rise and fall in amplitude, a slow, resonant beat known as yee-un.
The primary hum tone vibrates at approximately 64 Hz, while the fundamental strike tone is around 168 Hz. For the primary mode, the frequency difference that creates the beat is just 0.34 Hz, producing a consistent beat duration of 2.94 seconds. This effect is characteristic of Korean bronze bells from this period and distinguishes them from the simpler bells of China and Japan. The bell's massive scale, 3.75 meters tall, 2.27 meters in diameter, and weighing 18.9 tons, amplifies this complex sound, which could reportedly be heard for many miles.
Silla's acoustic engineering
The creation of the Divine Bell of King Seongdeok was a massive undertaking, spanning approximately 34 years and requiring about 72,000 kg of copper. The final bronze alloy is a specific ratio of copper and tin, which Silla artisans perfected for acoustic quality and durability. Unlike European bell metal with around 23% tin, Korean bells often used less, which prevented the alloy from becoming too brittle.
Several unique structural features contribute to the bell's sound. At the top, a hollow cylindrical tube called an eumtong is thought to help absorb high-frequency waves and tune the overall sound. Next to it, the suspension loop (yongnyu) is an cast dragon's head, a common feature of Silla bells. The designated striking point, or dangjwa, is a lotus-shaped medallion positioned to effectively excite the paired vibration modes that generate the beat. The bell's inscriptions, containing nearly 1,000 Hanja characters, detail its creation and purpose, linking the artisans of the 8th century. For preservation, the bell has not been regularly struck since 1992, but acoustic studies confirm its structural and sonic properties have remained stable over decades of monitoring.