Gateway to an acoustic marvel
The ruins of the Ak-Saray Palace, or "White Palace," in Shahrisabz are dominated by the remnants of its colossal entrance gateway. Construction, ordered by the Turco-Mongol conqueror Timur, began in 1380 and continued for over two decades. What remains today are two massive pylons, separated by a collapsed arch, that once formed an entrance of staggering proportions. The original arch had a span of over 22 meters (72 feet) and the gateway towers are estimated to have reached a height of up to 70 meters (230 feet).
This grand entrance was a symbol of power and an acoustic device. The specific geometry of the deep, recessed arch and the flanking pylon walls created a parabolic structure. This shape could capture sound waves originating from a focal point on one side of the gateway and reflect them across the expanse to a corresponding focal point on the other side. A person standing at one of these points could whisper and be heard clearly by someone at the other, a phenomenon called a "whispering gallery." The hard, glazed tile surfaces that decorated the facade increased this effect by providing a smooth, reflective surface for the sound waves, minimizing absorption and distortion.
The science of a whispering arch
The acoustic properties of the Ak-Saray gateway are a practical application of the physics of sound reflection. Sound travels in waves, and when these waves encounter a concave surface like the palace's arch, they reflect inwards at an angle equal to their angle of incidence. A parabolic reflector, the shape approximated by the gateway, has a unique property: all waves traveling parallel to the parabola's axis of symmetry are reflected to a single point, the focus. Conversely, any wave originating from the focus is reflected parallel to the axis.
In the case of Ak-Saray, the arch and its side walls created a three-dimensional acoustic mirror. A whisper spoken at the focal point would radiate outwards, strike the curved and flat surfaces of the gateway, and be reflected as a focused "beam" of sound toward the opposite side. This directed transmission of sound energy allowed a low-volume utterance to travel a significant distance with minimal loss of clarity, effectively turning the massive architectural feature into a communications device. This principle is the same one used in satellite dishes to collect faint radio waves from space and in some modern acoustic listening devices. While the main arch collapsed roughly 200 years ago, the scale of the remaining 38-meter-high pylons shows the scale of the engineering involved.