how curved ceilings and walls redirect and amplify sound


Created on February 19, 2020 08:45 AM.

Updated 03.23.

Author: Dmitry Pirozhkov.

The change in the audibility of sound, including its distortion, depending on the shape of the bends in certain rooms, was noticed and described by Edward Boyd in 1824.

In particular, he described the discovery of such an effect in the outline as follows: “In the Cathedral of Girgenti in Sicily, the slightest whisper is quite distinct from the great western door to the cornice behind the high altar – a distance of two hundred and fifty feet. Unfortunately, the confession was poorly placed: “Secrets that were never intended for the public hearing thus became known, to the confusion of confessors and scandal among the people … until one listener experienced a curiosity that was somewhat overestimated. Having heard the confession of his wife about her own betrayal, this strange feature became generally known, and the confession was confessed. “

However, the very fact that different shapes of surfaces have an amplifying or deforming effect on sounds was well described back in the 17th century by Athanasius Kircher, who studied the effect of sound echoes.

In addition to describing the giant auditory tubes that were built into the walls of the royal chambers and intended to be eavesdropped, Athanasius also described a truly satanic instrument known as the “cat’s piano” (Katzenklavier).

Sounds from such a “piano” were extracted by pressing the keys, behind which the cells were located, with … cats contained in them. The key was associated with a cat. When the button was pressed, a nail bit into the hapless animal’s tail. The animal emits a cry of a certain timbre. As a result of the action, the sadist musician could “play” a kind of melody. The task of such a “tool” was to influence patients in psychiatric clinics. In such a Jesuit way, they tried to change their behavior.

Despite the description of such “tools” Afanasy Kirchner created diagrams that illustrated a fairly deep understanding of the physics of the process. The way the elliptical ceiling amplified the sound, establishing a better connection between people.

Rice. 1. Drawing of an elliptical ceiling. Source:

Schematically, the figure shows the paths of the wave and the reflection of its sound from the ceiling. You can calculate the trajectory data using a regular ruler and protractor. Also, the trajectory of the cut-off sound can be compared to the movement of a ball that rushes to the target after hitting it with a cue, but the laws of gravity are not taken into account.

As a result of the experiment, it becomes clear that the sound directed towards the elliptical ceiling, after reflection, will reach the listener. That is why even a whisper is so well heard in such a large room.

However, the disadvantage of this scheme is that the location of the speaker and listener is clearly defined – these are the foci of an elliptical ceiling. If, for example, it was necessary to give a lecture in front of a group of listeners, then such an arrangement would be useless. Some people will have difficulty hearing the lecturer.

Another example. At the Royal Albert Hall in London, two science exhibitions were presented to a large number of children. The hall, created for musical performances, was equipped to popularize art and science. So, in this hall at one time the Prince of Wales gave an opening speech.

The prince’s clear reading was accompanied by echoes that twisted the words, giving them a whimsical accent. The reason most likely was the curved surfaces of the hall with an ellipsoidal ceiling. The reflections produced by such a Kircher ceiling depend on the overall dimensions of the room.

That is why the immense hall of the Royal Albert Hall generates terrible sound echoes, incredibly distorting sounds. Sound seems to flow not only from the stage, but from all directions in the room.

An interesting point is also related to the interpretation and integration of incoming sounds by the human brain. After all, if there were no “special processing”, one could simply drown in a huge number of sounds. Imagine even typing regular text on your computer at your desktop. Pressing the keys, the crackling from the buttons is reflected from the table on which the keyboard is located. From the monitor standing in front of, from the phone lying next to it, and even from the ceiling above the workplace and walls in the room. However, the brain clearly distinguishes the sound of keystrokes among everything.

Comparing the propagation of sound in a small Kircher room, you need to understand the difference between the processes. So in a room with an elliptical ceiling, the sound is reflected from it rather quickly. It is not very strong, so the human brain does not distinguish these reflected sounds from the main one coming from the speaker. But in the huge Royal Albert Hall, focused reflections reach the human ear much later in the form of sound echoes.

In the 19th century, the US Capitol was reconstructed. The famous whispering dome is no longer so. The Capitol was originally a near-perfect hemisphere covered in curved squares with a trompe l’oeil that gave the illusion of structure and texture.

Until the beginning of the 20th century, the domed space was very popular for tourists, although it caused dissatisfaction with the House of Representatives, since it was impossible and very tiring to debate in such a space.

Not experienced in the intricacies of acoustics, the speaker, moving during speeches, became a witness to the distortion of his own voice, which took on bizarre outlines and timbre. Whispers were interspersed with screams and screams, and quiet phrases met with loud exclamations.

It all ended at the end of the 19th century in 1898, when a fire practically destroyed the wooden dome. The former structure was replaced by a fireproof dome, and plaster chests were installed instead of clots. As a result of the reconstruction, the effect of the sound echo was greatly weakened and became almost invisible.

By the way, in comparison with domes, a spherical room further enhances the effect of sound reflection. A striking demonstrator of the effect is the three-story stained glass globe located in the Mary Baker Eddy Library of Boston. The structure is a 30-foot sphere. The huge spherical hollow structure is decorated with stained glass windows depicting seas and continents. Visitors pass through the center of the globe along a special path. Inside the structure, you can feel the original acoustics.

Rice. 2. The Mapparium Museum in Boston, USA. Source: Courtesy of Smart Destinations / Flickr

Thus, William Hartmann from Michigan State University and his colleagues have recorded acoustically illusions inherent in spherical rooms. For example, the speaker is in the center at a dead center, and the listener is 7 feet away on the bridge to his right. The speaker’s voice is heard quietly. However, as the speaker moves to the left of the center, his or her voice becomes louder. Until it is exactly 7 feet to the left of center and 14 feet from the listener.

Everything is explained quite simply. When the speaker is exactly in the center of the sphere, then his voice, reflected from the walls, returns to its original …