1. Why Flat Walls Bounce Sound in Only One Direction
In a concert hall, it is important that sound surrounds the audience from all sides. According to an explanation by Terashima Takane of Mie University, reflected sounds that arrive shortly after the direct sound help make the audio clear. Reflected sounds coming from the side (lateral reflections) help create a sense of spaciousness. However, reflected sounds that arrive much later than the direct sound become echoes and interfere with listening [1].
If a wall is smooth and flat, sound bounces back in a specific direction, similar to how light reflects off a mirror. This creates an imbalance where some seats receive strong sound while others receive very little. To fix this, designers use "diffusers." Diffusion means spreading sound energy widely throughout the room. According to Wikipedia, diffusers do not absorb sound but scatter it. Depending on the conditions, this can reduce unwanted coloring or echoes while maintaining the room’s reverberation [2]. So, bumpy walls are not just decoration; they function to break up the direction of sound.
2. How Grooves of Different Depths Change Sound Arrival Times
A common way to scatter sound is to arrange grooves of different depths on a wall. Because the distance to the bottom of each groove varies, the same sound returns from each groove at slightly different times. Since the waves may not align in phase, the sound does not bounce back in one direction like a mirror but spreads out in many directions [3].
Imagine rolling a ball down stairs: on a flat floor, it goes straight, but on an uneven surface, it scatters. However, sound is a wave, not a particle, so this analogy is only approximate. How big should the bumps be? University lecture materials suggest that for effective diffusion, the depth of the irregularities should be at least one-quarter of the wavelength of the target sound [3]. Wavelength is the length of one cycle of a sound wave. This is a design guideline, not a strict rule for entire halls.
Since pitch affects wavelength, larger bumps are thought to be effective for lower sounds, while finer bumps are effective for higher sounds. Older halls also used statues and recesses as these uneven surfaces.
3. Statues and Recesses in Concert Halls Over 100 Years Ago
The Great Hall of the Musikverein in Vienna opened on January 6, 1870. It is a rectangular "shoebox" shape, about 49 m (about 160 ft) long, 19 m (about 62 ft) wide, and 18 m (about 59 ft) high, often called the "Golden Hall." The designer Hansen reportedly relied on intuition rather than architectural acoustics research [4].
Wikipedia notes that reflective and diffusive surfaces in this hall create strong lateral reflections. However, the hall’s acoustics result from a combination of shape and materials. It is not possible to determine exactly how much the decorations contribute or if they were placed specifically for scattering [4].
The Boston Symphony Hall opened on October 15, 1900. Wallace Sabine, a young physicist from Harvard University, served as an acoustical consultant. Wikipedia describes it as one of the first halls designed using scientific acoustic principles. It is noted that the coffered ceiling (a grid-like pattern) and niches holding statues on three walls are said to help provide good sound from almost every seat [5].
4. How Math Helped Find the Best Shape to Scatter Sound
In the 1970s, Manfred Schroeder is said to have developed diffusers based on number theory. He reportedly gained inspiration from lectures by André Weil in Göttingen in 1977, which discussed properties of numbers like Gauss sums and quadratic residues [6].
According to a 2006 article, Schroeder asked about 30 years prior, "What wall shape maximizes sound diffusion?" He initially created diffusers using maximum-length sequences (a type of 0 and 1 arrangement) and later moved to designs based on quadratic residues and primitive roots [7].
The strength of this "recipe" is that the performance of a diffuser can be predicted by calculation according to design conditions. For the quadratic residue type, determining groove depths according to number sequences allows designers to predict how sound will scatter before building it. This type is still called the "Schroeder Diffuser" and is widely used [6]. While this shows how research can help in unexpected places, adding more diffusers does not automatically make a hall better.
5. How the Elbphilharmonie's 10,000 White Panels Handle Sound
The walls and ceiling of the Elbphilharmonie Great Hall are covered by a continuous "white skin." The panels are made of gypsum fiber concrete, with more than 10,000 pieces carved out. The hall has 2,100 seats [8].
According to the official website, sound reflects directly off the flat parts of the panels, while it scatters in the deep recesses. It is not all scattering surface; reflecting and scattering surfaces are combined within a single wall. No further details are given about the shape of individual carved pieces [8].
The design was by architects Herzog & de Meuron and acoustician Toyoda Yasuhisa. This is an example where architectural form and acoustic design were integrated into a single wall from the start [8].
6. Can Walls Have Too Much Bumpiness for Sound?
In *Acoustics Today* (2006), it is noted that one prominent hall designer argued that too much diffusion harms the tone quality of high-pitched string instruments. Conversely, other views suggest that poor acoustics in some large halls were caused by insufficient diffusion [7]. Therefore, "more diffusion is better" is not a fixed rule.
The method for creating lateral reflections changes with the hall's shape. The shoebox type is rectangular, represented by the Musikverein Great Hall. The vineyard type divides audience seating into stepped blocks like grapevines, creating many side reflection surfaces close to the seats through these steps. In Japan, Suntory Hall is cited as an example of this type [10][1]. Project materials from Nagata Acoustics state it was completed in 1986 with 2,006 seats and a reverberation time of 2.1 seconds when full [9].
Even though hall shapes differ, the importance of lateral reflections is common. The specific shape of wall irregularities varies by hall. A concert hall’s sound depends on the balance of direct sound, lateral reflections, diffusion, and reverberation. Wall bumps are just one adjustment knob for scattering.
7. A Home Experiment: Comparing Echoes by Clapping
You can compare how echoes differ by clapping your hands in spots with and without furniture. Clap once in front of a bare, flat wall and once in front of a bookshelf. This is not an experiment to test hall diffusion itself, but a way to notice how sound reaches you in your surroundings. A family member can discuss what you notice with you.
You can also try this in bathrooms or hallways. Differences in sound depend on room size and furniture, so you cannot say the effect is due to the bookshelf alone. If you do not hear a difference, that is also a result.
When looking at photos or videos of halls next time, look for whether wall and ceiling irregularities look like flat surfaces or scattering surfaces. Comparing old statues and coffered ceilings with modern panels can help you notice the differences in shape.