Carnegie Hall in New York City ha d been f a mous f or it s wa r m,resonant sound since it opened in 1891. After ninety years, however, the hall needed a major makeover. When it reopened in late 1986, something was wrong. Musicians said the sound was harsh, hollow, brittle, and dead.The hall tried several fixes, but nothing worked. Nine years later, the mystery was solved. Somehow during the renovation, concrete had been laid under the wooden stage floor. That had dulled the sound, especially the bass notes. In a concert hall, even unseen det ails can make a huge difference.
Architectural acoustics deals with how characteristics of a room or building affect sounds in it. And there’s a lot going on in a concert hall. The hall is the medium in which sound is transmitted. The musicians and their instruments are the source of the sounds. And the listeners receive and hear the sound. “So, you don’t only need to understand how musical instruments vibrate and how the sound travels in air or reflects from the wall, but also how we perceive it, ” says Henna Tahvanainen. She is an acoustics researcher at Aalto University in Finland.
Concert halls present a twist on the usual challenge of trying to make unwanted noise go away, says Densil Cabrera. He heads the Audio and Acoustics Program at the University of Sydney in Australia. “In concert hall acoustics we are aiming to make music sound good—in other words, to do something positive instead of just getting rid of a negative, ” he explains. “From a science perspective, it is an intersection between psychology and physics applied to the creative arts. “
The music must be loud enough to hear but not so loud that it’s unpleasant.Audience members want clarity. In other words, they want the sound to be clear enough that they can focus on different instruments. At the same time,listeners want a sense of envelopment.
Reverberation matters!
Reverberation plays a big role in all of that. Pop a balloon in a room,and after a loud bang, you’ll hear the sound decay. That happens because y ou hea r t he init ia l s ound f r om the direct source—the pop of the balloon. But surfaces (of walls, floor,roof, furniture, etc.) in the room also reflect, diffract, or absorb some of that energy. Reflected sound energy bounces off a surface while diffracted energy is somewhat bent by it.
In other words, you hear the sound directly from each instrument when it reaches your ears. But much of the sound is also reflected by a concert hall’s ceiling, walls, floor,seats, and other surfaces. Each of those reflected sound waves is not as loud as the initial sound when it r eached you. But t he mult iple reflections reach you from different directions. So it seems as though the sound has built up. Not all of the sound bounces off of a surface.Surfaces in the room may absorb some of the sound that hits them. The amplitude of the sound waves falls,and the sound fades or decays. This persistence of the sound after it’s produced until it decays and you no longer hear it is called reverberation.At the Musikverein (known also as Golden Hall) in Vienna, Austria, it takes between 1.8 and 2.2 seconds for the sound to decay, so that’s the reverberation time there. At Boston Symphony Hall in the United States,the reverberation time is 1.8 seconds.That concert hall opened in 1900.
Wallace Clement Sabine was a teacher at nearby Harvard University while Boston Symphony Hall was being built. He came up with a formula to figure out the reverberation time for a concert hall. In math terms, it says the reverberation time equals a constant amount, 0.163, times the value of the room’s volume and divided by the area of absorption. Or,more generally, the reverberation time is directly proportional to the size of a room. And it is inversely proportional to the area that absorbs sound energy.
T=0.163 A
T = time in seconds; V = volume of t he r oom mea s ur ed i n c ubi c meters; A = area of absorption of the room, measured in units called sabins.
Sabine used his work to guide the Boston Symphony Hall’s building committee. Music lovers today still rank the hall as one of the world’s best. That hall has a basic shape like a shoebox. And indeed, shape has a lot to do with reverberation in a concert hall.
Listener s don’t wa nt a long reverberation time in a lecture hall.Words would all blur together. For a concert hall, it depends on the type of music. For baroque music with its continuous rhythm, 1.6 seconds would be optimum. For the large orchestras and dynamic sound of nineteenth century romantic music, 2.0 seconds would be ideal. Symphony Hall’s 1.8 seconds is in the middle—a good compromise.
Shape matters ！
But rever ber at ion t ime isn’t the only factor that affects sounds in concert halls. Shape matters too.Leo Beranek did acoustics research at the Massachusetts Institute of Technology in the United States. One of his projects asked music experts to rank different concert halls. Ten of the top fifteen had a shoebox shape,including Boston Symphony Hall and the Musikverein in Vienna. Sounds can bounce off walls, ceiling and floor, possibly multiple times before reaching listeners’ ears.
“One of my favourite halls is the Vienna Musikverein, ” Tahvanainen says. The shoebox shape lets the listener hear a significant number of reflections after the direct sounds reach the ears. That lets the listener detect the note-to-note changes as music is played. A common place to measure this is about two-thirds of the way back from the stage and a bit off center. Further back, the echoes from the back wall would reach the listener only a tiny bit after the direct sound arrived. And closer to the front would have fewer sound reflections.
It is almost universally agreed that properly-delayed early lateral reflections add to the quality of sound in a concert hall. Listeners in controlled tests find that the source of sound is widened by these reflections. Professor Marshall at University of Auckland was the first to recognize their importance.The acoustical quality in a hall is better if a significant number of early lateral reflections occur before about one hundredmilliseconds after arrival of the direct sound. This requirement is better fulfilled in shoebox-shaped halls than in other shapes.
“The dominan tview is that shoebox-shaped halls perform best for audiences, ” Cabrera says. But he adds,”I find some of the less conservative approaches interesting—where the lateral reflections are provided by more complex designs than the shoebox.” The Berliner Philharmonie in Germany and Suntory Hall in Tokyo have a vineyard shape, for example. Seats are arranged on sloping terraces, like those in a vineyard, facing the music from multiple sides with the terraces and other forms in the hall to help provide acoustic effects. Still other concert halls have been built in a fan shape. Roy Thomson Hall in Toronto is an example.
To complicate matters, sounds from the orchestra don’t all have the same frequencies. The wavelengths hit and bounce off surfaces at slightly different times and in different ways.So some surfaces may absorb certain sounds but be more transparent to others. A lot depends on the size of something relative to the sound’s wavelength.
As a result, the first sounds you hear come directly from the instruments,plus some sound from the area around your seat. But those sounds may lack the bass notes (lower frequency). “After that, the first reflections from the side walls and possibly the ceiling reach you,”Tahvanainen says. “Then you receive the sound from more complicated routes, like bouncing between two walls, or three walls, and the routes become more and more complicated until you perceive the rest of the sound as reverberation,” she continues. That lets you fully appreciate the bass notes. If the concert hall’s geometry doesn’t give a lot of reflections, though, “some bass will be missing.”
Other features can add to or detract from a concert hall’s acoustics.The “seat-dip effect,” for example,deals with how seating in a concert hall can reduce the musical sounds for listeners. Tahvanainen and others wrote about this in the Journal of the Acoustical Society of America in 2017. The team recommends seats with air space underneath to reduce the effect. Mover, the presence of people in a concert hall can affect the acoustics, too.
In the lab
Of course it’s great to travel to grand concert halls to listen to music.But that’s not always practical for research. For that reason, Tapio Lokki,Tahvanainen and ot her s at Aalt o University often use a virtual acoustics lab. Scientists and engineers set up speakers in different concert hall spaces. They then play music through the speakers as if the orchestra were playing. And they record the music from one listener’s position in the concert hall. The microphone can pick up sounds coming from different directions. The lab’s team then uses computer programs to reproduce the sounds in a smaller listening room at the lab.
The lab’s setup even mimics how it sounds to have multiple instruments playing at t he same t ime. “Each instrument excites the room a bit differently, depending on their position on the stage and the played note, ” Lokki says. “In our ‘virtual lab’ we take this into account, and people listen to a simulation of a real orchestra with headphones or with a 3D loudspeaker array.”
The lab lets listeners compare music from one hall to another. It can also let researchers tweak different features to see what might happen.And listeners can report on the effects without being influenced by how the space around them looks.
“Many current halls are optimising s i g h t l i n e s a n d m a k i n g ‘ w o w ‘architecture, which unfortunately often leads to a situation that the sound is quite boring,” Lokki notes. An audience might watch an orchestra playing, but the sound might not really be very enveloping and engaging. “Our novel research results explain well what architectural features are important for sound.”
Some of Cabr er a’s wor k has looked at how musicians experience concert hall acoustics. He says, “If they can’t hear themselves well, then the hall is difficult to play in. On the other hand, if musicians can’t hear each other well, then timing, tuning, and balance become very hard to maintain.The question is what circumstances provide the right condit ions f or different instrumentalists.” Cabrera and colleagues from Korea reported on that work in 2010 in the Proceedings of the 20th International Congress on Acoustics.
More recently, Cabrera and other colleagues r epor t ed on chamber musicians’ perceptions of different playing spaces. That paper is in the June 2019 issue of the Journal of the Acoustical Society of America.
“The basic idea is that it’s not just the amount of sound reflections, but the direction that sound reflections come from that matters,” Cabrera says. “Musicians who are relying on acoustic reflections to hear each other and themselves feel better supported from reflections that come from the sides and back of the stage than if the same amount of sound reflection comes from the ceiling. So a high ceiling is a good thing in a concert or recital hall (up to a point).”
At Carnegie Hall, the problem was literally under the musicians’ feet. A follow-up renovation project removed the concrete subfloor. That solved the problem. Afterward, both musicians and audiences could applaud.
