IELTS Academic Reading passage: The Acoustics of Concert Halls

The Acoustics of Concert Halls

An orchestra can play the same symphony, with the same instruments and the same conductor, in two halls a few streets apart and produce two quite different experiences. Until the end of the nineteenth century nobody could say why, and the acoustic quality of a room was regarded as a matter of luck. The subject acquired a science almost by accident. In 1895 Harvard University asked a young physics instructor, Wallace Clement Sabine, to do something about a new lecture theatre in which a speaker was effectively unintelligible; a sentence, once uttered, hung in the air long enough to blur the one that followed. Sabine had no method to draw on, and spent years carrying seat cushions in and out of the room at night, timing by ear how long an organ pipe took to fade into silence.

Out of that tedious work came the founding quantity of architectural acoustics. Sabine defined the reverberation time of a room as the interval a sound requires to fall by sixty decibels, roughly the point at which it becomes inaudible beneath audience noise, and he showed that it can be predicted before a building exists: it rises with the volume of the room and falls with the total absorption of its surfaces and contents. The optimum depends entirely on what the room is for. Speech demands a short reverberation time, around a second, so that consonants are not smeared together; romantic orchestral music wants something close to two seconds, which supplies the warmth and blend audiences associate with a great hall; and a cathedral, at seven or eight seconds, is unusable for either but ideal for the plainchant it was built to carry.

Two shapes dominate the concert halls of the world. The older is the shoebox: a tall, narrow rectangle, of which the Musikverein in Vienna, opened in 1870, is the most admired example. Its proportions were arrived at without calculation, and its excellence was recognised only afterwards. The alternative was introduced in Berlin, where Hans Scharoun's Philharmonie of 1963 abandoned the rectangle altogether and terraced the audience in irregular blocks around a central platform, an arrangement that became known as the vineyard. The vineyard brings the audience nearer the players and seats more of them, which is why many recent halls have adopted it; the shoebox, whose side walls are close together, remains the form that acousticians reach for when sound quality is the only criterion.

The reason a narrow room flatters music has to do with what arrives early. A listener hears the direct sound first, then a scatter of reflections from the surfaces of the room, then a diffuse tail. Reflections arriving within roughly eighty milliseconds of the direct sound are not heard as echoes at all: the ear merges them with the original and registers the music as louder and fuller. What matters is where they come from. Reflections that reach the two ears from the sides, rather than from the ceiling, arrive slightly out of step, and the brain interprets that difference as spatial impression, the sense that the sound has width and that the listener is inside it rather than in front of it. A wide hall cannot deliver them, because its side walls are too far away.

Materials do the rest, and they are less glamorous than the geometry. Bass frequencies are only reflected by surfaces heavy enough to resist being set in motion, which is why the great nineteenth-century halls, with their thick plaster on masonry, sound warm, and why a lightweight partition, however handsome, drains the low end from an orchestra. The statuary and applied ornament of those halls, added for reasons unconnected with sound, scatter reflections in all directions and prevent the harshness a flat wall produces. The largest absorber in any hall, however, is the audience itself, which is why seats are upholstered to a carefully judged degree: the aim is that an empty seat should soak up roughly as much sound as an occupied one, so that the hall an orchestra rehearses in resembles the hall it performs in.

The modern designer is better armed than Sabine was. Ray-tracing software follows tens of thousands of sound paths through a proposed building and reports what a listener in a given seat would hear; physical models at one-tenth scale are tested with ultrasound; and auralisation lets a client hear a recording processed to sound as it would in a hall not yet built. None of it removes the risk. Several celebrated halls have opened to poor reviews and been altered afterwards, and many new venues are designed with variable acoustics: movable panels, retractable banners and chambers that enlarge the effective volume.

What no model captures is the part of the judgement that is not physical. Musicians need to hear each other on the platform, and a hall that is glorious in the stalls and hopeless on stage will be disliked by the very people whose playing determines how it sounds. Listeners disagree, consistently and irreconcilably, about how much reverberation is too much. And the numbers, however carefully computed, describe a room rather than a performance: the Musikverein was not designed by anyone who understood why it works. Acoustics can now explain a great deal about why a hall sounds as it does. It remains poor at predicting, before the first note is played, whether people will love it.