Academic Reading — All-Types Test 29

Reading passage

The acoustics of concert halls Walk into a great concert hall and, before a single note has sounded, the room is already at work. The shape of the ceiling, the material of the walls, the very upholstery of the seats are shaping how the music will reach the ear. Acoustics, the science of how sound behaves in an enclosed space, is what separates a hall in which an orchestra blooms from one in which the same players sound thin and muddled. For much of history this science was a matter of luck and imitation; only in the last hundred and thirty years or so has it become something that can be measured, predicted and, to a degree, designed. The single most important quantity in the acoustician's toolkit is reverberation time, the number of seconds a sound takes to fade to inaudibility after its source has stopped. It was first defined at the end of the nineteenth century by a young physicist who, asked to cure a lecture room in which speech was hopelessly blurred, spent his nights carrying seat cushions in and out and timing echoes with a stopwatch and an organ pipe. He found that the fade depended in a regular way on the volume of the room and the amount of sound-absorbing material it contained. A hall with too long a reverberation smears successive notes together into a wash; one with too short a reverberation sounds dry and lifeless, robbing the music of warmth. The ideal, it turns out, is not fixed but depends on the repertoire: around two seconds suits a symphony, while speech and opera call for something noticeably shorter so that consonants stay crisp. Reverberation, however, is only the beginning. A listener does not simply register a single decaying tail of sound; the ear is exquisitely sensitive to the pattern of early reflections, the first echoes that arrive from the side walls and ceiling within a few hundredths of a second of the direct sound. Reflections arriving from the sides, rather than from overhead, are what give a listener the pleasant sense of being enveloped by the music, and halls celebrated for their sound tend to be long, narrow boxes whose parallel side walls fold sound back onto the audience. The broad, fan-shaped auditoriums that became fashionable in the twentieth century, designed to bring as many seats as possible close to the stage, often disappointed precisely because their splayed walls sent those valuable early reflections away from the listener rather than towards them. If reflections can help, they can also ruin. When a strong reflection arrives too long after the direct sound, the ear no longer fuses the two and hears instead a distinct echo, the bane of large domed or curved spaces, which act like acoustic mirrors and focus sound onto particular spots while starving others. A related affliction is the standing wave, in which a sound of a particular pitch bounces between two parallel hard surfaces and reinforces itself, so that a single note booms unnaturally loudly at some seats and vanishes at others. Much of the acoustician's craft consists of breaking up such troublesome geometry, using ornament, panelling, coffered ceilings and irregular surfaces to scatter sound evenly rather than let it collect. Modern practice draws on tools the pioneers could only dream of. Before a note of concrete is poured, designers build a computer model of the proposed hall and trace thousands of simulated sound rays through it, watching where they gather and where they fade. Scale models, sometimes built at one-tenth size and tested with correspondingly high-pitched sound, offer a physical check on the mathematics. Yet no simulation has entirely replaced the trained ear, and the final adjustments to a new hall are still made by listening, often with movable panels and adjustable drapes that let the reverberation be tuned to the music being played. For all its instruments and equations, acoustics remains a craft in which the ultimate judge is the human listener, and a hall is not truly finished until an audience has sat in it and an orchestra has filled it with sound.