IELTS Practice Test 14

Transcript (on the real test you hear this once)

You will hear part of a lecture on sustainable building design.

Lecturer: Good morning. Today I want to look at a structure that has kept people cool for centuries without using any electricity at all: the wind-catcher. I’ll explain how traditional wind-catchers work and then describe some recent attempts to adapt the idea for modern buildings.

A wind-catcher is essentially a tall tower rising above the roof, with openings near the top. Traditional examples are found across a wide belt of hot, dry regions, and they vary a great deal in shape — many are square, some have eight sides, and a few are round. Height varies too: in some towns a particularly tall tower was a sign of a wealthy household. The principle behind all of them is simple. People tend to assume the air up there is much cooler, but in fact it’s barely cooler at all. What matters is that air at roof height moves faster than at street level, and it carries far less dust. The openings that face the prevailing wind catch this air and push it down a shaft into the rooms below. At the same time, warm air inside the building rises and escapes through openings on the other side of the tower, where the pressure is lower. So the tower acts both as an inlet and as an exhaust. And even on still days it keeps working. You might expect the household’s cooking fires to drive it, but they were usually in a separate courtyard kitchen; it’s the sun heating the tower’s walls that creates an upward draught, rather like a chimney.

Traditional towers were built of mud brick, which absorbs heat during the day and releases it slowly at night. Visitors often notice wooden beams sticking out of the walls and assume they’re decorative. In fact they partly strengthened the tower, and they also served as permanent scaffolding whenever the walls needed re-plastering.

The most effective designs went a step further and cooled the air with water. In some houses the shaft led air over a pool or an underground channel before it reached the living rooms. You might think the water works simply by being cold, but a pool in a sunny courtyard is often quite warm. The cooling comes from evaporation: as the water evaporates, it draws heat out of the air. Other households hung wet cloths in the shaft, or placed porous clay jars there, to get the same effect.

Now, wind-catchers weren’t perfect. People sometimes say the big problem was cold in winter, but most towers had wooden shutters that could simply be closed. Maintenance, too, is often mentioned, but re-plastering mud brick was part of the normal yearly routine. The real drawback was that an open shaft is also an open door — for dust during sandstorms, and for insects and birds.

In recent decades, architects have revived the idea. Modern versions are usually made of lightweight metal or plastic rather than brick, and some have a rotating cowl on top, which swings round to face the wind, rather like a weathervane. Others are fitted with filters and dampers, which deal with exactly that problem of dust and insects. Part of the attraction for modern buildings is that a wind-catcher brings in fresh air without anyone opening a window, which is useful on noisy city streets.

For one recent study, the Calder Building Research Group had hoped to use a secondary school, but its building was due to be demolished, so the team fitted wind-catchers to a primary school instead and compared its classrooms with those in a similar building nearby. The school was chosen partly because its classrooms faced the afternoon sun and overheated badly in summer. The team’s first report gave an average temperature reduction of four degrees, but one sensor turned out to be faulty, and the corrected figure was three degrees. Humidity barely changed. But the researchers themselves said the more important finding wasn’t about temperature at all: levels of carbon dioxide in the classrooms fell by about a third, and that matters a great deal for children’s concentration.