Barbara Bratman, age 13,of Barrington, I11., for her question:

What exactly is the sound barrier?

 On Oct. 14, 1947, a high flying plane flew faster than

758 m.p.h.  and created a resounding sonic boom. Headlines announced to the world that at last the sound barrier had been broken. Naturally lots of ordinary folk assumed that this so called sound barrier was some sort of solid wall.    Hence it was logical    to expect to hear a shattering racket when a plane bashed through it. Actually, this is not what happens at all.

The sound barrier is related to the speed of sounds, which travel through ordinary air at an average speed of about 1,100 feet per second. This is roughly one mile in five seconds. Naturally the early planes could not approach this speed  but the faster planes of World War II were coming close.

Then tragedy struck, again and again. As fast flying test pilots reached the speed of sound, around 758 m.p.h., their planes were shaken to pieces. For a time it seemed that the speed of sound created a barrier through which no plane could pass.

But the experts did not despair. Physicists explained how sound travels through the air, and designers created streamlined planes to overcome the problems. By the late 1940s, pilots flew safely through the so called sound barrier with sonic booms like claps of thunder. Nowadays, many planes can fly several times faster than the speed of sound  and survive.

The problem lies in the air, which is made of gaseous molecules. Sounds travel through the air in pulsing waves that expand and contract. And those pulsing waves are moving molecules. However, these molecules can move only so fast ¬and no faster. And a moving plane must shift them out of the way in order to get through.

The old slow going    planes had no problems. Their leading edges shoved the air ahead out of the way, and currents of crowded molecules flowed safely backward over the wings and tail. This worked fine, so long as the air molecules could move fast enough to get out of the way. But as faster planes approached the magical speed of sound, this became harder to do.

At about 758 m.p.h., masses of crowded air molecules pile up ahead of a plane's nose and along the leading edges of its wings. As the plane tries to get through, these masses of compressed air punch it with mighty shock waves. This happens at the so called sound barrier, the fastest speed at which ordinary air molecules can move themselves out of the way of a speeding plane.

The problem was solved with mighty engines and streamlined planes designed to guide the shock waves safely around the wings and tail. However, the shock waves trail behind in a cone shape path that reaches our ears with a resounding sonic boom.

Planes that break the sound barrier are said to travel at ultrasonic speeds, faster than the speed of sound. This varies somewhat with temperature and with height above the ground. The speed known as Mach I is around 758 m.p.h., the average speed of sound through ordinary air. Mach II is twice this speed, and Mach III is three times. as fast.