At Supersonic Speed, The Earlier Sound arrives Later

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

13 March 2005

So, what happens to the noise of a supersonic aeroplane? Of course the noise is left behind, but what do we here?

Suppose an aeroplane is flying 100 metres above the ground at a speed of 2,000km/h. At this height and on a cloudless day, the aircraft appears from the horizon when it is 38km away, and disappears at the same distance on the opposite side. Remember: the earth is round! Therefore the plane will take only 2 minutes and 15 seconds to traverse the sky from horizon to horizon. But from what distance can it be heard?

The engines of the supersonic jet produce about 150 decibels (dB) of sound when listened from point-blank range (one metre or less). This is loud enough to break the human eardrum. As the listener gets further away, the loudness decreases. When the distance increases buy a factor of ten, the intensity is reduced by 20dB. Thus at 10m from the plane, the noise level is about 130dB (louder than a car alarm siren placed next to your ear), and at 1km it is 90dB.

At 15km, the noise from the aeroplane is 65dB, which is about the same level as normal environmental sounds (cars on the street, people talking, birds singing etc all put together). Thus the jet will only be audible only when it is less than 15 km away. That is, a total of 30km in the sky (15km as it approaches and 15km after it flies past).

At 2,000km/h, the plane takes only 54 seconds to clear the “audible zone” of the sky. All the sound produced within the 30km distance is concentrated into a small region and heard at once within an interval of one minute. Because of this concentration, what we hear is a sudden “boom” a few seconds after the aeroplane has passed.

Now, there is another interesting effect produced by supersonic jets. Since sound travels at about 1,200km/h, the noise made when the aeroplane is 15 km away takes 45 seconds to reach us and that made from 10km away arrives after 30 second. But at 2,000km/h, the aircraft takes only 9 seconds to cover the 5km distance between the two points (15km – 10km).

Therefore, if at, say, 8:00:00am the aircraft is 15km away, the noise it produces reaches our ears at 8:00:45am. At 8:00:09am, the aeroplane will be 10km away and the sound it makes from this new position will get to us 30 seconds later, that is, at 8:00:39am. In other words, we hear the sound made at 8:00:09am BEFORE hearing the one produced at 8:00:00am.

That concept may be difficult to paint in the mind. To see a good simulation, visit the Internet website www.jillionlimited.com and click on the “articles” button. In the meantime, there is still one more question: What would happen if the aircraft travelled faster than light? That is a story for another day.