How waves fade with distance

Science · Waves, Light & Sound · ages 18-19

Name ______________________ Date ____________

What is intensity?
- Total energy ever emitted
- Speed divided by area
- Power per unit area
On a string wave, power is proportional to what?
- Amplitude squared times frequency squared
- Amplitude plus frequency
- Wavelength alone
Doubling the distance halves the intensity.
Circle one: True False
Intensity is 8 units at 1 metre. What is it at 2 metres, in the same units?
Answer: ______________
Distance doubles with no absorption. What is the new intensity?
- One half
- One quarter
- It doubles instead
A sound rises by 10 dB. What happened to its intensity?
- It multiplied by ten
- It added ten units
- It halved twice
A student says 30 dB is twice as intense as 10 dB. What is the error?
- Decibels ignore intensity
- Each 10 dB multiplies by ten, so it is one hundred times
- Loudness can only fall
Amplitude doubles on a string wave. What happens to power?
- It doubles
- It stays fixed
- It quadruples

Answer key

For grown-ups. Fold this page away before handing over the rest.

Power per unit area · Intensity tells how concentrated the flowing power is.
Amplitude squared times frequency squared · Both amplitude and frequency enter to the second power.
False · The sphere area grows fourfold, so intensity quarters.
2 · Quarter the start: 8 divided by 4 is 2.
One quarter · One over distance squared turns doubling into quartering.
It multiplied by ten · Each 10 dB step scales intensity by a factor of ten.
Each 10 dB multiplies by ten, so it is one hundred times · Two tenfold steps stack into one hundredfold.
It quadruples · Power follows amplitude squared, and two squared is four.