#### Answer

10,000 times more intense.

#### Work Step by Step

The decibel scale is logarithmic, based upon powers of 10. Adding 10 dB means the intensity is multiplied by a factor of 10. In other words, when the intensity of sound is increased by a factor of 10, the intensity level increases by 10 dB.
A sound of 40 dB has an intensity 10,000 times greater than 0 dB, the threshold of hearing.
A sound of 40 dB corresponds to an intensity of $10^{-8} \frac{W}{m^{2}}$, which is $10^{4} = 10,000$ times greater than the intensity of a 0 dB sound, $10^{-12} \frac{W}{m^{2}}$.
This is discussed on page 394, and shown in Table 21.1.