The walls of the halls built for music concerts should

The power of sound from the speaker of a radio is $20 \text{ mW}$. By turning the knob of the volume control,the power of the sound is increased to $400 \text{ mW}$. The power increase in decibels as compared to the original power is ..... $dB$.

If the threshold of hearing is assumed to be the reference $(0 \ dB)$,then the threshold of pain is taken to be $120 \ dB$. Let the corresponding sound intensities be $I_0$ and $I$ respectively. Then $\frac{I_0}{I}$ is:

In a test,a subsonic jet flies overhead at an altitude of $100 \, m$. The sound intensity level on the ground as the jet passes overhead is $160 \, dB$. At what altitude should the plane fly so that the ground noise is not greater than $120 \, dB$?

In a city,the sound intensity level increases uniformly by about $1 \ dB$ annually. How many years will it take for the intensity of sound to become double?

Tone $A$ has a frequency of $240 Hz$. Of the following tones,which one will sound least harmonious with $A$ (in $Hz$)?