Four sources of sound each of sound level $10\,dB$ are sounded together, there sultant intensity level will be ... $dB$

An engine approaches a hill with a constant speed. When it is at a distance of $0.9 km$ it blows a whistle, whose echo is heard by the driver after $5$ sec. If speed of sound in air is $330 m/s$, the speed of engine is .... $m/s$

A car $P$ approaching a crossing at a speed of $10\, m/s$ sounds a horn of frequency $700\, Hz$ when $40\, m$ in front of the crossing. Speed of sound in air is $340\, m/s$. Another car $Q$ is at rest on a road which is perpendicular to the road on which car $P$ is reaching the crossing (see figure). The driver of car $Q$ hears the sound of the horn of car $P$ when he is $30\, m$ in front of the crossing. The apparent frequency heard by the driver of car $Q$ is ...... $Hz$

A $10\, m$ long steel wire has mass $5\,g$. If the wire is under a tension of $80\, N$, the speed of transverse waves on the wire is   .... $ms^{-1}$

Two waves represented by ${y_1} = a\sin \frac{{2\pi}}{\lambda }\left( {vt - x} \right)$ and ${y_2} = a\cos \frac{{2\pi }}{\lambda }\left( {vt - x} \right)$ are superposed. The resultant wave has an amplitude equal to

The diagram shows snapshot of a wave at time $t = 0$. The particle at $x = x_1$ is moving upward at that instant. Direction of propagation of wave is