In the phenomenon of interference of sound by two coherent sources if difference of intensity at maxima to intensity at minima is 20dB, then the ratio of intensities of the two sources is

1.  $\frac{121}{81}$

2.  $\frac{11}{9}$

3.  $\frac{101}{99}$

4.  $\frac{10}{1}$

When a sound wave travels from one medium to another, the quantity that remains unchanged is :

1.  speed

2.  amplitude

3.  frequency

4.  wavelength

If a sound source of frequency n approaches an observer with velocity v/4 and the observer approaches the source with velocity v/5, then the apparent frequency heard will be-

1.  (5/8)n

2.  (8/5)n

3.  (7/5)n

4.  (5/7)n

The equation of plane progressive wave motion is y=a sin 2$\mathrm{\pi }/\mathrm{\lambda }$(vt-x). Velocity of particle is

1. $y\frac{dv}{dx}$

2. $v\frac{dy}{dx}$

3. $-y\frac{dv}{dx}$

4. $-v\frac{dy}{dx}$

A person speaking normally produces a sound of intensity 40 dB at a distance of 1 m. If threshold intensity fo r reasonable audibility is 20 dB, the maximum distance at which he can be heared clearly is:

1. 4 m

2. 5 m

3. 10 m

4. 20 m

The sound intensity level at a point 10 m away from a point source is 20dB, then the sound level at a distance 1m from the same source would be

1. 40 dB

2. 30 dB

3. 200 dB

4. 100 dB