93. The source being isotropic means A

sphere

= 4πr

2

is used in the intensity definition I = P/A. Since

intensity is proportional to the square of the amplitude (see Eq. 18-27), this further implies

I

2

I

1

=

s

m2

s

m1



2

=

P/4πr

2

2

P/4πr

2

1

=

r

1

r

2



2

or s

m2

/s

m1

= r

1

/r

2

.

(a) With I = P/4πr

2

= (10 W)/4π(3.0 m)

2

= 0.088 W/m

2

.

(b) Using the notation A instead of s

m

for the amplitude, we find

A

4

A

3

=

3.0 m

4.0 m

=

⇒ A

4

=

3

4

A

3

.