An organ pipe is closed at one end and has fundamental frequency of 1500Hz. The maximum number of overtones generated by this pipe which a normal person can hear is :

Critical hearing frequency for a person is 20,000Hz.
If a closed pipe vibration in N^th mode, then frequency of vibration
$\mathrm{n}=\frac{(2\mathrm{N}-1)\mathrm{v}}{4\mathrm{l}}=(2\mathrm{N}-1){\mathrm{n}}_1$
(where n₁=fundamental frequency of vibration)
Hence $20,000=(2\mathrm{N}-1)\times 1500\Rightarrow \mathrm{N}=7.1\approx 7$
Also , in closed pipe
Number of over tones = (No. of mode of vibration) – 1
= 7 – 1 = 6