How to Calculate Noise Level in Duct?

The duct dimensions and air flow rate are not required to know the noise level. You can measure noise level at any point by a dB Meter.

"The duct dimensions and air flow rate are not required to know the noise level." Please do not mislead people just to increase your postings. First of all, the OP wanted to know if there is an empirical relation, not what equipment to use. The noise can be calculated by Lw = 10 + 50 log( v ) + 10 log( A ) where Lw will be sound power level (dB) and v is the air velocity (m/s) and A is the duct cross sectional area in (m2) work it out and you will see that the noise generated inside a duct is small compared to that generated by fans.

"Please do not mislead people just to increase your postings." Sorry, it is totally non intentional,not for increasing my postings. Here I do agree with you about different answer. Thanks for providing an expected answer to the OP.

Can the same empirical relation will hold good for suction and discharge of a two lobe blower with circular pipe velocity range 22 to 30m/s? I presume that log base is 'e'.

Thank you for your reply.

Say i am not misleading.

If i know the duct dimension, I can get area.

I know my airflow rate, say from Q=A*V, I can get Velocity.

According to your equation given, now i can get Sound level.

may i know from where you got this equation.

http://www.engineeringtoolbox.com/ducts-noise-generation-d_1405.html