SOUND OF AIR

<how souund is generated and why>

Because the flow through the pipe is turbulent, and the turbulence causes the pipe to vibrate, further influencing the turbulence and the vibrations it produces. That's why tree leaves rustle and organ pipes vibrate when wind passes them.

<Is there any technique or principle to reduce such noise?>

A lot depends on local working hygiene regulations. In the UK, there's no absolute requirement to do this provided the workplace noise level is below 80dBA. If it's at or above this, and workers have to endure it for any length of time during the working day, them protective measures must be taken, with the exposure and intensity determining the effort needed to avoid prosecution.

Other factors may come into play: nuisance to neighbours might invoke action as organisations can be prosecuted for noise blight under various pieces of pollution legislation. Instead of keeping the sound within the equipment, deflecting it might be more appropriate; tyre noise from busy roads comes to mind, and there are installations where embankments or walls are built to make sure the noise goes upwards instead of sideways.

Acoustic lagging, as can be fitted to the inside of boat engine compartments, might be applicable to the room containing it and has been used successfully in the past, as would applying glass fibre pipe insulation to the outside of the pipe. Another possibility is to install some form of baffle into the pipe or into the vessel; a stainless steel mesh of the same sort of consistency as a washing-up scouring pad springs to mind though it hasn't been tried here. A third possibility is to increase the size of the pipe between compressor and receiver, thereby reducing the air velocity and with it, the turbulence that causes the noise.

Air compressor suppliers can sometimes offer advice in this area and it is certainly worthwhile contacting the local representative for advice. Many of them have seen similar problems countless times before.

Only local circumstances can determine the correct solution.

"Is there any technique or principle to reduce such noise?"

Pipe of 1.26" ID may be too small for your compressor output and receiver volume.

How many CFM is compressor?

Size the piping to be commensurate with the compressor capacity. A muffler may be of some help in reducing excess noise.

Thanks !

CFM - 112, Tank Capacity - 5 NM3

Is there any link or books that should be referred for muffler designs and options.

Whats the key to damping of noise?

Sir:

Increase comp. to tank connection at least one pipe size.

Make as much as is practical of high quality/high pressure rubber or equal reinforced fabric hose.

Provide as much of a non linear alignment as possible in hose (bend a "horse shoe shape in it) Subject to hose manufacturers radius restrictions/specifications.

Missing is indicated operating pressure......high/low.....

If at all possible do NOT mount compressor/motor assembly on the tank. Isolate them.

It appears you have a sympathetic harmonic induced from conductor/cross section changes vibration resulting velocity alterations in creating the sound. (not dis similar of the way a whistle works)

MR. GUY

Though you may be hearing mechanical vibrations through the walls of the pipe because of turbulence in the flowing column of air, it's more likely you are hearing what is commonly called an "edge tone"...as the compressed air reaches the receiver, it expands to fill the void, but passing by the edges at the end of the 1.25" pipe can actually generate a tone much in the same way a flute does.

This tone is a function of air speed, pipe diameter and the length of the resonating column. If the sound is loud and annoying, you can change the resonant frequency in several ways. One way could be to make the end of the carrying pipe very ragged...introducing jagged mountains and valleys to the edge where the pipe enters the chamber will raise any resonance factors up beyond the range of human hearing.

Another way would be to change the bore of the pipe, and one other way would be to change the length of the pipe and chamber (since that is the resonating body).

If you are not hearing a tone, but instead a whooshing sound or a white noise sound, you are probably hearing sound from the rushing air penetrating the walls of the pipe. In this case, wrapping the pipe with a sound absorbant material like foam rubber could dampen the sound below the annoyance threshold.