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I am designing an automatically played air instrument which consists of an air compressor, solenoid valves, a variety of pipes through which air flows and makes sounds, as well as an Arduino connected to a relay module for easily controlling the solenoid valves. When I apply electricity to solenoid valves, I hear (in addition to the usual clicking sound in solenoid) something like a popping sound (or maybe better described as a plop sound), probably caused by the sudden escape of compressed air. Is there a way to reduce this? I initially thought of driving the solenoid valves using PWM, so that it opens slower and the air makes less sound, but heard that this would not slow down the opening speed of the valve since solenoids cannot be slowed down... Any electrical or mechanical suggestions would be very helpful. The usual solenoid clicking sound is ok, I just need to reduce the air popping sound if possible...

(Below is the type of valve I am using; the pressure I use does not exceed 1-2 bars and I am using push-in connectors to hoses with 4mm inner diameter)

This is the type of solenoid valve I am using...

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    \$\begingroup\$ Reduce the air pressure? Or possibly put a reservoir (coke bottle? perhaps with baffles? some tuning needed) downstream of the valve to slow up the transient. \$\endgroup\$ – Brian Drummond Nov 26 '16 at 18:03
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    \$\begingroup\$ There's no reason not to at least experiment with PWM drive to the valve. If you want to open a valve very quickly, then lots of volts/current is necessary, so there's no reason to think the reverse wouldn't apply, though it might not be enough of an effect to be useful. Where, physically, do you think this noise comes from? \$\endgroup\$ – user1844 Nov 26 '16 at 19:08
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    \$\begingroup\$ I would try switching to a soft-shift valve. The soft shifting is usually accomplished with a small hydraulic reservoir with an orifice attached to the spool piece to slow its rate of travel down. \$\endgroup\$ – R Drast Nov 28 '16 at 14:36
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You are right in that PWM may not work since the solenoid was not specifically designed to be proportional. Still, it's worth a try to see what you get. You can also get proportional valves, although those will be more expensive.

One obvious answer is to add a muffler to the output of the valve. Perhaps even a old car muffler if you have the room, but it shouldn't be hard to make something yourself.

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Without measurement instruments, Plop and Pop are crude descriptive terms for the turn-on (and likely turn-off) transient event. It seems that your musical instrument propagates or transforms this transient into undesired sound that detracts from the musical note you desire. Slowing down the transient can help, but going too far may introduce other problems - I am thinking of prepping a bagpipe to operating pressure, and the squeals of protestations that can be produced.
Some valves might be activated by a valve element moving in a direction with or against the airflow. I can imagine that this exascerbates your problem. Others may have a valve element moving at right angle to the airflow. This kind of valve might give a more desirable-shaped transient.
Others have suggested modifying the air path, this could work too - but can also add artifacts - rigid reservoirs (like coke bottles) introduce resonances in the audio range that can interfere with your musical note. Consider flexible reservoirs too, and compliant hoses, perhaps of smaller diameter.

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