Air pumps. The smallest, quietest air pump is the one I'm after, since I don't need air pressure sufficient to run a wall o' bubbles or a bubble volcano: just a discrete sponge filter here and there in 10-gallon tanks. The smallest Rena and Whisper 10 each have their fans. We're living through a design phase where most consumer products, even cars, imitate either a computer mouse or a sneaker: Rena is vividly a mouse.
The Fusion 200 pump divides the air flow into two opposing chambers in an attempt to damp sympathetic vibration. As for bedroom-worthy quiet, all air pumps seem to develop vibration hum with time, in part as their diaphragms age and stiffen. Reducing back pressure extends the life of a diaphragm; if there is no dial to adjust airflow, you can install a bleeder valve in the air line.
Damping that hum. Air dampens vibration noise, but an air pump that shares a counter surface with a tank, or even one on the inside shelf of the cabinet stand, can transmit quite a roar. Though the pump may sit on its own rubber feet and hum quietly, if I put my ear to the tank, transmitted vibration makes it sound like the engine room of the Titanic in there. Setting the pump on a cork hot plate supported on several thick rubber bands lying on their sides helps muffle the hum. A stiff, unused kitchen sponge, scrubber face upwards, also makes a firm but sound-deadening base. But the best arrangement is to put the pump, with its sound-insulating base, on a bracket screwed to the wall. Make sure the bracket has a lip round it, because those pumps are made for walkin' and that's what they're gonna do.