Reduce noise, Mask noise, Cancel noise

There are three stratagies to counter noise:

1. Reduce noise
This is the most widely used method. Noise may either be reflected or absorbed by a medium between a noise source and a hearer; most barriers act by both reflecting and absorbing the sound. A wall reduces noise, largely by reflecting it before it reaches our ears. Indoors we are protected by the walls of the house and by closed windows. On motorways there are reflective walls with corrugated surfaces to reflect the sound. Curtains are good absorbers which reduce significantly sound from outside. Solid cement blocks in a wall are not good absorbers, hollow bricks are much more effective. A single pane of glass still allows a lot of sound to penetrate, double glazed windows are highly effective in blocking sound.
Ear plugs or ear muffs are effective sound absorbers at individual level.
The ultimate in blocking is achieved in a "quiet room" used for sound testing where the walls are made of absorbant material shaped to scatter any reflected sound in all directions.

2. Mask noise
Noise masking works by creating a continuous sound at a tolerable level in the vicinity of the hearer. Our senses filter out repetitive signals so that we are no longer aware of them. We cease to be aware of stationary objects in our vision field. We are no longer aware of the pressure of the seat moments after we sit down. After a while in a room with a ticking clock we no longer hear the sound of the tick. When we create a noise mask we soon block out the sound and with it the sounds of noise which have similar frequencies and sound level. The best noise mask is what is called 'white noise' which is a source containing a very wide band of frequencies and therefore effective against a wide variety of noise. An air conditioner or an electric fan can create a noise mask. For psychological reasons the playing of recorded sounds of nature such as sounds of sea waves or wind in trees may be effective. This method is not effective against random sharp sounds such as a barking dog! The effectiveness is limited by the need to keep the mask below a level where it would itself become a noise nuisance.

3. Cancel noise

Sound is a wave having peaks and troughs. If we add to it another wave of the same amplitude but shifted in time so that the troughs and peaks of the added wave coincide with the peaks and troughs of the original wave, the two cancel. This can be accomplished by earphones which include a microphone and electronic system to generate the second wave to cancel the first. It works well against a continuous sound such as that of an airplane or car engine, but not against the barking dog. Such earphones are on sale but the manufacturers are generally shy to quote the effectiveness in decibels. One claims to achieve total silence which is totally impossible!