Keeping out unwanted sound
So, if you try to block out noise with hard surfaces such as wooden boards or walls, you risk simply redirecting it. A muffling effect can be created with a wall of foliage -- leaves and stems will absorb noise. A well-placed leafy plant between you and that pesky air-conditioner across the street can work wonders.
How do you choose the right plants? Match leaf size to sound frequency: High frequency noises need a lot of small dense leaves packed close together. Lower tones are best muted by bigger leaved plants. But keep in mind, if you jam-pack your balcony with plants to deaden all noise your home will be claustrophobic and depressing.
As the urban Buddah says: "You live in a city - get over it."