The Black Canary’s main power is a scream of epic proportions that can do one of two things:

1. Incapacitation - Making it very hard to concentrate or do anything.

2. Produce  blast/shock wave àWhich Physically knock people off their feet with a wave of air.

 

First we need to understand the loudness of sound which is measured in decibels (dB). The decibel scale is a logarithmic scale, from 0 dB (near silence) to 10 dB (low whisper) the sound will be 10 times more powerful, 0dB to 20dB will be 100 times more powerful. 0 dB to 30dB will be 1000 times more powerful.

A normal conversation is about 60 dB, a car horn can be over 100 dB and a jet engine about 120 dB.

 

Incapacitation.

There are devices already in use that produce sound above 120 dB, which is where sound becomes painful to humans and makes it difficult to do anything but try and get away. These devices have been used in riot situations for crowd control. However at 140 dB the eardrum will be damaged and actual physical pain will be caused which means these devices can be very dangerous if misused.

 

Blast or Shock wave.

Can sound knock over a person? Kind of, if it becomes a shockwave much like an air blast from a bomb. If you can compress the air enough to send a wave with a big increase in pressure it can cause damage. A wave with pressure 1 psi (pound per square inch) above normal air pressure travelling at 40 mph can shatter glass. A compression of 2-3 psi above normal pressure travelling at 100 mph can cause structural damage and physical injury. 5 psi would be enough to level buildings but that would take the energy from a nuclear bomb.