When you decide to fight back, vocalization is of utmost importance. However, do not scream “Help” as nobody will come and help you and at the same time it confirms to the attacker that you are a soft target just as he had assessed you before initiating the attack. Psychologically it also puts you at a disadvantage because in your mind you are re-affirming that you are a helpless victim.
Rather scream something forceful such a loud “Get Back!” or “Back Off!” generated in the stomach. Coupled with posturing and a step or two forward with your hands high and ready to initiate the attack, you seem like less of a helpless scared victim. This creates doubt in the mind of the attacker whether you are indeed the soft target he thought. Bystanders are also more likely to pay attention and often times the phones come out to take videos. This is not good for the attacker as suddenly there are evidence and witnesses. Also it forces you to breathe and it makes you feel less like a helpless victim. Forcefully screaming also helps to generate power when striking. It is for that reason why karate practitioners scream when they strike and why some tennis players grunt/scream when they hit the ball.
Note: Avoid “Mock Charging”. Do not move forward and scream, only to back off again. This tells the attacker you are not serious about defending yourself and that you are just “making noise”. When you decide to fight back, you need to follow through and only stop when the threat is stopped.