This is the 14920th most frequent German word.
ruhige
Calm; quiet.
In this case, 'ruhige' characterizes 'Stimme' (feminine singular noun), describing its gentle and calming quality.
Die ruhige Stimme beruhigt alle.
The calm voice soothes everyone.
The word 'ruhigen' is the accusative singular feminine form of 'ruhig,' describing the street ('Straße') in the sentence.
Ich wohne in einer ruhigen Straßen.
I live on a quiet street.
Here, 'ruhige' modifies 'Atmosphäre' (feminine singular noun), indicating peace or calm.