Elephants are extremely sensitive to very low frequency sound and are able to detect such sounds over long distances. City environments, trucks and trains, generators and air traffic produce sound at the frequencies at which elephant hearing is most sensitive and also at the frequencies at which elephant themselves communicate.
Circuses are known to acclimatise young elephants to this noisy environment by keeping them penned or chained next to a continuously blaring radio. This is obviously not acceptable from an animal welfare perspective.
It is in any case neither healthful nor humane to keep elephants, or any animal, in an environment where they are continuously exposed to loud noise. The kakofoni of sounds from visitors is in itself a big burden for circus elephants.