Commercial Neighbor makes too much noise, bother my tenants

[Gary Erdoglyan]

I have a property in the Los Angeles, CA area it is in a multi zoning area.

There is a Zumba studio right in front of my property and they make loud noise.

My tenants call me to complain and that they wake up from the noise and they  threaten to leave. 

I contacted the landlord 4 months ago and the noise stopped, but it is back now.

I went and confronted the Zumba owner and they are claiming its not that loud. 

What else can i do?

(...i didn't know which subject forum to put it under)

[Jon Holdman]

What time of day do they make the noise?  Are they out of compliance with the relevant noise ordinances?