Audio Systems: Ambient Noise, ambient noise level, amplifier system

Question
I am designing audio system for a public hall of 150'x270'.
The ambient noise level (people talk to each other) is
around 70 dB. How much should be the power level for a good
intelligble sound.

Answer
OH, my goodness.  This is not so simple as an innocent question would suppose.  There are so many factors such as the volume of the hall, the materials on the walls, air handling system, lighting conditions, mounting locations for the speaker system, music or voice or both reproduction, location of the live microphones, crowd gathering purposes and so on.  For a room of this size I strongly recommend an acoustic or sound engineer who specializes in professional and public systems.

For good intelligibility the power and speaker system design and location play a major role.  A projection system is best for intelligibility, but that may not fit your requirements if the people will be moving around or the layout changes with purpose.  A distributed speaker system is much more expensive and requires overhead speakers all around to fill in the spaces.  

The amount of power from the house amplifier system is a function of the speaker system sensitivity and directivity.  For example, if you have speakers that are 89db sensitivity they will require twice the power input from the amplifier as a speaker system that is 92db sensitivity.

The ambient noise level (70db) is an important point, but more important is the frequency content.  If it is just crowd noise from people talking and the floor space is a hard surface (which has lots of reflection) you would want to get the reproduction level up to at least 100db.  That would give enough head room to avoid distortion and overload with loud inputs.

Surely you have checked with JBL or other professional sound companies who can give you some advice, also.  JBL has a professional sound department that will help you with system requirement on a customer/supplier relationship as a possibility of getting your business.  

One approach would be for you to draw up an RFQ and let a few companies come in and bid on the system. To get the system right I strongly urge you - somewhere along the way - to obtain the services of an experienced professional who can come into the space and make sure, based on experience, that you will get a good working system.

Wishing you the best. Hope this helps.
C