RECOMMENDED GAIN STRUCTURE
System headroom and operating levels are normally defined at the line output stage of the operating console, while system noise floor is defined at the microphone input stage.
The total dynamic range of the system is thus established and cannot be improved upon later in the audio chain.
However, through careless down-stream gain structure it can be degraded.
As an absolutely safe procedure we recommend that a music or speech reinforcement system be setup to provide a nominal 20 dB of operating headroom over the normal “zero level” calibration.
This should apply across the board, so to speak, to all electronic elements in the chain.
Basically, once the headroom value in dB has been determined, the precise relationship between headroom and operating level should be maintained through all following line level electronics.
At the end of the chain the power amplifier-loudspeaker combination must be considered as a separate entity, and adjustments made so that a given signal level (e. g., 0 dBu) is assigned a given sound pressure level in the house.
This process is shown in Figure14 for a relatively simple reinforcement system.
Figure 14: Setting gain structure in a speech reinforcement system.
Our recommendation is that a VU meter reading of “zero” at the output of the operating console be assigned a nominal level mid-way in the seating space of about 72 dB SPL. You may wish to change this value slightly, depending on local requirements.
This standard approach simplifies normal system operation; all the operator has to do is raise or lower the input fader of the console to attain a nominal zero dB reading in order to ensure consistent speech levels in the listening space.
This article is excerpted from JBL Audio Engineering For Sound Reinforcement, used by permission of JBL Professional.