Gap Ambient Level Sensor
Purpose
The Gap Ambient Level Sensor device measures the ambient sound level in the room during gaps in the program material. By only measuring the room when no signal is being injected by the sound system, the device avoids mistaking its own output for ambient sound. It applies gain to the input signal and switches a router based on the detected ambient level. This allows different equalization settings to be selected as the ambient noise level rises in the room.
NOTE: While the Gap Ambient Level Sensor still works as advertised, there is now a full-time sensing version of this control available: the Ambient Level Control.
Inputs
Input 1: ‘Unprocessed’ Program Material
Unprocessed program material is wired to the first input of the device, where the signal is examined for silence (gaps) that indicate when it is safe to measure the ambient noise sense signal.
Input 2: Ambient Noise Sense Signal
An ambient noise sense microphone must be wired to the second input of the device. The device measures the signal level at this input only during periods of silence at the first input. Based on the level detected in this manner, the device chooses to route one of the processed program material inputs to the output. The device also applies gain based on this level, as described in the controls section.
Inputs 3 and greater: ‘processed’ program material router inputs
The third input and all that follow (the number varies with the size of the device) form a router. The system designer wires various processed versions of the program material to these inputs, and the device selects from among them, depending on the level detected at the second input. This allows, for example, totally different equalization settings to be in effect when the system is operating in quiet, normal and loud ambient environments.
Check out the Recommended Setup Procedure for tips on setting up this device.
Controls
Ambient Level Meter
This meter indicates the ambient sound level detected at the sense microphone input.
Ambient Threshold Level
This is the threshold (of the signal level received by the ambient microphone) above which the device begins to add gain to the program material.
Blue LED
Lights whenever the sensing circuit is active.
Bypass
This button bypasses the device, routing the program input of the device to the program output of the device with unity gain.
Gap Threshold Level
This determines the threshold of the program material above which the device does not sense signals from the sense microphone. If the program material level falls below this threshold the sensing circuit becomes active.
Hysteresis
This control adjusts the "width" of the guard band around the Router Threshold Level settings. If a signal is in a particular level range it must go the amount of the hysteresis above or below its range before changing to the next router selection. This prevents a signal that is hovering around a threshold level from frequently switching between router inputs.
Maximum Gain
This is the maximum gain that the device can apply to the program material.
Minimum Gain
This is the minimum gain that the device can apply to the program material.
Ratio
This is the ratio of the gain change applied to the program material to the level above the ambient threshold level of the ambient signal received by the sense microphone input. For instance, if the ambient level increases above the ambient threshold level by 3 dB and the ratio control is set to 1.5 dB, the program material’s level will increase by 4.5 dB.
Response Time
This adjusts the response time of the circuit. A short response time will result in a signal that responds very quickly to changes in the ambient level in the room. A long response time will average the signal over a longer period of time, resulting in a slower, but smoother effect. In most applications a short response time is not desirable.
Router Threshold Levels
These controls adjust the threshold of the ambient signal level received by the sense microphone input above which the router is switched. The green LED’s located between each router threshold level control indicate which router input is currently in the circuit. It is a good practice to set the left most control to the lowest threshold level and progressively set the thresholds higher, with the right most control set to the highest threshold level.
Sense Average Time
This control adjusts the time constant of the detector.
Sense Delay Time
This is the amount of delay time after the program material falls below the threshold level (set with the Gap Threshold Level knob) that the device waits before beginning to use the signal from the sense microphone. This delay allows the acoustical signal level in the room to decay sufficiently so that the program material is not detected by the sense microphone.
Adjust the Gap Threshold Level to be approximately 10 dB above the noise floor of the program material. Setting the Gap Threshold Level too high will cause program material to leak into the ambient noise sensor. Adjust the Sense Delay Time to allow for the program material to decay at least 10 dB below the ambient noise level in the room. The blue LED indicates whenever the level controller is sensing ambient noise. The Ambient level meter shows the ambient noise level in dB.
Bypass the level controller in order to set its gain to unity and use a Level device cascaded with the level controller to adjust the nominal program material level under nominal ambient noise level conditions.
Release Bypass. Under low ambient noise level conditions, use the Minimum Gain control to adjust the minimum program material level. Under high ambient noise level conditions, use the Maximum Gain control to adjust the maximum program material level. Depending on the ambient noise level the controller varies the gain between Minimum Gain and Maximum Gain.
Adjust the Ambient Threshold Level to the ambient noise level above which the controller should start adding gain. Adjust Ratio to the desired amount of dB program material gain change per dB ambient noise level change. Adjust Response Time to the desired time. The Gain meter shows the amount of gain in dB (either positive or negative) that is applied to the program material. Note that this meter does not indicate unity if Bypass is pressed, although the actual gain applied does equal unity.
Adjust the Router Threshold Levels to the desired levels. The green LEDs indicate which router input is currently selected. Hysteresis provides a guard band around the Router Threshold Levels in the event of ambient noise hovering around a threshold. A good starting value for Hysteresis is 0.5 dB.