Using Level Control
Level Control is a function that can control the output level of a sound system, based on the sound level detected in the room.
Level Control works by monitoring the
sound level from a microphone (connected to the left audio input of an
iOS device), and applying attenuation to a sound signal present at the
right input and passing it to the iOS device output.
Note that as a minimum, a two-channel
audio input device is required, and so Level Control will not work with
the built-in iOS device hardware. Also, since the sound system signal
comes into one input channel, Level Control only works to control mono
sound signals.
iAudioInterface2 is an excellent audio
I/O device for this purpose, as it includes an XLR microphone input, a
second balanced line input, and a balanced line output.
Once installed, Level Control is
available in several modules in AudioTools, including SPL Traffic
Light, Dual SPL Traffic Light, and SPL Graph. The dB level that
triggers the level control is set in each module differently. It is the
red trigger level (or levels) in the traffic lights, and it is
controlled from the notifications in SPL Graph.
Note that each module has its own Level Control settings for attack
and release times, so you can use different settings depending on the
module.
Setup
On your two-channel audio interface, make sure the
microphone is connected to the left input (sometimes called Channel 1),
and a mono feed from the sound system is connected to the right input
(which may be called Channel 2).
The output of the audio interface is then sent to the amplifer in the
sound system. It is also possible to use the iOS audio interface as an
insert in the console. This would be inserted after all gain and
limiting, and just before the output to the amplifier.
You may need to adjust your interface
gain settings to prevent clipping on the input, and to provide the
proper signal level on the output..
Connect a microphone to the left input (or iAudioInterface2 XLR
connector), and make sure you have calibrated the microphone so that
you are getting good SPL readings.
Operation
Once you have the
hardware set up correctly, you can proceed to the software setup. We
are using SPL Traffic Light as the module in this example.
Settings
Open SPL Traffic Light, and tap the Setup button to
get to the settings page. Now tap the "Sound Level Control" button to
open the Level Control settings page.
Input Gain Range
If you have iAudioInterface installed, you will see an
input gain range control. This applies to the 1/4" Balanced Input. In
most cases, Mid Range is appropriate, but if you experience clipping on
the line input, select High Range.
If you are using another audio input device, set the right input level to match the level coming from your mixer or console.
Attack Time
Select the desired attack time. This will be the
exponential attack time applied to the sound level attenuation. Faster
times will correct excessive sound levels faster, while longer times
will be less noticable.
Release Time
Select the desired release time. This sets the exponential
release time for the sound level to return to 0 dB attenuation. In
addition, there will be a delay of this time before the sound release
starts.
For example, if you set a 20 second release time, and a loud sound is
detected that applies an attenuation of 10 dB, the attenation will stay
at 10 dB for 20 seconds, and then slowly decrease to 0 dB with an
exponential time constant of 20 seconds. Thus, it will be about 60
seconds before the attenuation completely disapears.
Trigger Levels
The level at which attenuation is applied is set differently, depending on which module you are using.
SPL Traffic Light
In this module, the level control attenuation depends on the difference between the current SPL and the red trigger level.
For example, if you set the red trigger level to 85 dBA, Slow, and the
traffic light shows 90 dBA, 5 dB of attenuation will be applied to the
line level signal. This will then slowly decrease back to 0, depending
on your attack and release times.
If another overage is detected before the attenuation is removed, more attenuation will be applied.
Dual Traffic Light
The dual traffic light module works similarly to the traffic
light, except in this case two conditions are monitored to detect
excess SPL.
For each traffic light, the current SPL is compared to red trigger
level. If either exceeds the red level, attenuation is applied,
If both traffic light SPL levels exceed their red trigger levels, the higher SPL overage will control the attenation.
Thus, you can use two entirely different criteria and trigger levels
for controlling sound levels. One channel could use C weighted Slow
SPL, while the other is using 32+63Hz low frequency bands as a running
10 second Leq.
SPL Graph
Sound Level Control is available on every notification in SPL
Graph. This allows a very flexible setup, where different trigger
levels and measurement criteria can be in effect at the same time, or
the criteria can be based on time of day or day of the week. Also, you
can send email notifications if desired, to document level control
instances, from the same notification setup.
See SPL Graph for more information about setting up notifications. On
each notification, you can enable or disable level control, so you can
decide which ones you want to be used to control the sound level. Note
that sound level control must also be enabled on the Sound Level
Control settings page accessed from the SPL Graph Setup page, which is
also where the attack and release times are set for level control
events triggered from the notifications.
If multiple notifications are in effect which have level control
enabled, the highest SPL above its notification trigger level will be
used to derive the attenuation.