Sound Level Control

Controlling the sound level in a club, bar, restaurant, or music venue, is becoming more critical as regulations become stricter, and more government entities get involved in policing sound levels. Up to now, the only equipement available to control sound levels automatically has been expensive and difficult to set up and maintain.

Studio Six Digital is now making automatic Sound Level Control available at a much lower cost, and with an easy to set up and operate system.

Sound Level Control is now available as an in-app purchase in AudioTools. Note that a 2-channel audio interface, such as iAudioInterface2, is required to use Sound Level Control.

Using Sound Level Control

Soubd Level Control is a function that can control the output level of a sound system, based on the SPL detected in the room.

Sound 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.


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.


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.


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.