Loudness Calibration

To compensate for the changes in human perception of varying volume levels, loudness applies a predetermined filter curve to the signal in response to output volume values. As the volume is lowered, perception of certain frequencies is progressively diminished and returns to a more flat response as volume increases. Therefore, loudness boosts the frequencies to the highest degree at low volume, decreasing the boost as volume increases.

Inserting loudness into the signal chain can be a set-and-go operation. Loudness can also be fine-tuned "by ear", or calibrated using an SPL (Sound Pressure Level) meter and either pink noise or program material. First, it is important to set up the basic gain structure of the system. See the Loudness Overview section for more information on the controls used to calibrate loudness.

Setting Input and Output Gain Structure

Before calibrating loudness, set up the system gain structure (see the Optimizing Audio Levels section).

A prerecorded track of pink noise (played through a device such as a DVD player) or a signal generator with pink noise is preferable for this purpose. Otherwise, you can use program material (using material that you are familiar with is recommended).

The pink noise should be recorded at -20 dBFS. Alternatively, set the signal generator to output -10 dBu and set the gain in DSP Configurator so that the input meter reads -20 dBFS. If you are using a recorded source and the player has an output level setting control, set the output of the player to its maximum, or 0 dB of attenuation. For program material, set the input level to meter at approximately -15 dBFS, with peaks safely below 0 dBFS.

Make an audio tie from the source to the speakers that will carry program material in the room to be calibrated. With the basic gain structure now set up, you can calibrate loudness using an SPL meter or by ear. You can also set loudness using an SPL meter, then fine-tune it by ear.

Calibrating Loudness

To calibrate loudness, use a sound pressure level meter set to "C" weighting.

1.  Double-click the LOUD block. The Loudness dialog box opens.

2. Ensure that the Bypass option is engaged (the Bypass button is red).

3. Generate pink noise or start your program material playback.

4. Measure the SPL in the room.

5. In the Loudness dialog box, click and drag the Loudness Compensation slider to the left or right until the value in the SPL readout field matches the reading on the SPL meter. Loudness is now calibrated.

6. Click the Bypass button again (button is gray and bypass is disengaged) to hear the compensation.

Theoretically, you can perform a calibration with the output channel volume and post gain level set to any comfortable listening level, but a relatively loud volume (well above the ambient noise in the room) that can be easily measured, is preferred.

An alternate method is:

With the Loudness Compensation slider in its default center position and the output channel volume fader at 0 dB (100 percent volume), adjust the amplifier until the SPL meter reads 90 dB. Loudness is now calibrated. This method works if 90 dB is an acceptable amplifier or volume limit for the room.

Setting Loudness By Ear

When setting loudness by ear, it is also essential that the system gain structure be set up first. Sit in an average listening location.

To set loudness by ear:

1.  Double-click the LOUD block. The Loudness dialog box opens.

2. Ensure that the Bypass option is engaged (the Engage button is red).

3. Set the output volume fader in DSP Configurator to a relatively quiet listening level (see the Output Volume section). Filter compensation from the Loudness processor is most prominent at low listening levels. Use program material that you are familiar with and set the levels as described in the Setting Input and Output Gain Structure subsection above.

4. In the Loudness dialog box, ensure that the Loudness Compensation slider is set to 0 dB (center point).

5. Click the Bypass button again (button is gray and bypass is disengaged). You should experience a moderate enhancement to the program material, with more accentuated bass frequencies (below
   500 Hz), and more brightness in the high frequencies that carry harmonic content (above 7 kHz). You can engage and disengage the Bypass switch in order to identify the difference between loudness being off and loudness being on.

6. To experiment with less loudness compensation, click and drag the Loudness Compensation slider to the left, in the direction of less compensation. For more loudness compensation, click and drag the Loudness Compensation slider to the right, in the direction of more compensation.

7. Any adjustment you make to the Loudness Compensation slider will carry through to all listening levels. Set the output volume fader in DSP Configurator to a relatively loud listening level.

8. Engage and disengage the Bypass switch in order to identify the difference between loudness being off and loudness being on. At a loud listening level, this difference should be minimal or barely perceivable.