Blue Sky

Note: The calibration test files, available for download on this page, have recently been updated and improved. Changes include; reduced hum & distortion, which although it didn’t negatively impact the accuracy of the previous version of the test files, the noise was somewhat annoying. Additionally, the crest factor on the 40 to 80 Hz test file has been reduced to help improve accuracy when measuring this test signal with a SPL meter.

The Instructions

Instructions for electro-acoustic calibration of 2.1 and 5.1 audio systems using an SPL meter and Blue Sky’s test files

Before starting, download the BlueSkyTestFiles.zip (15 MB). To download the zip file, “Right Click” and select “Save Target As” and the file will begin downloading, or click the link at the foot of the page.

Once downloaded, either burn the test files on to a CD or import them into your DAW and follow the instructions below.

Additional Required Items

• 2.1 or 5.1 Monitoring System
• SPL Meter – such as the SPL meter sold by RadioShack in the U.S.

BlueSkyTestFiles.zip Includes 4 files:

• 1000Hz SINEWAVE -20dBFS.wav – a 1kHz file recorded at -20dBFS for electrical calibration
• 40-80Hz PINK NOISE -20dBFS.wav – a 40Hz to 80Hz bandwidth limited pink-noise file recorded at -20dBFS
• 500-2.5kHz PINK NOISE -20dBFS.wav – a 500Hz to 2500Hz bandwidth limited pink-noise file recorded at -20dBFS
• Pink Noise full bw -20dBFS.wav – a full-bandwidth pink-noise file recorded at – 20dBFS

These test files are all mono files. Please make sure you hard assign them to the left and then the right, not both channels at the same time. If you are using a CD / CD player use only one channel of the CD player.

Theory
The purpose of calibration is to adjust the overall electro-acoustic system gain so that 0dBVU of electrical signal level equals a certain acoustic level at the listening position. Since most recording media is now digital, the reference electrical signal level is usually –20dBFS with 20dB of headroom. The reference SPL level however can vary based on the delivery media and speaker type.

Please note that the bandwidth limited signals that have been provided, limit many of the room interaction affects often associated with measuring SPL and broadband pink noise.

*Also note that the LFE channel gain in 5.1 formats varies from 0 to +10 dB depending on the encoding format used. Since the LFE channel is not calibrated as a separate entity, the LFE gain will not affect system calibration. For more information about the LFE channel, follow this link.

The common calibration levels are listed below:

All test signals are recorded at –20dBFS including the 1 kHz sine wave tone. The sine wave tone is used to set the electrical output level throughout the signal path, right up to the point you get to the speakers. While the various pink noise signals are used for acoustic measurements and calibration.

The following procedure assumes you are calibrating the system to 85dBC SPL. If you are calibrating to TV, etc. substitute the appropriate level from the above chart.

1. TURN OFF THE MONITORING SYSTEM (until step 4)
2. Remove all eq and dynamics from the signal path and set all controls to zero / unity gain.
   Play the 1kHz Sine Wave, hard assign it to the left channel only, and adjust the output fader so the so the output meter reads -20dBFS. If you are using an analog console, set the output level to 0 VU. Then hard pan the signal to the right channel output and repeat for the right channel. For a 5.1 system assign the channel to each output and repeat the adjustment procedure. Once calibrated do not move the output faders.
3. Mute everything and make sure the 1kHz tone is OFF.
4. Now that the system has been electrically calibrated, turn ON the SUB and SATs.
5. Assign the 500-2.5kHz pink noise signal to the left channel only. Make sure there is nothing coming from the right channel (or any other channels). Because this signal is bandwidth limited, you don’t have to worry about turning the sub off. There are two methods of setting the levels.
   1. If you have a master monitor level control, you can set the sat gain control at reference and then adjust the monitor gain control for 85 dBC. Then mark the monitor level as your reference position.
   2. The other method is to set master monitor level to the position you want as a reference level and then use the volume controls to set 85 dBC. If you use this method you should mark the knob position with a grease pencil so you can always go back to reference level if the knob gets moved.
      For either method:
      SPL should be measured at the mix position, with the SPL meter at arms length, with the microphone at seated ear height, angled at approximately 45 degrees, and pointed at the center point between the left and right speakers.
      Once the left channel is set to 85dBC, repeat this step for the right channel (and C, LS & RS in a 5.1 system).
6. Feed 40-80Hz pink noise signal to the left channel only. Adjust the subwoofer level control until the subwoofer reads 85dBC (slow) at the mix position. The meter will bounce around a little, so you will need to do a mental average (I tend to filter out the peaks in my mind, so I don’t set the sub too hot). The right channel should measure about the same and no additional adjustments need to be made.
7. You can play the full-bandwidth pink noise, assigning it to the left and then the right channel (not at the same time). You should measure about 85dBc. It may be a little higher, because below 30Hz the room may have a little extra gain.
   No adjustments should be made with Full Bandwidth pink noise, unless you have an RTA (real time analyzer).
8. You are finished and the calibration process has been completed – enjoy!

If you have any questions, please don’t hesitate to contact us.