How to Do An Audio
System Noise Floor Test
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Technical Support Information
Be certain that all test equipment with a three-prong AC power cord is plugged in to a properly grounded AC outlet, check the outlet with an outlet tester or an AC voltmeter to be sure. If possible, use dedicated circuits with isolated grounds to power all equipment involved in the test. This will insure that you have a true earth ground as a reference for your measurements. Unplug all equipment in the facility that is not being tested to avoid possible sources of stray interference.

Turn on all equipment involved in the test and let it warm up for at least 30 minutes.

Create a signal path and a gain structure that represents a typical working situation as if one were doing a session.

In a multi-track facility, send audio from the console or pre-amps to the tape machine or digital workstation at unity gain and return it to the console. Assign the multi-track outputs to the stereo buss and send the console's stereo output at unity gain to the DAT machine or other two track. You can also assign effects to the stereo buss. Monitor the DAT's or other two track's output through the console and studio monitor system. Listen to the stereo output with studio monitors and headphones. Connect audio test instruments to the output of the DAT or other two track.

If a tone generator is available it can be used to establish a unity gain structure throughout the system. Set it at 1 khz and +4 db or -10 db, depending on your system reference level and run a tone through the entire system. Then, set all inputs and outputs at unity gain -- all the way through and out to the test equipment. Proper gain structure is an essential part of accurate noise floor testing.

Measurements can now be made without signal present -- but with all audio paths at unity gain. Record some blank audio then play it back checking the noise level on both the multi-tracks and the two tracks. Also, A/B testing can be done this way. Various different types of wiring, grounding and AC power sources can be evaluated accurately.

Listen critically to the system, increase the gain on the DAT's or the two track's input to raise the level of background noise until it is both audible and visible on the meters. In a typical project studio with 24 to 32 tracks of MDM and an inexpensive console the noise floor is visible at about -40 db on the meters of the DAT machine when the input of the DAT is turned all the way up. The noise floor will be audible long before that. With the DAT or two track gain up, A/B test various wiring, grounding and AC power systems by listening as well as measuring.

When A/B testing balanced AC power with unbalanced AC power the use of an isolated ground outlet to power the system under test is highly recommended, be certain that all equipment with a three-wire AC cord is properly grounded and most importantly, make sure that all interconnected equipment involved in the test, including the test instruments are running on balanced power.

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