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How to Null Test Your Gear: Part 2

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This post is part of a series called How to Null Test Your Gear.
How to Null Test Your Gear: Part 1

In the first part of this tutorial I explained  the concept of null testing, the benefits and how to go about testing software. While this is all well and good, audio is not just about computers.

Between microphones, speakers, amps, and other equipment there are of potential pieces of physical equipment that can be tested. Of course testing some equipment is easier than others, but with a little patience it should be able to null test anything.

Null Testing Explained

Null testing is a way to show the difference between two pieces of gear using the exact same source signal. 

By inverting the polarity of one of the signals, you cancel out the original signal and are left with only the difference between the gear. 

This result allows you to see how similar or dissimilar two pieces of like equipment are.

Null Testing Analog Gear

Null testing physical equipment is almost as easy as null testing software plugins, it only takes a few extra steps. 

Remember that one of the key aspects in setting up a null test is to ensure that you have a consistent signal. In the software domain it is easy to create a noise or sine wave track that can be flawlessly repeated. 

Assuming you always use the same converters, with the same gain settings, you can use a DAW as the signal source for the analog domain. 

Remember, null testing is that it does not care how good a signal is, it only cares that it is consistent.

FOH Racks
FOH Racks (Photo credit: PhilyG)

Rack Gear

Rack gear is probably the easiest physical equipment to null test. This is because you can feed the signal out of the computer and back with no worries about their physical placement. 

This is how to set it up:

  • Start by creating a sine wave or noise track. If you are unsure which to use, see the previous tutorial
  • Using an Aux Output on the audio interface, route the signal out of the DAW (generally any output that is not Output 1-2)
  • Make note of the gain settings on the track and the physical output. If anything is too loud or too soft adjust it now
  • Once the output gain is set do not touch it until the test is over
  • Next connect the aux output to your chosen piece of rack gear
  • Adjust the settings accordingly for the desired test. Again, refer to the previous tutorial for more info
  • Route the output of the rack gear back into the audio interface. While in most cases it would be better to use a line level input, a mic level input can work
  • Set an appropriate gain for the input back into the DAW and do not touch it. Even when switching between various pieces of rack gear, this input gain should not be adjusted

As you can see here, the biggest trick is not to mess with the gain settings once established. 

This is because varying the audio levels will cause incorrect phase cancellation and result in an inaccurate null test. Which defeats the object of what you are doing.

This method also works for guitar pedals. It may not sound as pleasing as a guitar, but you're are not going for pleasing. You're going for facts.

Speakers and Microphones

This is a tricky part of null testing. Until now, everything I have null tested as been either digital or electrical. Software and equipment. 

Microphones and speakers, however, also deal with acoustical properties and presents a whole new set of variables.

An anechoic chamber
An anechoic chamber (Photo credit: JP Roche)

The biggest issue you'll run into with microphones and speakers are room tone, gain, and placement. 

Simply by having the microphone or speaker change angles could potentially throw off your test.

This why acoustical null testing would be traditionally done in an anechoic chamber. These rooms are completely devoid of sound reflections which takes a lot of the acoustical variables out of play. You can still get a rough idea at home, however.

Speaker Null Testing

Null testing a speaker is straightforward enough in theory, it is the practice that becomes tricky.

  • Set up the desired testing signal—noise for frequency response, sine wave for THD and noise
  • Route the signal out of the DAW into the amplifier and speaker
  • Position the microphone—ideally a measurement mic—at the desired distance and height. Note that some tests are better served at specific locations. If you are going for a general comparison without adhering to a specific standard, then set the microphone at the listening position (where you sit while mixing or mastering)
  • Take note of the mic position and do not move it
  • Position the speaker to directly face the microphone. Only one speaker is necessary since you are not measuring anything in stereo
  • Playback the test track and set an appropriate level for the microphone. Again proper gain staging without adding too much noise is crucial
  • Setup a recording track inside the DAW to record the speaker. Ensure you mute the tracks output so you do not cause a feedback loop
  • Record the output of the speaker, ideally without being in the room, no fan noise, no street noise, etc. This is because any additional noise that gets into the recording will obscure the null test. This is why this test is so tricky
  • Replace the speaker with the other and ensure that you're facing the exact same way as the original, and that the microphone did not move
  • Check the output level of this speaker. Assuming there are no tremendous gain changes, you should be good to record. If the speakers are way off in gain, try boosting the test signal before adding any gain on the microphone
  • Record the second speaker

Microphone Null Testing

Setting up microphones for null testing is almost the same as the speaker null test. There are only a few changes:

  • Only ever use one speaker. Changing them out will change the frequency response.
  • Ensure that when you change the microphones out that you do not change the microphone's position. Always measure from the microphone capsule to the speaker

Otherwise, that is it. You'll still be recording the test signals and will want a really quiet environment. The big trick is the microphone gain and position relative to the speaker.

Final Notes on Analog Null Testing

Up until now you've been told to never ever touch the gain settings once they are set. 

You may, however, have noticed that not every compressor, EQ, speaker, microphone, has the same output level even if the settings are match exactly. This of course would lead to an incorrect null test.

While it would make sense to match them with the input gain, it is much easier to match them in the DAW. This is because you can peak match or RMS the results with exacting detail. Try doing that with a gain knob.

Unless you have two very drastic output levels from the gear in question, the slight differences in bit-depth (from A/D conversion) will be negligible.

Good luck with your analog null testing, and I hope your learn something new about your equipment.

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