IMD = Intermodulated Distortion = Distortion caused by playing more than one tone at a time. Typically tested with just 2 tones: a low frequency tone near Fs and a higher frequency.
For example a speaker driver plays 30Hz at the same time it plays 200Hz. Any distortion artifacts created by the sum and/or difference of the speaker playing both tones at the same time is a result of IMD. For example, the speaker is supposed to play ONLY 30Hz & 200Hz at the same time. Thanks to IMD, it plays 200+/- 30. Second order IMD would be 200+/- 30Hz = 170Hz & 230Hz. Third order IMD would be 200+/- 60Hz = 140Hz & 260Hz. If these 'side bands' are high enough in output then they are heard as distortion. If not, they are not. Simple version. I'll create a video on this later that will hopefully help those who don't quite understand the concept.
With that in mind, I am using Klippel's IMD module to test mid/woofers in two ways:
- "Bass tone" fixed at the driver's Fs, with a "voice sweep" where the 200Hz - 6kHz region is played, one tone at a time (over 50 individual tones).
- The same as above, but the "bass tone" fixed at 80Hz.
The above is tested at 3 voltages each: 2.83v, 5.92v, 9v. This makes my life easier and gives us an idea of what happens as output increases.
The purpose of me sweeping voltages is to show what happens as the drive unit is pushed for more output.
The purpose of me testing with two methods (different bass tones) is to see the difference between what happens when the driver plays with high excursion vs when a typical HPF is used.
With all of that said, I have a few options for presenting the results.
- "EVERYTHING, SEPARATELY". I can just dump all the charts (6 total) and you guys can figure out the differences.
- "COMPARE BASS TONE CHANGE". I can provide a gif that compares the difference of the Bass Tone's effect directly to each other. What happens when you emulate a HPF of 80Hz (with a brick wall). 3 graphics: one for each voltage (SPL output).
- "VOLTAGE STEPS COMPARED WITH SAME BASS TONE". I can provide a gif of each bass tone method. 2 total graphics. One for each bass tone, showing the difference as voltage (SPL output) is increased.
Which would you prefer to see? 1, 2 or 3?
Examples are attached. #2 is the first gif. #3 is the second gif.
Remember, the multitone testing is what I am using to determine max SPL through a bandpass region. The IMD test is really more for further understanding but isn't intended to provide a final number of SPL. Just to give us an idea of the performance of a speaker in regards to what happens when you push the excursion envelope.