Intermodulated Distortion (IMD):
Klippel’s
3D-DISTORTION MEASUREMENT (DIS) module is used to calculate the Intermodulated Distortion for this drive unit.
Measurements were completed in the nearfield. Multiple output levels were tested to provide the trend of distortion component profiles and to provide a comparison against other drive units I have tested. The SPL provided is relative to 1 meter distance, averaged in the noted bandpass region.
Unlike Harmonic Distortion - which is a measure of only harmonics from a single tone - IMD is tested with two tones at the same time: a low frequency “bass tone” near Fs and a higher frequency “voice tone” much greater than Fs. 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. In this example, the speaker is supposed to play
only 30Hz & 200Hz at the same time. Thanks to IMD, it plays 200Hz ± 30Hz. 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.
With that in mind, I used Klippel’s DIS module to test this drive unit 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 purpose of me testing with two methods (different “bass tones”) is to see the difference between what happens when the driver plays with high(er) excursion vs when a typical HPF is used. All similarly sized and similarly purposed speakers are tested in the same manner. For better or worse. This means a 6-inch midwoofer is tested the same way an 8-inch midwoofer is. Ultimately, this is for my sanity, because having numerous measurement methods for all sizes of speakers would muddy the waters quickly and wouldn’t give us an idea of when performance is great (say, a 6-inch midwoofer that has much less distortion than an 8-inch) or vice-versa.
The above is tested at 3 voltages each. The first voltage is always 2.83v. The other two voltages are increased to provide higher output (usually targeting at least 96dB for one). As is the case with the multiple HD tests, the multiple IMD levels provides an idea of what the speaker’s IMD profiles look like as the output of the speaker is increased.
Results are provided in GIF form to help understand how the increased output levels impact the distortion profiles and levels. Also, I have provided the (calculated) excursion at the provided tests’ output level so you can see how much excursion the speaker is under at the bass tone. Naturally, with a lower frequency there is higher excursion than with a higher frequency bass tone. This also helps when you are comparing performance against another drive-unit because different speakers have different response profiles; one drive unit may have 2x the excursion from 30Hz to 80Hz while another may have 3x the excursion. Just something to keep in mind.
Test 1: Bass Tone at Fs, Voice Tones from 200-6000Hz
Test 2: Bass Tone at 80Hz, Voice Tones from 200-6000Hz
Maximum Long Term SPL (Multitone Distortion Testing) & Compression
Klippel’s
Multi-Tone Measurement (MTON) module is used to calculate the maximum SPL for this drive unit.
Unlike IMD testing, Multitone testing incorporates a complex stimulus, used to emulate actual music. However, it is a bit more limited in evaluation purposes so I am using it here to characterize the maximum SPL of a drive unit at 1 meter distance as well as provide how much compression it exhibits with an increase in voice coil temperature.
The below data provides the metrics for how Maximum Long Term SPL is determined. This measurement follows the IEC 60268-21 Long Term SPL protocol, per Klippel’s template, as such:
- Rated maximum sound pressure according IEC 60268-21 §18.4
- Using broadband multi-tone stimulus according §8.4
- Stimulus time = 60 s Excitation time + Preloops according §18.4.1
Each voltage test is 1 minute long (hence, the “Long Term” nomenclature).
The thresholds to determine the maximum SPL are:
- -30dB Distortion relative to the fundamental
- -2dB Compression relative to the reference (1V) measurement
When the speaker has reached either or both above thresholds, the test is terminated and the SPL of the last test is the maximum SPL. In the below results, the legend indicates the average SPL within the same passband as is provided for the Frequency Response measurements. The highest SPL in the legend is the maximum SPL referencd to 1 meter distance. I provide the data showing which threshold was exceeded. The graphic(s) with a red line indicate the threshold that failed and capped the maximum SPL.
This measurement is conducted twice (with a 30-minute break between to let the voice coil cool down):
- “Typical” - 80Hz to 1600Hz
- “Extended” - 40Hz to 3200Hz
The purpose of me testing with two methods is to see how a driver performs in a more “typical” passband vs when it plays an “extended” passband. All similarly sized and similarly purposed speakers are tested in the same manner. For better or worse. This means a 6-inch midwoofer is tested the same way an 8-inch midwoofer is. Ultimately, this is for my sanity, because having numerous measurement methods for all sizes of speakers would muddy the waters quickly and wouldn’t give us an idea of when performance is great (say, a 6-inch midwoofer that has much less distortion than an 8-inch) or vice-versa.
You can watch a demonstration of this testing via my YouTube channel:
Test 1: Typical
Multitone compression results.
Multitone distortion results.
Test 2: Extended
Multitone compression results.
Multitone distortion results.
The maximum SPL referenced to 1 meter for each test are:
- Max SPL for Test 1 is 100.6dB. The compression threshold was exceeded above this SPL.
- Max SPL for Test 2 is 100.6dB. The compression threshold was exceeded above this SPL.
Does this mean the speaker will not play above this level? No. It simply means, within a set of limits, the above values are the maximum the speaker output is. Above that SPL, the limits are further and further exceeded.