CEA-2010 standard
The CEA-2010 (Consumer Electronics Association) is a standard which defines a method for measuring the performance of powered subwoofers. It present tone bursts centered at 1/3 octave frequencies in 20 - 63 Hz range. I also measured some additional frequencies below and above that range. The test is performed by increasing the input level until the SPL is limited by prescribed frequency-dependent distortion threshold (staircase function) or compression (won’t go any louder). The test signal is 6.5 cycle long, shaped (Hann window) sine wave burst. So the length of the tone burst stimuli goes longer as the frequency decreases. The staircase function defines the allowed distortion level for each harmonic. The staircase function allows higher SPL for harmonics closer to the fundamental, and lower for harmonics further to the fundamental. This is because human hearing has a decreased tolerance for distortion components at higher harmonics (based in part on studies of distortion audibility and masking [Shorter (BBC 1950’s), Harman]). Allowed SPL by harmonic (compared to the fundamental): 2nd -10dB; 3rd -15dB; 4th and 5th – 20dB; 6th – 8th -30dB; 9th and above -40dB. CEA-2010 rating is defined by calculating an average distortion or compression-limited SPL (GP @2m, dB RMS values) for each of two one-octave low-bass performance ranges - Ultra Low-Bass: 20, 25, 31.5Hz and Low-bass: 40, 50, 63Hz. If max SPL is not measurable or the S/N ratio becomes too low through some portion of ultra-low band, then the measurer may choose to state “NA” for that band.
Intermodulation distortion
IMD is also one of the variables which hasn’t been used in subwoofer reviews before. By definition the intermodulation distortion occurs when the non-linearity of a device or system with multiple input frequencies causes undesired outputs at other frequencies. IMD is measured by inputting two stimulus frequencies using certain spacing between them, a few common stardards are 250 Hz / 8020 Hz (with a 4:1 amplitude ratio), 60 Hz / 7000 Hz (with a 4:1 amplitude ratio) and 19 kHz / 20 kHz. These stardards were designed for measuring electrical devices such as amplifiers. Naturally they are not suitable for subwoofers, so again I and my peer Ed Mullen had to start from the scratch with this test too.
Eventually we ended up using fundamentals (or carrier signals) at 30 Hz and at 72 Hz. 30 Hz isn’t too low for most subwoofers to produce and 72 Hz is still below the commonly used 80 Hz crossover frequency. This combination also doesn’t share any THD/IMD harmonics. The Excel spreadsheet we made calculates all IMD harmonics up to 6th order, which was more than enough because most subs didn’t have any 5th or 6th order IMD harmonics (already below background noise floor at ~30-40 dB, system's noise floor is at much lower level). There isn’t a very large database yet, but these first results suggest that subwoofers with high output capabilities and low THD levels also have low IMD levels. One should carefully examine if IMD is even worth measuring with subwoofers due this relationship.
Ideally IMD should of course be as low as possible at all levels.
IMD tests haven't been performed since Round 3.