Image Dynamics IDMAX v4 12"

Overall summary

In this sample and under these test conditions, the 1 volt baseline distortion is pretty low over the intended passband and remains controlled into the lower midbass. Around 20–30 Hz it rises modestly but stays around 1 percent in this nearfield setup. H3 distortion is slightly dominant below ≈35 Hz, while H2 distortion is slightly more evident from ≈40–120 Hz. No narrow, level invariant spikes appear in the 20–120 Hz band at this drive.

It’s a different story at the near limit sweep of 16 volts. Distortion increases as a broad rise instead of as narrow peaks. THD is ≈10 percent around 20 Hz and finally drops below 5% in above 50 Hz. H2 distortion is dominant below for pretty much the entire usable frequency range. There are no obvious narrow, level invariant spikes in the 20–120 Hz band at high level.

Large signal data show a BL 70 percent one way limit of 21.56 mm, CMS 50 percent not reached within the ±22.39 mm evaluation window, and the 17 percent inductance criterion also beyond ±22.39 mm. The test engineer notes good CMS symmetry and good Le(x), but moderate BL asymmetry toward outward stroke. These traits align with the distortion pattern: the moderate BL asymmetry is consistent with even order H2 distortion as drive rises, while the stable inductance behavior tracks with limited H3 distortion growth outside the very lowest band.

For sealed use, the manufacturer positions the IDMAX12 for sealed enclosures, with this project’s data computing a 1.0 ft³ nets a Qtc of 1.175 on this sample, while approximately 7.1 ft³ would be required for a 0.707 Qtc using large signal cold parameters. That means most realistic sealed installs will land at a higher Qtc with a peaky response and low end roll off, but an infinite baffle installation will yield its best results.

Manufacturer's suggested use case

The manufacturer describes the IDMAX series as high excursion subwoofers with a field replaceable cone assembly. The sheet lists 500 W RMS, 19 mm one way xmax, and sealed and ported enclosure guidance, with sealed options of 0.65, 1.0, and 1.8 ft³ (Qtc ≈ 1.19, 1.01, and 0.83 respectively). Sensitivity is listed at 85 dB and the IDMAX12 is available in Dual 2 Ω and Dual 4 Ω. Essentially, they suggest its use case is for average sized sealed enclosures that have a good amount of output capability.

Our suggested use case

Based on the data gathered for this sample, the IDMAX 12 V4 performs best in large sealed or infinite baffle installations. Its compliance and low frequency behavior indicate that it benefits from generous air volume, allowing it to operate within its linear range. Smaller sealed boxes raise Qtc significantly and restrict the subwoofer’s ability to play well without extreme roll off in the bottom octave. A large sealed cabinet or infinite baffle environment provides smoother low end extension, better transient response, and mechanical headroom. For listeners focused on infinite baffle or trunk baffle installations where relatively low power levels are going to be used, this is where the IDMAX 12 V4 excels.

Testing and linearity limits vs. what is advertised

What it took to reach our high-level sweep limit, and how that compares to the published specs.

High-level sweep rule: Set just under the BL 70 percent point from LSI

High-level sweep limit for this sample: 16 volts

Approximate electrical power at that limit at 20Hz: ~130 watts. Real power varies with frequency and impedance. volts

Rated power (published): 500 watts.

Power used to hit the standardized limits in free air, relative to their xmax rating free air: ≈42 percent. Hits 21.5 mm of xmax in free air with 210 watts of power. Real power varies with frequency and impedance.

Claimed Xmax vs. measured at BL 70%: 21.56 mm measured vs 19 mm claim, ≈113.5 percent of claim.

Xmax @ 50% CMS: > 22.39 mm - limit not reached within the evaluation window.

Xmax @ 17% Le: > 22.39 mm - limit not reached within the evaluation window.

Manufacturer suggested sealed enclosure size (and its resulting QTC): 1.0 ft³ nets a Qtc of 1.175 on this sample.

Required sealed enclosure for 0.707 QTC: 7.1 ft³ nets a 0.707 Qtc.

Xmax @ 50% CMS: > 22.39 mm - limit not reached within the evaluation window.

Overall performance snapshot

This is our subjective interpretation of the objective data. How we derive these scores can be found on the home page of the testing section.

Distortion & frequency response - TRF measurements

Method recap: Method recap: Nearfield mic positioned at 1/10th the cone diameter plus 2 inches, on‑axis. Response measured to 1 kHz and THD to 500 Hz. 1/6‑octave smoothing. Two drive levels, 1 V baseline and a high level set at 16 V per the under‑BL‑70 rule derived from LSI for this unit. This sample in this test on this voltage level hit ≈16.8 mm at 20 Hz in free air, below the accepted standard of 70 percent of BL. Distortion reported both as percent and by harmonic.

At 1 volt - baseline

Distortion stays low over most of the 30–120 Hz band, with THD generally around ≈1 percent and about ≈1–1.5 percent near 20–30 Hz. H3 distortion is slightly dominant below 35 Hz, while H2 distortion is more evident through 40–120 Hz. The curve is smooth in this band and shows no narrow, level invariant spikes between 20 and 120 Hz.

At high level voltage (16 volts)

Distortion increases broadly rather than in discrete peaks. THD approaches 10 percent at around 20 Hz, then trends in the 4–6 percent range through much of 30–120 Hz. H2 distortion becomes the dominant component through the entire usable range. No narrow, level invariant spikes are visible in the 20–120 Hz band at this level.

Delta - 1 volt distortion vs. high level distortion

Operation for the high‑level sweep was set just under the BL 70 percent point and remained below the CMS 50 percent point on this unit. Compared with 1 V, the 16 V sweep shows higher THD across essentially the entire 20–120 Hz band, with the largest relative increase from ≈30–120 Hz where the baseline is near ≈1 percent. H2 becomes the most dominant across the entire usable frequency range. This rise in even‑order distortion with drive is consistent with the moderate outward‑stroke BL asymmetry noted in LSI, while the good Le(x) behavior aligns with the limited odd‑order distortion.

What this means in practice

In this sample and under these test conditions, THD is about 10 percent near 20 Hz and roughly 4–6 percent through 30–120 Hz at the 16 V drive, with H2 distortion dominant across the usable band and no narrow, level‑invariant spikes visible. At 1 V, H3 distortion is slightly dominant below ≈35 Hz and H2 distortion dominates above ≈40 Hz, so the change with level mainly reflects an even‑order rise rather than new resonances. The practical clean one‑way limit for this sample is set by the BL 70 percent point; CMS 50 percent and the 17 percent Le variance were not reached within the evaluation window.

Motor & suspension linearity - LSI measurements

Method recap: Klippel LSI large-signal identification for this unit, cold and used for enclosure computations. Standard thresholds in this project are BL 70 percent, CMS 50 percent, and a 17 percent inductance variance criterion. Commentary below ties the large-signal behavior to the acoustic results.

Bl(x)

Bl(x) shows how much motor force a speaker produces as the voice coil moves, B is magnetic field strength and L is the wire length in that field. A high, wide, symmetrical BL curve means linear control and low distortion, a steep or uneven drop means earlier output limits and rising distortion, which is why BL(x) is often the most telling single Klippel LSI indicator of real performance.

Bl(x) window and shape

BL 70 percent is 21.56 mm one way. The BL curve presents a broad shelf with gradual falloff, indicating maintained motor force through the main stroke region observed here. This is consistent with the absence of BL‑driven narrow artifacts in the TRF results.

Bl(x) symmetry

The symmetry plot and engineer note indicate moderate outward‑stroke asymmetry at high stroke. That asymmetry can elevate even‑order H2 distortion with level, which is consistent with the trend seen in the high drive distortion measurement on this sample.

Cms(x)

Cms(x) is suspension compliance versus displacement, the inverse of stiffness. When the curve is broad and symmetrical, motion is linear and distortion stays low. Early roll off or offset indicates progressive stiffening or mis-centering, which adds mechanical distortion and caps clean excursion.

Cms(x) window and shape

CMS 50 percent was not reached within the ±22.39 mm evaluation window, which is favorable for mechanical headroom in this band. The CMS magnitude shows progressive stiffening near the extremes only.

Cms(x) symmetry

The engineer note calls out good CMS symmetry, which aligns with the lack of obvious CMS‑driven anomalies in the acoustic data. Though, there is some asymmetry at lower excursion.

Inductance - Le(x) and Le(i)

Le(x) and Le(i) measure how a subwoofer’s voice coil inductance changes with position and current. These curves show how stable the motor’s magnetic field is under real movement and drive conditions. When inductance varies heavily, it causes distortion, uneven response, and a loss of upper-band clarity, which is why Le(x) and Le(i) are critical for evaluating how clean and consistent a motor’s behavior really is.

Level and shape

Le at rest is 0.58 mH on this sample. The 17 percent Le variance criterion was not reached within ±22.39 mm one way, and Le(x) is described as good, so inductance does not impose an earlier limit than BL or CMS in this evaluation window. This aligns with the modest H3 distortion.

Current dependence

Le(i) shows some variation across the tested current span.

Qts(x)

Qts(x) is the driver’s total damping versus excursion, combining electrical and mechanical losses. Stable, symmetrical Qts(x) means consistent control, while large variation or asymmetry signals uneven damping that can shift response, raise distortion, and cause compression.

Qts stability

Qts near center is 0.54 cold and tends to rise with stroke, indicating some reduction in control at the extremes as excursion increases. The change is moderate near the operating center and grows toward larger displacements. The QTS curve is also not the most symmetrical, most likely due to the BL asymmetry.

LSI takeaway

The earliest limiting mechanism on this sample is BL at 21.56 mm one way. The BL shelf is broad with moderate outward‑stroke asymmetry, which is consistent with increased H2 distortion as drive rises. CMS 50 percent is beyond the ±22.39 mm window and symmetry is reported good, which is favorable for mechanical headroom. Inductance swing is low with good Le(x) and stable Le(i), aligning with the modest H3 distortion. Qts drifts upward with stroke and is asymmetrical, pointing to gradually reduced and uneven damping as excursion increases.

Enclosure alignment calculations

Manufacturer sealed enclosured recommendations and the resulting QTC: 1.0 ft³ sealed nets a Qtc of 1.175 on this sample.

Sealed volume required for 0.707 QTC on this sample: 7.1 ft³

Applicable for infinite baffle? While the manufacturer doesn’t explicitly recommend it, this seems like an excellent candidate for infinite baffle use due to its large enclosure requirements, a QTC that is between critically damped and flat response, and relatively high xmax.

T/S parameters