Limiter Settings for Subs, Worked Through

Limiter settings for subs come from three numbers: the driver's continuous rating, its impedance and your amplifier's gain. A high-pass filter handles excursion; an RMS limiter handles coil heat; a peak limiter catches transients. Set them once from the arithmetic below, lock the DSP, and the drivers are protected however hard the night runs.

Two failure modes

• Over-excursion: content below the box tuning or horn cutoff unloads the cone. It travels past Xmax, the coil leaves the gap and the suspension or former is damaged. See Xmax and excursion.
• Heat: sustained average power raises coil temperature and resistance. Output drops (power compression), then the adhesive or wire fails. See voice coils and heat.

Step 1: high-pass filter

A reflex box stops controlling the cone below its tuning. Set the high-pass a little below Fb with a steep slope (24 dB/oct): for a box tuned to 40 Hz, start around 30 to 35 Hz. For a horn or scoop, use the design's stated cutoff.

Step 2: limiter voltage

Limiters act on voltage, so convert the rating: V = √(P × Z). For the peak limiter, use the peak rating, which for a 2:1 peak-to-continuous rating sits 10·log₁₀(2) = 3 dB above the RMS threshold.

dBu = 20·log₁₀(V / 0.775). The 4 Ω row assumes the same power rating on the 4 Ω version; confirm it on the spec sheet. Two identical drivers in parallel each see the full channel voltage, so the channel limit stays the same. Two in series split it, so the channel limit doubles in voltage (+6 dB).

Step 3: subtract the amp's gain

Most DSPs set thresholds at their own output, so take off the amplifier's voltage gain. At 32 dB gain (×39.8, a common setting):

• 8 Ω, 120 V: 43.8 − 32 = +11.8 dBu RMS threshold (3.0 V at the DSP output); peak +14.8 dBu.
• 4 Ω, 85 V: 40.8 − 32 = +8.8 dBu; peak +11.8 dBu.

MC2's limiter application note recommends setting the RMS threshold 1 to 2 dB below the calculated figure as a margin, and the peak threshold at least 2 dB, typically 3 dB, above the RMS threshold. If the amp's gain or sensitivity switch is set differently, redo the sum from the manual's figure, and confirm with a meter and a sine tone at low level.

Step 4: attack and release

Attack and release must be slow relative to the waveform, or the limiter distorts the bass it is controlling. MC2 derives its automatic RMS-limiter times from the high-pass frequency, with release at 16 × attack:

Use these as a reference if your DSP has no automatic mode. If it offers only one limiter, set it to the RMS threshold and rely on the high-pass filter for excursion. Processor options are compared in budget DSP for sound systems.

Step 5: lock it

Password-lock the DSP and fix and mark the amp gain controls. A limiter calibrated to 32 dB of amp gain gives no protection once someone turns the amp up.

Headroom you are paying for

Both Wavemark 18s take 1800 W continuous and 3600 W peak, so a 120 V RMS limit leaves the full rating usable rather than holding the driver back. The WM1850/8 has a 125 mm coil and a vented motor for £275 (15.3p per rated watt); the WM18X451 has a 115 mm copper coil and double spider for horn and scoop loading at £345 (19.2p per rated watt).

Sources: MC2 Audio, Delta DSP amps limiter settings application note, checked October 2026; Wavemark WM1850/8 spec sheet and WM18X451 product data. Voltage and dBu figures calculated and rounded. Follow your processor and amplifier manuals, and never exceed the driver's published ratings.