Installation Guide

You've got your custom box, your subs, and your amp. Now it's time to install everything and tune the system so it actually performs. This guide walks you through it in the exact order things should happen.

Step 1 — Seal the Box to the Vehicle (Walls, Forward-Firing, and Up-Firing Builds)

Not every box needs sealing to the vehicle, but the ones that do can't skip this step. If pressure radiating from the front of the enclosure is allowed to leak behind the front plane of the box — back toward the rear of the vehicle — it will induce cabin cancellation and your bass will sound weak. The goal is to keep all pressure forward of the front plane of the box.

You need to seal your box to the vehicle if it's:

• A wall (B-pillar or C-pillar) — seal every edge to the vehicle interior: roof, floor, side panels, and around any pillars the wall touches.
• A forward-firing trunk box (sub firing forward and/or port firing forward or to the side) — seal the top and sides of the box to the trunk opening so the trunk lid area is isolated from the cabin.
• A no-wall SUV or hatchback box with subs and port firing UP — seal the top and front of the box to the surrounding cargo area / hatch opening so the up-firing output can't wrap around and cancel itself.

How to seal:

• Secure a trim panel or wall face against the front (or top, for up-firing builds) of the box.
• Use foam, weatherstripping, or sound deadener to close every gap around the perimeter.
• Even small gaps hurt performance. Be thorough — this is where a lot of "why isn't it hitting?" problems come from.

Step 2 — Reinforce the Cabin (If Needed)

When a subwoofer moves a lot of air inside a car, panels flex. Flexing panels waste energy that should be making sound. Reinforcing the cabin means adding stiffness to weak spots so more of that energy stays as sound.

Where cabins usually flex:

• Roof (especially on SUVs and vans with big flat roofs)
• Firewall (the panel between engine bay and cabin)
• Top of the windshield (the roof-to-windshield brace)
• Doors and floor pan

Common fixes:

• Wood sheets attached between the roof crossbars
• Heavy sound deadener over large flat areas
• A vertical support pole between the roof and the floor (sometimes called a "stripper pole" in the audio world) — stops the roof from flexing on very loud systems

Step 3 — Wire Everything Up

Wire your subs to the amp based on your amp's stable impedance (ohm) rating and your subs' voice-coil configuration. If you're not sure what wiring diagram to use, message us your amp model and sub model and we'll send you the exact diagram.

Double-check:

• Positive (+) to positive, negative (–) to negative on every sub
• All connections are tight (crimped or soldered, not just twisted)
• No exposed wire touching bare metal

Step 4 — Pop Test (Verify Every Sub Moves Together)

The pop test confirms all your subs are wired correctly and moving the same direction at the same time. If one sub moves backward while the others move forward, they cancel each other out.

What to do:

1. Turn the amp off (or leave the vehicle powered down).
2. Disconnect the speaker leads from the amp so the subs are isolated.
3. Touch the 9V battery's + terminal to the + speaker lead and the – terminal to the – speaker lead, briefly.
4. Watch the sub cones. They should all pop outward (or all pop inward) at the same time.
5. If one moves opposite the others, its wiring is reversed. Fix it before moving on.

Step 5 — Head Unit Settings

Your head unit (radio) is what sends the signal to the amp. Set it up to send a clean, flat signal:

• Sub level: 0 (neutral) to start
• Bass boost: Off
• EQ: Flat — do not boost anything below 125 Hz on the head unit's global EQ (it distorts the signal at high volume)
• Loudness: Off

Step 6 — Crossover Settings (LPF, Subsonic, and Phase)

A crossover is a filter that tells your amp which frequencies to play and which to ignore. There are two crossovers we care about for a subwoofer setup:

• LPF (Low-Pass Filter): blocks frequencies above the number you set — makes the sub only play bass, not vocals or midrange.
• Subsonic Filter: blocks the very lowest frequencies below what your box was designed to play — protects the subs from over-excursion (moving farther than they safely can).

Best practice — set crossovers globally, not on the amp

The recommended approach is to leave the amp's filters wide open (LPF turned all the way up, subsonic switched off) and set your LPF and subsonic filter globally at the head unit or DSP instead. Two reasons:

• No amp-to-amp crossover matching. If you have multiple amps (or ever add one), you don't have to configure identical crossover points on each amp — one global setting covers everything downstream.
• No unintentional filter stacking. If the head unit or DSP is already applying a crossover and your amp is also applying one at a similar frequency, the two filters stack and roll off your bass more aggressively than you meant to. Setting crossovers in one place eliminates that risk entirely.

If your head unit or DSP does not have crossover functionality, you'll have to set the LPF and subsonic on the amp itself. In that case follow the same starting-point numbers below.

LPF starting point

• All enclosure styles: LPF around 80 Hz, slope 24 dB per octave.
• This works as a starting point for sealed, ported, 4th-order bandpass, 6th-order bandpass, and T-Line builds alike. Adjust by ear from there if you want to blend more tightly with your front stage — dropping to 70 Hz or raising to 90 Hz is fine.

Subsonic filter starting point

• As low as possible, or off entirely. The subsonic filter is not something you set once and forget — it has to be tested and dialed in for the specific box + sub + power combination you built.
• Start with subsonic off (or as low as your amp / DSP will let it go). Play the system at your normal high-volume listening level and listen carefully. If the subs sound uncontrolled, flappy, or like they're bottoming out on deep notes, inch the subsonic filter up a few hertz at a time until the subs sound clean and happy at maximum output.
• The goal is to raise it only as much as you have to, and no more. Every hertz you add takes real bass output away.

Phase / Polarity

Most sub amps have a phase switch (0° or 180°). The only thing that matters here is that every sub amp is set the same — all 0°, or all 180°. Don't mix. I prefer all 0° as the default.

If you want to actually experiment with flipping polarity between the subs and the front stage, do it in the DSP, not on the amp. That way it's a clean, reversible signal-level change and it doesn't accidentally put your sub amps out of phase with each other.

Step 7 — AC Voltage Sweep (Verify the Signal Is Clean)

This step confirms that no hidden filter or EQ setting is quietly cutting your bass. It matters because if the head unit or a processor is silently dropping the volume at, say, 30 Hz, your sub can't play deep no matter how good your box is.

What to do:

1. Temporarily bypass every filter in the signal path — set subsonic to OFF and LPF to MAX (both on the amp and on the head unit / DSP if you set them globally). This is for the test only; you'll put them back after.
2. Disconnect the speaker leads from the amp.
3. Set your multimeter to AC voltage.
4. Play test tones at consistent volume: 20 Hz, 30 Hz, 40 Hz, 50 Hz, 60 Hz. Measure the amp's speaker-output voltage at each frequency.
5. All the readings should be nearly the same. If one frequency is noticeably lower than the others, something in your signal path (head unit EQ, DSP, factory processor) is cutting it. Find it and fix it before moving on.
6. When done, put your LPF and subsonic settings back where they belong (from Step 6).

Step 8 — Set the Gain (Match the Amp to the Head Unit)

Gain is not a volume knob. Gain matches the amp's input sensitivity to the head unit's output voltage — setting it wrong causes distortion (which kills subs) or leaves output on the table.

Use an SMD DD-1 or an oscilloscope

This is the only method worth using. Don't try to set gains by ear or by the amp's LED clip indicator — you'll set them wrong every time. Get a DD-1.

1. Play a 0 dB, 40 Hz test tone through the head unit.
2. Turn the head unit up until the DD-1 shows the head unit is just about to clip (distort). Note that volume level.
3. Switch to a –5 dB, 40 Hz test tone.
4. Turn the head unit back to the max clean volume you noted.
5. Slowly turn the amp gain up until the DD-1 shows the amp is just about to clip. Stop there.
6. If you have a bass knob, keep it on max the entire time.
7. If you have multiple amps, match their AC output voltages to the lowest amp's voltage — within 0.1 volt of each other.

Handy reference: DD-1 setup and downloads at damoreengineering.com/products/dd-1

Need Help?

If something isn't sounding right after following this guide, take a short video of the issue and message EZ DB's Designs. Most tuning problems get caught in 2–3 messages back and forth.