What tuning frequency should your subwoofer box have?

For most car subwoofers a tuning frequency of 32-36 Hz is the best all-round choice; go lower (28-32 Hz) for deep, accurate bass and higher (38-45 Hz) for loud, punchy SPL. The right value also depends on your driver's Fs and Qts, and every ported box needs a subsonic filter a few hertz below its tuning.

Fb vs Fs: two different numbers

Fs is the driver's free-air resonance, a property of the subwoofer you bought. Fb is the box's tuning frequency, set by the net volume and the port dimensions. They are related but not the same, and Fb does not have to equal Fs.

What matters is how the driver and box work together. A good starting point comes from the driver's Thiele/Small parameters. A classic QB3-style alignment (Keele's fit, the "Classic (Keele)" start in the calculator), which gives a fairly flat response, suggests:

Example: a 12" with Fs = 30 Hz, Qts = 0.40 and Vas = 45 L gives Vb ≈ 48.7 L (1.72 ft³) and Fb ≈ 28.7 Hz. That is a flat, deep starting point for a home system. In a car, many builders would move it up a few hertz (more on that below). See the Thiele/Small parameters guide if the terms are new.

Tuning frequency by music style

Lower tuning extends deeper but makes the bass around 40-60 Hz a little leaner. Higher tuning trades deep extension for more output and "hit" in the upper bass. Typical car ranges:

These are typical ranges, not laws. A large box with a very capable driver can tune lower and still play loud; a small box usually can't.

Car vs home tuning

A car cabin is small, so below its transition frequency (roughly 40-70 Hz depending on size) it pressurises and lifts the bass at about 12 dB per octave. That cabin gain fills in the bottom octave for free. A box tuned to 28 Hz that measures flat on paper can sound overwhelming and slow in a sedan, while the same driver tuned to 33-35 Hz often sounds tighter and louder where most music actually lives. Read more in our car cabin gain guide.

Rooms give little or no gain at the lowest frequencies. At home you generally need the box itself to reach deep, which is why home theater subwoofers are often tuned far lower than car subs.

What happens below tuning

Near Fb the port does most of the work and the cone moves least; this is the ported box's big advantage. Below Fb the situation reverses. The port and cone go out of phase, the box stops acting as an air spring on the cone, and the driver behaves almost as if it were in free air.

• Output drops at about 24 dB per octave below Fb.
• Excursion rises fast, often well above its value at Fb, even with modest power.
• The driver can bottom out, causing mechanical noise and eventually voice-coil or suspension damage.

Content below tuning is common: low synth notes, pipe organ, film effects, and road and wind rumble. You hear little of it, but the cone still tries to reproduce it.

Setting a subsonic filter

A subsonic filter is a high-pass filter set below the tuning frequency to remove the content that would only cause excursion. Most car amplifiers and DSPs have one.

1. Set the frequency 3-5 Hz below Fb. For a 33 Hz box, that is about 28-30 Hz.
2. Use the steepest slope available, ideally 24 dB/octave.
3. Check by ear with a test tone sweep at moderate volume: excursion below tuning should no longer grow visibly.

Setting it too high cuts into real bass; setting it too low leaves the driver unprotected. A sealed box doesn't strictly need one because the air spring controls the cone at every frequency, though it can still help at very high power.

How to change the tuning

Tuning depends on net volume, port area and port length. To lower Fb, lengthen the port, increase box volume or reduce port area (which raises air speed). To raise Fb, shorten the port. Because length scales roughly with 1/Fb², a small change in tuning changes the port length noticeably. The port length guide shows the exact formula with a worked example, and the port length calculator does it for you.

Some builders make the port adjustable during testing, for example with a removable slot extension, then glue in the final length once they are happy.

Don't forget port area

Lower tuning in the same box needs a longer port, and people often shrink the port to make it fit. That raises peak air speed. As a rule of thumb, keep port area around 12-16 in² per ft³ of box (about 2.7-3.6 cm² per litre) and peak velocity below about 17 m/s for quiet operation. Above that, chuffing becomes likely. Our port noise guide explains why.

Bottom line

Start from your driver's parameters, shift the tuning up a little for a car or down for a room, choose within the range that suits your music, and always protect the driver with a subsonic filter just below Fb. The BassDesign calculator lets you try several tunings and compare predicted response and cone excursion side by side before you build. It is free, has no sign-up and runs in your browser.