How much cabin gain does a car add to a subwoofer?

A typical sedan adds approximately 10-12 dB of gain at 30 Hz relative to the anechoic response. The gain decreases as frequency rises, reaching approximately 0 dB around 50-60 Hz where standing wave modes take over. Smaller cabins provide more gain; larger vehicles like SUVs provide less.

Should I account for cabin gain when designing a subwoofer box for a car?

Yes. Cabin gain adds 10+ dB below 40 Hz, so a box designed for flat anechoic response will sound bass-heavy in a car. Sealed enclosures benefit most - their 12 dB/octave rolloff combines with rising cabin gain for a naturally flat in-cabin response. Vented boxes are often tuned higher (32-38 Hz) to work with cabin gain.

Why does bass sound different in the front and back seats?

Above about 50-60 Hz, standing waves develop in the cabin with position-dependent pressure nodes and antinodes. Different seat positions sit at different points relative to these modes, producing different perceived bass levels. The effect is most pronounced in the 60-150 Hz range.

Education12 March 20265 min read

Cabin Gain: The Free Bass You're Not Accounting For

If you are installing a subwoofer in a vehicle, the single biggest factor in your bass response is not the box. It is the car.

What cabin gain is

Cabin gain (also called cabin transfer function or vehicle transfer function) is the acoustic amplification that occurs when a subwoofer operates inside a small, sealed space like a car cabin.

At low frequencies where the wavelength is much larger than the cabin dimensions, the cabin acts as a pressure vessel. The subwoofer pressurises the entire cabin volume, and the resulting sound pressure level is higher than what the subwoofer would produce in open air.

This gain is substantial. A typical sedan cabin provides 10–12 dB of gain at 30 Hz relative to the anechoic (free-air) response. Some vehicles provide even more.

Why it happens

Sound waves at 30 Hz have a wavelength of about 11.4 metres. A car cabin is roughly 2.5 m × 1.5 m × 1.2 m. At frequencies where the wavelength is much larger than the cabin, the air pressure is essentially uniform throughout the space - there is no time for the wave to develop spatial variation.

In this regime, the cabin volume acts as an acoustic compliance (a spring), and the subwoofer is a pressure source driving into that compliance. The smaller the cabin volume, the stiffer the spring, and the more pressure per unit of driver displacement.

As frequency increases, the wavelength shortens and eventually approaches the cabin dimensions. At this point, standing waves develop - the pressure is no longer uniform, and nodes and antinodes create position-dependent variations. The transition typically occurs between 50–80 Hz depending on the vehicle size.

The 3-axis standing wave model

Above the transition frequency, cabin gain becomes position-dependent. The three internal dimensions of the cabin each support their own standing wave modes:

Length (front to back): First mode around 60–80 Hz for a typical sedan
Width (left to right): First mode around 100–120 Hz
Height (floor to roof): First mode around 130–160 Hz

Where the subwoofer is mounted relative to these modes determines how strongly each mode is excited. A subwoofer in the trunk (rear of the cabin) sits near the pressure antinode of the length mode, exciting it strongly. This is why trunk-mounted subs have a pronounced bump in the 50–70 Hz range.

The listener's head position also matters. Sitting near a pressure antinode means you hear more output at that frequency. The driver's seat and front passenger seat are at different positions relative to these modes, which is why bass can sound different in each seat.

How this affects your box design

Cabin gain changes the game for enclosure design in several ways:

You need less low-end extension. An anechoic response that rolls off at 35 Hz will be flat to 25 Hz or below once cabin gain is added. Designing for deep anechoic extension means over-emphasised deep bass in the car.

The effective system peak shifts down. Cabin gain adds more boost at lower frequencies, which pulls the perceived peak lower than the anechoic peak. A system that peaks at 45 Hz anechoically might sound like it peaks at 35–38 Hz in the car.

Sealed enclosures benefit enormously. Sealed boxes roll off at 12 dB/octave, which combines with the cabin's rising gain to produce a nearly flat in-cabin response. This is one reason sealed enclosures sound so good in cars despite their lower anechoic efficiency.

Vented enclosures need careful tuning. A vented box tuned to 28 Hz will have its port-region output amplified by 10+ dB of cabin gain, potentially creating boomy, one-note bass. Many car audio installers tune vented boxes higher (32–38 Hz) specifically to work with cabin gain rather than against it.

How much gain are we talking about?

The boost is strongest at the lowest frequencies — in a typical mid-size sedan, you can expect significant gain (10 dB or more) in the deep bass region below 30 Hz. The gain tapers off as frequency rises, reaching roughly zero around 50–60 Hz where standing wave behaviour takes over and makes things position-dependent.

These values vary significantly between vehicles. An SUV with a large cabin volume has less gain than a compact car. A convertible with a soft top has less gain than a hardtop because the flexible roof absorbs energy.

How RokketBox models cabin gain

RokketBox includes a cabin gain simulation with adjustable cabin dimensions. The model uses a three-axis standing wave approach: uniform pressure loading below the first modal frequency, transitioning to standing wave behaviour above it.

The dashed SPL line in the simulator shows the combined response with cabin gain applied. Use this for vehicle installs rather than the anechoic (solid) line - it is a much better predictor of what you will actually hear.

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