Subwoofer: How to implement it well

Relation Between Emissive Surface and Enclosure Volume

Woofer size (diameter) plays a role, but for similar sized woofers, the enclosure volume significantly affects how deep the bass extends.

A larger enclosure typically allows a woofer to reproduce lower frequencies more efficiently.

The optimal enclosure size depends on the specific woofer’s parameters and desired response.

Here is four 15" compared in low end with the same amount of energy (2.84V):

woofer low end

We can see the effect of sensitivity on the speaker’s output, but it doesn’t impact low-end response below 50hz for the same amount of power amplification if the box volume is the same (all drivers have the same diameter).

We prefer to say, in this case when volume and driver size are the same, that some drivers have a “flatter” response in low end than others, rather than talking about bass extension — because below 50 Hz, the bass extension is actually the same for all of them.

When we use the recommended enclosure volume of 200L for this subwoofer, we see that we gain significant SPL in the very low end.

Woofer & Subwoofer Characteristics

Sensitivity indicates how loud a woofer gets with a given amount of voltage. A higher sensitivity woofer will produce more SPL at lower amplifier levels. However, sensitivity doesn’t directly translate to how much low-end a woofer can handle.

Low-frequency distortion is primarily linked to cone movement (extrusion) — the more the emitting surface moves, the more distortion we get. Sensitivity or EQ only change the amount of energy needed to reach that movement. This gives a clear advantage to larger woofers or subwoofers in terms of distortion at a given SPL.

Even if we equalize a high-sensitivity woofer to match the low-end response of a lower-sensitivity one in the same box volume, there is likely no significant difference in distortion, assuming the drivers share the same size and load (enclosure volume). Below 50 Hz, the SPL will be roughly similar for the same input voltage.

In this context, it’s not a problem to increase the low-end slightly with EQ on high-sensitivity woofers to achieve a flatter response, since the distortion remains primarily tied to cone excursion.

Of course, using a vented enclosure instead of a sealed one allows for higher SPL by recycling the rear energy produced by the driver. This is a clear advantage, even though vented enclosures require more volume.

Vented vs Sealed

When choosing a subwoofer design, two main options come up: sealed (acoustic suspension) and vented (bass-reflex) enclosures. While both have their place, some persistent myths and misunderstandings deserve to be cleared up — especially now that modern DSP and room measurements make deeper integration possible.

Bass Extension: Vented Wins

A vented subwoofer enclosure will always provide better low-frequency extension compared to a sealed one, assuming the same driver and enclosure volume. Thanks to the tuned port, vented boxes reinforce bass around the tuning frequency, offering more output at lower frequencies.

“Tight Bass” comes from proper modal control — not from the type of enclosure.

A common belief is that sealed subs produce “tighter” or “faster” bass. In reality, this tightness comes from the room’s modal response, not the enclosure type, of course the vented enclosure goes lower so require more attention to room correction with DSP. Below ~200 Hz, your room dominates the response with resonances and standing waves. If these aren’t managed — through acoustic treatment or DSP correction — the bass will sound boomy or smeared, no matter how “fast” your sealed sub is supposed to be.

Today, with affordable measurement tools and DSP, it’s possible to correct room modes precisely. This means that a well-designed vented subwoofer, placed and calibrated correctly, can sound just as tight and controlled as it should be — while going deeper and with greater efficiency than a sealed one.

The Port: Design Matters More Than You Think

Not all ports are equal. Many designs use slot (rectangular) ports, which are compact but far from optimal. Slots tend to generate higher air friction due to greater surface area contact, leading to more turbulence, chuffing, distortion and power compression.

The best-performing ports use a circular cross-section, which minimizes wall contact and supports smoother airflow. Among circular ports, the geometry of the flares is critical. For example, the hourglass port design featured a flat velocity port , delivers a flatter velocity profile minimizing noise, distortion and compression even at high SPLs. It’s one of the best examples of how careful port shaping can outperform conventional designs.

Final Thoughts

In the age of DSP and accurate measurement, the sealed vs vented debate shouldn’t be about subjective impressions anymore. A well-engineered vented subwoofer, with a proper port and correct room integration, can offer deeper, cleaner, and equally “tight” bass.

The key lies in system design, not dogma:

Good bass isn’t about sealed or vented. It’s about control, design, and integration.

Another Myth: Heavy Cones Are “Slow”

It is often believed that lighter woofer cones produce “faster” bass, while heavier cones sound slower. In reality, a loudspeaker has no inherent “speed” — it faithfully reproduces the input signal. If it were truly “faster,” it would alter the frequency or timing of the signal, which constitutes distortion.

A heavier cone primarily lowers the system’s resonant frequency, enabling deeper bass extension, but this comes at the expense of reduced sensitivity because more energy is required to move the increased mass. Conversely, lighter cones typically have higher sensitivity and lower moving mass, which allows them to generally reproduce higher frequencies with less mechanical stress and distortion, but they cannot achieve the same deep bass extension or control at high excursion levels as heavier cones.

Ultimately, perceived “tightness” of bass is much more influenced by proper room integration and modal control than by cone mass alone—similar to the vented versus sealed enclosure debate see upper.

Some advanced woofers cone, with an optimized balance of mass, damping, and rigidity, can be relatively heavy yet still achieve excellent high-frequency extension, better than some dedicated midrange ones, with low distortion. This is sometimes accomplished through sandwich cone constructions (like those from AudioTechnology) or/and by pairing the driver with highly advanced motors (such as PURIFI designs). Therefore, generalizations about cone weight and performance are very difficult to make.

Ultimately, precise measurements with flat EQ, and bass extension simulation remain the definitive criteria to evaluate performance.

Subwoofer Setup and Integration

Even if we have a FIR DSP, so linear phase filtering, we don’t use it here as it will be very costly in TAPS as the frequency is lower and useless about phase at this frequency. We will search to cut between 50 and 80 hz optimally.

Measure the delay in cm and convert it to ms if the subwoofer is far, be sure to not have a reversed phase as we said for the woofer, you can check it with the same techniques.

You can correct the woofer response at the listening area by doing several measures by moving the mic in the listening area, not at just one position.

A regular EQ in minimal phase (“IIR”) placed on a modal effect with a quality coefficient (Q) equal to the modal effect will not only improve the frequency response but also the Time-domain behavior, reducing trailing bass and improving global bass experience.

Clear cancellations should not be corrected, as the nulls move in frequency depending on microphone placement:

eq
Thanks to these EQ adjustments, the temporal behavior is clean and allows for very good bass reproduction:
eq
Here we can see that the temporal behavior is clean, except at the frequency where the null appears due to the room and subwoofer placement — around 55 Hz. Unfortunately, there is nothing we can do here, and acoustic treatment will not be effective at fixing such a low frequency.

You can add an electric (in DSP) Butt 12 on your natural woofer natural fall to have a real (acoustic) Butt 24 in minimum phase filtering, linear phase filtering is not necessary at these frequencies.
Then add Butt 24 on the subwoofer as we have do in our speaker implementation article with woofer:

crossover

Note: Acoustic treatment will not solve or significantly reduce modal acoustic issues. The affected wavelengths are very long (several meters), and standard treatments are not deep enough to be effective in this range. Modal behavior is primarily determined by the room’s dimensions, geometry, and the placement of subwoofers or other low-frequency sources — the listener or microphone position, only affects how the modal field is perceived or measured at a given point or area.

Subwoofer and Masking Effect

Be careful when you correct or “boost” your subwoofer to not introduce a masking effect. A subwoofer should continue at the same existing SPL as the woofer.
If we want more bass — and more importantly, more kick — we can increase the SPL starting from around 200 Hz, gradually, to avoid inducing masking.

Increasing the subwoofer gain alone can lead to a vicious circle: the more gain we add to a subwoofer cut at 60 or 80 Hz, the more it will mask the kick in the upper range. That’s why, when we want more impact/kick, we don’t increase the subwoofer gain alone — we increase both the subwoofer and woofer SPL with EQ starting from 200 Hz accordingly.

If you boost the subwoofer in your system, you might notice a loss of impact or “kick.” This happens because excessive sub-bass will mask the upper bass and low midrange, where the kick drum energy typically resides.

To mitigate this masking effect:

Is Group Delay (GD) Audible in a Bass Reflex System?

It depends on the cycle length. Generally, it’s recommended that the GD should not exceed 1 or 1.5 cycles of the frequency, as this is related to its wavelength.

The formula for calculating the maximum acceptable GD is:

Δt max = n / f

Where:

Example:

You can use VituixCAD Box Simulation to visualize the GD according to the box volume and port, and consider this information in your design and choices.

In practical cases, we don’t exceed the recommended GD limits if we follow standardized vent alignment so it will not be audible.

Positioning Subwoofer

Subwoofer position can be hard, we want room gain but without room modes, but the first come rarely without the second.

We should place subwoofer at 25/20% of each side wall on the speaker wall as here, if we have several subs we regroup it by pair:

speaker position

From : getting the bass right - Harman

The alternative of this is to put a line on sub horizontally on the ground, if possible integrated to the wall, completely from one side wall to the other, each subwoofer unit as close as possible from each other, it can require a lot of 18", it will allow to create a plane wave radiation subwoofer.
It’s more technical and very expensive. It can be coupled (same chanel) with Divatech style wall of sub.

We have to choose, depending on our room and SPL max goal, between one or two 12",18",21":

If we prioritize max SPL we go for pro audio subwoofer, if we need flat responding subwoofer so non pro speaker as car audio.

If we place the subwoofer on the center of the room we will have less mods but also less room gain so a flat responding subwoofer will be better.

Generally, except very big room we search for flat responding subwoofer.

Here is a list of soms, for vented ones we will search a Qts between 0.33 and 0.36 and a low Fs:

Vented:

Sealed:

SBA and SBA

This particular positioning of sub is mentioned in our wavefront-propagation article

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