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.

Finally, when it comes to woofer breakup modes, choosing a driver with higher breakup frequencies ensures less distortion. As discussed in our breakup article, some tricks — such as using air-core inductors combined with active filtering — work especially well with high-sensitivity PA drivers.

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

Subwoofer and Masking Effect

Be careful when you correct or “boost” your subwoofer to not introduce masking effect. A subwoofer should continue at the same 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.

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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