Archive for June 16th, 2025

Detailed Cabinet Volume Calculations and Theory

Posted By Andy Kos

Ready to dive in a little deeper?

The full formula for calculating ‘optimum box volume’ is:

Vas, Qts and fs are all known numbers, and are available from the TS Parameters published for the speaker driver you want to use. However to use the above formula, you need to determine fb which is the tuning frequency of the ports in the cabinet. Its generally considered that you get the best results by selecting a tuning frequency just slightly higher than fs – it gives the best combination of performance improvements. The aim of the ported cabinet is to extend the bass response, and enhance the lower bass output, getting the right cabinet volume and correct fb also helps maintain damping and give a smoother bass roll off, whilst at the same time controlling excursion at bass frequencies to prevent over-excursion. If you’ve read about fs you’ll already know this is the frequency that the woofer naturally moves easiest, if it moves too much, it will get damaged. The ports reduce the cone excursion around fb so its sensible to keep this near fs to keep the overall cone excursion under control.

At this stage we are just trying to work out an approximate target volume for the cabinet based on the desired tuning frequency. The final tuning frequency of the box, fb is determined by the port dimensions and length with respect to the box volume. We are not setting fb with the box volume, we’re just estimating a target box volume that’s suitable for the driver to get good results.

The above formula would prove useful if you are aiming to get a small compact cabinet with optimum damping, but sacrificing bass extension. If you have enough headroom to compensate with DSP, this would allow for some very compact designs, particularly if punchy bass is the target.

Assuming we are looking to achieve good bass extension, common in large format PA applications, it would be simpler to just use fs = fb to get a typical cabinet volume, this simplifies the equation to:

Our recommendation is that this would normally be a good target volume for good bass response, and you could adjust this according to your design objective. You don’t want to make it too large, as this will compromise damping, so perhaps up to 30% larger for deep bass. If you want a compact cabinet, 10%-15% smaller if you are willing to have some compromise on bass extension to minimise cabinet volume.

Here are some example calculations for a popular 18″ woofer:

  • Vas = 212 litres
  • Qts = 0.35
  • fs = 34 Hz
Vb = 20 × 212 × (0.35)3.3 ≈ 132.7 litres

This gives a practical starting point for designing a PA bass reflex enclosure. It aligns closely with real-world cabinet sizes — allowing for bracing, port volume, and tuning flexibility — while remaining efficient and well-controlled for live sound applications. Remember this is the REMAINING volume after subtracting handles, ports, braces, and the volume of the driver itself. Your unloaded cabinet would probably need to be around 150 litres when designed.

To make the most compact cabinet, you could reduce this to 125 litres, and accept reduced low frequency extension, but potentially the ability to tune for tight punchy bass above fs

If you want more deep bass, and size isn’t an issue, a 200 litre cabinet would give more options for lower tuning, deeper bass, but most likely worse damping (sloppy or less responsive bass) and lower efficiency.

So you can see, cabinet volume can vary a lot, the driver will still work, but the results will also vary.

Filed in Speaker Design | Tagged: , , , , , ,

Cabinet Volumes for Ported Speakers

Posted By Andy Kos

When designing a speaker enclosure, especially a ported (bass reflex) box, it’s a common misconception that there’s one perfect cabinet volume for every driver. In reality, there’s a range of suitable volumes, each delivering a slightly different result depending on the design goal. Cabinet volume has a direct effect on bass response — not just in terms of depth, but in how the bass feels: tight and punchy vs deep and smooth.

The internal volume of the box interacts with the driver’s compliance (represented by Vas) and contributes to how freely the cone can move. A very large enclosure provides less air resistance (acoustic spring), allowing the cone to move more easily, which can improve low-end extension but may reduce control. Conversely, a smaller enclosure increases acoustic stiffness, restricting cone movement — which can tighten response but raise the system’s resonance.

It’s worth noting that modest changes in box volume aren’t inherently problematic. There’s no need to obsess over a perfect number — cabinet volume isn’t a razor-sharp target, but more of a zone you want to stay within. Problems tend to occur only when the volume is way off — such as using a box far too small or unnecessarily large for the driver. In practice, modern speaker designs aim for the smallest box possible that still delivers strong bass extension, reflecting a shift toward compact but efficient systems.

There’s always a trade-off between bass extension (how low it goes) and bass response (how tight or efficient it sounds). With that in mind, here’s a quick and practical method to estimate your starting box volume for a ported speaker:

  • Vb: Recommended internal cabinet volume (in litres)
  • Vas: Equivalent compliance volume of the driver (litres)
  • α: Alignment factor, typically between 1.5 and 3

What Does Alpha Mean?

The value of α (alpha) determines how large or small your cabinet will be, and how the bass response is shaped the value of α in the formula above is a scaling factor that reflects how the compliance of the air inside the box compares to the compliance of the driver’s suspension (Vas). The speaker cone and the air inside the cabinet form a mass-spring system, and the box volume determines how stiff the air spring is.

  • α ≈ 1.5 – A softer air spring, leading to a larger box. The cone moves more freely, resulting in better low-frequency extension. It’s close to a maximally flat alignment (like QB3 or SBB4), but physically bigger than most other options.
  • α ≈ 3 – A stiffer air spring, giving a smaller box. This restricts cone movement more, often creating a bass hump and a stronger punch in the upper bass, at the cost of earlier low-end roll-off. It’s closer to alignments like SC4 or EBS, often used for compact or punchy systems. It’s closer to alignments like SC4 or EBS, often used for compact or punchy systems.

Choosing your α value is about balancing size and performance. Lower values favour flat, extended bass; higher values prioritise compact design and punchy response. We recommend sticking between 1.5 and 3 for purposes of calculating ball park figures. Choosing α is essentially choosing your compromise: deep and flat but large vs compact and punchy, with less deep bass.

Example

Say your driver has:

  • Vas = 100 L
  • Qts = 0.35

This driver suits a range of box volumes depending on your design goals:

  • Flatter, deeper bass: Vb = 100 / 1.5 = 66.7 L
  • Compact, punchy box: Vb = 100 / 3 = 33.3 L

So your practical range is between 33 to 67 litres — smaller if you prioritise space, larger if you want extended bass depth.

Please note, the above is a SIMPLIFIED calculator to estimate suitable cabinet sizes without getting too involved in the science. Here is the full formula:

This gives a more precise prediction based on Thiele-Small parameters and your chosen tuning frequency. Let’s break down what each part means:

  • Vb: Recommended internal box volume (litres)
  • Vas: Equivalent compliance volume of the driver (litres)
  • Qts: Total Q of the driver (combined electrical and mechanical losses)
  • fb: Desired box tuning frequency (Hz)
  • fs: Driver’s free-air resonance frequency (Hz)

What It Tells You

  • Higher Qts values usually call for a larger enclosure
  • Lower tuning frequencies (fb) increase the required volume

This formula is useful as a starting point for simulation tools. It’s more accurate than the simplified Vas/α method, but assumes a traditional alignment without DSP-based adjustments to compensate for bass response. It also allows you to specify your target tuning frequency. You should also note that Qts is quite significant in determining suitability for bass reflex enclosures. The useable range for Qts is typically 0.28 – 0.45 with a ‘sweet spot’ at 0.37-0.38 which tends to gives many of the best ‘all round general purpose’ woofers for ported enclosures. The above calculations will give you Vb results for Qts values outside of this range, but these will most likely result in poor performance, or give unrealistic volumes that are too large or too small.

Filed in Speaker Design | Tagged: , , , , ,