Speaker Enclosure Software Guidelines

In a speaker system, the speaker enclosure is designed for more than just a mechanical fit and installation of loudspeaker drivers. For optimized performance, the enclosure is also designed to work with the loudspeaker driver’s electroacoustic characteristics in a mutually dependent manner. When done properly, low-frequency reproduction is optimized for acoustic power and bandwidth. The science and math to design an enclosure for specific drivers is somewhat complex but thanks to the work of Thiele, Small, Beranek, and many others it is fundamentally straightforward. With the help of simulation software, acoustic performance, electroacoustic, and electromechanical attributes under various input conditions can be modeled.

Before beginning to design a loudspeaker enclosure, the intended loudspeaker driver characteristics must be available. Before the works of Richard Small and Neville Thiele leading to the Theile/Small (T/S) parameters, electromechanical parameters were used to calculate the interaction between the driver and enclosure. Over the years the list of electromechanical characteristics has been expanded, for example by David Clark, Wolfgang Klippel, and others.

Read more: OEM Speaker Design: Enclosure Assembly and Final Testing [VIDEO]

Speaker Enclosure Design Software

Software for the design of speaker systems has been evolving since computers were first available. Some of the first iterations were basically iterative steps to be programmed into a mainframe and then, hand-held calculators. Now, there are numerous software packages available.

Not intended to be a comprehensive list, the software listed in Table 1 includes freeware and commercial programs. Some are standalone programs, others are based on spreadsheets. Most run on Windows, some on Mac, a few on Linux and some are cross-platform compatible.

Table 1

While the T/S parameters will guide you to an optimal enclosure, trade-offs are involved. The designer will need to use experience, preference, and practicality considering the application.  Let us look at a design example using BassBox Pro and a ToneSpeak TSB-15-250 bass guitar woofer. This is not intended to be a how-to guide for the software, but a platform to expand on some key principles of enclosure design. 

Once BassBox Pro is open and we manually enter the T/S parameters for the woofer, we can begin the design. 

The first step in the design presents us with a decision to make on enclosure type.  See the image below for all the enclosure types BassBox Pro will allow you to model. Any speaker can be modeled in any enclosure type, but experience, speaker knowledge, and the application type will lead us to the best option. For this bass guitar application with this woofer, a vented enclosure (aka bass reflex) is most appropriate.

The first step in the design presents us with a decision to make on enclosure type.  See the image below for all the enclosure types BassBox Pro will allow you to model. Any speaker can be modeled in any enclosure type, but experience, speaker knowledge, and application type will lead us to the best option. For this bass guitar application with this woofer, a vented enclosure (aka bass reflex) is most appropriate.

Next, the enclosure volume is determined. BassBox Pro will allow manual entry or options for “Optimum” or “X-Bass” (extended bass), where the software calculates the enclosure volume based solely on T/S parameters. It is simply doing the math for us and creating a resultant model. The trade-off with volume is low-frequency extension versus mechanical power handling. A larger enclosure increases low-frequency extension/output but at the expense of the speaker power handling. A small enclosure is less mechanically demanding on the speaker, so higher power handling, but at the expense of low-frequency extension/output.

The application must also be considered. Not many musicians are going to lug around a refrigerator-sized speaker enclosure. After modeling and considering the crucial variables of the design (more on all of that in a moment), 4.19 cu. ft. of internal volume was decided. It is a practical volume for a 1X15” bass guitar enclosure. It handles decent power. The vents are practical sizes and not prone to unwanted noise. The frequency response and low-frequency extension of the speaker in this enclosure are ideal for bass guitar. 

Let’s take a closer look at each variable. Notice in the above image that Fb (resonance of the enclosure or tuning frequency) is 51.5 Hz and F3 (lowest audible note) is 47.99 Hz. Lower frequencies could be achieved by increasing the enclosure volume but at the expense of power handling. These values were good compromises to maintain decent power handling. Another designer might choose different compromises with enclosure volume versus low-frequency extension versus power handling.

In the image below, the vent (or port) amount and size are determined. The keys here are what vent sizes will fit the enclosure, what is practical to fabricate, and whether the vent velocity ia low enough so that no unwanted vent noise occurs. A good rule of thumb is to keep vent velocity below 24 m/s at rated speaker power. In this example (second image below), that has been satisfied. A higher vent velocity than 24 m/s might chuff and add distortion to the sound when the speaker is pushed. In regards to vent amount (4) and size (3” diameter x 3.313” long), PVC pipe was used, so a 3” diameter was easy to find at a local hardware store, and 3.313” of vent length easily fit inside the box. Different vent amounts, materials, or shapes might also have been chosen by a different designer, like one single rectangular vent with a shelf to achieve depth inside the cabinet for tuning. Of course, the vent velocity would need to be checked with the different vent variables and modified accordingly.

Now let’s look at how much the speaker is moving to determine what this model sacrifices in power handling. The TSB-15-250 is rated at 250 watts continuous. That is mostly a thermal limit of the speaker, tested with an industry standard method in free-air. The enclosure is more of a mechanical influence on the speaker. The mechanical limit of the speaker will never exceed the thermal limit of the speaker, as the speaker would fail from heat first. Notice in the image below, that the travel of the speaker at 250 watts is highest around 60-90Hz and the line is faded. This is an example of the speaker exceeding its mechanical limits (Xmax or maximum linear excursion). In the second image below, the power was reduced to 150 watts to achieve a solid line. So in this enclosure design, the TSB-15-250 is mechanically limited to 150 watts continuous. Why limit the power handling so much, you ask? Well, we are achieving good low-frequency extensions for this particular woofer and application. Plus, BassBox Pro allows us to look at another variable, the SPL or output at 150 watts. The third image below says we are reaching maximum levels of over 121dB with this design. That’s respectable output! A different designer, however, might decide to maintain more power handling. This is achieved by decreasing cabinet volume and/or decreasing the tuning frequency. Again, this will be at the expense of low-frequency extension.

Let’s look at the frequency response shape in the image below. You will notice a significant bump around 60Hz-90Hz. Remember where the speaker was moving the most? This increase of output at that range is nice for bass guitar, as it will sound very punchy and articulate. If this was a pro sound or home hi-fi application, a flatter response would likely be more desirable. The response can be shifted by changing the enclosure volume. The image below compares this example in red (4.19 cu. ft) versus the same speaker in an 8 cu. ft. enclosure in orange.

In conclusion, there are a lot of software options to assist in designing an optimal enclosure for a speaker. With some basic knowledge of speaker parameters and the trade-offs involved in the aspects of enclosure design, anyone can design like a pro with all of the tools readily available. It can even be fun to learn, as you watch how performance changes with changes to the enclosure details.