Guide to Woofer Speaker Design

When it comes to sound, a good woofer can give you the feels. Woofers produce the full bass tones that make audio systems feel big and impressive. They bring sound to life. They will also consume up to 90% of the energy that goes into your audio system. 

As you consider your audio needs, these are the things to keep in mind when it comes to incorporating a woofer. 

Get the Frequencies Right 

Typically, a woofer is designed to handle frequencies between 20Hz and 2kHz. However, it also depends on the other speaker drivers you’ll be pairing a woofer with in your system. If your woofer will be working alongside a subwoofer it won’t need to handle the lower end of that range as much. Conversely, if you go with a 2-way design that features a woofer and a tweeter (one of the most popular audio system designs) your woofer will need to handle frequencies up until the point where that tweeter can take over. 

The crossovers in your audio system make sure the correct frequencies are sent to the right speakers. Knowing the resonant frequency of the tweeter and upper frequency response limits of the woofer determines where those crossovers should occur. 

A woofer (or any speaker) may use either passive or active crossovers. Passive crossovers rely on capacitors, inductors, and resistors to filter the frequencies in a woofer. They don’t require power, but do consume a small amount, and are a good low-tech solution that has been around for decades. However, adding these components to a speaker adds cost and a system can quickly become cost prohibitive if it is too complex.

An active crossover relies on digital signal processing (DSP). This is a powerful technology that gives you a lot of control over a speaker. You can program much steeper crossovers with DSP, which wouldn’t be possible if you relied on physical components to achieve them. 

Bass Needs Space

Low frequency sound waves are longer (some are more than 50 feet long). A woofer needs to be big enough and have adequate space to pulse. This comes into play when you’re considering the speaker enclosure.

The parameters that help determine what kind of enclosure you need for a woofer are: the Thiele/Small parameters (T/S); the free-air resonant frequency (Fs); and the volume of air space that has the same compliance (“springiness”) as the woofer’s mechanical parts (Vas). 

A woofer’s cone is moving significantly farther to create soundwaves when compared to a tweeter and the T/S parameters tell you how its electrical and mechanical components are performing at low frequencies. The free-air resonant frequency is where the moving parts of your speaker move easiest. The volume of air space is exactly what it sounds like–how much space is in your enclosure.

Working with a speaker manufacturer, these data points can help you determine the best type of enclosure to optimize the performance of your woofer. You will most likely be deciding between a sealed enclosure, a ported (or vented) enclosure, and a passive radiator enclosure. Both ported and passive radiator enclosures can help lower the resonant frequency of your woofer, and extend low frequency response, and may be solutions for overcoming limitations around how much space is available for your application.

The material of your enclosure also comes into play with woofer design. The go-to option is medium-density fiberboard (MDF) which is an engineered wood product that’s been used for decades in hi-fi and pro audio systems. It’s inexpensive, durable, and acoustically dead (meaning it won’t “ring” as your woofer kicks out long soundwaves at high volumes).

Another option is ABS injection molding (think plastic). This requires tooling to create the prototype so there are extra initial costs for setting up the manufacturing. However, if you need a large quantity this could be the most economical option. 

Turn it Up to 11

How loud does your woofer need to get? When you tell your speaker manufacturer you’re working with the answer to that question they’ll be able to do some backward math to determine the power handling capabilities that will be required.

First, you need to determine if your woofer will be reproducing constant tones (such as alarms), or music and voice. If constant tones are to be used, it is important to know if the constant tones are sine waves or square waves. For music or voice, the frequency range needs to be defined. This helps to select a woofer with the correct resonant frequency and frequency range.

Next, determine how much output (dB SPL) the woofer needs to produce and at what distance. Typically, speakers need to be about 15 to 20 dB louder than the ambient noise level of the environment they are deployed in. This helps to select a woofer with the correct sensitivity.

Determining the correct woofer power handling is done by selecting a woofer that can safely achieve the desired output at the desired distance. This can be done by looking at the woofer’s sensitivity specification, which is measured at 2.83V (1W into 8 ohms, 2W into 4 ohms) at one meter. 

If the woofer is designed to create 90 dB at 1W/1m, and the target is 100 dB at 2 meters, then we would need to increase power by 40 watts to increase the speaker’s output to the desired level. The first 10 watts is needed to increase SPL by 10 dB (90 to 100 dB), and the additional 30 watts are needed to offset the doubling of distance, since you lose 6 dB of output per doubling of distance and only gain 3 dB per doubling of input power.  

• 1W to 10W = SPL increase of 10 dB (90 to 100 dB)
• 1 meter to 2 meters = SPL decrease of 6 dB (100 to 94 dB)
• 10W to 20W = SPL increase of 3 dB (94 to 97 dB)
• 20W to 40W = SPL increase of 3 dB (97 to 100 dB)