Mono subwoofer versus stereo woofers

This topic has been done lots of times by different people, but I think it deserves to be revisited as there are a couple of things that often seem to be overlooked.

SPL, maximum loudness and whatnot...

This angle is usually pretty well covered. If you want lots of bass, then you need to work out how much air a speaker can move. As a rough indication, multiply the surface area by the maximum linear displacement, usually referred to as Xmax. However, exactly what constitutes "linear" is debatable. One person's 10mm Xmax might be another's 2mm, depending on the amount of distortion they're willing to accept.

Here's a hypothetical example comparing the displacement volume of dual 25cm (10") woofers with a single 30cm (12") woofer:

A modest 0.5cm Xmax * 330cm² * 2 woofers = 330cm³

0.7cm * 540cm² * 1 woofer = 378cm³

However, as they say, "actual performance may vary". There are numerous other criteria to consider so it'll be far more accurate to do a simulation, especially since the peak displacement levels appear to be pretty similar at first glance.

Down-mixing a stereo signal to mono

This is where it gets tricky. Most music sources consist of more than just one channel, so if you're building an active crossover, or at least some kind of gain control, and you want to produce a mono signal for a subwoofer then you're likely to encounter a few unexpected issues. Contrary to popular belief, it's not as simple as taking a stereo signal (from a CD source or whatever) and summing the amplitudes. Why not adjust the gain by 0.707 so that the output powers are summed? Well that doesn't work either.

Well, what exactly is the problem? Take for example a pair of bass speakers placed 1.5m apart:

Initially only one of the speakers is plugged in, then the second one is connected and fed exactly the same signal. What happens then?

Well it depends on the frequency. At some frequencies the average acoustic power delivered to the room will double, while at extremely low frequencies the amplitude will be doubled (almost). The lowest frequency that can undergo strong cancellation is approximately 340/1.5/2 = 113.3Hz, where the 1.5m distance corresponds to a 180 degree phase difference. Below that frequency, the speakers will begin to undergo constructive interference that exceeds the average power doubling that occurs at higher frequencies.

This means that a pair of woofers are acoustically coupled together below a certain corner frequency, which produces a natural 3dB bass boost. The corner frequency is inversely proportional to the physical distance between the woofers. However, a mono subwoofer doesn't have that effect at all.

While there is no right or wrong answer, mono subwoofers aren't hot-swappable with large stereo woofers in full-range loudspeakers. Equalization and level-matching in active crossovers can be a tricky issue that is influenced by the satellite placement.

Vented vs. Sealed boxes

Vented/ported boxes are slightly different from sealed ones in that they have a hollow tube (or some other type of port or vent) connecting the internal air volume to the outside air.

At low frequencies, ported loudspeakers have:

• Two masses – the speaker cone and the air mass.
• Two springs – the speaker suspension and the air spring.
• Two damping devices – electromechanical damping of the speaker, and a tube that opposes the velocity of the air flowing to/from the box.

A ported design has several advantages over a sealed design:

• The two resonances can be tuned to provide a lower cut-off frequency than with a sealed design.
• The excursion of the speaker cone at low frequencies can be much smaller for the same output, resulting in lower harmonic distortion and higher maximum output.

However, it also has disadvantages:

• The system is more sensitive to variations in temperature, humidity and other environmental or design factors.
• The system has 4 poles instead of 2. Thus, 'ringing' (boomy bass) is a common side effect due to the relatively high q-factor of one of the pole pairs.
• Sounds at midrange frequencies may leak through the port.
• At high amplitudes the port may produce audible hissing.

Comparison of cone materials

This seems to be a common question when choosing a speaker driver: what's the best cone material? Well, I don't think there's really any such thing as a "best" material – they all have strengths and weaknesses.

One difficulty that speaker manufacturers have to contend with when designing their speakers is that they have to operate across a wide range of frequencies. At low frequencies (such as 100Hz) the cone generally has to move as a stiff, cohesive unit, while at relatively high frequencies (such as 3kHz) it's sometimes preferable if only a small, low mass section of the cone vibrates and the rest remains still. So, what should they do?

There are two popular schools of thought regarding the design of speaker cones or diaphragms:

1. The cone should be as stiff as possible and operate like a piston throughout its usable range.
2. The cone should be stiff enough to operate like a piston at low frequencies, but also be flexible and have enough internal damping to behave in a well-controlled manner at high frequencies.
   

Both varieties have drawbacks.

Stiff cones:

1. They tend to have very little internal damping and therefore ring prominently at their resonant frequencies.
2. The relatively large radiating area introduces "beaming" at high frequencies, whereby most of the sound is projected in a forward direction along the cone's axis of vibration.

Flexible cones:

1. They tend to introduce distortion due to hysteresis and intermodulation effects when the cone flexes and a variety of different frequencies are reproduced simultaneously.
2. Minor resonances occur across a wide range of frequencies due to imperfect absorption of transverse ripples that travel across the cone's surface.

Stiff cones are usually constructed of materials such as: aluminium/magnesium alloys, titanium, ceramic, and sandwiched composites. Some of the manufacturers include: Seas, Visaton, Accuton, Alcone, Eton, Aurasound, and HiVi. Manufacturers of flexible cones tend to use materials like paper, polypropylene, and a variety of composites. Some manufacturers: Vifa, Seas, Audax, and Manger.

Of course, this isn't the "be all and end all" of speaker cones. It's just the beginning! Have a look around on the 'net and you'll find lots of cool variations, techniques and alternative technologies.