Box Basics: Essentials - Enclosures Bass Guide

Passive RadiatorThat leaves passive radiators and bandpass boxes. Passive radiators are very similar to a vented box, but in order to not suffer from the same non-linearity problem you find in typical vented boxes, you need the radiator to be larger than the woofer. A 12" woofer needs at least a 15" radiator and that means a lot of baffle real estate to work properly. They can sound good but have a sound quality that's somewhat different than vented boxes. Group delay for a passive radiator box is greater than vented boxes or sealed boxes.

BandpassThis brings us to bandpass enclosures. These are larger than typical sealed or vented boxes because they require two box volumes in one enclosure, but have a really cool efficiency bandwidth tradeoff thing going for them. You can make them a little less efficient and go very low. If you make them with ports, you still have the same port tube linearity problem, and group delay is far worse than the any other type of enclosure.

So what's the bottom line here? I think that for car audio, if you know what you're doing, you can make any of these types of enclosures work well and sound more than acceptable. Sealed boxes are still the most popular box in car audio. You can get a large (12" to 15") high-excursion woofer work into a small box (1ft3 to 2ft3) and produce and low-frequency roll-off of 40Hz to 50Hz, perfect considering the acoustic lift in the average car, truck, RV or van, and with good transient performance. Likewise, a well-damped Qb3 vented box can also be made compact and roll-off in the 40Hz range with a transient performance almost as good as a sealed box, although you still have the port nonlinearity issue.

So which woofer is best for which type of box? A good criterion is called EBP or Efficiency Bandwidth Product and was first suggested by well-known audio engineer Richard Small (yeah, the Small in Thiele/Small parameters). You can calculate any woofer's EBP by dividing the resonance frequency by the driver electrical Q (Qes). If your woofer has an Fs of 20Hz and a Qes of 0.50, EBP is 40. EBP numbers in the vicinity of 50 or lower generally are best suited in sealed boxes. Numbers 100 and higher are best suited to vented boxes. Numbers between 50 and 100 are often able to offer good performance in both sealed and vented boxes. Like all rules, this one can be broken. Any EBP 50 and lower woofer can be put in a vented box, although it's typically a vented box with seriously high group delay. Likewise, any EBP 100 or higher woofer can be put in a sealed box, but will usually have a rather high f3 roll-off frequency. It's less about finding the perfect box, than knowing how make any given woofer do its best.

SealedThis must mean that sealed boxes are better? Again, no free lunch. Sealed boxes are likewise easy to design and also very easy to get good transient performance. Coming up with a 12" woofer with a low box Qtc of 0.7 is a piece of cake, but getting that same box to go down to 20Hz - an easy task for a vented box - is a serious struggle for a sealed box. You generally need a very low-resonance woofer, which creates mechanical design issues or you need some kind of amplifier manipulation in the form of low-frequency boost or servo control.

Dual Reflex BandpassDual reflex enclosures are very similar in design to single reflex. The difference comes with a placement of a vent or vent in the rear chamber of the enclosure. This addition makes use of both the front and back wave produced by the speaker to improve the overall efficiency over that of a single reflex. The additional vent also allows for extended low-frequency response. In a dual reflex, the cone motion of the speaker is generally very well controlled within the bandwidth. But, as with everything there's a downside. This is apparent in the low-frequency roll-off that can be sharp, often in the range of 18 to 24dB per octave. This steep roll-off in the low frequency implies that a dual reflex usually has to be larger in size to produce the same low-frequency extension of a single reflex design. The low frequencies are a direct effect of the roll-off of the enclosure design; and as the enclosure is made larger to compensate, the transient response can be degraded considerably.

Since all of the sound is poured through the vent, bandpass systems are vulnerable to the same issues as a vented enclosure. Nonlinearities in the vent mean higher distortion levels and a high group delay, making them less than ideal for sound quality.

In comparison to more conservative box designs, bandpass enclosures can be very complex to design and construct. Their complexity leaves little or no room for error without suffering consequences. The slightest miscalculation in volume size or vent proportions can create drastic changes in the response of the system. It's highly recommended that the design and construction of these boxes should be left to shops or people with extensive enclosure experience.

So where did the term "bandpass" derive from? This is really pretty simple. The term refers to the fact that the enclosure will only allow a certain frequency range (band) to play (pass) into the listening environment without the aid of a crossover. In order for this to happen, the frequency response of the speaker is acoustically limited by the enclosure.

IBs are also termed "free air" systems because the trunk space - now the enclosure - typically contains a greater volume of air than is depicted by the VAS parameter of a speaker.

Unfortunately, trade-offs come in many forms when working with IBs, most of which relate to efficiency. Of these, structural integrity, leaky divisions and low bass extension top the list. Without a solid structure surrounding the woofer(s), it's difficult to reinforce the audio. Trunk interiors are notoriously designed with flimsy panels, thin sheet metal and often a spare tire - each of which can diffuse the energy produced by the subwoofer. This diffusion can contribute to vibration of the trunk's contents, causing cancellation, distortion and unwanted noises.