In this first-in-a-series FRONT of HOUSE Tech Feature, author David K. Kennedy answers some FAQ’s relating to sub drivers and enclosures that are well suited to the requirements of concert sound production.
Q: Do woofer and enclosure sizes and types matter?
A: Woofer and enclosure size are very important; assuming the use of a quality driver, the woofer cone size, voice-coil size (diameter and length) and the box/enclosure size and type are primary factors in predicting maximum sound-level-output, and how low in frequency a subwoofer can play — in combination with sensitivity/efficiency.
Primary factors in predicting a speaker’s maximum sound level output and F3 (-3 dB down point) include: a woofer’s resonant frequency (Fs) along with other Thiele/Small parameters, such as Bl (magnet field strength); Xmax (excursion); Pe (max power); and Vas and Qts (effect box size).
Classic pro-sound woofers are very efficient, but required very large enclosures to provide any low bass. Some newer pro-sound woofers work in smaller enclosures by adding some mass — somewhat like home hi-fi woofers, but with larger cones and voice coils — and/or by requiring equalization to get full low-frequency extension (40 Hz or less). Some new state-of-the-art (SOTA) concert subwoofer drivers have amazing/superior power handling and excursion — implying higher-output subwoofer performance. However, Thiele/Small bass-box modeling software predicts that most of the new/SOTA subwoofer drivers don’t quite equal the output of other high-quality, (same brand) lower-power drivers, in larger boxes, like I design for venues.
Fortunately — for anyone getting back on tour — the newest higher-power SOTA subwoofer drivers do work very well in much smaller box/enclosures, proving that it may be possible to bend Hoffman’s Iron Law.
Modern home subwoofers tend to be in small-sealed enclosures with a built-in amplifier and equalization to get full LF extension. While a small subwoofer can sound decent at low sound levels, the notion of a small yet high-output subwoofer is basically a contradiction in terms. Hofmann’s Iron law is elegant in its simplicity. The Iron Law focuses on the interplay between enclosure size — LF extension and sensitivity/efficiency. Note, while often misused interchangeably, there is a difference between sensitivity and efficiency and we will look at this in an upcoming column.
A small subwoofer cannot be made to do all three of these:
These three features are mutually exclusive in a subwoofer enclosure. It’s like the old sign in many quick-print shops: “Quality, low-price, speed; choose any two…
Pro-sound and mass-market consumer (home-stereo) subwoofers have been getting smaller and lower-cost. Many people don’t seem to notice that the sound quality goes down with the size and lower price.
Q: What subwoofer enclosures are best for live music reinforcement?
A: Multiple band-pass, large-tapped or large-folded horn subs, or…
B: A greater number of ported/vented boxes (with large/curved vents) — ideally in an end-fire configuration or 1/3 reversed cardioid subs added, for directional control.
While the list of bass-box types in the following section is available online (for the most part), there is not good consensus on the list, much less are the best choices explained or suggested. Subwoofer choices should be based more on the differences in application, cost, performance and size rather than brand name.
Q: What are some types of subwoofer enclosures?
A: To achieve full low-frequency extension, some sort of loudspeaker enclosure is needed. As the box affects mostly bass frequencies, “bass-box” is a more all-inclusive label than “subwoofer.” However, subwoofer is a more modern term — we will use both terms interchangeably here.
There are several types of bass/sub-boxes; they are well documented in several loudspeaker books and on several loudspeaker web sites including: The Audio Engineering Society (aes.org); diysubwoofers.org; and wikipedia.org/wiki/loudspeaker_enclosure. So, our comments here will be brief as possible, limiting history, and with minimal technical details.
Most of the hi-fi speaker/subwoofer box types in mass production are sealed (infinite baffle) or vented boxes. Most of the studio and concert-sound bass/subwoofer box types in mass production are vented boxes. We will look at the most common types of bass-box systems and we will not discuss the more unusual (in pro applications) designs such as acoustic labyrinth, aperiodic, in-wall, isobaric, passive radiator, sealed or Tapered Quarter Wave Pipe (TQWP) types.
Vented (bass-reflex or ported) boxes add a port/vent — which allows the movement of air in and out of the box/enclosure — adding to the low-end output of the woofer. They are more efficient (play louder) than sealed enclosures, but are larger and usually have inferior transient response (group delay) using a similar driver. Predictable vent size and tuning changes the low-frequency extension, cooling and amount of resonance. There are many possible alignments (variations of box size and vent dimensions) for a ported system with a given driver, each with its own advantages and disadvantages. This type of bass box is the most common for pro-sound and is also very common for hi-fi systems; it provides the maximum deep-bass output for a given enclosure volume and is economical to build. If not high-passed, distortion rapidly increases below the cutoff frequency however, as the driver becomes unloaded. Look for flared or oversized ports to reduce chuffing noise. The compact size, lower cutoff frequency and high-power handling — within the system’s passband — often make ported systems the subwoofer box type of choice for most people.
Band-pass enclosures have two chambers. The dividing wall between the chambers holds the driver. Sometimes, only one chamber is ported (4th order), or the enclosure on each side of the woofer has a port in (6th order). These are considerably harder to design and tend to be very sensitive to driver characteristics. They are more efficient (play louder) than sealed and most ported enclosures, but most have a limited range and worse ringing issues (don’t sound “punchy”), thus are not typically used for the most critical applications.
Bass Horns use a flare to better match the driver cone to the air — this improves the coupling and controls/confines dispersion, both improving efficiency and transient response — thus, sounding “punchier” (on-axis), partly due to the lower moving mass, and lower Le of the compatible woofers. Properly designed horns for very-low frequencies are massive, dozens of feet. A few factories still produce smaller folded low-frequency horns — but these are still large and expensive compared to vented bass boxes with the same F3. Smaller models don’t go low enough to be considered subs. However, they can be stacked in clusters/arrays to improve the LF extension (lower F3) of the subs.
Tapped Horns have both sides of a long-excursion high-power driver inside of a tapped horn enclosure ported into the horn itself, with one path length long and the other short. These two paths combine in phase at the horn’s mouth within the frequency range of interest. The series of Danley subwoofers is especially effective at very-low frequencies and is smaller in size than most low-frequency horns — but is larger than sealed, band-pass or vented bass/sub boxes — yet yield more output.
Transmission Line enclosures shift the phase of the driver’s rear output by at least 90°, thereby reinforcing the frequencies near the driver’s Fs (and low-end). Transmission lines tend to be larger than ported enclosures of comparable performance, due to the size and length of the guide required. Good designs are often described as non-resonant, and some designs are sufficiently stuffed with absorbent material that there is indeed not much output from the line’s port. But it is the inherent resonance (typically at ¼-wavelength) that can enhance the bass response in this type of enclosure, albeit with less absorbent stuffing. Much information about transmission line speakers is available on the Internet — but unfortunately, a lot of that information is either based on “classical” transmission line design (which is basically obsolete) — and/or is just flat-out inaccurate. Thankfully, there are quite a few software tools these days (including the freeware Hornresp tool) that can be used to simulate transmission lines, which makes the design process easier. Some audiophiles and loudspeaker engineers consider the TL to be the ideal, but most complex enclosure with which to load a woofer; providing higher sound level and quality than sealed or vented boxes. The author would like to collaborate on such a TL sub design.
Multiple Woofers not only increase the maximum output of the sub system, but dual or quad opposed (manifold) mounting can lower both cabinet vibration, and also lower woofer distortion.
Q: Are pro-sound subwoofers directional or omni-directional?
A: Basically omni-directional. A compact single subwoofer box — such as a typical vented (reflex or ported) box — has no directional value at the lowest frequencies and only a couple dB of directivity at 80 Hz. Most of the various types of subwoofer boxes — outlined above — can be clustered into large enough arrays, or as few as three subwoofer boxes can be arrayed and processed to make their dispersion cardioid (single-box cardioid subs are available).
While this is describing a desired dispersion rather than a specific type of sub array, it has become common in the sound industry to describe gradient type subwoofer arrays as cardioid. An end-fire array is so named because three to four subs are arranged in a line and delayed so that they “fire” in order, starting at the end farthest from the audience. Many of us find end-fire subwoofer arrays the most useful in providing high-output directional control of sub frequencies. But a smaller gradient type subwoofer array can provide uniform directional control of sub frequencies at a lower cost and output.
Both of the above sub array methods create the desired off-axis attenuation, so some important questions are: at what direction, over how wide of a frequency range, at what size and cost? Search my past FRONT of HOUSE articles at (fohonline.com/?s=david+kennedy) for more great quotes, graphics and expanded coverage of directional sub arrays.
More to Come
In upcoming installments, we’ll delve into the history of Hoffman, his Iron Law, Thiele-Small parameters of woofers (to predict woofer performance), the differences between sensitivity/efficiency and provide expert input on subwoofer venting tradeoffs.
For more on live-sound design in large spaces visit the author’s consulting sound design website at www.D-K-A.com.