- More than meets the Eye (Part 2)
(...and what meets the eyes)
Speakers come in all shapes and sizes, for both aesthetic and acoustic purposes. The "shell" onto which the speaker drivers are mounted, that which also houses the critical crossover, is called the speaker cabinet or enclosure. This is an important aspect of building a good loudspeaker, thus any respected speaker manufacturer pays close attention to the R&D of their speaker cabinets. (Though, there are some good speakers which do not have "cabinets", but their way of functioning is a different story altogether...)
Why do we need speaker cabinets? It acts as a platform for mounting the speaker drivers. It also prevents the sound "back wave" (generated by the drivers) from interacting with with the "front wave". The cabinet also "loads" the bass driver, tuning the bass driver to the designer's specifications.
Cabinet designs may vary from manufacturer to manufacturer, but common ones are usually of the sealed box (infinite baffle / acoustic suspension) or bass reflex (vented, ported, tuned enclosures) category. (Some claim that the best speaker cabinet is actually the infinite baffle, which is an infinitely large panel onto which the drivers are mounted. This is impossible however, and the closest that people get to this is to mount drivers onto a wall with the backs of the drivers firing into another room.) The sealed box is usually a compromised version of the infinite baffle. There is also another type of sealed box called acoustic suspension. These require specially designed drivers to work together with the box. Bass reflex enclosures have an opening in the cabinet which helps to produce low bass. The air in these enclosures are tuned to resonate at certain frequencies, so as to augment the woofer's deficient low bass. Our bookshelf speakers are usually of this type (eg. Dynaudio 1.3mk II, Jamo Concert 8, Tannoy M2.5, etc.). In general, a bass reflex speaker can produce lower (or more) bass than a sealed box speaker using woofers of the same size. Sealed box speakers are also larger. But a sealed box speaker tends to have better transient response (i.e. can handle climaxes better) and have better defined bass. Ok ok, enough about this ... this article was supposed to focus on evaluating cabinet construction and also tweaks (yes!).
So what makes a good cabinet?
No one can say for sure. Different manufacturers seem to have different "formulas". Let's look at the materials used to make speaker cabinets. Many manufacturers swear by MDF (medium-density fibreboard) for making rigid cabinets, others use hard-wood, some metal and others using resin and minerals. It seems to me that there are 2 kinds of design approaches that are taken here. One approach is to tune the resonances of the speaker cabinet to assist the drivers in producing sound. This is usually achieved using hard-wood cabinets. This approach is generally more rare (Italian speaker manufacturer Sonus Faber uses such an approach). The other kind is to design a cabinet which tries to damp and virtually eliminate all cabinet resonances (This is naturally an impossible task, but the keyword is that they "try"). Most manufacturers adopt this second approach (But there are also those who adopt this approach and fail miserably). Some designs lie in between the 2 approaches.
Let's see how this second approach of resonance-damping cabinets is usually achieved. A sturdily-built cabinet with internal bracing is critical. Internally, damping materials are also glued to the cabinet walls. Give the various panels of a speaker cabinet raps of the knuckle. A dead "thud" sound means that the cabinet is well-damped. (A more technical note: In general, the more rigid the cabinet, the higher the resonance frequency, and the lower the amplitude of this resonant peak. This resonant frequency should be away from the critical mid-range band of frequencies, where our ears are most sensitive.)
Of all the panels, IMHO, the front baffle (panel) is the most important, which is why manufacturers pay special attention to this part of the speaker cabinet. Sound produced from the drivers will be muddled with sound from the front baffle if it is not well-damped. This is the reason why some manufacturers make the front baffle out of thicker and more expensive materials than the rest of the cabinet (For eg., some budget speakers use MDF for the front baffle and less-dense particle board / plywood (less damping) for the rest of the cabinet). Some manufacturers even go to the extent of using mineral loaded front baffles that sound solid as a wall when knocked on. Most speakers also have front baffles with bevelled edges. This is to prevent "edge diffraction", which is sound waves being radiated from the front baffles sharp edges and these get confused with the sound waves produced by the drivers. The drivers themselves are also flush-mounted into the front baffle for more than just aesthetic purposes.
Generally speaking, the bigger the speaker cabinet, the less rigid it is. Thus, more efforts are needed in the design of a floorstanding speaker to make the cabinet more "dead", so to speak (usually achieved through internal bracing). Beware of floorstanders that are very light for their size!
Tweaks to control the resonances of your speaker cabinet are available from DIY and Hifi dealers. Internally, speaker cabinets can be dampened by sticking sheets of dampening materials such as Deflex panels (from Spectra Dynamics), bitumen sheets, lead sheets (tell me if you can get these!) to the cabinet walls. Fibreglass wool and acoustic foam are other forms of cabinet treatment. (Note: Be careful with adding internal treatment, make sure they are firmly fixed / glued to the cabinet walls. Also, try to KEEP THE INTERNAL VOLUME OF THE SPEAKER THE SAME as before adding the treatment) A speaker cabinet can have one or more of these treatments inside. Externally, there are other vibration control devices such as teknasonic vibration absorbers which try to absorb the vibrations, without you having to pry open (sometimes permanently) your beloved speakers. Other vibration absorbers come in the form of feet for your speakers. Of all these treatments, internal treatment tends to have the greatest effects, but that is not to say they make the most improvement. Experiment with different resonance control tweaks to suit your musical taste. Effects also vary with the original design of the speakers themselves. Some speakers sound thin and "dead" when over-damped while others respond well and gain tighter bass and better imaging.
Supporters: Pick and choose from a variety of designs and sizes of the best book shelf speakers at The Speaker Company!