GUIDELINES FOR CABINET DESIGN
A unique area Of loudspeaker design
A loudspeaker is usually a transducer, designed to faithfully reproduce the acoustics of the signal presented to it. An electric guitar speaker however is a creative part of the music, contributing its own character and tonality. The ‘instrument’ an electric guitar player uses is really a combination of guitar, amplifier and speaker. All three parts are vitally important to a good sound.
As guitar speakers are different, so their cabinets are different to hi-fi or PA cabinets. Deep, thunderous bass is not required (the low E of a standard-tuned lead guitar is 82Hz). High frequency reproduction is a positive disadvantage, allowing unpleasant harmonics and electronic noise to be heard. Distortion-free sound would be a disaster.
Two elements, a drive and a box
The box design is acoustically less critical than that for hi-fi or PA systems, but proper construction is essential. The cabinets should be solidly built to ensure no joint vibration (unpleasant buzzing), and be of adequate strength to withstand hard use. Remember that guitar speakers are quite heavy and amplifiers that sit on top of guitar cabs are even heavier. Internal bracing is generally not required, but battens inside the joints are good if your woodwork skills do not extend to complex corner joints, and a central bracing post can be advantageous in a 4x12.
Most quality cabs use 15-18mm plywood for the main cabinet, with MDF for the baffle (the part where the speaker(s) are mounted), but they can be constructed of any material. Many budget cabs are made of chipboard (cheap, but poor in terms of strength, ruggedness and sound) or MDF (easy to machine, but heavy and dead). Maple, mahogany and walnut are often used for high quality cabinets.
The important characteristics of the cabinet material are strength, sound and ease of use. Lively resonant materials, such as plywood or real woods, vibrate in sympathy with the speaker and enhance the sound, but should be at least 13mm / 0.5" thick, or they will unduly colour the sound. Most woods or wood composites will be strong enough at this thickness. When considering price, you should also consider cabinet finish. Cabinets can be painted or stained, or covered with a vinyl or carpet finish. Real woods finished with a stain can be very exclusive and expensive looking!
Generally, open back or sealed boxes should be used.Open back gives a looser low end with less depth, and 'figure 8' directivity (sound field looks like an 8 when looked down on from above the cab). Sealed boxes give tight, deeper low end but are more directional, giving less spread of sound. Vented / tuned / ported / reflex boxes do not generally offer an advantage to lead guitar speaker operation. If such a loading is required. it is recommended that the alignment should be chosen with care as inappropriate reflex designs can cause speaker damage. The box size is not critical. The baffle size is more important in open back boxes (larger = more low/mid presence), and for closed back boxes larger volume means deeper but looser bass. Do not use internal acoustic wadding, it is inappropriate for guitar cabs, reducing sparkle and life.Mount the speaker securely using bolts into T-nuts, not self tapping screws. Do not overtighten so the housing rim bends. Ensure the speaker is protected from the front, as the cone is easily damaged. The speaker can be front or rear mounted.
In Summary
Plywood or real wood construction is preferable
Strong, rigid construction means no buzzes or rattles
Size is not critical
Ensure the speaker is adequately mounted and protected
Avoid air leaks if using sealed box construction
CAUTION
We do not advise mixing different impedances of driver within the same cabinet. This can lead to uneven power sharing between speakers, causing one speaker to be overdriven and damaged, while the other is underdriven.
Thiele-small parameters
Thiele Small parameters are useful for controlling the low frequency response of sealed or ported cabinet systems by changing the cabinet internal volume, and port dimensions. However they are of limited use when designing a guitar speaker cabinet.
Electric guitar speakers do not reproduce 'low' frequencies (the low E string of a lead guitar has a fundamental of 82Hz) and so the frequencies at which Thiele Small parameters have significance are mostly below the operating range.
Also, the parameters are measured at very small signal levels. Guitar speakers become non linear at very low levels compared to other types of speaker, greatly reducing the significance of Thiele Small parameters in actual speaker use. Using the Thiele Small parameters of a typical guitar speaker, you will find that halving or doubling the cabinet size makes minimal difference to the response.
They have no relevance to open back cabinets.
Care should be exercised designing ported (or reflex) cabinets for guitar speakers as the increased cone excursion below the tuning frequency can cause speaker damage.
The cabinet size, shape and construction are of far higher significance than the internal volume. Cabinet design using Thiele Small parameters ignores these most fundamental aspects. Important factors include the material you make the cabinet from, the panel sizes and shapes, how they are joined, how the cabinet is finished, the mounting of the speaker, etc. These, not Thiele Small parameters are the critical factors in the design and ultimately the sound of a guitar speaker cabinet
