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Pt.2: Sub-Woofer Design Car Sound In any current state-of-the-art car sound system, a sub-woofer is de rigeur especially amongst those inter­ested in either competition or lots of bass. In Pt.2 this month, we look at some of the trends in sub-woofer design. By JULIAN EDGAR In the past, car sound systems used loudspeakers mounted on the rear parcel shelf as their major bass units, with 6 x 9-inch designs most common. The cone area of a 6 x 9 speaker ap­proaches that of an 8-inch design, but without the packaging difficulties associated with fitting such a large speaker into this sometimes-cramped location. When fitted by the car’s origi­nal manufacturer, 6-inch speakers were most frequently used. Placing the speakers on the rear deck allowed them to use the whole of the boot volume as their enclosure. In fact, with boot volumes of hundreds 4  Silicon Chip of litres, the speakers were effectively being used in infinite baffle form. Since the resonant frequency of high quality 6 x 9-inch loudspeakers is down in the 60Hz range, quite good bass could be developed. Other locations commonly used for speakers have included the front doors, kick panels, dashboard and rear quarter panels. Each of these locations pose major problems in terms of bass response, with the most constraining factor being the lack of volume behind the speaker. A speaker mounted in a very small sealed enclosure will have a high resonant frequency because of the stiffening effect which the small air volume has on the compliance of the cone suspension. Sub-woofers When bass below 60Hz is wanted, it is necessary to match specialised drivers to a detailed enclosure design. Initially, most woofers were mounted in a similar way to the other speakers; ie, mounted on the rear deck and using the full boot volume. However, as woofers in cars increased in size and even more bass performance was demanded by customers, sub-woofer enclosures within the boot space were constructed. The enclosure volumes were usually based on the manufacturer’s recommendation and were usually acoustic suspension (sealed box) designs. In the United States, “sound-off” competitions started becoming popular and these caused a dramatic change in the expec­ tations of the consumer. Part of the competition judging involves the use of a spectrum analyser to measure the in-car frequency response and so the demand for a flat response down to below 32Hz increased. Previously, any bass was deemed to be good but when variations in response of ±3dB or more could be read off the judge’s printout, consumers became far more exacting in their demands. Sub-woofer design Most top-quality car sub-woofer systems are now designed using the computer software package “Term Pro”. This program has been devised specifically for sub-woofer enclosure design in cars and follows on from the very successful “Term One” package. Steve Burgess of the Adelaide car sound company Cartronics took me on a guided tour of the package. In addition to the traditional two designs of loudspeaker enclosure (ie, bass reflex and acoustic suspension), the program also produces bandpass and isobaric designs. In bandpass enclosures, the speaker is mounted on the dividing wall of a two chambered box. Vents may be used in either one or both of the chambers. By contrast, an isobaric design uses two drivers mounted concentrically in close proximity. Generally, the speakers are mounted face-to-face and so are driven out of phase. In Steve’s own Commodore demonstration car, the boot-mount­ed sub-woofer uses a 6th order bandpass ported design, with the central chamber tuned to 100Hz and the outer two chambers to 38Hz. The details of the system are easy to see because the enclosure is constructed entirely of Kenwood’s HQW-300 sub-woofer driver has a maximum power handling of 300 watts RMS. The voice coil diameter is 80mm & the speaker uses a diecast aluminium frame. Claimed frequency re­sponse is 18Hz - 2kHz. transparent polycarbonate! When using the Term Pro software to design a sub-woofer enclosure, the first question that Steve asks of the customer is the type of music that he or she likes to listen to. Although a flat response can be engineered down to almost below audible level, that may not be what the customer wishes to hear. For example, tight, punchy bass of the sort encountered in current “rap-techno” music is best answered by the use of a sealed enclosure. This will also require the use of a power- ful amplifier, as this sort of enclosure provides low efficiency. Other types of music require different enclosure designs – a ported single chamber for classical music, for example. For one cost-no-object system, Steve asked the customer to bring in his 10 favourite CDs. An analysis was then made with a spectrum analyser to determine which bass frequencies were most common in this music. In this case, almost all of the bass mate­rial fell into the 80-120Hz range and a design capable of strong­ly reproducing bass in this area was duly built. Powerful amplifiers are used to drive car sub-woofers. This Earthquake amplifier has an output of 50 watts for each of its four channels, at less than 0.15% THD. February 1995  5 An elaborate 6th order bandpass enclosure is used in this demon­ stration vehicle. The enclosure, which is built into the boot behind the back seat, is made of clear polycarbonate. The whole system (obviously more than just the sub-woofer) took three months to develop and cost $5000. Driver selection The next step, after looking at the type of bass response wanted, is to select the driver. If you have not looked at car woofers recently, the range available is quite staggering. As an example, the “Earthquake” line-up includes a 10-inch unit priced at $239. It boasts a 2.5-inch diameter voice coil, a power han­dling capability of 300 watts, a 1.9kg magnet and a sensitivity of 96dB at 1 watt/1 metre. Most top manufacturers are also quot­ing Thiele/ Small parameters like Qts, Vas and so on. It’s this that allows the software to work so well. The program has the specifications of 612 drivers loaded into it, with space to store the specifications for up to 1000 drivers. Selecting These two photographs show the types of enclosures which the Term Pro software package is capable of designing. 6  Silicon Chip from one of those available, Steve decided to use a top-quality $600 Soundstream unit. Its specifications include a nominal diameter of 10 inches, an Fs (resonant frequen­cy) of 35Hz, a Qts (total Q) of 0.376, a Vas (equivalent com­pliance air volume) of 1.8 cubic feet, and an Xmax (maximum cone excursion) of 0.087 inches (the program can run in either metric or imperial units, with the latter still used most frequently in speaker design). The sensitivity was quoted as 90dB. The program was asked to design an enclosure which would give the flattest frequency response (dubbed the Maximally Flat design). The result was a sealed high-pass enclosure of with an internal volume of 0.71 cubic feet (20 litres). Furthermore, the program predicted that the response would be virtually ruler-flat from 120Hz to 1000Hz – see Fig.1. The predicted low frequency response was -3dB at 66Hz and -10dB at about 38Hz. Next, a ported high-pass enclosure was tried. Given that the program had already recommended a sealed high-­ pass design for the flattest response, improvement in this area obviously could not be expected. However, the -3dB point was substantially low­ered to 40Hz, while the -10dB point now occurred at about 30Hz (meaning that the roll-off was also much steeper) – see Fig.2. This enclosure design required a box volume of 38 litres – almost double the volume of the previous design. Whether or not it could be physically fitted into the vehicle would be another factor in determining the usefulness of this approach. Finally, a purposely mismatched enclosure design was picked. The isobaric 4th order bandpass box substantially reduced the efficiency of the loudspeaker – it was about 7dB down com­pared to the other two designs. A much more powerful amplifier (over Fig.1: the Term Pro enclosure design software was used to design an enclosure which would give the flattest possible response from a specified Soundstream driver. The result was a sealed enclosure with a volume of 0.71 cubic feet. Fig.2: next a ported enclosure was tried. The bass roll-off is now steeper but the -3 dB point has dropped to about 40Hz. Cartronics’ Manager Steve Burgess using the Term Pro software to design a car subwoofer enclosure. February 1995  7 rectangular boxes can be designed. The resulting bass response achieved by designs based on this package and using high-quality drivers is exceptional. Amplifiers & crossovers Fig.3: a deliberately-mismatched isobaric 4th-order bandpass enclosure was also tried. In this case, the speaker efficiency markedly dropped, while there was no improvement in bottom-end response. Fig.4: port design can also be carried out by the software. Here the port diameter has been user-fixed at 50mm, with the program then calculating the length. twice the power rating) would therefore be required to get the same sound pressure level. As well as the loss in efficiency, this system has a steeper bass roll-off than either of the other two proposals. Port design The software can be also used to design the size and length of the port required in vented designs. The internal diameter of the port can be specified by the user, allowing common sizes (2-inch, 3-inch, etc) to be entered, with 8  Silicon Chip the program then calculat­ing the appropriate length of the vent. If the port diameter is too small, then the speed of the air flow back and forth within it can make an audible (chuffing) sound. The predicted port velocity is provided by the program, so that this figure can be kept appropriately low. Finally, the dimensions of the box can be listed. Certain of the box dimensions can be fixed by the user, with the program calculating the others so as to retain the same internal volume. Wedge-shaped as well as Due to the ear’s poor sensitivity to low frequencies and the low efficiencies encountered in some sub-woofer enclosure designs, a separate power amplifier is generally specified to drive the sub-woofer. Fortunately, the omnidirectional nature of bass notes means that only one sub-woofer is required. This also means that a stereo amplifier can be used in bridged mono config­uration to provide the extra power required to drive the sub-woofer. An example of an amplifier that’s suitable for sub-woofer duties is the US-made $500 Earthquake PA2030 which has a mono output of 150 watts into a 4-ohm load, accompanied by a maximum total harmonic distortion (THD) of 0.015%. An alternative ap­proach is to use a 4-channel amplifier, with two channels driving deck-mounted two- or three-way speakers, and the second pair of channels run in bridged mode to drive the sub-woofer. In my own car, for example, a $440 Coustic 45-watt x 4 AMP268 is used, with one pair of channels bridged to give around 90 watts. While these sorts of power outputs initially appear exces­sive (how loud do you want it?), in a moving car which has extra­neous tyre, wind and exhaust noise, the bass notes can be easily lost. Add in low-frequency panel resonances and the power re­quired to drive a sub-woofer to audible levels in a moving car can be quite high. Finally, a crossover network must be employed to prevent unwanted frequencies from being fed to the sub-woofer. For this reason, many amplifiers have a built-in sub-woofer output with a variable crossover point. Indeed, the Coustic amplifier mentioned above has both low and high-pass crossovers built-in, with the low-pass design variable between 32Hz and 400Hz. Either passive crossovers can be used or electronic parametric equali­ sation modules like the Audio Control EQX unit can be employed. As well as having 13 equalisation controls, this unit has a 24dB/octave sub-woofer SC crossover output.