I’ve had many people ask me, “What makes a good SQ subwoofer?” I am finding that this is a sorely misunderstood topic, and the sheer lack of understanding behind the science of it is overwhelmed by the amount of strictly anecdotal evidence available. Making a decision on the purchase of a subwoofer for sound quality purposes is certainly not an easy one. The intention of this article is to serve to assist you in making that decision. I will only cover subwoofer drivers in this article. Look out for another article on enclosures. Please note, I won’t go into specific motor technologies or the presence of shorting rings and such. This is intended to be a fairly basic article to give someone a general understanding.
An SQ Subwoofer (one that is designed for sound quality) is able to reproduce a linear frequency response in a generous frequency range, while maintaining excellent transient response and transparency. Today we will look at a few features that can often be found on high-quality SQ subwoofers for mobile audio use.
By far my greatest interest in SQ subwoofers is moving mass. Given an identical motor structure, take two subwoofers of varying moving mass (for example, the Dayton HO 10 and the Dayton HF 10), and the subwoofer with the lower moving mass will always sound tighter, more musical, and more transparent. What we need to consider is the effect that moving mass has on the way a subwoofer sounds. A high moving mass subwoofer will not sound “tight” or have an excellent transient response simply due to the sheer amount of weight that needs to be oscillated in high repetition. That moving mass contains inertia, and reversing its direction continuously will prove to be difficult in all but the strongest subwoofer motors. For this reason, I often dismiss aluminum-coned subwoofers with a few exceptions where a powerful motor or an impressive design are used. A lighter cone will have the ability to respond to immediate oscillations accurately and with authority. I would compare an SPL sub to a semi truck, which has incredible amounts of power, and an SQ sub to a Mazda Miata, which can swing around corners like lightning. With the exception of subwoofers that are powered by neodymium motors, I don’t even look at subwoofers that have a relatively high moving mass. I would quantify high as anything higher than 0.4 grams per square cm of cone area.
Another thing to be considered in the choice of an SQ Subwoofer is not just moving mass, but also motor strength. The ratio of motor strength to moving mass is of great importance, and while related to the above point, is still worth mentioning. A classic example I use is a comparison between the Alpine Type-R subwoofers, between the 8″, 10″, and 12″. On initial appearance, these simply appear to be the same subwoofer in varying sizes. However, the R10 and R12 are built more for general purpose duty, while the R8 can be effectively used for SQ duty. The reason for this is a very high motor strength to moving mass ratio. The R8 shares very little tonal quality with the R10 and R12.
For car audio SQ purposes, my experience and observation has been that a low Qts subwoofer of will generally be indicative of great sound quality. Factors that can be attributed to the Qts of a particular driver include the suspension compliance, moving mass, and motor strength. You will find that, when any of those are compromised, Qts begins to rise. Take an Image Dynamics IDMax10 V3 subwoofer, for example. This subwoofer has an incredibly powerful motor, an 1000w RMS power handling, and impressively, a moving mass of right around 120 grams. The result is an impressively low Qts of .328. This is a subwoofer that can play with extreme accuracy and authority up to 125hz, reaching up into the higher bass frequencies that would normally be reserved for your door speakers. The incredible motor strength to moving mass ratio allows this subwoofer to be very articulate and detailed, but also have the ability to produce high output levels. Furthermore, low Qts drivers generally work well in exceptionally small sealed boxes, which are preferable for a variety of reasons for SQ installs. Their low space requirement allows us to use them in multiples to create output while maintaining space. I would say anything lower than ~0.40 to be a decently low Qts.
Efficiency & History
One dead giveaway for identifying a SQ subwoofer is efficiency. Generally speaking, I’ve found that a subwoofer’s efficiency is directly related to it’s tonal accuracy and general sound quality. I will go into length on the history of this topic because it will bring to light some options you may not have considered. There are a few ways to improve efficiency: increasing cone area, increasing motor strength, and reducing moving mass. For the purposes of this article, we will not be discussing impedance with respect to efficiency.
Back in the 90s, high efficiency was a requirement, as power was expensive. One could purchase 200W RMS of Class AB power for around $200 and consider it an excellent deal. By comparison, you can get 1200W RMS with $168 these days. Back then, in order to get loud, you needed to have efficiency. That meant finding subwoofers that had a low moving mass, high motor strength, and high cone area. You could reach painful SPL levels in those days with two CerwinVega 15″ Strokers in a vented enclosure on merely 200W of power. Obvious benefits of augmented enclosure output aside, that’s over 1600 square cm of cone area! The requirement for high efficiency subwoofers was prevalent due to the sheer expense of power, both financially and electrically on a vehicle’s electrical system given the inherent inefficiency of Class A/B architecture compared to Class D. The low amplifier efficiency also meant you needed massive heatsinks that could dissipate all of that wasted heat.
As Class D architecture came into play, power became cheaper in all respects, and packaged in smaller heat sinks. Thus, power wars began, with manufacturers creating the most powerful amplifiers they could. To meet the capacity of those amplifiers, titanic subwoofers were also made to handle all of that power and create incredible levels of SPL in vented or bandpass alignments.
Somewhere along the way, the art of efficiency and accuracy was lost in a race for SPL and power. When the vast majority of ‘enthusiasts’ wanted to “live it loud” and be the loudest kids on the block, what purpose does an SQ oriented subwoofer have? The old school Kicker subs of the 90s were phased out for subwoofers with heavy cones (300+ grams) that could reproduce a 30hz test tone at earthshaking volumes. In this current day, we find that most manufacturers produce subwoofers to be abused; subwoofers that can handle copious amounts of power and produce high levels of output. After all, why would you want a low-excursion, low power handling subwoofer that couldn’t play synthetic bass notes without bottoming out when you could buy these monstrosities on every retail stores shelf to satisfy your bass-head desires?
So, where can I find an SQ Subwoofer?
The market dictates what sells, and unfortunately, SPL sells well. To get what one would consider high fidelity, you would be left choosing between subs such as the Morel Ultimo or one of a myriad of Focal subwoofers that would set you back a minimum of $800 in a 12″ format. Not to lose faith though; there are some companies out there that have valued true sound quality above all else. One of those is Image Dynamics. As I’ve mentioned before, their IDMax and IDQ subwoofers have maintained an excellency in sound quality competitions that is greatly feared. An IDQ10 can be had for $210, while an IDQ12 can be had for around $260. These are purpose built SQ subwoofers, and every aspect of their specifications makes that undeniable. They are, however, being phased out for ID’s V4 lineup of subwoofers, and are becoming difficult to find brand new. Personally, I use one of their older IDQ15 V2 subwoofers; a subwoofer true to the high efficiency, extremely low moving mass, and high motor strength designs of that era. I have yet to give anyone a demo in my car who isn’t absolutely blown away by the accuracy and transient response of this subwoofer, which I purchased used for $125. They simply cannot believe that I am using one 15″ subwoofer, and not an array of 8″ subwoofers. However, these subwoofers are becoming very difficult to find, as anyone who has one and knows what it can do will not be likely to let it go.
If you are looking for a high fidelity/sound quality subwoofer these days, you need to look beyond your retail store shelf, beyond the over-saturated forums and online reviews full of anecdotal evidence, beyond the limited Klippel results you may find, and into other realms such as home and pro audio.
Allow me to briefly provide two examples of SQ subwoofers.
In an 8″ subwoofer format, you’d be hard pressed to beat the Tang Band W8-740P.
This subwoofer sports a very strong motor, very low moving mass (94 grams, or .29g/cm2), and a low Qts (0.30). As a bonus, this subwoofer features a very high excursion (for an 8″) with 12mm of xmax. At the time of writing, this subwoofer can be purchased for $78 plus shipping and will work beautifully in .4 cubic feet sealed gross. It may not be very pretty, but it works excellently. Power handling is 150W RMS
On the other end of the spectrum, we have a sub like the Peavy 15″ LowRider.
This subwoofer is a bit unusual, being a pro audio subwoofer, but it has a cult-like following in certain car audio SQ groups, and for good reason. In a 15″ format, this subwoofer’s moving mass comes in at 142 grams, or .17g/cm2. What lacks in xmax (9.5mm), it makes up for in sheer cone area (840 cm2). Qts is .342. This subwoofer sells for $200 and provides unparalleled sound quality. I would use one in 2.25 cubic feet sealed; respectable and perfectly manageable when you consider the added height would not otherwise be used in your trunk with a smaller subwoofer.
Well, that’s all I have time for today. If you have any questions or need any recommendations, feel free to leave a comment.
To learn more about designing an SQ subwoofer box, click: Designing an SQ Subwoofer Box.