A lot of music store subwoofers have sensitivity or efficiency ratings around the 90dB SPL mark to the 102dB SPL professional region. These are based on a theoretical one-meter distance away from the cabinet and a one-watt audio source in the sub-woofer frequency band. That is why I shun the smaller cabinets, and choose the larger dual driver models with more size and efficiency. The ultimate aim is get close to, or more than, 130dB SPL at 1-meter capability per subwoofer.

To figure out that number, take the amplifier power rating at the subwoofer impedance (usually 4-ohms) and take the base ten logarithm of that rating. Then multiply by ten to get the dB watts your amplifier provides at maximum capability. For example a 1200 watt at 4-ohm amplifier is 3.08 when base ten logged or 30.8dB watts. Then add this number to the subwoofer sensitivity/efficiency value to get the maximum SPL rating out of the cabinet. This calculation will also help when comparing powered subwoofers with un-powered versions needing external amplifiers.


I tend to avoid powered subwoofers only because most are in compact configurations, and the large cabinets need kilowatts of audio power and that means tens of pounds of extra weight to lift. In my gig rig, I always keep a two-wheeler or hand cart to move my subwoofers in and out of the venues. And some manufacturers are also offering 4” casters on the back of cabinets or tilt-back caster-wheels for one-man transportation.


I just do not see two musicians busting their backs dead-lifting a subwoofer cabinet without wheels or roadies. From the above math, subwoofers will chew up a lot of watts since they are the least efficient speaker that you will haul around. Be prepared to purchase corresponding audio power amplifiers that have per channel ratings of 1400 watts to 3000 watts for typical dual-18” subwoofer cabinets. And the speaker manufacturers seem to be not helping by de-emphasizing efficiency a bit to offer more power handling capability. To me this makes subwoofer cabinets better space heaters than acoustic transducers as only about 20% of the input power conveys to acoustic power (SPL), with the rest heating air.


Like speaker top boxes, your amplifier sizing has to be between the subwoofer’s continuous and program power ratings for the best performance. Too low of power and the amplifier may clip out too early and burn up the driver voice coils. Rarely done, too much power will punch the voice coil and former out of the magnetic gap and/or mechanically damage the drivers.

In summary, do not take a half-hearted step into subwoofers. Instead, save up for professional grade cabinets and properly sized power amplifiers. And do not forget the required speaker processor or crossover to segregate the sub frequencies from the top-box frequencies. On my example list of nice subwoofer cabinets, check out the double-18” offerings from EV (Eliminator KW, QRx218, Phoenix subs, Tour-X subs), the SRX728 from JBL, and the Peavey QW218 subwoofer offerings. There are many more great offerings, so let the grand beauty contest begin.


Originally posted 2009-02-09 01:27:46.