Some subwoofer notes

Some thoughts

I have a nice pair of large three-way speakers which are making glorious sounds in a smallish suburban living room. Why am I starting to think about building subwoofers?

  • Improved low frequency extension. A loudspeaker cabinet which is optimised for good imaging and soundstaging needs to sit well out in the room. It is then in full view and has to be domestically acceptable (read slim...) and is therefore difficult to design with a low resonance frequency without active equalisation. A separate subwoofer placed relatively inconspicuously can use a larger driver and a bigger cavity.
  • Reduced low frequency distortion. More driver surface area means a smaller cone displacement is needed to move the same volume of air. This in turn means that the magnetic system is working in its more linear region, and hence producing less distortion. The importance of low frequency distortion is underappreciated, partly since many people who are not used to clean low bass are more impressed by distorting woofers which produce harmonics rather than the fundamental, and are perceived as having more "kick". The real motivation for having low distortion at the lowest frequencies is to do with the response of the human hearing system, which falls off steeply with decreasing frequency. This means that when a subwoofer is fed 20Hz and is producing plenty of third harmonic at 60Hz, it is the harmonic that will be much more audible, and the deep fundamental which gives the spaciousness and power of real low frequency response is masked out.
  • Higher power handling. Smaller drivers have smaller volume displacement, as a more or less universal rule, and are less efficient. Adding a larger driver can seriously increase maximum SPL levels. It's not that sheer volume is a main priority for me, but increasing headroom will improve the ease of delivery at lower levels.
  • Reduced low frequency phase errors. Whenever a driver is approaching its fundamental resonance frequency the falloff in its frequency response will have a phase error (a delay) associated with it. The lower the resonance frequency and the lower the rate of falloff with decreasing frequency (in other words you're better off with a closed box rather than a ported box), the smaller the phase error is at the frequencies you want to hear. Keith Howard has written a couple of interesting articles on the audibility of phase distortion in Hi-Fi News (October and November 2000).
  • Better use of corner loading. A subwoofer placed on the floor in a corner will have an enhanced efficiency due to reinforcement of the immediate reflections from the walls and floor, which is not available for a free-standing loudspeaker.
  • One lump or two?

    Two subwoofers are better than one.

    Why? Certainly not because you or I can hear much of a stereo image below 40Hz. There are several advantages, however, to having two widely spaced subwoofers in your system.

  • Increased power handling. Doubled drivers and doubled enclosures mean the system can put twice the power into the room for the same linear displacement.
  • Less excitation of room modes. A single subwoofer, particularly if it is placed away from the walls, will excite a whole set of standing wave modes in the wall, depending on the three principal dimensions of the room. Placing it in a corner will help things, but putting another subwoofer somewhere else in the room will result in partial cancellations of many of the modes (and of course enhance one or two others, but you can't have everything...).
  • Increased efficiency at the bottom end. Using two drivers spaced less than a wavelength apart gives a 3dB boost in efficiency (provided they are reproducing the same signal, of course).
  • Better integration with the main speakers. Even though we can't hear stereo at true subwoofer frequencies, there is normally still output from the subs at 80-100Hz, at which point we can marginally detect spatial cues. Using a separate feed from each stereo channel to the subwoofers and placing each sub more or less behind its respective main allows the blend to be made much more seamlessly without compromising the soundstaging and imaging from the main speakers.
  • Closed, ported, passive radiator or coupled cavity?

    The choice of low-end loading for the driver determines the roll-off rate for the speaker and also the response and power handling at the lowest frequencies. The closed box has, for any given driver, the highest -3dB frequency, but the shallowest rolloff below it, namely 12dB per octave. This gives the most output at very low frequencies, but not much power handling. The ported enclosure by contrast has a lower f3 and a steeper rolloff (24dB per octave). The passive radiator is similar, but does not suffer from the uncontrolled cone excursions of the ported box below resonance. The fourth-order bandpass is again similar, but with higher efficiency and power handling, and the disadvantage of complexity in design.

    What needs to be counted against increased output at low frequencies is the effect of room loading, which increases the efficiency and hence the response of a speaker at frequencies where the wavelength is longer than the size of the room. The flattest frequency response will be from a sealed box whose f3 is chosen to match the corresponding increase in room response.

    Because of this, and strongly motivated by the additional advantages of simplicity and ease of design for a simple driver in a box, I'm inclined towards sealed boxes.

    At this point my plans are still inchoate, but I do have plenty of ideas and a pair of Maplin's 150W MOSFET amplifier circuits waiting for a home. My intention is to build a pair of closed boxes, probably using the 10" Scan-speak driver linked to below (which I notice ProAc are now using in their new ER-1 subwoofer).

    Some subwoofer drivers

  • Scan-Speak's impressive 25W-8565-01 paper and carbon fibre woofer.
  • The Adire Audio (formerly Avatar Audio) Shiva woofer. A legend on the BASS list!
  • Some recommended subwoofer-related links

  • REL's home page, with FAQ and various good articles.
  • Lynn Olsen's article on building and using subwoofers
  • The Contrabass Corner - an original approach to deep bass with low distortion.
  • Brian Steele's Subwoofer DIY Page, with design equations for all sorts of configurations.
  • Ingvar Ohman's article on Subwoofer placement (Ingvar has some other original and very interesting articles on this site too).
  • Some subwoofer projects

  • Geert Meddens' dipolar subwoofer.
  • Dave Atkins' NHT1259-based subwoofer.
  • Back to DIY page.