How Many Subwoofers Do You Actually Need?

Even though you could get away with just one, how many subwoofers do you actually need to achieve natural sound at a reasonable cost?

By: Ringo Bones

Believe it or not, subwoofers were already sold by hi-fi retail stores around the mid 1970s. Sadly, those early subwoofers were very notorious for sounding as if an electric bass guitar connected to a loud bass guitar amp is playing along with the music in your listening room – even when playing small-scale chamber music. After the early design excesses were weaned out, subwoofer in the home has undergone a hiatus that made it desirable again in a few years time.

The subwoofer gained popularity again in the 1980s due to a great number of affordable small loudspeaker systems designed with very small woofers / main drivers – i.e. 5 to 7 inches or so and, sadly, most pain-in-the-ass interior decorators find them attractive. Sadder still, the growing impracticality for many hi-fi enthusiasts to own loudspeaker systems with 12-inch or 15-inch or larger main drivers for reasons of cost, space or room aesthetics (thanks to your pain-in-the-ass interior decorator) finally made subwoofers a viable option when it comes to system building.

During the early 1990s, a partial, but interesting solution to the problem was the introduction of the three-piece system. This generally consists of two quite small loudspeakers that cover the frequency range down to 100-Hz or so. Such a loudspeaker system with a 5-inch or 6-inch main driver are fairly easy an inexpensive to design and manufacture. But a loudspeaker system with the bottom two octaves missing is not very satisfying to the seasoned audiophile, so a third component is added – the subwoofer.

The rational of using a subwoofer is that the main front left and front right loudspeakers can be designed to give appreciable power output and have optimal performance at very low frequencies because the two main left and right front loudspeakers no longer has to reproduce the very low frequencies. And while the subwoofer no longer has to reproduce the higher frequencies assigned to the front left and right loudspeakers, that one subwoofer box can now – in theory – be placed in a more or less out-of-the-way location in the room since the low frequencies – i.e. below 100-Hz – have very little directionality. It was generally thought – since the mid 1960s – that a single monaural channel for frequencies below 100-Hz was needed and also because frequencies below 80-Hz were, allegedly, mixed to monaural anyway. This latter factor was a result of the limitations – and still is - of the mechanical reproduction process used in making vinyl records and back then, open-reel tapes are a way, way more expensive alternative to vinyl playback. But when properly connected, two subwoofers operating in stereo absolutely sounds more gorgeous that a system with just one, even in casual listening.

The assumptions that justified using a single subwoofer has some basis in fact because three piece systems have been, and are still popular when space and room styling (thanks pain-in-the-ass interior decorator) take precedence over other acoustic considerations. Audiophiles who preferably listen to rock music usually place the subwoofer to the left because the bass guitarist is usually mixed to the left of the soundstage, while those who listen to large scale Classical music place the subwoofer to the right because in a majority of orchestral recordings, the low frequency instruments tend to sound as if they are located on the right of the soundstage. However, over the years, I have some concerns and reservations about the technical justifications for the three-piece system.

It can also be quite a shocker to know that not all, or even most, loudspeaker setup absolutely needs a subwoofer or two to make them complete in the low bass. To my ears, it might be the wisest decision to get a truly full-range loudspeaker in the first place, since it will have been designed as a unit and have a smoothly balanced response over its full range. However, in the case of smaller bookshelf-type loudspeakers, considerable advantage can be had by removing the lower two octaves of the sound from the two main left and right loudspeakers and diverting them into a good subwoofer. The advantages are that the subwoofer now handles the hard-to reproduce very low frequencies due to much higher power requirements freeing the smaller loudspeakers from the greater power and displacement demands of reproducing the very low frequencies. This can result in lower distortion in the small loudspeakers and considerably improved sound quality from them. It also allows planning for system growth – that is if you first invest in a good, small pair of loudspeakers, you can augment them later by increasing bass capacity with a subwoofer.

But the question now is whether one, two or perhaps more, subwoofers should be added. If only one subwoofer is added, the result would be a three-piece system with monaural bass response below 80 or 100-Hz. If two subwoofers are added, the result is the two bookshelf-sized loudspeakers virtually becoming two full range loudspeaker system with a full stereo low bass capability. But does it matter if we have stereo or monaural low bass? To my ears and my over 30 years experience in being an audiophile, I believe that it does for at least two reasons. The first is that we have a much better program material today from the Redbook Compact Disc, various high resolution digital audio formats like DSD and 24-bit 192-KHZ sampled PCM audio – and even low rate data online streaming in MP3 and the like compared with entry level vinyl in the low bass frequencies.  

The second reason is not quite so obvious, but very important if you want your system to accurately reproduce low bass frequencies in a timbrally accurate manner, whether it is from a Fender Mustang electric bass played through an Ampeg GS-12R equipped with 7591A output tubes, an upright acoustic bass, or even a full-sized church organ with 64-foot pipes. Believe it or not, there is considerable interaction between the room and the low frequency loudspeaker units. We all know that placing the bass loudspeakers in near a wall or in a corner of the room changes the amount and distribution of the bass frequencies in the room. At low frequencies, strong and widely spaced room modes are occurring. These modes manifest itself by the fact that as we move about the listening room, some locations have a lot of bass while other locations have a lack of bass. Also, the locations where more or less bass frequencies are heard also moves around the room when you shift the position of the low frequency loudspeaker.

The problem with a three=piece system is that we only have one loudspeaker producing the low frequencies, thus the room is only excited from one point and we have a set of modes – i.e. those parts in your room where the low frequencies are either too strong or too weak – that are quite audible. When two bass loudspeakers – or two subwoofers – are placed in a room and are excited by a full stereo bass signal, each will have its own set of modes. But the benefit this time is that it is not as likely that the lack or excess of bass from one loudspeaker will fall exactly the same point as for the other loudspeaker. This would result in a considerably better uniformity of bass response is obtained when the room is excited from two low frequency sources – in comparison when excited from just one low frequency source. But if using two subwoofers is better than using just one, would using more than two provide even far better results?

A fellow audiophile of mine has a set-up that uses eight REL Acoustics Q100E subwoofers in the frontal 60-degree arc of his hi-fi rig. Four were connected to the left channel and the other four were connected to the right. According to him, this set-up was a quantum leap in terms of performance from his previous set-up when he just used two REL Acoustics Stadium II subwoofers – about four times larger in terms of volume than the REL Q100E. But how does it sound? To my ears, it is probably the most timbrally accurate set-up when it comes to producing bass below 100-Hz. Not only that, on some minimalist chamber music recordings, it transformed his listening room into a medieval era chapel where the music was recorded!!! Probably due to the room modes are now vanishing low.

Motional Feedback: The Future of Loudspeaker Design?

Best known as the working principle behind Velodyne’s patented High-Gain Servo System-equipped subwoofers, is motional feedback truly the future when it comes to hi-fi dynamic loudspeaker design?


By: Ringo Bones


Seasoned audiophiles, more often than not, had their first hand encounters with motional feedback technology via adverts – and hopefully purchases – of Velodyne’s world-famous servo-controlled subwoofers. These famed subwoofers are famed for their very low levels of harmonic distortion and coloration – when compared to their competitors’ offerings – that doesn’t use Velodyne-style High-Gain Servo System technology. But before we proceed any further, here’s a primer on what is motional feedback.

Motional feedback uses a second voice coil on the drive unit – typically a large woofer in stand-alone subwoofer systems – that provides a signal in which a tiny millionths of a second later is fed back into the amplifier, correcting distortion. The result – if correctly implemented – is amazingly powerful articulate bass / low frequencies devoid of coloration and distortion.

In those famous Velodyne subwoofer adverts of the 1990s, the company points to their use of a low mass – 2.5-gram accelerometer – that form the brains of their High-Gain Servo System. Velodyne mounts this amazing device directly on the voice coils of their subwoofer drivers, and measures the actual movement of the driver. The information is then sent back to a circuit, which makes corrections for any deviations from the pure output signal 3,500 times a second. Resulting in a virtually distortion-free bass even when the subwoofer is set at its highest frequency setting of 120-Hz.

My encounter with Velodyne’s FSR-18 subwoofer – one of the very best subs that we (me and my audio-buddies) can still afford when we still have Clinton administration era prosperity money during the 1990s. It can easily be described as a technological tour-de-force, imagine very loud and very clear bass subtle enough to portray the acoustic structure of the venue where the music being reproduced was originally recorded. Although early samples of the Velodyne FSR-18 subs were notorious for having loose wiring that came loose and slapped against the moving cone producing a tapping sound – evident when What’s Left by Lunachicks – from their Pretty Ugly album - was played during that particular listening session. Thus resulting in another flight to Hong Kong to our very friendly hi-fi dealer that gladly replaced our faulty subs.

In reality, motional feedback-type loudspeakers are a pain to design and make to work properly – tweaking one should only be a job for very gifted electrical / electronic / mechanical engineers – i.e. probably those skilled enough to make a humanoid robot that can run a hundred-meter dash in less than 10 seconds. It is not just Velodyne who had faced such problems developing and perfecting their very own motional feedback-equipped loudspeakers. Celestion and once-upon-a-time-loudspeaker-manufacturer Philips also experienced first-hand challenges one will likely encounter when flirting with motional feedback technology.

Graham Bank of Celestion once stated in a hi-fi magazine interview that a lack of positional reference in their prototype motional feedback loudspeaker design resulted in an enormous crack from the cone as it attempted to leave the chassis on musical peaks. Each time it did this, the connecting braids carrying the signal from frame to the voice coil broke, even though they were long enough to cope with the movement. Both Graham Bank at Celestion and Paul Mills at Tannoy have worked on motional feedback-type hi-fi loudspeakers during much of the 1970s and were somewhat convinced that that there were some deep seated difficulties in its application.

Even a notable demonstration of motional feedback technology by Philips – back when they were still making hi-fi loudspeakers – at their headquarters in Eindhoven, Holland, that motional feedback gave the sort of fast, tight and even bass most hi-fi enthusiasts dream about. Fantastic bass quality - provided that the user kept the volume down, otherwise, the driver instantly destroyed itself. The technicians at Philips also noted in their motional feedback loudspeaker experiments conducted during the 1970s is that much of the correction by the accelerometer and the servo system was being applied to signals above 100-Hz.

So is motional feedback the future of high-end hi-fi loudspeaker design? Well, remember how Velodyne’s FSR-18 subwoofer compares the deviation from the pure output signals and the one sensed by its low-mass accelerometer at 3,500 times per second? This is just with an audio bandwidth that rolls-off at 120-Hz – although it works beautifully, imagine this concept applied to the full Redbook spec CD audio bandwidth of 20,000-Hz. Corrections would be applied at 600,000 times a second – that’s just with a 20,000-Hz bandwidth. Imagine the difficulty with DVD-audio or Super Audio CD / SACD that reaches out to 100,000-Hz – a motional feedback hi-fi loudspeaker with a 100-KHz bandwidth would be doing servo corrections at 30-million times per second. Full audio bandwidth motional feedback hi-fi loudspeakers may be the future if its quirks are ironed out, but for now – and for simplicity’s sake – they’re only practical for subwoofers that only play notes as high as 120-Hz.