[Paul Spencer]
What is a subwoofer?
A subwoofer is a specialist speaker designed to reproduce low frequencies, usually down to 20 Hz crossing over to the main speakers (aka mains) somewhere between 40 - 120 Hz. While some subwoofers are designed for significant 15 Hz output, and some even as low as 10 Hz, most don't attempt to operate this low since even movies noted for deep bass content usually place more emphasis on content above 20 Hz. A more compelling reason to not go this low is that four times the displacement is required for every additional octave lower you wish to reproduce. Suppose you had a single sealed subwoofer with 1 cubic feet of volume and 300w amplifier power, capable of 100 dB @ 20 Hz. If you want to achieve this output at 10 Hz, you would need four of these subwoofers and some additional equalisation (eq). If you only had the single subwoofer, you would probably only sacrifice 12 dB of output to attempt to get down to 10 Hz. 88 dB @ 10 Hz probably won't be noticed.
Subwoofers have been made popular by an interest in home theatre. A subwoofer will play a full octave lower than typical floorstanding speakers and will reproduce bass with far more authority owing to their capacity to move more air.
| Tip: A quick rule of thumb to compare the bass output capability of a driver down low is to find the swept volume (VD). Multiply the effective piston area (SD) by the one way maximum linear excursion (xmax) x 2. eg. a typical 12" high excursion driver with an SD of 500 cm2 and a one way xmax of 15mm will have a VD of approx 1.5L. There are more factors to consider, but when comparing the output capability of drivers in the bottom octave (20 - 40 Hz), this is the first thing to consider. |
A subwoofer allows you to use five smaller "bookshelf" speakers in a surround sound format, yet to achieve greater bass impact than would have been achieved with larger speakers by the use of a subwoofer.
Subwoofers for music?
There are many who believe that subwoofers are only suitable for home theatre special effects, and that subwoofer drivers are inferior reproducers of music. This is quite often true, but not for the reasons which are often assumed. It's a matter of design.
Some factors to consider:
* Subwoofers play lower:
This means they reproduce output you are not accustomed to hearing. Listen to TV with a subwoofer and you will be surprised how many commercials have the "pffft" boom that you normally hear in PA systems where the mic picks up unwanted sounds. The presence of deeper bass in music can make it seem heavier and even make the bass appear a more "slow."
* Subwoofers add distortion
Subs play lower and this usually means distortion components will be added. While many argue that this is not significant down low due to the insensitivity of the ear to low bass, this is more significant than you might think. A 30 Hz signal will have associated harmonic distortion. This means if you play a 30 Hz tone, the speaker will introduce sounds that are not in the original. 2nd order harmonic distortion will add 60 Hz content, 3rd order 90 Hz, 4th order 120 Hz, 5th order 150 Hz. It is quite common for even "high end" $5000 commercial subwoofers to have harmonic distortion which is more audible than the fundamental from which led to the distortion!
* Subwoofers move more air
If a subwoofer moves ten times as much air as a woofer you may compare it to, this is hardly a fair comparison! It's like comparing a car to a bus for the same price. The same money spent on a luxury car might buy a used bus with damaged seats, no air conditioning, a noisy engine and bumpy suspension!
Subwoofers can enhance music. They can add depth and authority. In space-limited situations they can also achieve significant depth and output which could not be achieved in the past with size limitations.
It's all about compromise
Hoffman's iron law shows us that there will always be a compromise between box size, low frequency extension and efficiency.
[More on Hoffman's Iron law]
Suppose you decide that you want a small box and 20 Hz extension. You will then be forced to accept that the efficiency will be low, ie you will need more power. If you can live with a larger box, then you will need less power, and you will most likely also achieve more output. Choose two of these factors, and the third becomes a given. You can reduce the compromise of the third factor by spending more money, by choice of quality parts and the use of technology, but there are limitations imposed by physics!
Should I add a sub to my system?
There will be situations and enthusiasts who will be more satisfied without one. If you don't intend to do it right, then perhaps the best answer to that question should be
NO! If you have a music-only system and you are happy with every aspect of its performance except bass extension, then you
may be happier adding some bi-amped woofers.
An alternative for music-only systems
This may also suit those who are more excited about preserving the character of acoustic double bass than they are about extreme low frequency effects. Suppose you have some bookshelf speakers with extension to say 60 Hz. You could add a subwoofer and cross it over at 60 Hz with a 4th order (24 dB/octave slope) low pass filter to match the rolloff of the vented mains. The sub will give extension and more bass impact, but it won't actually do anything to improve your mains, and they may in fact remain the limiting factor in output.
Here is an alternative. Use a simple two way active crossover, add a stereo power amp and a pair of woofers. The woofers will run up to about 200 - 300 Hz. This will reduce the excursion of the mids - now they will operate as true midrange drivers, reducing IMD (intermodulation distortion) and increasing dynamics and overall maximum SPL. The power handling of the midrange will now be determined by the thermal limits of the driver, rather than by its limited excursion capability. Harmonic distortion in the midrange will also be lower as a result of less excursion.
The extension could range from 20 - 40 Hz depending on the drivers, alignment and the room.
Drivers could range from a number of 8" or 10" units to a 12" or larger. If you are crossing at 120 Hz or below, you can put the drivers in the side of the box, make them stands matching the width of your mains. Otherwise, with a higher crossover point, you should place the driver in front.
The active crossover can be made as a diy unit, or you can purchase either a PA unit, or for more power and flexibility a digital active crossover could be used. If you choose the latter, you may even wish to convert your system to active.
Q: When is this alternative better than a sub?
A: When you don't need the brute force of a sub.
A woofer will give you a higher useable response and will do more to improve the midrange by virtue of a higher crossover point. A subwoofer will have more brute force displacement, but it is more expensive to combine this with a useable response to 80 Hz and above. A driver as good as the Peerless XLS is useable up to 120 Hz but many are not. Using a subwoofer up to 300 Hz is not likely to be a good idea. You may have success with it, but it's more likely that you could do it with a cheaper woofer. The downside to this approach is output down low as the output will be displacement limited. While there are a growing number of ultra high excursion subwoofer drivers with advanced motor systems designed for low distortion, it is very expensive to provide a large displacement, low distortion and a wide bandwidth.
Nirvana ?
If your system includes an AV amplifier with bass management you can exercise both options. Bi-amp your mains to give always on extension down to eg 35Hz for music. Set your bass management to send only the Dolby/DTS LFE (Low Frequency Effects) channel to a big ported sub for extra oomph down to 15Hz.
Conclusion
So are subwoofers for home theatre only and bi-amped woofers for music?
NO! It is rather a matter of performance goals, and how you think they are best achieved. You may prefer to use subwoofers for music, or you may prefer to use a number of woofers for home theatre! Neither pertain only to a particular use. Also keep in mind that movies contain music and usually have more convincing voice recordings. There are so many different factors to consider that will be different for each person, that it's not possible to suggest appropriately which is best. My advice is to consider you options, give it some time, get informed and then do what feels right and what you think will meet your performance goals.
