Am I ready For A Subwoofer?
May 08 '03
The Bottom Line Subwoofers aren't all bad. But look for "punch" and not "thump"
Preface
When DVD came about, there was a lot of hype surrounding the sound formats, such as Dolby Digital and DTS. In the day DTS was rare, but leaving that, both formats brought out 5.1 channel sound information for 6 speakers. Normally, each channel would consist of a corner of the room, and one left over for a random placement. These five channels would concentrate on the whole sound spectrum, and the other channel would concentrate on a twentieth of the spectrum. This channel was the LFE, or, Low Frequency Encoding. Subwoofers are designed to patch into this sub-sound spectrum and turn those sounds the other five speakers can’t handle into life-like shakes and rumbles.
Understanding Sound Frequencies
Sound frequencies are a way of representing the waveforms of sound. A sound wave is produced by vibration, this vibration causes a wave that looks like the mathematical graph plot of “sin x” or “cos x”. Within each sound wave are wavelengths, these are measured from, say, the top part of the wave to the next top point, or the middle part etc. A frequency is how many wavelengths pass in one second. A high frequency sound is one that is high in pitch, and a low one is low pitch. Low frequencies that we are talking about in the LFE channel, are below approximately 120Hz. Humans like you and me can hear frequencies of around 20 to 20,000Hz, the gap gets closer with age.
The Difference Between Subwoofers & Normal Speakers
Subwoofers are designed to produce a mono sound that is well below normal speakers capabilities. The reason you usually get one subwoofer instead of two, is that the sound they produce (low frequency) is very unidirectional, that you cannot generally tell where it is coming from. A subwoofer has a large driver (this produces the sound), the reason behind it is to create more air movement. Normally you can find subwoofers with drivers as large as 12” in diameter. Naturally these ones are equally large in dimensions. Ordinarily the boxes are much thicker than regular speakers, and often have a port, which makes them a bulky thing to add to a room full of speakers.
When Subwoofers Are Needed
It depends on your situation, your equipment and what budget you are willing to put aside for one. Subwoofers are a very mild impact to a good home cinema system, if you’ve paid many hundreds for a good sound system, you could easily ruin it by adding something like a subwoofer. Even so, adding these boxes tends to make only small differences. The only place you will want a active subwoofer is during movies. Subwoofers are notorious for ruining music replay, unless you have a really good one. Nevertheless I don’t advise adding them to dedicated Hi Fi systems. If you are wanting more bass from your speakers, buy bigger speakers. They are going to offer much better integration and will sound better in the long run.
Music replay aside, with movies they are quite important. But considering the facts such as room size and equipment, you will have to decide whether one is necessary. The first, room size is easily solved; you don’t want a subwoofer if you are in a small room, and avoid them in square rooms. Bass tends to get amplified when they are placed near walls, and worse with corners. You will need room to put it so that it won’t be suffocating behind a couch and not damaging the foundations in a corner. Now, your equipment. If you still have your booklets for the front speakers lying around, it may be a good idea to look up what sort of specifications they have. Ignore this if you are using bookshelf speakers. Floorstanding speakers are much better at providing bass than smaller bookshelf units, if you find the sound-frequency range in the books; anything that goes below 50Hz is plenty for you. If you already have speakers that are quoted to go below 50Hz then you are not going to see major leaps from a subwoofer, yes there is that 30Hz going to waste but, believe me, your speakers will produce that sound even though they aren’t quoted it – just not as loud as the rest.
Another factor to consider is whether you have a dedicated Subwoofer output on your DVD player or receiver/multi channel amplifier. If not then you will have to buy a subwoofer with pass-through sockets (discussed later). The output sockets for the subwoofer channel carries that LFE, but in analogue, so if you have on then you have no problem; without one, you will need to buy some more speaker cable. That is just about it, unless you have bad neighbours then you should be okay for getting a subwoofer.
The Right Sub
Having gone through the equipment part of the last section, you should know whether you have got a subwoofer line output on the DVD player or receiver. If you do, then you will have no problem looking up for a good subwoofer. If not, there is a solution, and you may find this more effective than the former.
Not a great deal of subwoofers have them, but if you can find a unit with “Pass-through” sockets then you are best off getting one of these. Pass through sockets take the two front-channel speaker output on your receiver/amplifier, and pass them into the subwoofer, and out of the subwoofer. The sub will filter out the channels it needs and return the channels the front speakers need and without the parts it took out. This basically routes the speaker signals through the sub, however the subwoofer doesn’t use the amplification. These type of subwoofers will let you “tweak” the filtering so you can adjust the cut-off limit and tune it to the room. The pass-through system does not lose anything compared to the LFE channel, since the LFE and the regular 5 other channels have the same bottom limit, that being 20Hz.
Active subwoofers are practically the only type of sub you can find. For a short time Passive subwoofers were available, but there is not much point unless you intend to buy a Subwoofer amplifier. As long as the subwoofer is active, that’s the only other thing you need to worry about as far as features are concerned.
You can usually find that they come with a subwoofer-line input, a set of stereo RCA line-level inputs, and a power socket. Try to get one with build in rubber spikes.
As for design there are a few favours to try out. Port and sealed subwoofers are the general term. Ported designs, basically have a port, that lets the sound and pressure escape to create more volume, at the cost of being “lose”. Sealed types need a lot more power, but are much more “tight” in the sound character. The best-ported designs have the driver faced downward to a dispersion plate and have the port face out the side.
How To Install A Subwoofer
With the subwoofer unpacked, you can begin to incorporate it into the room. First hook it up temporarily to the system you have and play some music with just the subwoofer on (do this via the receiver), put the box in the centre of the room. Walk around the room a few times listening to the same track; you will notice that in parts of the room the bass sounds louder than others. These areas are the best place to put the subwoofer in a large sized room. For smaller rooms you can do the opposite. If you are after a seamless integration I recommend you buy a Sound Pressure Level meter and take a reading from the centre of the room with the test signal on in the receiver.
If you are going with the pass-through solution, then you will need to get some extra speaker cable, especially as this requires additional lengths if the sub is going a fair distance from the amp. On average in this situation, you will find that between the two front speakers, or behind the screen is the best place for the subwoofer, as it will integrate with the front sound better than if it were at the rear.
Final Tips
- Demo the equipment before and check it doesn’t vibrate (touch the cabinet).
- Try to keep it away from optical electronic equipment (CD/DVD players).
- Calibrate the cut off frequency by ‘ear’ sense.
- Run the subwoofer for at least 24 hours playing a CD.
- Do not place them in a cabinet or a wall enclosure, keep them in the open.
- Face the port away from the wall.
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