What is The Purpose of Subwoofers?

A popular question that seems to be on the minds of audio customers is, “Why do I need a quality subwoofer, what is the point?”. Subwoofers are what gives you more bass. It is what extends the sound of your music. Of course, if you properly implement the right kind of subwoofer, it can improve the overall sound and experience of your main speakers. High-quality subwoofers give you a more precise and amplified sound. The lower quality “boomy” subwoofers are basically just loud at certain frequencies and the difference in sound quality is very noticeable. The right subwoofer will help produce the full, rich, dimensional sound that so many people love but are not currently getting from their music.

If you are considering investing in a higher-end subwoofer for your in-home theater/media room/man cave, you will want one that has the ability to deliver lots of low-end impact. What is a “low-end impact”? Low-end impact refers to the bass-frequency signals that are below 250 Hz. 

The size of the room is what has the biggest influence on the impact.  If the room is larger, then a bigger subwoofer is needed and maybe even more than one. Installing multiple subwoofers in a room will essentially even out the bass response for all of the seating positions in the theater room and it will distribute an even sound to improve your listening experience.

Sealed Subwoofers

It’s pretty self-explanatory, but a sealed subwoofer is a woofer that has either its front or rear surface inside a sealed box. Generally, this type of configuration can be put into a smaller sized box and this gives it an easier integration into the listening environment with minimal visual impact on the décor of the room. Sealed enclosures are typically smaller and easier to build because there is no port to tune and the choice in woofer box really does matter to the sound.

They also reproduce the low frequencies more accurately than ported enclosures do because the air inside of the box acts as a shock absorber, allowing the subwoofer to move back and forth with more control. These subwoofers are the simplest to design and construct. Sealed subwoofers consist of a driver, an enclosure, and an amplifier; the driver is what’s responsible for 100% of the system’s output. 

The cone inside of a sealed subwoofer takes more power to produce the same volume as it would in a comparable ported box. A sealed sub rarely roars, thunders, or booms. That's because a sealed sub has a very flat frequency response and tends to play tight, full bass that provides a level low-frequency foundation to your music.

A well-designed sealed subwoofer will exhibit less phase rotation (an intentional distortion to the waveform to attempt to make asymmetric signals more symmetrical), lower group delay, and a reduced ringing in the time domain. All of these characteristics are why audio enthusiasts refer to these subs as sounding “tighter and more articulate with less perceived overhang”.

The sealed air inside one of these enclosures acts like a shock absorber, smoothly modulating the subwoofer's cones back and forth motion, so all the notes get produced and are heard evenly.

Timing

If you are a music enthusiast and are looking for a high-quality sound while listening to music, you need to consider the timing. Timing is important because you want the lower notes that are being sent to the subwoofer to sync up as closely as possible to the notes coming from the main speakers.

The sealed subwoofers resemble most other speakers (the woofer is what reproduces low frequencies). Being able to line up the notes and sound requires a high-quality subwoofer and some light tuning. Some subwoofers even give you the ability to adjust timing within the amplifier settings.

And The Bass Roll-Off?

The roll-off switch removes (or decreases the volume of) frequencies that the subwoofer has a hard time reproducing, or shouldn’t be playing. When an audio clip is playing and the subwoofer hasn’t had its roll-off set up correctly it will play frequencies that will cause the track to sound muddy and unclear. Sealed subwoofers usually provide a softer more consistent bass roll-off than ported subs do. 

In addition to sealed subs, there are also some subwoofers that are in a sealed box but have something called a passive radiator, which helps to extend the bass instead of having a port. A passive radiator is actually moved by the back wave of air coming from the main woofer. This movement of air is coming from the passive radiator and is more than what would come from a port.

Ported Subwoofers

Ported subwoofers are not built as simply as the sealed subwoofers because of the port. A ported subwoofer contains the woofer and one or more ports, which allows the air to escape out of the box (vented). This is what gives you the bigger and louder sound out of one box because there are two different mechanisms that are moving the air -- the port and the woofer.

Porting augments the system output at the vent’s resonant frequency, which extends the subwoofer’s response and allows increased output capability at the turning point, similar to a sealed subwoofer. These subs can move a lot of air and fill up a very large room with sound. However, be aware that some subsonic filters are fixed and some are adjustable in frequency. 

Subsonic filters should be set close to the tuning frequency in order to limit the frequencies that are below the tuning of the enclosure. If they are set above the tuning frequency the ported sub will perform similarly to a sealed sub.

Subsonic Filters

A subsonic filter is basically a high pass filter for very low frequencies. If the woofer is below the tuning frequency of the enclosure, then there is no acoustic suspension for the woofer and this results in a lower performance similar to an infinite baffle. Essentially, this can damage the suspension of the woofer because of excessive cone travel (called “unloading”).

All ported enclosures should contain a subsonic filter to restrict low frequencies when it is needed and many amplifiers do have a subsonic filter built into them. These filters are harder to tune to a lower frequency due to the small vented enclosure, so they are usually tuned to a higher frequency. The vent length and area are too big to fit inside the smaller vented enclosure for lower tuning. Because of this, it limits the power handling of a smaller ported subwoofer due to the increased susceptibility to unload at lower frequencies. 

Many users have said that they do not hear a change in sound quality that is noticeable. These filters are usually just meant to protect your subwoofer and speakers. It is only to remove the unwanted woofer travel from warped vinyl and other sources, when the woofers are fully extended they cannot properly reproduce the normal range. Leaving the subsonic filter on will not hurt the woofer and in the end, may improve the sound every once in a while.

Timing

Timing, unfortunately, can also be an issue with ported subs too. Jumping into physics, the air that is moved by the woofer will sync closely with your personal speakers, but the air coming from the port will be a little behind. The port in a ported sub is more difficult to design correctly due to its need to get it exactly right with the woofer and box size.

And The Bass Roll-Off?

Ported subs will roll-off extremely fast compared to sealed subs, to put it simply the ported subs have a much steeper roll-off. If you are choosing to go with a ported sub, be sure to get one that will provide a roll-off that will best compliment the speakers you are partnering with. If there is any gap between the roll-off between the speakers and subwoofer, you chance missing out on parts of the track the artist wanted you to hear!

Still Not Sure?

Both sealed and ported subwoofers have good attributes but which type of subwoofer is best for a specific customer and the application depends on several different variables.

Room Size Matters

If you are looking to fill a bigger room, the bass demand will increase significantly. Place one smaller sealed subwoofer in this big room and it will certainly not be enough. Just to match the perceived sound of having one sealed subwoofer in a small room, you will need larger and potentially more than one subwoofer for this big room. Remember that the room size can always be mitigated by using two or more subwoofers and using two smaller subs versus one large one and is often the smarter solution to allow for a more even bass output throughout the room.

If you are looking for a subwoofer to put into your home theater or media room, you want one that will let you hear the difference between the booming sound of thunder or the clashing and smashing of cars in a car crash. A majority of home theater applications require a ported subwoofer due to the size of the room and what kind of audio experience you are looking for (rattling and shaking the walls of the room).  A large room (above 2500 cubic feet) will probably need either ported subwoofers or large sealed subwoofers, and more than one preferably.

Conclusion

This is clearly not an easy decision to make but whether you decide to go with sealed or ported, choosing to add a subwoofer to your home audio system is one of the best decisions you can make.  It will upgrade and bring you a new level of energy to all of your music, home theater, and any other audio experiences. By adding a subwoofer to your current audio system you"ll be guaranteed a better listening experience along with the ability to play your music as loud as you want without any type of distortion.

TruAudio has both sealed and ported subwoofers available. If you have questions and need to be put into contact with a dealer in your area just jump onto the TruAudio website.

 Always remember that when it comes to the selection process that the room size, system usage, playback level, decor and aesthetic appearance, floor space, and limitations on the overall subwoofer size, are all things that go into consideration. 

If you already have some smaller speakers that perform pretty well and you enjoy listening to music as much or more than watching movies, then a sealed sub will work better for you. On the other hand, if you really enjoy the booming rumble of deep louder bass and have a big enough room, the ported sub is going to make you much happier and make the most sense.