Articles and whitepapers

Getting the Best from Your Sub Bass System (2/12/2004)

By Phill Mason, MJ Acoustics

The desire for the most natural sound reproduction is top of the list for most audiophile and home cinema enthusiasts. Careful time spent on configuring your system will pay dividends to your listening pleasure and your pocket.

So, you have your amplifier and speakers, but there is still something missing. Enter the subwoofer. The addition of a good subwoofer to your system will enhance your overall sound stage considerably. Adding a subwoofer does not just add more bass, it expands the frequency bandwidth. You will enjoy tighter, more defined bass lines and punchier bass drums. A whole host of other instruments come alive with a sense of space and realism due to cleaner mid frequencies. A sense of depth becomes apparent when using a musically-accurate active sub bass system.

A subwoofer is an essential part of a multichannel setup. The sub is the '.1' part of the surround system (e.g. 5.1, 7.1 etc), and handles the LFE (low frequency effects) from a DVD that has been encoded in Dolby Digital, DTS or any other multichannel format.

A subwoofer that is slow and woolly will spoil your setup, so choose wisely and look for a subwoofer that is fast and musically accurate. This type of sub works extremely well with both music and movies.

Types of Subwoofer

A subwoofer comes mainly in four guises: active, passive, sealed and ported. A passive subwoofer comprises a cabinet loaded with a driver and no electronics. An active subwoofer is the most common of the two. It is a self-contained powered system that has the addition of its own amplifier, and generally comes in two types of cabinet, namely sealed or ported. A sealed box tends to have a tighter, faster bass response, whilst a ported cabinet usually offers a higher SPL (Sound Pressure Level) and a lower frequency output.

A typical active sub bass system

Connections

Connectivity is usually via low-level RCA (phono) inputs or high-level Speakon or speaker bindings posts. The best subwoofers will have both high and low connection onboard the amplifier panel. Occasionally, professional balanced XLR sockets for high-end connections are used.

Crossover settings

An active sub bass system takes a signal from your sound source and amplifies the signal in much the same way as your stereo or multichannel amplifier amplifies your main speakers. A crossover frequency is set on the subwoofer at a point where it continues the frequency bandwidth beyond that of your main speakers. An example for a two-channel setup would be setting the subwoofer to around 50Hz if your main speakers roll-off at around 40Hz - 45Hz. A setting above your main speakers of approx 5Hz - 10Hz allows for a better musical integration with your main speakers. If setting up for movies, 80Hz - 120Hz setting is the norm for most DVDs' LFE channel. If your sub has the choice to set the crossover to LFE, this is another option that bypasses the subwoofer's onboard filter electronics and uses the predetermined crossover setting from your AV amplifier.

The difference in frequency settings between two-channel and multichannel can be vast. There are new subwoofers on the market that offer two totally independent crossovers that allow you to have a stereo and multichannel setup from a single sub, without having to adjust the crossover when you change between movies to music. A good subwoofer can handle both source types well.

Choosing a subwoofer

When deciding which type of sub would best compliment your system, you need to look at what choices are available. If you are to use a passive subwoofer, you will need an amplifier to power it. If using an active sub (recommended), then all that is required to start enjoying your sub is a simple connection from your sound source. This could be CD, DVD, vinyl, digital TV etc. Whatever your pleasure, a good subwoofer will make it all that more enjoyable.

Flexibility, simplicity and control should be up there on your wish list. A subwoofer with limited control should be avoided. An on/off switch and gain control is not enough to ensure that a subwoofer will seamlessly integrate into your system. Look for a sub that offers independent gain controls for both high- and low-level inputs, and a variable frequency contour with a useable bandwidth, e.g. 200Hz down to 20Hz selectable. This more than covers the subwoofer bandwidth required for a 2.1 audio setup and is also the frequency range used for the LFE (.1) channel in surround sound formats. A variable phase control to ensure the sub works in time with your main speakers is also very important - you do not want your sub to be playing catch up!

Digital displays on subs are becoming very popular. A display provides visual confirmation of your settings and simplifies your setup. Some of the latest active sub bass systems, such as the MJ Acoustics Reference 200, have twin independent crossovers allowing the user to have a totally independent setting for both music and movies.

Typical panel controls for adjustments

Amplifier power

The amplifier power of a subwoofer is usually a concern for most prospective buyers. You need to ascertain what your listening room requires to be correctly loaded. If you live in the Albert Hall, then lucky you - you have just become every manufacturer's dream customer, but if like the majority of us your listening environment is your living room, then you will want your subwoofer to be discreet, but powerful enough to load the room correctly. A subwoofer ranging from 100 to 250 watts is usually sufficient to cater for most average-sized rooms.

Installing the subwoofer

When installing your subwoofer, its location can make a considerable difference. Do you need a forward- or downward-firing sub? Look at where you would like your sub to be positioned, behind your TV, beside your speakers or sofa? If it were to be placed behind your TV, a forward-firing sub would not work as well as a downward-firing model. A forward-firing sub has the driver facing outward from the front of the box, and can be harder to integrate when the frequency setting is above the 50Hz point, as the elusive omni-directional myth of sub bass frequencies becomes apparent. We can hear frequencies well below this 50Hz point but they are harder to hear and less directional. A downward firing sub has the driver facing downward out of the bottom of the box, and is easier to integrate at all frequencies - making it the more popular choice.

Transparent, seamless integration is the key to a good sound system. Think of it as not adding a subwoofer, but extending the bandwidth of your existing speakers. Your aim is to get all frequencies into all areas of the room - this is known as loading the room correctly, and it will give you a better listening experience.

If you can hear too much sub bass coming from one particular area, the chances are that you have your sub turned up too loud. Remember, a subtle underpinning is all that is usually required from a subwoofer. As ever, quality not quantity gives better results. You may want to adjust the frequency contour and gain, as a different setting might be better suited to the room. Also try your sub in different locations to possibly improve on the sound stage.

Summary

The acoustics of a room do play an important part with any audio equipment, and should be considered when purchasing a sub. Heavily-carpeted and furnished rooms tend to require more power from your system than an open plan apartment with wooden floors. Bass-light or bass-heavy rooms can be improved with the use of a good subwoofer. More often, two smaller subs can help a bass-heavy room rather than one larger sub. Bass-light rooms benefit from the use of a sub or two due to the added frequency bandwidth. This can help reduce the harshness of high frequencies, give a more even sound stage and add bass weight at lower volumes.

A subwoofer that is setup correctly can give more enjoyment than one would expect. A well-integrated sub will place your audience right inside a movie, and audiophiles will experience richer solo instruments and cleaner, more defined lower octaves when listening to their audio collection over and over again with fresh ears.

My advice to someone considering a subwoofer is to shop around. Read the reviews, but remember, your ears are the best judge when it comes to audio. Nobody knows what you perceive to be the best. Audition if you can, if not speak to someone who can explain what features will benefit you. And remember, a dedicated subwoofer manufacturer puts all of its efforts into designing only subwoofers. It should be your first port of call as you are likely to get more sub for your money.

Phill Mason is a Partner in MJ Acoustics, manufacturer of sub bass systems.

www.mjacoustics.co.uk