Elephant Traps in Home Cinema Setup #3: Subwoofer setup

Subwoofer setup
SUB2pianoblackgrillonWe have come across a number of installations (not performed by us!) where bass is a bit of an issue – either far, far too much of it, mismatched compared to the other speakers, or just uninspiring.  As with many systems there are a number of different ways to connect subwoofers, mainly caused by manufacturers wanting to cover all of the bases (no pun intended). However here are a few guidelines to get near ideal results:

1 Placement

The subwoofer setup position in a system can have a dramatic impact on the delivery of low frequencies to the listening position(s). Whilst it is fair to say that below 80Hz or so the human ear does not detect the direction that the sound is coming from, the bass frequencies bounce around the room, and can excite the walls ceiling, doors and windows, which can all have an impact on the resultant effect. The position the sub is less crucial than other speakers, and is often governed more by aesthetics than sound. However it’s position affects the efficiency it drives the room, and the standing waves that are set up in the room.

In general we have these recommendations:

  • If the speakers are small (largest drive unit <~12cm diameter) it is best to keep the sub close(ish) to the front speakers.
  • For a very large room, often the bass fills it most efficiently when the sub is in a corner (though not always the case)

One of the best ways to find the ideal subwoofer setup position, is to make sure you have a long enough cable, and to physically sit the sub in the listening position. Put on some music with plenty of bass, and then walk around the room listening to the music close to the floor (after all that’s where the sub will be). Where you hear it loudest or most clearly, will be one of the optimum places to put the subwoofer, and means that if you place it there, you’ll get the least amount of overspill outside of the listening room.

2 Connections

Subwoofers often come with e bewildering array of connections on the back to cope with all eventualities. Here’s a brief summary of the most popular:

  • Line in (left/right) – connects to the pre-amp output of the AV processor. This often goes through an internal filter or signal processing in the subwoofer amplifier (in case the AV receiver doesn’t have any filtering). you can connect one or both of these to the sub output on the AV processor (connecting both only makes it a bit more sensitive)
  • LFE in – Low Frequency Effect input – this connects to the Sub output on an AV processor and typically has no internal filtering in the Subwoofer amplifier. This is the input to use for most good quality AV processors if available.
  • High Level input (left/right) – This is designed for reinforcement of speakers where there is no dedicated sub or pre-amp output. It connects to the speaker output of the amplifier in parallel with one or more of the speakers, and is usually filtered in the subwoofer amplifier. Using this connection is not recommended unless there is no other option as the quality is affected by the other speakers.

Most good quality AV processors will have a dedicated Sub output (usually an RCA connection), and a single good quality screened RCA cable is all you need to connect either the LFE  or Line input of the Subwoofer to the AV Processor.

3 Level / setup

SPL image_miniSome Subwoofers have internal signal processing that can provide room correction to counteract the resonances that can occur within a room. These boost some frequencies and cut others to try to level out the bass within the room. If you run this room correction, it is generally only valid for that particular subwoofer setup location, and will need re-doing if you move furniture around etc. We normally would only recommend using room correction if your listening room has particularly noticeable problems in terms of booming frequencies or nuls. Either that or if you are using more than one Subwoofer.  There are some good DVDs that run test tones / sweep frequencies that will highlight any problems e.g. Sound and Vision Tune-up DVD or the Spears and Munsil set up Blu-Ray disk.

The phase the sub works at makes a difference, and this is where setup disks can be useful. If you don’t have a setup disk, get a helper to flick the phase switch (or turn the knob) on the sub whilst you listen to music – you’ll find that one direction sounds louder than the other – this is the setting it should be on. The Phase governs the direction the speaker drive unit moves when a signal is first applied – i.e. if a positive half cycle is applied to the input, does the drive unit move out into the room or inwards. This should be the same as for your other speakers.

Distance setting in the AV processor. Sound takes time to travel from the speakers to you ears (around 3mS per metre) , and the timing setup adjusts the speaker timings, so that the sound arrives from each speaker to your ears at the same time. This doesn’t have a huge effect on the sub performance, and obviously cannot work for all listening positions, however it all helps the consistency of the whole.

In general most AV processors come with detailed instructions on setup, including balancing sub woofer levels. We find that there is a few dBs difference in personal taste when it comes to subwoofer setup levels, as some people prefer more or less bass in their music, however this aside the setup procedure is fairly common: The speaker levels are set to be at a consistent volume at the listening position by playing white (or pink) noise generated from within the AV processor. you can either measure the output with a sound pressure level (SPL) meter (set to slow response “C”-weighted), or just listen, switching between the speakers. We tend to prefer our own ears, as SPL meters can be thrown off by all sorts of things, that you might not hear. Take your time and go around all of the speakers several times.

The cut-off frequencies set within the AV processor should depend on the frequencies your speakers are capable of generating – you can use the frequencies published for your speakers as a guide, however exercise some caution here as manufacturers tend to be a bit generous with their numbers. A disk with a frequency sweep on it will highlight if there is a gap in response between where the subwoofer leaves off and the main speakers take over indicating if the main speakers don’t go down quite as far as needed. THX uses 80-Hz as the cross-over frequency, and this is reasonable for most medium to small speakers. If you have large floor-standing speakers for the front left and right channels, you may want to set these to “Large” and with no cu-off frequency in the AV Processor – a little experimentation might be required for optimum performance, and to get the best balance between subwoofer and main speakers.

 4 Listen Listen Listen

You may need to go around the levels / setup loop a couple of times, but after each iteration put on some music you’re familiar with and listen to the system. You shouldn’t hear the subwoofer specifically, it should just blend in with the rest of the system and all you should be listening to is the music. If there are any holes or distractions in the system, then you may need to go around the loop again – bear in mind that adjustments of phase or cut-off frequencies will affect the levels too.

5 More bass effects

There are a number of transducer options available if you really want awesome effects. These are typically heavy motors that are attached to the frame of your sofa or seat, and physically vibrate the seat with the ultra-low frequencies. Be warned though, these are quite fun in action films, and can certainly add another dimension, but you’ll be wanting to turn them off for listening to music, as personally I think they can get quite wearing. See the Buttkicker or Clarke Synthesis for examples