A basic guide to bass bliss

One of the most challenging aspects of your sound system to get right is the bass. With most components, all you have to do is choose wisely then plug and play, but this is almost never true with bass. You can't simply plug and play a subwoofer, and you can't rely on high end loudspeakers to give great bass, no matter how good they are. To get accurate bass in a domestic room requires tuning - there is no way around it.

Update

I strongly recommend that you aim to add bass traps to your room. Not convinced it's worth it? Take the free bass trap test.

A few things to consider

Music or home theatre? Either way not having subwoofers is a compromise. While the design of the subs will be different for a music only system, for the most accurate bass you need at least one subwoofer.

• How low do you want to go?
• How loud do you want to go?
• Are you happy with just one sweet spot?

Aren't high end floorstanders enough?

There is very little chance that you will get the best bass with placing speakers in the room without tuning the system. From a bass point of view there are four types of spaces, and a domestic room is usually the most difficult one to get right:

1. Very small room (car) - uniform bass
2. Small room - typical - room mode problems
3. Very large room (commercial cinema) - no room modes problems
4. Outdoors (half space) - no bass

Room modes are the problem with bass in a room. Let me illustrate. This is a subwoofer measured so that you don't see the impact of the room:



It's called a nearfield measurement. The mic is so close that it only "hears" the driver.

This is the same subwoofer measured as you would hear it in a normal room. This room is my room, and it's better than most.



The grey band shows the +/- 3 db range the speakers are designed to work within. You can see what the room does to it!

Along a sound wave there are successive nodes and antinodes - peaks and troughs in the response. In the midrange these are so densely spaced that they are not a problem, but in a domestic room, which can be considered as acoustically small, there is a point where they become space far enough apart to create a problem. The result is peaks and dips in the response. To make matters worse, they are different as you move around the room. EQ can be used to compensate for one position, but the result may be that other seats are still a problem, and they may even be made worse. There comes a point where the wavelengths become so large that they are not able to cause modes and thus as they approach the dimensions of the room the bass becomes uniform. In a very small room such as a car interior, the dimensions are so small that room modes don't create this problem. Outdoors we have a different problem - lack of room gain, so we simply need more output to compensate. In a very large room such as a commercial cinema, we still have room gain but room modes are less of an issue. So in a domestic audio room we have the most difficult of all possible spaces to get right.

We can change the bass response by physically moving the bass source and the listening position. As the listening positions will be fixed, what we need is the flexibility to choose the best locations for a bass source. This is why floorstanders alone will rarely be enough. They should be placed for optimum midrange performance and for best imaging - away from walls with an appropriate toe in.

7 steps to bass bliss

1. Place the main speakers for best imaging and midrange performance

Ideally they should be set up so that they overlap the subs. (Yes, I realise this is very different to what you have been told).

Placement should look something like this diagram above. It was created with open baffle in mind, but applies equally to most speakers.

2. Find a position for your first subwoofer

You'll need to place the subwoofer in the listening position, then place the mic in every position that you could consider using. Measure each one with a calibrated mic and then choose the position which gives the most low frequency extension and output. Ideally you should repeat this process for each listening position. This subwoofer should be the most powerful and be responsible for giving you all the extension and output that you desire as the other subs won't add much output.

3. Find a position for your second subwoofer

The main function of this sub is to smooth out the response. Place this sub in the position that will compensate for the in-room response problems of the first sub. Keep in mind how the subs will sum together as it doesn't happen as you expect. The curves do not simply add or subtract. Where one has a peak and the other has a dip, they don't average out as flat. The summed response will be that of the peak and the dip won't subtract. Where one is flat and the other has a dip, the dip will be removed. As a result, give priority to removing dips. The only exception to these rules is where phase shift occurs.

After choosing a position, place the sub there and measure both together, experimenting with different phase settings until you get the smoothest result in all positions.

4. Repeat the process for the third subwoofer

Do you really need 3 subwoofers? There is no one size fits all solution, but typically 3 is a good number. Normally with two subs the room won't be fully optimised, a third will yield an improvement but going beyond that won't yield much improvement.

5. Review bass filtering on the main speakers

The main speakers will add two bass sources and it's often best not to set them to small and high pass filter them so that they provide little or no bass. The bass should overlap. This way the room will benefit from five discrete bass sources. Review the measurements and they will tell you where the mains provide a benefit. In my room I find that they help remove dips down to 60 Hz. Below that point there is no need for bass output from them.

Note: I know what you are thinking!
This goes against what all the experts have told you. They will probably say that you want to bi-amp with the subs at say 80 Hz so the mains are "relieved" of bass duties. This does have some sense, but bass quality will suffer. A better option exists and the main reason the experts have not suggested it is that a) they didn't think of it and b) most speakers are lacking in oomph so the standard advice gives them a hand. But if you are dedicated enough to have a dedicated room, then a better solution is to use speakers with more capability overall that don't need to be relieved. The mini monitors have to go!

6. EQ

Now that you have achieved a reasonably flat response in-room, it's likely you'll benefit from a little EQ. Aim if possible to remove the peaks first. You may still need to boost some dips, but you should do this carefully. Quickly the headroom in your system will disappear so there is a trade-off between headroom and flatness to consider. Every 3db of boost will double the power required at that frequency. Every 10db will multiply it by 10. Effectively a 1kw sub will act like a 100w sub.

7. Measure

Measure it all again to be sure you've got it right. You might need to tweak a little and keep measuring. Once you get it all flat, keep in mind that the result will be underwhelming if totally flat. While peaks and dips should be avoided, the level of the bass should be higher than the midrange. Movies are mixed to add 10 db of extra gain to the subs, but music tends to benefit from boosting the bass level. This is a matter of personal taste.

Conclusion

The most common mistake audio enthusiasts make is to think that it's all about getting the best speakers or the best subwoofer. That's a good start, but nothing more. It's highly unlikely that you will find out what your speakers and sub can really do unless you have gone through this kind of process. It will take quite a lot of time but there is no way around that. The good news is that with moderately priced subs done right you can embarrass much more expensive subs, which are rarely fully optimised.

Before you replace your big sub with three small ones, it's important to realise that the first sub will determine the output level. If you add a second identical sub stacked right on top you will get 6db extra output, but no improvoment of smoothness. In a multisub arrangement the additional output isn't anything to get excited about. You might not gain any extra output. You might get 3db more than just one sub, you might get 1.5 db! In the rare instance where your room measures perfectly flat, you only need one sub, or a number of subs clustered together for higher output. The only way to know is to measure. Fortunately for those who have never measured anything audio before, it's simple and low cost equipment will do just fine. Purchase a Behringer ECM8000 mic and a Behringer Xenyx502 mixer and some cable, download the Room EQ Wizard (REW) which is a free download and visit the Home Theatre Shack where you will get all the help you need on how to use it.

This article is based on a combination of online forum posts by Dr Earl Geddes and my own experience and opinions. I've also learnt from other enthusiasts in online discussions.