December 13, 2010

Confessions of a bass trap Luddite

For some time I believed bass traps were for those with a problem room and for perfectionists. As I considered myself a perfectionist who had a pretty good room for bass, bass traps were low on my list of priorities. Until it suddenly hit me that I already had the materials to throw some together. You can read about my weekend experiment that changed my views on bass traps here

Cutting to the chase, my mission in this article is to convince you that bass traps are essential. They will make such a big difference that you will wonder how you ever lived without them. In fact, unless you have bass traps, you haven't yet discovered how good your speakers or subs are for bass. 

There are plenty of excuses for not using bass traps, but I'm about to sweep them to the side and at the end you will have no excuses left.

Excuse #1
My room is very good for bass already

This was my main excuse. I considered my room to be one large bass trap. Plasterboard lining with a cavity (or drywall for those in the states) acts as a membrane bass trap and is quite effective. It's certainly better than concrete or masonry. My reasoning was that if the entire room acts as a bass trap, how much will adding 2% more achieve except waste time, money and space?

What is needed is broadband absorption and it's unlikely that any domestic room will have it unless designed professionally for ideal acoustics. Without bass traps, a typical room will lack broadband absorption so the result will be quite hit and miss. The only other exception would be a room that is unusually lossy. I have measured one room that would not need bass traps. It was a demo room in an audio retail store. It had a wall of glass on one side, another was open to the shop which was quite large and the ceiling had "acoustic tile" which is virtually transparent to bass. There was so little enclosure offered by the boundaries that bass trapping was a non issue. This would be extremely rare in a home environment. If you have such a room, you are very lucky indeed and you have my permission not to use bass traps. Otherwise, excuse #1 is out for you!

Excuse #2
I believe careful placement is enough

You are right that placement is important. Good idea to pay attention to both the speaker and listening positions. However, this won't do the whole job.

Placement is the first step, but it isn't the only step. It will affect how the speakers couple to the room, having an impact on room gain. Placement will affect how room modes are excited. Where you position the speakers will determine where peaks and dips are located due to room modes. Where you sit will determine which of those peaks and dips you will experience.

There is no way you can avoid those room modes having an impact. There is no placement that can do it. The modes will also create time domain problems, such as modal ringing. You can see this here:

This is quite smooth and better than most. A typical room will fit within a 30 - 40 db range, but in this case this room is +/- 10 db. I should admit that I did use a little parametric EQ at 45 Hz to tame a peak.
Placement is the first step, but it isn't the only step. It will never replace bass traps.

The modes are marked with red dots (not all included). Notice how the modes decay slower in time? One is marked in particular where it has only a very small peak that would hardly be noticed. Yet in the time domain, it takes much longer to decay. If the modes decayed at the same rate, then the response shape would not change over time. You can see that this is clearly not the case.

Excuse #3
I have a magic EQ box

Fix the frequency response and you will fix the time domain as well. This is the argument that is made, but it does not work. The response shown above does have some EQ and I have shown previously that the modes decay at a different rate. This can't be fixed by EQ alone.

I'm certainly an advocate of EQ as a useful tool, but it certainly doesn't do the same job as bass traps. If you are relying on EQ, you will often need to use some boost and this will quickly kill the headroom. The amount of power and excursion required is not practical. You would need to double up on subwoofers just to allow for 6db of boost, or double up again for 12db.

Bass traps and EQ are best used together. You can EQ a room with bass traps and it will probably be an improvement. If you bass trap a room that already has EQ you will see a bigger improvement. in many cases, a much bigger improvement and it will be global.

Don't make the mistake of thinking EQ can eliminate the need for bass traps. If you use both, you will see the difference in measurements and hear it clearly in listening tests.

Excuse #4
Bass traps are expensive

If buying bass traps presents a problem, consider building your own. It's not difficult and it's not expensive. If funds are really tight, then next time you see a dodgy couch on the side of the road, think "free bass traps." The cushions will work just fine. Add a little inventiveness and I'm sure you can put make something that will work just fine.

Excuse #5
I don't have a dedicated room

I've left the most difficult one 'til last. With a little creativity, you can come up with something that can get past the interior design committee. Consider ways that you can get them to blend in. Membrane traps are the easiest to integrate in this way, but it's possible to do the same with typical broadband traps. Imagine that your room had duct work built into the corners. Would your partner insist that those heating or AC ducts have to go or else? Would your partner insist that the fireplace chimney has to go because it's just too big? What about that huge freezer?

These things are accepted because the function is understood and given a priority. You can't have a fireplace without the chimney. Just like you can't get high end bass without bass traps. I don't expect every partner will accept that idea, but if you can tackle the challenge creatively and make it work visually, you have a better chance. And if all else fails, bribery in the form of jewlery can be a wise investment.

"Ok, you've convinced me..."

You can find quite a few bass trap plans online, but first you need to understand how they work and how the various types are different. My bass trap primer will save you reading through many different pages on the internet that will still leave you wondering about the basics.

Bass trap primer >

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