November 4, 2010

What is bafflestep compensation?

An issue that many don't quite understand is bafflestep compensation. It relates to where a speaker should be placed in a room. A speaker that is mounted on a wall, such as a surround speaker, won't require bafflestep compensation. One that is placed into the room away from walls, such as a stand mount, will require bafflestep.

What is bafflestep?

Bafflestep is a compensation in the crossover for the loss of bass and lower midrange that you get when a speaker is moved away from a wall.

Why is it needed?

Mount a speaker flush with a wall and the wall itself will reinforce the sound. Imagine a speaker suspended up in the air. That is called full space radiation. It radiates in all directions and so gets a bit lost. Now put it on the ground and that is now divided up in half. We call that half space. That's a bit like a surround speaker on the wall. Now move it away from that wall and it no longer has that reinforcement. The bass and lower midrange efficiency response will go down, however, above a certain point in the midrange, the speaker baffle will reinforce the sound. This depends on the width of the baffle, where a wider baffle will reinforce the midrange to a lower point, below which the response will step down. This needs to be compensated for in the crossover, otherwise the bass will be weak and the lower midrange will sound thin. The speaker will sound thin and overly bright.

Two ways to compensate

There are two ways to deal with bafflestep:

1. 2.5 way crossover

Let's say you have a 2 way speaker with a tweeter and mid. You can add a second woofer to compensate for the bafflestep loss. You simply apply a first order low pass filter at say 200 Hz.

2. Bafflestep compensation in the crossover

You can also simply alter the crossover design so that there is some EQ to reduce the midrange above a certain point, then the tweeter would be padded to reduce it's level. The downside is that efficiency is reduced by around 3 db.

How does your speaker handle bafflestep?

If you intend to build a diy speaker design someone else has published, it's important for you to know how it was designed.

Is it designed without bafflestep? In that case, you can use it for a surround speaker or against a wall without any change. If you want to use it as a stand mount placed well into the room, then you need to compensate.

Is it designed with bafflestep? Then you can place it into the room without change. If you want to put it on a wall, say as a surround, you will need to remove bafflestep.

How to add bafflestep

1. Convert a 2 way to 2.5 way

Add a matching woofer as described previously.

2. Bi-amp a 3 way speaker

That allows the woofer to compensate by increasing the gain. You might need to change the crossover point.

3. Modify the crossover

You will give up some efficiency in doing this and hence need to drive it with more power.

How to remove bafflestep

Essentially, this is the reverse of all previous suggestions

1. Convert a 2.5 way to 2 way

Take out the second woofer

2. Change the gain on a 3 way biamped speaker

3. Modify the crossover

Further reading

Rod Elliot describes bafflestep clearly, along with some charts that will make it more clear:

ESP Bafflestep article >

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