BSC made simple (and why it may be important to you)

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by Jim Salk

in Speaker Design

While baffle step compensation (BSC) sounds complicated, it is really quite simple once you understand what happens when sound waves emanate from a speaker. Here is a slightly over-simplified explanation:

the nature of sound

Sound, by its very nature, wants to travel in all directions. When sound is generated by the woofer in a speaker, for example, that sound not only projects forward to the listening position, but also travels around to the rear of the speaker.

You can confirm this with a simple experiment. Stand behind a speaker and you will still hear sound, although note that the highs will be lacking.

Congratulations, you have just unlocked the key to understanding the mystery of BSC! Let’s look at what is happening here.

details

What happened to the high frequency sounds? In short, they were reflected forward when they bounced off of the front baffle of the speaker.

This phenomenon is related to the wavelength of the sound at various frequencies. Just so you don’t have to do any math, here is a rough table (bear with it…this will not get too technical):

Frequency
Approx. Wavelength
20,000 Hz
.67″
10,000 Hz
1.35 “
5,000 Hz
2.7 “
1,500 Hz
9 “
1,000 Hz
13 “
750 Hz
18″
440 Hz
(A above middle C)
30 “

I have highlighted two entries in the above table. We will see why in a moment. And you will understand the theory behind BSC when that moment arrives!

baffle width and BSC

Let’s take a look at what happens when sound is generated by speaker drivers that are mounted in the middle of a baffle that is 9″ wide.

When the 20,000 Hz signal in the table is generated by the tweeter, it tries to move in all directions. But after one wavelength, it has traveled only .67 inches. So although a portion of the energy tries to move to the rear of the speaker, it can’t. It hits the front baffle and is reflected forward towards the listener. The same is true of the 10,000 Hz and 5,000 Hz signals in the table.

The 1,500 Hz signal (highlighted), on the other hand, has a wavelength of 9″ and reaches the edge of our 9″ baffle where diffraction can actually cause a slight rise in response levels at the listening position. At 750 Hz (also highlighted), the sound actually begins to travel around to the rear of the speaker.

So basically, any frequency lower than 750 Hz will be able to travel around the speaker (creating a roll-off of 3db for two octaves at the listening position), while frequencies above that will not. You heard this in the listening experiment above.

Note that increasing the width of the baffle will simply reduce the frequency where this behavior difference occurs. So BSC must be designed for the specific baffle in question.

the problem

OK. So, in our example, most of the sound at frequencies above 750Hz is directed forward toward the listener. Sounds at lower frequencies are not only directed forward, but also pass around the speaker to the rear. In fact, nearly half of the sound pressure is lost to the rear of the speaker.

So think about this: if the tweeter and woofer generate the same volume, high frequencies will be twice as loud as low frequencies at the listening position. (While this sounds like a huge difference, keep in mind that a doubling of sound pressure is about the smallest volume differential humans can detect.)

At any rate, the result at the listening position is sound that will be thin and lacking in the bottom end.

the solution – BSC

The solution is to design a circuit in the crossover that shapes the sound to compensate. It basically rolls off the higher frequencies so that they are in line, volume-wise, with the lower frequencies at the listening position. Baffle step compensation saves the day!

You should now understand the theory behind baffle step compensation. Congratulations!

but…

What happens when you mount this speaker in a wall?

Well, you have now increased the width of the front baffle. It now becomes the entire wall surface, so you have essentially created a baffle of infinite width. In this case, even the low frequencies cannot move to the rear of the speaker.

Since you rolled off the highs with BSC, you will now have too much low frequency energy directed forward. The result will be a boomy, uncontrolled bottom end.

The same would be true, although perhaps to a lesser extent, if you backed the speaker up to the wall. In both cases, an excess of bass energy (in relation to higher frequencies) is directed at the listening position.

This is why speakers designed to be free-standing (which require BSC) should not be mounted in a wall.

congratulations!

You now have a working understanding of baffle step compensation. Your friends are bound to be impressed, but as to whether that is in a positive or negative fashion remains to be seen. You are now officially a “speaker geek.”

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