Questions on speakerbox building (baffle mainly)

[NoDestiny]

I have been building speaker boxes for some time (mainly for car subwoofers, used to run a car stereo shop a few years back). The construction is pretty easy, been using WinISD to design most of them with pretty ideal output.

But I find that there is more to it with building speakers as I read through some of these. For instance, I see the Modula MT's (which I am interested in building) are designed for a 9" wide baffle. What happens if you go larger or smaller? Does any of the other dimensions really matter? Also, does thickness/type of wood change this? Generally I use 3/4" MDF...

Thanx for any input you can give me.

[ThomasW]

There's a freeware program called Baffle Diffraction Simulator. With it you can see what happens for a given driver as the baffle size is changed. A significant change in width will require a redesign of the crossover.

Adding depth to the box isn't a problem, usually the height can be modified as well

Depending on the driver using thicker wood can require back beveling of the baffle so the rearwave off the driver isn't blocked.

[NoDestiny]

Interesting. ill take a look at that program. So the hight of the baffle is not the issue, its the width?

Edit: also, I looked up a bit into baffle step compensation.. Im assuming this is what its all about? If so, A Xover built without one for a wall-mounted system might not be as strict with the width of the enclosure?

[fbov]

You're working with a continuum here, between two extremes of a) a point source in empty space radiating in all directions and b) a point source mounted on an infinite baffle where the radiation in one hemisphere is reflected into the other (I'm ignoring the back wave.)

Changing the baffle size moves you on this continuum. The speaker designer optimized for a specific geometry, adding the amount of BSC needed for that the design baffle width to sound right in a free-standing application in one case and wall mounted in the other. The wall-mounted version is just as strict; it wants a really big baffle, like a wall. It will be lacking in low end when mounted away from a wall.

In my case, I initially used a BSC design for my center channel, mounted in an entertainment center - effectively in-wall. Bass was boomy. I changed to no-BSC and things smoothed out. (The BSC XO's are in the L/R towers.)

Have fun,
Frank

[---k---]

You were originally talking about Baffle Diffraction. To learn about this, you use the freeware program called Baffle Diffraction Simulator (BDS). This is different than Baffle Step Compensation.

For tweeters, the distance from the center of the dome to the edge of the speaker will change the response. This is because the length of the wave length for tweeters is very short. So, the sound bounces off the baffle. The wider the baffle, the more the sound bounces and the more funk you can get in your frequency response. In general, a narrower baffle with big roundovers or chamfers produces less ripples in the frequency response than the wide baffle.

So, the distance to the top and sides of the enclosure should be kept the same for the tweeter. Because of wavelength of the midrange and woofer is much longer, the width of the baffle generally doesn't matter.


Baffle Step refers to how far from the back wall you are. The closer you are to the back wall, the less baffle step you require. This is because of the polar radiating pattern of speakers. If you pull a speaker far away from the wall, the bass will radiate in all directions around the speaker, including behind speaker. While the tweeter and midrange will mostly radiate forward. As you move the speaker back towards the wall, the bass can't radiate as much behind the speaker, but is reflected back forward, so you pick up more bass the closer to the wall. Speaker designers must decide how far from the wall they want the speaker to be in order to get the bass right.

Speaker Building 201 is a good, easy to understand book. I recommend it if you want to understand some of this more thoroughly.

[NoDestiny]

Cool, ill look that book up. Thanx for your help, gang!

[cjd]

Baffle step is related to the baffle and driver-edge distances most directly, not distance-to-wall (which we do have to consider because it DOES potentially play a role in how much to compensate for baffle step).

So, there are two things messing this all up - diffraction, and step.

[NoDestiny]

OK so the width of the box is going to tell the tale of baffle step and the distance to the rear wall (or lack of) is the diffraction?

[TacoD]

OK so the width of the box is going to tell the tale of baffle step and the distance to the rear wall (or lack of) is the diffraction?

In short: No.

In more detail:
The bafflestep is the transition of predominantly 2pi sound radiation to 4 pi sound radiation. The total acoustical output is not only radiating forward (2pi) but also to the sides and back (4pi). This occurs when the wavelength becomes long compared to the baffle width. The sound wave does not "feel" the baffle at all and easily radiates to the sides and the back of the speaker. This transition is gradual depending on the wavelength of the sound, so the loss in forward radiating energy is frequency dependent and baffle width dependent.

Diffraction on the other hand is caused by the same sidewards moving acoustical energy, primarily surface waves which reflect from sharp edges, the abrupt change of a baffle edge (or bad recess of a tweeter) causes a reflection traveling back and interfering with the source. This is also wavelength dependent, high frequencies (i.e. short wavelengths) feel this abrupt changes and are therefore more sensitive.

The back wall or the room boundary can reinforce the sound energy by reflecting part of the backward radiating sound energy. Therefore, in some cases you do not need to fully compensate for the baffle step loss as part of the 'lost' backward radiated energy is recovered.

[NoDestiny]

In short: No.

In more detail:
The bafflestep is the transition of predominantly 2pi sound radiation to 4 pi sound radiation. The total acoustical output is not only radiating forward (2pi) but also to the sides and back (4pi). This occurs when the wavelength becomes long compared to the baffle width. The sound wave does not "feel" the baffle at all and easily radiates to the sides and the back of the speaker. This transition is gradual depending on the wavelength of the sound, so the loss in forward radiating energy is frequency dependent and baffle width dependent.

Diffraction on the other hand is caused by the same sidewards moving acoustical energy, primarily surface waves which reflect from sharp edges, the abrupt change of a baffle edge (or bad recess of a tweeter) causes a reflection traveling back and interfering with the source. This is also wavelength dependent, high frequencies (i.e. short wavelengths) feel this abrupt changes and are therefore more sensitive.

The back wall or the room boundary can reinforce the sound energy by reflecting part of the backward radiating sound energy. Therefore, in some cases you do not need to fully compensate for the baffle step loss as part of the 'lost' backward radiated energy is recovered.

Excellent information. Thank you.