Audioclinic (Apr. 1980)

Designing an Infinite Baffle Speaker System

Q. I intend to build a 9.5-cubic ft., infinite-baffle enclosure, employing a 12-in. woofer, an 8-in. midrange, and a 3-in. tweeter. I will use crossover points at 800 and 5,000 Hz.

I have a friend who has this identical setup, and 1 like the tight, gutty sound his system produces. I feel, however, that some male voices sound slightly "honky."

My friend, not being an engineer and unable to obtain the necessary information, took pot luck with a separate compartment which he built around the midrange and tweeter speakers. This compartment measures approximately 20 x 7 inches, with a partition separating the two compartments. No absorbent material is used in the tweeter side, and only two walls of the midrange side are lined with 1/2-in. thick material.

The rest of the enclosure is adequately lined with 3-in. thick absorbent material. I believe that the "honky" sound might be caused either by insufficient space in the midrange compartment or by improper or inadequate use of absorbent material. Should more absorbent material be used? If so, on how many walls?

- Name withheld.

A. There can be a number of causes for the "honky" sound you have mentioned. Such a sound is probably the result of some kind of peak in the response. A term such as "honky," how ever, is vague, so there isn't much clue as to where the peak may be occur ring. It may be the result of cone breakup in either the woofer or the midrange speakers. It also could be the result of a lack of absorbent material in the midrange compartment of the en closure. This lack of insulation may be more of a problem if the midrange compartment is rectangular in shape.

The more a compartment approaches such a rectangular shape, the more likely it will be for a resonant frequency to occur directly as a result of the shape of the compartment--an organ pipe effect.

It may not even be necessary to sub divide your speaker enclosure, especially in those cases where the mid range and tweeters are of the "sealed-back" design. Even if such a design is not employed, if the cones are relatively stiff, they will not be affected significantly by the excursions of the woofer cone.

Assuming, however, that it is necessary to retain the midrange compartment, I suggest that you do not partition it to provide a tweeter compartment. Further, I suggest that you use some sound-absorbent material on all four walls, the exact amount of which will depend upon the acoustical results you hear.

The "honky" or "chesty" sound of which you speak may be the result of the woofer having a resonant frequency which falls within the mid-bass portion of the audio range, say 90 Hz.

There may be no cure for this unless you are willing to try some mass loading of the cone. This means putting some clay, or other substance which can be readily removed, in the center of the dust cover. Sometimes it is a good idea to spread out the added mass over more of the cover which helps reduce cone breakup as well as lower the resonant frequency of the woofer. Mortight or similar material works well for this application.

If woofer resonance is not the problem, it is likely that the cabinet itself contributes to the sound. Perhaps there is not enough bracing on the back panel or even the baffle. These panels should be very solid. If they vibrate, they will add sound of their own.

If the cabinet is not the problem, chances are that the sound-absorbent material is either inadequate or insufficiently applied. Be certain that the material used is intended for the absorption of sound. A lot of experimenting must also be done to insure that you have used the right amount.

Sometimes a boomy quality can be the result of a simple thing like using a wire gauge which is too small. This will reduce the damping and, in turn, will exaggerate any resonant peaks that the woofer may have.

Even though it does not relate to the problem as you have stated it, I must mention this: Many of the woofers available today have such low, free-air resonances that their cone motions cannot be controlled in cabinets of 9.5-cubic ft. volume. The result could be premature bottoming of the voice-coil on strong bass passages.

(Audio magazine, Apr. 1980; Joseph Giovanelli )

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