Question about speakers...

Hi all!

I have a question concerning speakers. I have heard that if more than one tweeter is used in a cabinet it is considered a poor design because of comb filtering.

Does this apply only to point source speakers or to line array designs as well? I ask because line arrays have multiple tweeters in them (usually ribbon tweeters from what I have seen). Does comb filtering have anything to do with the design of the tweeter (ribbon vs. silk dome, etc.)?

What's the scoop?

Whether or not a speaker design is successful depends on a lot of factors combined. I wouldn't necessarily say that two tweeters is bad - it depends on the specifics of the design. The devil is in the details, but so are the angels.

Lobing and comb filtering occur when two drivers' outputs overlap in frequency and angluar coverage, and might be considered to begin at frequencies where the drivers are more than 1/4 wavelength apart and get progressively more pronounced as the wavelengths get shorter (meaning that the drivers are getting acoustically farther apart, as measured in wavelengths). Comb filtering usually occurs in a line array, but it's taken into account by the designer.

A ribbon tweeter usually has wide dispersion in the horizontal plane but narrow dispersion in the vertical plane, so a vertical array of two or more ribbon tweeters will have less comb filtering than an equivalent array of dome tweeters.

The material that a tweeter is made of has negligible effect on comb filtering. It's the radiation pattern and driver spacing that play the biggest roles.

Obviously, a rear-firing tweeter is not going to have any comb filter effects on the front-firing tweeter.

dealer/manufacter/has been known to use more than one tweeter
Hey there!

Thanks for the response! There sure is a lot to know about that stuff.

Don't believe everything you hear.

It's not possible to sit at an equal distance from all the tweeters of a line array.
Not only the distance between drivers, but also the size of the driver's radiating surface relative to wavelength will cause interference patterns. You'll get comb filtering and beaming with both line arrays and ribbons. (Think of a line array as a discontinuous line source and a ribbon as a continuous line source.) Beaming has the advantage of limiting dispersion and therefore reducing room interaction. Conversely, beaming limits the listening area that has full linear frequency response, and consequently causes the reverberant field in the room to be less linear, affecting the overall tonal balance.

I used to own a pair of ribbon hybrid speakers. The vertical listening area was extremely limited—less than 12" at listening position. I couldn't recline while listening without loosing significant amounts of treble. Panels have this problem to some degree with both vertical and horizontal dispersion.
My Daedalus DA-1.1's use two tweeters to very good effect. The Eton tweeters are offset by 10 degrees so this may have something to do with their superior performance. My understanding is that the idea is to provide more uniform frequency response off axis. The result is a very wide "sweet spot" and a speaker that still images very well.