Tweeter Above Enclosure - Diffraction Issues?

I'm in the process of building a 2-way speaker and wonder if placing the tweeter directly on top of the enclosure would be better or worse than flush-mounting it on the front baffle? In this scenario I'm talking about a standard tall, narrow style bookshelf enclosure with rounded edges.

Looking at diffraction theory, would I get two dips in response - one based on the width of the baffle, and another based on the width of the tweeter? B&W uses this style in many of their speakers and I am wondering what, if any, benefit is this design?

http://www.bowers-wilkins.com/display.aspx?infid=2272&terid=2277
 
tschanrm said:
I'm in the process of building a 2-way speaker and wonder if placing the tweeter directly on top of the enclosure would be better or worse than flush-mounting it on the front baffle? In this scenario I'm talking about a standard tall, narrow style bookshelf enclosure with rounded edges.

Looking at diffraction theory, would I get two dips in response - one based on the width of the baffle, and another based on the width of the tweeter? B&W uses this style in many of their speakers and I am wondering what, if any, benefit is this design?

http://www.bowers-wilkins.com/display.aspx?infid=2272&terid=2277

I measured a system brought to one of the DIY New England events that was made this way. The tweeter was mounted in a small circular baffle supported at the top. There was a diffraction problem, but only a single serious one. It was related to the space that is between the tweeter and the top of the box. It doesn't look like it, but it's enough of a cavity that a dip of several db occurred, centered around 7KHz. It was easily reduced with a quick test of felt placed there.

Dave
 
tschanrm said:
Thanks for the link, unfortunately nothing much talking about hwy they use a top-mounted tweeter design.

Dlr, you mentioned a gap between the tweeter and the enclosure - are you saying the tweeter was resting on the enclosure, or that it was permanently mounted on the enclosure?

It was mounted on the enclosure, not flush with the front baffle, but slightly offset to the rear to bring the acoustic centers closer to aligned. The overall response was good with the exception of the dip in response.

It seems odd at first that there could be some reflections here, but tweeters having wide dispersion below about 10K, enough to create the problem, though it can be dealt with.

Keep in mind as well that there will still be a "baffle step" for the tweeter, only it will be much, much higher. Just the faceplate alone creates a baffle with diffraction, though it will be primarily the step portion. The low end extension of the tweeter will be reduced due to the higher step.

Dave
 

Caferacer

Member
2007-04-23 2:49 pm
"I measured a system brought to one of the DIY New England events that was made this way. The tweeter was mounted in a small circular baffle supported at the top. There was a diffraction problem, but only a single serious one. It was related to the space that is between the tweeter and the top of the box. It doesn't look like it, but it's enough of a cavity that a dip of several db occurred, centered around 7KHz. It was easily reduced with a quick test of felt placed there. "



I have built some speakers that looked like a B&W 801. Small box with a 6.5 mid in it on top of a 3ft sealed box with a 12" in it. And then I used a Morel "top mount" MTD41 on the top. It needed some felt on top of the mid box for optimal sound.

Wow, can I say "top" any more!