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Re: Trapezoid enclosures [message #69551 is a reply to message #43890] Mon, 19 September 2011 13:59 Go to previous messageGo to next message
Maxjr is currently offline  Maxjr
Messages: 57
Registered: August 2011
Baron
Wayne Parham wrote on Mon, 09 February 2004 14:34
There's no harm in making your cabinets this shape as long as the cabinet volume is kept the same. All box volume calculations are the same - The Helmoltz resonator formed by the bass-reflex cabinet doesn't care if the volume is formed by a box, a trapezoid or a bottle.

Wayne, I've been thinking about this lately and going toward the direction of building trapezoid cabinets for my 4Pi pair. I wanted to see if anyone has actually implemented this and what their thoughts are, as well as if this is still your recommendation. My reasoning for the trapezoid enclosure is since they will be toed in 45 degrees, this shape will make them more visually appealing as well as functional as one of them will be right next to a walkway.

I would keep the baffle width and height and keep the internal volume and port dimensions the same. The end result would look something like a standard JBL PA cab.
Re: Trapezoid enclosures [message #69552 is a reply to message #69551] Mon, 19 September 2011 14:32 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 17658
Registered: January 2001
Illuminati (33rd Degree)

In general, you can make the shape of the cabinet anything you want. The bass tuning doesn't care, unless it becomes very long and thin, in which case pipe modes setup in the bass, making it a transmission line. But in general, you can make the shape anything you want and that won't change bass response.

But this isn't true at higher frequencies. Midwoofers used in cabinets this size have standing waves in the midrange. Maintaining the Helmholtz frequency keeps the bass response right, but the midrange will be changed when you change cabinet shape, or even the positions of the drivers and port within the cabinet.

It is widely believed that trapezoid cabinets have a tendency to stagger standing wave frequencies, but depending on that really gives a DIYer a false sense of security. It's true, but it doesn't really matter. It only takes one standing wave mode in the midrange to mess up response, and to tell the truth, that's usually the case. So even if you use a cabinet like that, you really should make measurements to see what the response looks like.

The real benefit of the trapezoidal shape is in prosound, where sometimes it is desirable to splay cabinets. You can hang a number of trap cabinets to form a curved line array. Beyond that, I don't see any real benefit. But if it looks cool to you, then go for it. Just be sure to make measurements of your loudspeaker, looking in particular for peaks and dips in the 80Hz to 200Hz range. If it has some, rebuild with the drivers and ports in different places.

Re: Trapezoid enclosures [message #69553 is a reply to message #69552] Mon, 19 September 2011 14:54 Go to previous messageGo to next message
Maxjr is currently offline  Maxjr
Messages: 57
Registered: August 2011
Baron
Wayne,

Thanks for the quick response! I do like the looks of the trapezoid cab. Since I'm building the cabinet from drawings, I thought I would add something different to it. I hope response doesn't change much. I did a search here, on several other audio forums and even google, and didn't find any results of a trapezoid 4Pi cabinet. Are you aware of any previous users making one?
Re: Trapezoid enclosures [message #69554 is a reply to message #69553] Mon, 19 September 2011 15:02 Go to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 17658
Registered: January 2001
Illuminati (33rd Degree)

There have been soooo many builders here over the years, several of them doing their own mods and twists. The most common are just slight dimension changes, but some are towers. Some have made slanted front baffles and some have made angled sides, forming the trapezoid shape you're talking about. But I don't recall seeing any acoustic measurements of any cabinets like that.

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