official product request

[DrWho]

It's a gross calculus equation to calculate polar response, but I just use charts to get me close.

 

Something like this:

 

"ka" is an acoustics term that refers to relative wavelength. A 15" diameter driver has a ka of 1.8 at 1kHz. Here is the calculation:

1132 ft/sec / 1000 Hz = 1.132 ft wavelength, which is 13.584 inches. The 15" driver has a radius of 7.5 inches.

 

13.584 / 7.5 = 1.8

 

At 45 degrees off-axis, you can see that the polar response is almost 6dB down. In reality, the polars are a bit narrower because a 15" driver isn't a perfect piston - it's actually an inverted cone, and that causes the beam pattern to narrow a bit further (it imparts some inward facing velocity is how I think about it). I know this because I built a few cabinets and measured polar response in an anechoic environment

 

 

Also, these polar plot charts are assuming a piston in an infinite baffle. In order for the infinite baffle assumption to be valid, it needs to be 10x the size of the wavelengths in question. At 1kHz, that would be a baffle that extends more than 10ft in each direction....which is probably bigger than your room. Now the corners and walls and ceiling are acting like a poorly coupled horn and all of the polar calculations go out the window....

 

This pushes us into trying to minimize comb-filtering, which is accomplished by keeping boundaries << 1/4 wavelength away (the first boundary cancellation happens at 1/4 wavelength). However, as we go lower in frequency, that distance increases and we eventually reach a point to where the reflection doesn't have enough energy to create a null. This is because the sound level attenuates as you get further away, and the only way you get a deep null is if you have equal amplitude with opposite phase. The opposite phase is still there, but not the equal amplitude portion. Basically, if your cabinet is wider than it is deep, then the diffraction effects from the corner of the cabinet are reduced if the cabinet is placed up against the wall....because the wall is continuing that "infinite baffle" thing for you. It's only when you get a tall and narrow, but deep speaker that you start to have problems - like all of the modern Klipsch floorstanding speakers. Sound just wraps around those cabinets all day long. This is why they sound better pulled out into the room (it gets those reflections attenuated so they can't comb-filter). It also improves the time-arrival nature of the acoustics too (a cleaner direct sound).

 

At the end of the day you still see a 1 to 2 dB ripple in the frequency response from a wide/shallow cabinet, but this in on par with the ripples in the driver's raw response. Up at the top end of the passband where you're crossing over to the tweeter, there really isn't much change.

 

The bigger difference in my opinion is what happens when you pull the speaker out from the corner, or push it back into the corner. The nearfield loading of the room has a much larger impact on the lower frequencies, and that will most certainly affect how I voice a system. However, the equalization employed for those effects occur at the lower frequencies way below the xover frequency...trying to fix those effects with the xover design is more of a band-aid in my opinion. I know people do it all day long and are happy with the results, but I think that's why those results aren't experienced the same way by everyone (because we all have different rooms and different music).

[DrWho]

If you can live with a 12" deep speaker, Klipsch already has what you're looking for in the KPT-325-B.

 

I bet that sounds really good.

 

If you wanted to go shallower than that, then the tweeter would get shorter and you'd end up with less horn and more direct radiator in the system...that's what I was trying to get at earlier about wanting bigger speakers. More of the passband should be hornloaded.

 

One could always recess the speaker into the wall if they want it to be shallower. You could probably drop that 12" down to around 8" without much effort. Heck, bringing the screen 12" closer just makes the image bigger....and leaves some room for acoustic treatment behind the screen too.

[Paducah Home Theater]

One could always recess the speaker into the wall if they want it to be shallower.

The problem I ran in to with my room is that right in the center of the wall behind the screen there is a stud, that has plumbing in it no less, and on top of it there is a beam for my hip roof pressing right up against it. Cutting it out and making a header makes me nervous. I really needed a standalone enclosure that went outside of the all. Ended up building out from the wall 8". May try to re-engineer the joist though, that would gain me 3.5" if I could take it out without destroying the drywall on the other side, which also makes me nervous. I don't see how I can get the thing out safely with no drama.

Edited December 16, 2015 by MetropolisLakeOutfitters

[Paducah Home Theater]

You can't talk about baffle step compensation outside of the polar response of the drive units themselves. A 15" woofer at 1kHz is going to have the same acoustic output regardless of the baffle because the driver itself provides a 90x90 polar pattern (no sound is hitting the baffle). You really gotta get down to about 300Hz before you start seeing any appreciable baffle gain from a 15" driver.

Just now figured out how to calculate this. 13,512 / 15 = 900 hz. So, above 900 hz it will begin to start beaming, everything below is apparently fair game. Not entirely sure how to figure out just how much it is beaming as you go up in frequency.

http://www.soundandvision.com/content/dispersion-show-and-tell#y1vmTv9HsKGEFdmK.97