How much does Center-to-Center distance matter

[scottjoplin]

Go for it, I'd be interested, in a newer study.

 

[MerlinGS]

Thanks for the discussion, it was very helpful. Time to get on with my project.

 

[gadut]

for woofer-midrange ctc distance that has around 70cm like used in YG Hailey, i calculate the 1/4 wavelength rule gives 120Hz xo point. any downside of using 1/2 wavelength rule applied for higher xo point 240Hz?

 

[GM]

Not if you're far enough away for it to 'measure' a ~ > 1/4 WL, so obviously will degrade as one moves closer in.

 

[planet10]

..will degrade as one moves closer in.

basic trigonometry.

dave

 

[GM]

Only I failed it, big time, so please do the math to calc the distance required.

 

[planet10]

Draw a triangle between 2 drivers and the ear. As the ear gets further away the angle becomes greater and the math says that the delay between the 2 drivers decrease in relative terms.

dave

 

[gadut]

with my seating distance 2.5m then i should not worry rite, both woofer and midrange should already blend nicely

notes : i don't do the trigonometry calculation yet

 

[Sonus Praestantia]

How would this be possible in my scenario?

I have a 22” Radial Horn and a separate enclosure with an 18” Driver for Midbass in a two way system.

No matter the way I do the math, the 1/4 wavelength issue means I must set the crossover point lower than the horn will load.

Isn’t this more for say a bookshelf speaker with a 5” driver and 1” tweeter?

1/4 wavelength separation ensures that even at 90˚ off axis (the worst case example), the two drivers will still sum to an amplitude at -3dB relative to the on-axis response, which is still acceptable.

By the time you get to 1/2 wavelength driver separation, there would be a null created 90˚ off axis (assuming both drivers have the same off-axis response), with varying degrees of attenuation between 0˚ and 90˚ from the axis. This isn't likely to be a serious or even noticeable issue for most situations but it is advisable to avoid such design flaws if possible.

Using a 1 wavelength separation or more is asking for trouble as the nulls and attenuation quickly become an issue within 45˚ of the axis. This can degrade the direct sound quality as well as the reflected sound. See diagram for a visualization of nulls and amplitude variation:

 

[planet10]

Isn’t this more for say a bookshelf speaker with a 5” driver and 1” tweeter?

No.

dave

 

[planet10]

The midTweeter has to be heroic to achieve a quarter wavelength centre-to-centre. Or clever cabinetry.

What is the lowest the horn can go?

dave

 

[Sonus Praestantia]

@planet10 Ok- so how do I physically cross over a 22” horn and an 18” Woofer at a 1/4 wavelength correctly then?

I’m struggling to understand this concept when you move to larger drivers

 

[Sonus Praestantia]

The midTweeter has to be heroic to achieve a quarter wavelength centre-to-centre. Or clever cabinetry.

What is the lowest the horn can go?

dave

I have the best response using REW at 388hz. It’s an Oris 250

 

[planet10]

What is the lower limit of the horn. How deep is it?

dave

 

[Sonus Praestantia]

I believe 300hz is pushing its limits- and it’s 60 Centimeters deep (23.6 inches)

 

[planet10]

388hz

About 9”.

Depending on the shape of the horn, and the driver basket, i can imagine 12-15” (guess) centre-to-centre.

It would be easier to achieve the goal with 2x15” or even better 4x12”. With even numbers you can load them push-push and take advantag eof active force cancelation.

dave

 

[planet10]

I’m struggling to understand this concept when you move to larger drivers

its all related to wavelength and wavelengths get longer at lower frequencies.

dave

 

[AllenB]

I believe 300hz is pushing its limits

If you are prepared to cross it this low then you may get 1/4 wavelength. I wouldn't want to use it this low and would be expecting to cross it without 1/4 wavelength spacing.

I have a 22” Radial Horn

This is not a radial horn, but is axisymmetric. I cannot find a description of the flare.

Polar measurements will tell you when this is at 90 degrees, which is a region where it becomes useful to cross.