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RE: Opinions on the dual concentric/coaxial driver approach used on some box speakers ? (nt

Posted by tomservo (M) on July 6, 2025 at 05:34:32

In Reply to: Opinions on the dual concentric/coaxial driver approach used on some box speakers ? (nt posted by peppy m. on July 4, 2025 at 06:56:28:

Hi
A coax configuration can satisfy the conditions needed to radiate as if a single simple acoustic source when the dimensions are suitable.
The rules are the same at any frequency and so how things work are like with two subwoofers at low frequencies.

Two identical subwoofers when closely spaced produce 4 times the sound of a single sub (mutual coupling increases the efficiency) and they radiate spherically as they have no directivity at these dimensions.

AS the separation distance is increased, by the time they are 1/2 wl apart or larger, they are independent sources.
This coherent addition can be seen when you invert one of the two subwoofers, when less than 1/4wl apart they cancel each other out in every direction BUT when the spacing is 1/2 wl or more, they radiate independently and the inversion only changes the direction of the lobes and nulls in the interference pattern that is produced.

Wavelength (in feet) is about 1132 feet per second / Frequency.

The lobes and nulls are part of what allows one to localize the distance, a complex radiation pattern (clues that arrive differently at your two ears giving you the ability to hear the source's distance).
This complex radiation can also come from a driver operating above the size where it stops being a simple source. These clues can also be produced by a re-radiation near a small source and is why absorption is often placed around tweeters etc and partly why horns are used for pattern control.

Here below is a way to make a coax where the mids combine with a horn at suitable dimensions, the holes are like a normal synergy horn, they are an acoustic low pass filter and allow the mid to combine where the dimensions are less than 1/4 wl where they combine with the hf compression driver into a single source..

If you are a diy'r, you can make a speaker that radiates as a single point up to a high frequency.
Mount a pair of these drivers on a padded flat baffle at least a foot across. These make a strong stereo image with little source identity, you may not even be able to locate them as a source in a mono phantom image.

https://faitalpro.com/en/products/LF_Loudspeakers/product_details/index.php?id=401000100

• acts as a single driver.   (Open in New Window)

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