SL2 impedance scans and questions

Posted by: jfritzen on 30 May 2018

Hello,

perhaps someone can give me a hint:

As part of a small project, I've recently made impedance scans of my SL2s. Tweeter and mid/bass driver were measured separately, i.e. without passive crossover, and in-place.

So far the scans look sensible to me. However, there are some questions regarding the curves for the mid/bass driver (left and right driver, the impedance in the picture is shown on the right y-axis):

 

I think one can see that the drivers are relatively closely matched.
 
Two questions:
 
If I understand correctly the mid/bass driver in a ported speaker is supposed to show two large impedance peaks. However, there is only one large peak. Does this mean that the SL2 is not a ported speaker, despite the gap between the two enclosures, and should be seen as a sealed speaker? Or is my measurement incorrect?
 
And, just out of curiosity, there is a small peak at around 570Hz. Where does this come from? Certainly not from the port? The peak can be seen in both drivers, so the reason must be in the design and not a fault of the driver.
 
KR
Jochen
Posted on: 30 May 2018 by james n

Jochen - have a read of Colloms measurements in the link below which mentions a diaphragm resonance at a slightly lower 540Hz

http://www.tomtomaudio.co.uk/reviews/Naim%20SL-2.pdf

Posted on: 30 May 2018 by Richard Dane

Jochen,

in spite of the gap between the top and bottom boxes on the SL2, it behaves like a sealed box - the gap is tiny and precisely defined between two perfectly flat interface plates (and a PAR filter) and means that a gasket between the separate boxes is unnecessary. It's a really clever design.

Posted on: 30 May 2018 by jfritzen

James, Richard, thanks for the answers.

The 540Hz resonance in the article indeed seems to match the peak in the scans. I wonder if the dent in the frequency response is noticeable; so far it hasn't annoyed me.

And Martin Colloms also refers to the design as sealed box, so I will treat the SL2 as a sealed speaker. It really seems to be a clever design, the SL2s have a very musical bass.