Taming the Lambda 1.5khz peak

Since I acquired my Lambda NB a few years ago and removed the rotten material behind the drivers, I was always curious about how it influenced the sound. The various Stax threads on the other forums were not a big help, this is what my NotebookLM instance primed with the Stax threads on HF and HC says about the topic:



Once I started accumulating different Lambda models and measuring them, I became convinced that Stax used rockwool or foam damping to tame the 1.5khz peak common to most Lambdas. I then started idly looking for the best material to replicate the effect. I tried a few different ones with not a lot of success until I happened to use some generic foam in one of my orthos, and it did exactly what I was looking for: tame the region between 1khz and 2-3khz.

Testing the approach on a Lambda took me a while as life happened, but finally the other day I brought out my Nova Basic which have the worst peak across all my Lambdas together with the 207. I only had 10mm foam, and that worked so well as to completely crush the 1khz+ area. Back to the drawing board, I found some 3mm foam and ordered it. It arrived today, and this is the result (green unmodded, red modded):



The peak has disappeared, and the 2-3khz area has been smoothed out. The Nova Basic now sound remarkably similar to both my L500:



The 5mm foam is too thin to stay close to the driver, so I had to add some acoustically transparent foam behind it to act as a spring. In this pic you can see the black foam (it's so open you can see through it), the 3mm foam, and the much thicker foam I had used before which completely killed mids:



Happy to hear about others' experiences taming the peak, or on the effect that different materials have on frequency.

Edit: the rockwool in my NAD RP-18 has a similar effect, centered slightly lower in the frequency range; it should be the same vintage as the one used on the Lambda NB.

 

Could you share a link to the 5mm foam you ordered, and/or its specs? 1-3kHz can indeed be a tough area to deal with, it'd be good to have more material options.

 

I actually misremembered, it's 3mm. This is the ebay item I bought, probably not convenient to have it shipped from italy but I'm sure there's plenty similar in craft stores there.

https://www.ebay.it/itm/115458649762