Possible to reduce peak in fullrange driver?

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True. OTOH, that applies with equal relevance to electrical correction, be it passive or active. Technically speaking it's preferable to address as many issues as possible mechanically; if you can linearise the inherent behaviour without causing problems elsewhere, so much the better. The skill-set and experience needed to do it well however are beyond a lot of DIYers today, though it was more common back in the '40s - '60s. Lots of practice, lots of wrecked drivers, often a fair bit of money. Not a cheap learning curve for the most part. ;)
 
Hi-Fi loudspeaker factories are so incompetent they never used this proven tweak which are used by car audio hobbist since 1990s ever with plenty of success, its even an old trick, but hi-fi ''engineers'' are completely unaware.

A few minor issues with that:

-Most 'loudspeaker factories' are not incompetent. They are in fact very competent indeed at producing product to a specified set of requirements. There are certainly incompetent designers and manufacturers, but not many, since they quickly go out of business, or improve their design and marketing skills.

-You don't mention which 'proven tweak' you are referring to, but looking at the thread you link, I see a number of items stated, so since I have a minute, it's worth going through them.

Firstly, it's worth noting that midrange or HF peaks in a wideband drive unit measured under IB conditions are inherent to the driver, and are neither caused by, nor significantly affected by, what goes on inside the vast majority of enclosures. There are a handful of exceptions, but in most cases this applies.

-Rounding box internals does absolutely nothing to the inherent direct forward radiation of a moving coil wideband drive unit unless it happens to redirect a very strong reflection away from the moving components, or is in such close proximity to said driver that it is playing a roll in mass-loading the driver and preventing it moving or resonating as intended. If that is the case, you are addressing the effects of poor box design, not issues with the driver's own response.

-Painting the inside of a box does nothing practical to the internal air load at audio frequencies. It will provide a tiny amount of mass-loading to the panels, how much depending on the type and quantity of the paint or dye used, and an equally tiny amount of absorbtion in the former case, which is miniscule compared to, say, acoustic damping material. This has zero relevance to or effect upon any midband or HF response peaks that are inherent to a given drive unit. If we were talking stealth technology and the radar-absorbing paints applied to certain aircraft, e.g. some models of the U2, the defunct SR71, F117A etc., then certainly such coatings are beneficial. This doesn't really apply to the majority of loudspeaker enclosures though, and the relatively long wavelengths involved.

-Careful damping of the magnet system can indeed reduce reflections, although care must be taken not to cause problems in doing so, e.g. restricting the spider / suspension, blocking airflow excessively & causing excess heating etc. This may, or may not, affect midband or HF peaking depending on what the cause of this is. Most of the time they are the result of cone or suspension resonance rather than other factors. The appropriate material depends on the available space and the frequency / frequencies you are attempting to absorb.

-Stuffing the box will reduce internal reflections & resonances. These are rarely a cause of midband or HF peaking however, and definitely not the cause if said peaking is inherent to the driver. The main effect of stuffing a vent is to simply reduce output from a vented box, which large amounts of internal stuffing will also do. Excess stuffing in close proximity to a drive unit can mass-load the moving components and prevent it functioning properly, potentially causing additional unwanted response deviations.

-Open cell foam in a box does not raise bass response. Since it is a form of damping material, it damps output. Closed cell foam, which merely takes up volume, will in most cases reduce lower bass output while possibly putting a peak in the mid / upper bass, depending on what the original alignment was.

-Adjusting the toe-angle to obtain a desired balance, or closer to that, is good practice and well-worth experimenting with.

-Car damping material will damp output rather than increase it, how much and where depending on its absorbtion properties, the quantity used and the location within the enclosure.
 
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