T/S Measurement Question - Delta Mass

[Amphiprion]

Hey guys,

For those of you that use delta mass to measure T/S on large woofers (10", 12", and 15") what would you say the 'ballpark' mass is that you use? I know it will vary due to suspension compliance to get the desired ~30% shift. I usually use nickels (as in five cents) as a 5g incremental masses when doing 5-8" woofer T/S measurements. What is a practical mass increment for subs? Or should I just do delta compliance? Thanks!

[ThomasW]

JohnJ from AE Speakers has an interesting take on this ...

http://www.aespeakers.com/phpbb2/viewtopic.php?f=4&t=927

[Amphiprion]

I'd heard that delta mass had problems on large drivers, and those graphs are enough to convince me I've got to go to delta compliance. Every now and again I'll see a wrinkle on my T/S impedance measurements for smaller drivers because a nickel wasn't blue-tacked down properly; it looks like it gets much worse for larger drivers. Time to build some boxes!

[Dave Bullet]

I've done 12" JVC drivers with reasonably smooth impedance curves. I find what helps is:
1. Holding on the mass well (as you noted)
2. Clamping the driver magnet and have it in the horizontal (normal) firing position
3. Removing any reflective surfaces (front and back) from the driver

I recall using about 40-60gms for a 12"+ driver (not knowing its mms).

I try and use larger coins for the larger drivers. Less blu-tak to remove.

The problem reported with delta mass is usually the loweing of Fs is sometimes problematic for some soundcards below 20Hz. Not on mine however (and mine is some cheapo PCI-16 SB clone).

I've heard deltamass is more volitile when it comes to humidity and temperature variations than delta compliance..... but when it comes to determining box alignment does it matter? I've found with the limited testing I've done, Vance Dickason's comments about T/S parameter ratios hold, even though they may not match manufacturer specs and the box design curve ends up being within a few percent of both manufactuer and measured deltamass specs.

David.

[JoshK]

Timely....I was planning to see if I could measure the T/S of the 3 Lambda 15TDX's I have on hand. One pair were reconed and have a different cone then original, the 3rd is the original, so I wanted to see how much that changed everything.

I am not sure I will be able to build the appropriate enclosures for delta compliance measurements though, but it is still interesting to read John's comments.

[JonP]

There's a stuff called "Quake Hold" which is a "museum putty" type of product.

It's pretty heavy in its own right, for smaller drivers you don't need extra weight. But it's very sticky, better at holding weights than other stuff I've tried, and it's extremely non-contaminating, leaves next to nothing behind. Might help things to hold better... since it all gets harder as you scale up the size and weight used.