How is fs measured?

[IPunchCholla]

So I was reading the do speakers burn in thread. While I’m of the camp that they don’t ( anyway if one is going to invoke science the burden is on the people that claim it exists to prove it since it is impossible to prove its absence) one point came up that was interesting. Fs shift in woofers.

But I wasn’t able to find how changes in fs are instrumented (how is it actually measured). Or what the units are, since by definition it should include hz, mass, and maybe power?

Are there any good threads in fs for casual audio people?

 

[Adam_M]

There's DIY rigs you can build, but the shortest path to Fs is to buy a DATS from Parts Express. If you want to know the Fs of the woofer itself, you'll have to pull it out of the box and (ideally) disconnect it from the XO. IF you want to know the whole system, you can see the tuning of a vented box as the low point between the 2 impedance spikes, and a sealed box the tuning frequency will be the top of the single impedance spike.

 

[IPunchCholla]

Thank you! That helps. Definitely will look into getting one of those as I get closer to building my own speakers.

I plan on getting one of the books recommended for those of us starting out on this journey, but for now, I’m still trying to pick up on what all the lingo means.

Am I correct in understanding that the DATS will measure fs by measuring resistance against frequency for different voltage levels?

 

[Adam_M]

It doesn't (by default at least) measure at different voltage levels. It measures at "small" signal levels. I'd have to read the manual to see if it can alter voltage levels (I suspect it doesn't) and what voltage level it measures at.

Woofer break in is real*, and it happens very quickly - within an hour or 2 of being driven to about 75% X-Max signal near Fs, but after that, it's stable in my experience.
*This is different than speaker break in, which I've not experienced in any speaker - commercial or DIY.

There are things that'll change it like voltage drive level and voice coil temperature, series resistance before the woofer in the XO, but in general, I haven't found it necessary to design around any of those although if the box modeling tool allows it and I have an idea of the resistance of the series inductor, I'll include it. ...Don't put a resistor in series with the woofer.

I run the woofer at it's stated Fs for a couple hours, measure the T/S parameters, and design away. Sealed box tuning normally comes out almost spot on perfect, while ported is often juuuust a bit lower (I'm talking literally a couple of Hz) which I'm led to understand is caused by turbulent flow along the port walls. This is magnified on slot ports, especially ones with a wide aspect ratio. I've actually started making my ports arbitrarily slightly shorter to try to nail the tuning. It depends by the port though.

 

[IPunchCholla]

Thank you, that is incredibly helpful!