thiele and small parameters

[rab1991]

So I've recently accuired a pair of vintage bass/wide range drivers that on early listening seem promising. I'm not sure if the cabinets they came in are original or to be honest exactly what the drive units are. Is there a way of measuring the thiele small parameters of drive units at home so i can start looking at what sort of cabinets they would work best in?

 

[rab1991]

Pictures of said drivers. 15inch with large alnico magnets. Labeled RCA but has made in England cast on the chasis

 

[tuga]

If you have the right RoomEQ will calculate the T/S parameters for you:

https://www.roomeqwizard.com/help/help_en-GB/html/thielesmall.html

Here you'll find a more pedestrian approach.

 

[rab1991]

Thanks for those, i will take a look through tomorrow when i have more time.

 

[karma67]

i use dats v2 which is now superceded by v3.

https://www.daytonaudio.com/product/1650/dats-v3-computer-based-audio-component-test-system

 

[rab1991]

so small update, managed to find someone with a dats who kindly measured the thiele small parameters of these. seem to be well matched for approximately 65 year old drivers





so after some calculations it looks like a 150 ish litre box gives a -3db point of 50 hz and a 250 litre box is -3db at 41hz

blue line is 150 litre box, red is 250.



I've put them back in the original cabinets for now and they do seem to drop off quite early, I've orderded a measurement microphone so can hopefully run a sweep through them and check what's actually happening in a couple of days.

I've also found this entry in the 1958 hifi year book which appears to be the same speaker.
The issue i have is the quoted response of 29hz. It doesn't give a -db figure but going of internal dimensions by the time you take the bracing and driver out it does equal about 150 litres which is what the calculations are suggesting but with -3db at 50hz at 29hz it's -18db!

I wonder if the parameters have changed over the years, but almost seem too well matched for them to have changed equally.

They never played down to 29hz and the year book isn't correct.

Or the t/s parameters measured aren't correct.

Hopefully with the microphone i can check how low they go in the cabinets but i would say at a guess around 50 ish is correct.





I have also Found some Vitavox gp1 compression drivers which im hoping might make a nice hf unit to go with them, if any one has any suggestions on some horns to go with them let me know.

 

[rab1991]

So microphone arrived a couple of days ago and finally had time to try it out today.

Think I've managed to get it set up ok and started today measuring each of the vitavox gp1 compression drivers today that i picked up for this project. I will try and measure the bass drivers at some point next week,

Below is each drive unit with the same settings and all placed in the same position with the microphone approximately 55cm away.

As can be seen there's a huge difference between most of the drivers and I'm not actually sure if there's a decent "pair" in them. I guess this is the peril of vintage equipment or ignorace is bliss.