Weird T/S measurements

[thirdicomplex]

I ordered a pair of MCM 55-5670 8 inch woofers the other day. They look good on paper and they meet my design criteria. I tried to shoot some measurements with my DATS V3 but I keep getting this little bump at around 800hz. The impedance curve looks right. But the software considers this little bump the Fs and sometimes gives really high Q numbers and sometimes nothing for Q. Twice I've been able to get some realistic numbers. I played music through them for about 6 hours today and then tried measurements again and got the same results. Has anybody had this happen to them? Do I just need to break them in more? Or is there something wrong with these woofers?

 

[AllenB]

It might be something simple like a spider resonance, eg. nothing you need to do other than be aware of it when designing.

The problem seems to be the intelligence of the measuring software. If you can't get around this you could do it manually.

 

[thirdicomplex]

Ok. Thanks! I'll keep breaking it in and then take some more measurements. I plan to cross it to a midrange at around 400hz so I don't think it should be a problem in the end

 

[Joel Wesseling]

I'm very interested in the numbers that you come up with for Qes,Qms,Qts and Vas. I've entered in to Winisd the TS Param for about 80% of the MCM drivers and thinking about buying a pair. A few people on forums stated that for certain models their measured Qts was double the manufacturers specs. Somewhat higher Qts is not unusual but double is a little hard to accept. I like the 7.5mm xmax of 55-5670. Cheers!

 

[thirdicomplex]

From one of the sweeps where I was able to get some real numbers: Fs 37.31, Qts 0.4656, Qes 0.6554, Qms 1.607, Vas 84L. With these numbers it looks like I'll need a 56L box to hit the low 40's vs. 28L when modeled with factory parameters

 

[celef]

Can you limit the bandwidth of the measurement to exclude frequencies above ca 750hz?

Also make sure that nothing is blocking the airflow around the driver while measuring

 

[Joel Wesseling]

A few people on forums stated that for certain models their measured Qts was double the manufacturers specs

I should have said "Double the Volume"

Fs 37.31, Qts 0.4656, Qes 0.6554, Qms 1.607, Vas 84L
With these numbers I get a 2db rise with 56L

I'm thinking sealed mid bass crossed low in a 4way, or 3way.

 

[thirdicomplex]

@celef - I'm not sure if I can limit the measurement bandwidth. I'll check later when I get a chance.

@joel - I get a 1db rise in 56L tuned to 35hz. In any case, I may be going sealed as I didn't want to go that big for the enclosure

 

[Joel Wesseling]

Yes, I get 1db rise tuned to 35Hz - with the reduced low end extension.

Wondering if Vas is actually lower.

When I limit the bandwidth when using my software, there is no significant difference with the results.. what should I be seeing?

 

[thirdicomplex]

Limiting bandwidth was for me during measurement sweeps. Celef's idea was to limit the measurement bandwidth so that the 800hz bump doesn't affect the measurement

 

[Joel Wesseling]

Thanks!

 

[GM]

FWIW: MCM Audio Select T/S measurements — MAC/DIY

GM

 

[GM]

They look good on paper and they meet my design criteria. I tried to shoot some measurements with my DATS V3 but I keep getting this little bump at around 800hz.

No experience with DATs, but tall, relatively stiff [at least when new], rubber surrounds can create fairly strong eigenmodes with its odd harmonics being peaks = ~13543/3/800 = ~5.64", so is this ~the inside diameter of the surround?

GM

 

[Joel Wesseling]

Below is what He's showing over at MAC/DIY
If accurate, I want 4 of them!


Spec 55-5670 8"
Re 6.465
Fs 31.63
Le 0.88
Qes 0.4
Qms 1.253
Qts 0.303
Mms 24.31
Vas 59.86
SPL 89.14

 

[thirdicomplex]

I'm measuring just under 5.5" for the inside diameter of the surround. I did a couple hours of break in yesterday and doing a couple more today. I will remeasure either later today or tomorrow morning. Hopefully I can get some numbers more in line with factory spec

 

[GM]

OK, the resonance will occur somewhere between the surround's base and top, so likely the reason.

Maybe better to do a current measurement: Read Research - Articles - Measuring Thiele-Small Parameters - Page 3

GM

 

[thirdicomplex]

After a healthy dose of nothing but low frequencies, yesterday and again today, I was hoping for some lower numbers. But the woofers measure the same. At least both woofers measure pretty close together in spec. The resonance at 800hz is still there but I've found if I have the driver at a slight angle, I can get a valid measurement

 

[AllenB]

Sometimes these resonances happen, are you expecting it to disappear completely? If only the software wasn't fixated on it..

 

[thirdicomplex]

I thought with some break in it might have been at least reduced to the point of not affecting the measurement. Just wishful thinking

 

[AllenB]

All I'm saying is that it's anyones guess what the software is 'thinking', and that it is not a blight on the validity of your data.