Investigating my speaker cab...

[LukeFRC]

Inspired by some of the threads recently and my wanting to understand a bit more about speaker cabs armed with a screw driver I attacked my cab tecamp m210 today!
Initially I planned to look at WinISD and plug the numbers in there but after opening it and starting I realised that the whole thing was a bit beyond my ability at the moment...

But I found out....

My cab weighs about 18kg give or take... I knew this already - what I didn't realise is that about 14% or 2.5kg of that weight is made up from the front grill - which seems mental to me! it's more metal than holes if that makes sense.

The box is quite well braced with spars going back to front around the edge panels and two more in the middle.
Taking off the handles and the bracing I would say it has a volume of around 100l

The drivers are Sica Z006900 and the horn has no stamping or anything on it.

The vent is a rectangle with semi circle ends cut in the front baffle, area about 289cm[sup]2[/sup] and 15mm deep. I have no idea what tuning this gives the cab as I don't have a working amp at present I can feed into the cab.

the cab doesn't have very much damping material- there is some egg crate foam damping material, but only on the back wall, not on the sides. What effect would that amount have? I've seen other cabs ranging from speakers loaded in a ball of fluff right up to empty with no damping material.

so while it may be of no interest to you if you've read this far... but it was quite interesting looking inside. I'm intreated what adding more damping material would have to it, and may try and work out a way of making a lighter grill..

[Balcro]

The SICA speaker is on their website. A Neo magnet version rated at 250w. Fundamental resonance "Fs" is quoted as 54Hz. The horn is probably also by SICA as the letters N and TW appear under the heading "Studio Monitor tweeter". Tecamp also refer to an NTW1 horn.

I had a play with the main unit in WinIsd and it has an exaggerated upward twist in the lower end frequency response. Tuning is probably in the 60's (Hz).

I've no idea about that small quantity of foam.

Balcro.

Edited December 21, 2013 by Balcro

[LukeFRC]

[quote name='Balcro' timestamp='1387666025' post='2314453']
The SICA speaker is on their website. A Neo magnet version rated at 250w. Fundamental resonance "Fs" is quoted as 54Hz. The horn is probably also by SICA as the letters N and TW appear under the heading "Studio Monitor tweeter". Tecamp also refer to an NTW1 horn.

I had a play with the main unit in WinIsd and it has an exaggerated upward twist in the lower end frequency response. Tuning is probably in the 60's (Hz).

I've no idea about that small quantity of foam.

Balcro.
[/quote] exaggerated as in it shoots up before it drops down on the graph? hits zero at about 60 and -3 at about 40? if that's the case maybe I wasn't being so stupid with winISD as I thought I was

[Balcro]

Hi Luke,

Nice to see someone else who also finds WinIsd results occasionally surprising.

I presume that the SICA units were in there from new.

I tried a different tack and re-ran WinIsd using the "super boom-box" option under the Choose Alignment option. That produced more conventional results as far as the response curve was concerned. WinIsd left to its own devices came up with a 41 litre box tuned to 54 Hz. If you tweak that up to 65 litres and a tuning of 68Hz the graph looks smoother with very slight rise near 85Hz (a square vent 6cm x 24cm). Probably average for a bass guitar cab - just around the frequency of Low E or F.

Check out Maximum Power & Cone excursion!

These things sure ain't hi-fi!

Balcro

[LukeFRC]

[quote name='Balcro' timestamp='1387723746' post='2314892']
Hi Luke,

Nice to see someone else who also finds WinIsd results occasionally surprising.

I presume that the SICA units were in there from new.

I tried a different tack and re-ran WinIsd using the "super boom-box" option under the Choose Alignment option. That produced more conventional results as far as the response curve was concerned. WinIsd left to its own devices came up with a 41 litre box tuned to 54 Hz. If you tweak that up to 65 litres and a tuning of 68Hz the graph looks smoother with very slight rise near 85Hz (a square vent 6cm x 24cm). Probably average for a bass guitar cab - just around the frequency of Low E or F.

Check out Maximum Power & Cone excursion!

These things sure ain't hi-fi!

Balcro
[/quote]

Ok, so copied what you did with box volume and tuning


and got the same response with the peak at 85Hz



I then Increased the box volume to 100l which is what I estimate mine is (measuring the inside minus handles and struts) and then added my port dimensions.... it's a rectangle with a semi circle on each end, so converting the area of the circle into a rectangle we end up with a square port 30.5x9.5 with a depth of the wood at 1.5 cm.

Which gives me a tuning freq of 80.76Hz,
the graph then gives me a not very flat at all peak of 6db at 88Hz going down to -3 at 67Hz - so that's what I can hear!!!!

I had a look at max power and cone excursion - but I don't really know what I should be expecting to see to compare it... wiggly lines but don't know what they are supposed to look like -max power: steep steep curve up off the graph at about 75Hz, dip down at about 100hz and then up again

[Balcro]

I tried the same as you but ended up with a minus 0.77cm. Is that really a port? So, I changed the port
length to 1.8cm, the thickness of the front baffle, and the tuning dropped to 73.93Hz. The frequency
response peak is now about +3.5 at 80.5Hz. That's a punchy unfretted Lower E.

With a 120watt input signal it reaches xmax at about 60Hz and power handling dips to 227w @ 93.0Hz!!
Less than half the quoted specification, but it's still a normal response. Mind you, with 120w input it's
turning out nearly 113dB(a)!

The pattern of your Maximum Power graph is very much the same. That dip you report at about 100Hz
is the dip in power handling that I've mentioned above. It's slightly different because of the change in tuning.

Balcro.

[Phil Starr]

The power handling curve is what you would expect, it's the sums telling you what we have been saying in words on this forum. If you look at the max power above 150Hz ish then it is a straight line, constant max power. This is the thermal limit, how many watts of power the speaker can disperse before it overheats. This is, or should be, the rating the manufacturer gives a speaker.

At the bottom end the limit is how far the speaker can move, or Xmax. It moves further for deeper bass at the same volume. Try doubling Xmax in the parameters and the power handling will go up on the graph (though sadly not in real life). The big dip comes in as soon as the excursion goes over Xmax.

Because it is a reflex speaker this then has an extra effect at frequencies around the tuning frequency. Here all the energy you put in is spent on moving the air in the cab and the port and the cone almost stops moving and gets nowhere near Xmax, so you are back to heat dispersion being the limit.

[LukeFRC]

thanks for help Phil and Balcro!

[stevie]

The ideal volume for two of those drivers is about 50 litres - so I'm not sure what's going on there. If the box is tuned to 70 or 80Hz, that's not very good either.

As far as internal damping is concerned, you need to line more than just the back panel. Just keep the foam well away from the ports.