Excellent.....I started many years ago because I couldn't find an amp that I could
afford or liked the tone of. And ever since the struggle has been to overcome
the technical assumptions and fill in the gaps, like most DIY.
Love the thread where you pick up on something..... more tools, more shop goals...
Isn't that the point, tune the combo to the speaker, you see those dumble based
things with the whacked out rear panel, find the reason why.
The application in the glass audio was for a Hegeman sub woofer.
The thing was tuned with different length stub tunnels, quarter wave length stops
at specific freq. So to build one you have to adjust the Q of each stub to
flatten the response curve. Why not use it to figure a guitar combo.
FWIW- I've tried to calculate the Dumble cabs according to modern T/S based calculations, and the results are lightyears out in terms of a reflex cab.....
Seems more to be based on old assumptions about reflex cabs - port area equal membrane area. It it is really more of an open back cab, although somewhat restrained.
In order to get all the necesseary param's for an element, you also need a closed box of known volume. There are prog's out there that does all the measurements, - and cab design,- in one package. You need a PC with a reasonable sound card and a measurement jig and a small amp. Speaker Workshop is one of several free ones, - much more if you pay...
Aurora wrote:In order to get all the necesseary param's for an element, you also need a closed box of known volume.
Alternatively the params can be calculated by affixing a known mass in the vicinity of the voice coil and repeating the tests. I've used modeling clay. Either way the test requires some preparation. Getting the data from the manufacturer is the easiest by far.
I still think it can utilized by a DIY to figure what is happening to a driver in a combo.
Any data is better than no data, even if you have to test outside to eliminate room
interference. knowing the resonance of the combo can help tune the amp.
say you know there is going to be a nappy bass resonance by looking at the
drivers response sheet, You can accommodate for it with the dimension and
the material choice of the combo, and then test for it. And/or tweak the amp for it. and test for it.....
two celestion g 12 m on a pine speaker baffle in a open back pine combo
have a resonant peak around 72 Hz, not too far off from the specified 75 Hz.
Simply closing off more of the back of the combo dropped the resultant
resonant frequency to around 65Hz.
Seems like small change... but on the table of harmonics, it's the difference
between "D" and "C". What does this do to the musical presentation of the cabinet?
This makes for an interesting approach the looking at speakers.
If you assume that a driver will follow the same behavior; the open back should
bring the resonance down to a little less than the free air resonance of the driver.
and then you should be able to bring the resonance down critically with
partial closure of the combo back.
so lets assume you got say Jensen C12Q, the resonance is 87.3Hz, (F#).
with installation and a partial back you should be able to get the system
down to around 82.4 Hz (E) , seems like tuning a kick on a drum set.
Even if its a totally misguided set of assumptions, you have a way to test it.
Yes, I have often wondered how the rear oval port idea came about.
I know on Hi Fi speakers you can make or break how the speaker sounds by not having something right.
Has anybody here experimented with that design to see what different sized ports sound?
I wonder if they were calculated out or somebody (like HAD) just thought it was a cool idea and built them?
By varying the size of the hole on the backside, you alter the point where the lower frequencies manages to get around to the front and short-circuit acoustically... Like one does with an bass-reflex port on a hifi speaker...
It should be calculatable with the t/s parameters, volume of the cabinet and size of the port...
Here's a thought experiment using an Eminence 12" Cannabis Rex. T/S params were from the data sheet and the plot was done with WinISD.
The cabinet is a 1x12, using Lopoline dimensions of 17Wx11Dx15H which works out to about 1.15 cu ft internal with 3/4" construction. So I used 1 cu ft for the internal volume.
Two examples were done. One closed cabinet and the other with a 6x15.5x0.75 inch port, like you might get if you made a convertible or open back cab with a 6 inch gap in the back.
I don't know how accurate this thing is with such a large port. Anyway here we go: The yellow plot is the closed back cab and the green plot is the open back cab.
In order to get a non-peaky response out of the open back cab, and flat to about 80 Hz the volume needs to be increased to 4 ft or more with tuning at 80 Hz. As-shown tuning is around 170 Hz. Generally guitar speaks are selected for something other than bass response.
