Help with tuning bass reflex speakers

[normende]

Hi all,

This project is about replacing a two-way configuration on old speaker cabinets for a four-way one.

To do this I had to cut new holes and I set up two tweeters, one mid 8" and one woofer 12".

Now I have to tune the bass reflex and finish the design. I don't know the parameters of the speakers, their coils were replaced also. The cabinets are already built. So, what I understand I need to do is go for the empirical way of tuning a bass reflex based on the resonance frequency measured on the woofer.

The dimensions of the interior of the cabinets are approximately 82cms x 43cms x 35cms (32" x 17" x 14").

There will be silly questions for someone who is into designing, sorry for that. That's why I'm here

These are the questions related to the design and the tuning:

1) The woofers' resonance frecuency is 37Hz, should I tune the cabinet for this frecuency or something close but up?

2) In the 8" mids, should their back be covered to avoid bass to interfere modulating the cone?

3) The absorbing material, should be placed on 5 sides of the cabinet (not the front) or should I ommit another one also?

4) Where is it more convinient to place the tuning tubes, down to one side or somewhere else?

5) About the diameter and lenght of the tube, knowing only the volume of the cabinet is there a way to get initial values without the speaker data?

6) How do I know if I should use one or two tubes?

Ay tips will on any of the questions will be greatly appreciated!

Thanks in advance.

 

[Binkman]

Greetings;
Appears to me you don't have enough cabinet volume (even if I knew the drivers by manufacturer. assuming nothing )

So really we need to know what these drivers are but I think you can eliminate one tweet and go for a taller cabinet. Any data on the wattage of the drivers? wattage is additive but how many watts do you intend on pushing from the amp?

 

[mhardy6647]

Measuring (to a reasonable approximation) the Thiele-Small parameters of a driver (woofer) isn't difficult -- especially with downloadable "signal generator" apps or data files.

http://sound.whsites.net/tsp.htm

Armed with T-S parameters, a decent box alignment should be calculable using either 'cookbook' formulae (e.g., see Vance Dickason's Cookbooks) or web-based calculators or a spreadsheet (or purpose-built loudspeaker enclosure design s/w).

Otherwise, it's hit and hope.

Crossover design is another matter, as will be matching the output levels of the (new?) drivers to the existing ones.

I will say that I don't understand your driver (no pun intended) for this project, but I assume you have a good reason to be doing what you're doing.

 

[normende]

Oh...

And to make things worse, I measured again the interior and I got smaller values: 80cm x 41cm x 30cm (31.5" x 16.4" x 12").

I can not modify these old beauties (I uploaded more pics) so I will have to deal with what there is.

So if the volume is not enough this means I will not be able to tune the speakers to the woofer's resonance frequency?

About the drivers. They are Leea, an Argentinian brand but I had to replace the 8 coils because all of them were burnt so I read that even if I had them the parameters of the speakers will change after such a radical change. The mids are rated 12W, the woofers 20W. I used to tweeters in series so they can split the power among themselves, I saw this configuration in another speakers (original design) which have the same four drivers I am using in this project. That's all I know.

About the amp, it delivers 55Wrms on 8ohm but I will not play them that loud. I listen to Jean Michel Jarre, Vangelis, Alan Parsons, Elo, stuff like that hehe.

Supposing I will never get to tune them, should I just leave them with no window like an infinite cabinet?

 

[normende]

Oh mhardy I just saw your post after replying.

Yes my driver is to save these beautiful cabinets from keep sounding like an MW radio as they were intended to do when they were born!

With a circuit exactly like the one you posted I measured the resonance frecuency of the woofers, just adding a voltmeter to the terminals of the speaker and looking for the frecuency determining the highest voltage on the instrument. And to tune the cabinet I would use the same circuit but leaving the frecuency fixed in that value and playing with the window area and tubes lenght until getting the lower voltage on the instrument.

Interesting link, thanks. I will read it after work and see if I can start following the steps there.

The crossovers I am using right now I already had them. They have two positions for each driver, so I chose where to connect each of them according to my taste which has always remained the same over the years. Lots of highs, lots of lows, not too many mids. Questionable, but this how music sounds more pleasant to my ear.
The results so far have been very satisfactory in mids and highs with this config. Because I still didn't even start working on the lows, I can not tell the level of the final result I can achieve with what I have. No damping material and no tuning and no back cover so far.

 

[normende]

This is the "before":

View attachment 1069820

And this is the "after":

View attachment 1069822

I had to adjust the position of the drivers to make the edges of the previous holes match the edges of the new holes so I could not play a lot with the distribution but I am happy with what it came out.

 

[mark warren]

those are beautiful!