Designing bookshelf speakers

Sealed, vented, bandpass, etc.

Designing bookshelf speakers

Postby calfinn » Sat Mar 06, 2010 12:42 am

I want to build a pair of bookshelf speakers. I'm thinking a ported enclosure with a small mid/bass driver (4 or 5 inch) and maybe a soft dome tweeter maybe a 30 watt one. I've made sealed enclosure hi-fi speakers and guitar speaker cabs before but nothing too complicated. I could use some help with tuning the enclosures particularly what frequency to tune the enclosures to and any calculations I need to use. I could also use help figuring out what sort of cross over to use.
Thanks in advance.
calfinn
 
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Re: Designing bookshelf speakers

Postby Klaus Stock » Sat Mar 06, 2010 8:59 am

Hi,

calfinn wrote:I want to build a pair of bookshelf speakers. I'm thinking a ported enclosure with a small mid/bass driver (4 or 5 inch) and maybe a soft dome tweeter maybe a 30 watt one. I've made sealed enclosure hi-fi speakers and guitar speaker cabs before but nothing too complicated. I could use some help with tuning the enclosures particularly what frequency to tune the enclosures to and any calculations I need to use. I could also use help figuring out what sort of cross over to use.


I suggest that you try WinISD (http://www.linearteam.dk/default.aspx?pageid=winisd). It's free. I'm not sure about the "Pro" version, never tried it myself and it's still in an early stage of development.

Next, try to find a few candidates for your misbass driver. You'll need their Thiele-Small parameters, which about every reasonable manufacturer will provide in brochures or on some website. Enter the driver's parameters in WinISD and use the "New Project" assistant to generate a standard vented enclosure. WinISD should come up with the "natural alignment" solution for the specific driver. This is always the best possible solution for that driver, resulting in a maximally flat bassband.

Now, that "best solution" may, of course, be worthless to you. It may turn out that the speaker may not be able to cover the required frequency range. I guess that a bookshelf speaker should go down to around 60Hz, unless, of course, it's designed to work together with a subwoofer. Or it may turn out to be too big.

It is possible to deviate from the "natural alignment". WinISD allows you to play around with box size and tuning, while you watch frequency response and group delay. This will always make the design worse - from the HiFi viewpoint. However, there are popular designs which deviate quite a bit from the "optimum design". For example, it's possible to design a "bass hump" into the frquency response. The speaker may not go as low as with the optimum design, but the "hump" (which could be as high as 100Hz) can trick the ear into believing that the speaker has "substantial bass".

When tweaking the vent, keep the airspeed below 10% of the speed of sound. WinISD will display the mach number, which should stay below 0.1 mach.

Finding a suitable tweeter is less complicated. You#LL need one with similar or higher sensivity as the midbass driver achieves in your box. If the tweeter has higher sensivity, it can be padded down (with an "L-Pad"). As the bass typically consumed much more power than the tweeter, padding the bass is often not practical. The tweeter's frequency respose must overlap with that of the midbass, obviously.

For the crossover, the "usual suspect" is a 12dB/oct passive crossover. WinISD can also help here. The crossoverover point should be (at least) one octave above the resonant frequency of the tweeter. Note that for the crossover calculation, you need the driver's impedances at the crossover frquency. Which is definitly not the "nominal impedance" (well, it may be close in the case of the tweeter). If you don't find impedance plots, you can flatten out the impedance curves by adding a "Boucherot cell" aka "Zobel network".

Best regards, Klaus
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