Bookshelf Speakers

I will present you here a 2.1 speaker project made from some leftover materials. Nothing fancy but i will try to get the most out of everything. In this first part i will give information about the overall design and the first work on the satellite speakers. Drivers i used were those i had laying around, a pair of 8″ woofers with nothing written on them, probably Chinese stuff (I will have to measure some of its parameters), a pair of 6.5″ mid-bass drivers i saved from some commercial speakers and a pair of horn-loaded soft dome tweeters from PAudio model PCT-300.

I plan on using the two 8 inchers for a subwoofer, i will have to measure some parameters to determine what box to build. The satellite speakers will have one mid-bass driver and one tweeter in a closed box. The box should not be bigger than 10l in volume.

Some of the Thielle – Small parameters of the 6.5 incher mid-bass are as follows:

Fs: 65.41Hz, Qts: 0.62, Qes: 0.76, Qms: 3.43, Vas: 18.9l, Sd: 150 cm^2, Xmax: 3mm, Re: 7.4 ohms, SPL: 90dB/w/m, RMS Power Handling: 50W

6.5 inch mid-bass

A closed 10l speaker models with a -3db band starting 85Hz with a Qtc little over 1. Below 85hz there is a 12db per octave attenuation so it will be pretty easy to match it with the subwoofer. I can lower the Qtc with heavy fill hopefully to a value below 1. Because of small dimensions of the cabinet the driver will never exceed maximum excursion.

predicted lf response

Predicted low frequency response in WinISD

Now off to the wood work. Tools needed: Router, Jig Saw, Circular Saw, Electric Drill, Clamps, Screws, Glue, etc.

Lucky me i found a pair of old speakers measuring about 9l of internal volume. Almost perfect for my application. I only kept the top, bottom and the side panels. I make a new back panel and cut hole for the speaker terminals. I want the option of using separate amplifiers on highs and mids so i use bi-amp speaker connectors. I plan on painting the front baffle black and finish the rest in some dark colored carpet.

9l old speaker router back panel back panel connector

You can see here what i kept from the old speakers, the router getting ready, the back panel in position and the back panel with the connectors installed.

Now follows the routing of the front baffle. I use MDF for it’s easy to work with and has pretty good acoustical properties. First i prepare the router. I make the compass from a thinner board of MDF and i attach it to the bottom plate of the router. After this i measure the desired radius and make a hole in the board. This hole will be the center of the circle being routed.

router w/ compas cutting baffle baffles front baffle

Better measure 5 times and cut once…

The drivers are flush mounted and the front baffle’s edges will be rounded to minimize diffraction. The distance between the acoustical centers of the two drivers is 17cm. This distance corresponds with the wavelength of approximately 2kHz, which will be pretty much in the middle of our crossing point. This alignment avoids phase problems in this region. Also loading the tweeter with horn provides time alignment with our woofer.

Before talking about crossover lets look at the high frequency driver a bit:

pct-30

As you can see it is pretty sensitive at 93dB, mine actually reaches flat region at 92 dB. Because of the waveguide there is a big boost on the higher mid-range. Also note the fs of 1.4Khz. This means we should cross this tweeter not lower than 2.8khz. I decided to cut it at about 4Khz with 12db/ octave slope and this way make it linear down to approximately 2kHz. Considering the gain provided by waveguide i will cross the woofer at about 1.4kHz 2nd order Butterworth again.

This is how crossover looks like… on paper for now:

xover schematic xover graph 1 xover graph 2

In schemtic you will see the Zobel network in parallel with the woofer for impedance equalization and the L-pad on the tweeter to adjust for the increased sensitivity.

To be continued:

Finishing cabinets, round-over the edges of baffles

– Building crossover, install and voicing