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Old 8th May 2013, 06:04 PM   #1
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Join Date: Jul 2010
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Post The New Member Noob Newbie Questions Thread

In an attempt to collect any new member questions that will fly under the radar with the collected long-term members, I am starting this thread to answer and collect those questions new members to the hobby will have.

I do not know the answers but will help out those who ask and in doing so will move my own education along in the DIY field.

Anything goes, nothing is too basic, too simple, too obvious, or too rudimentary to ask.
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Old 11th May 2013, 09:49 AM   #2
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1) The -6dB per octave point on an Open Baffle OB is calculated at:
Fequal = .17 * 343 / D

Fequal is the frequency of 1/4 wave cancellation commencing at 6dB/octave.

.17 is in there to approximate a sealed box enclosure when listening to an OB. I do not know why the fudge factor is o.68 X .25. 1/4 wave, .25 right? Nope.

343 is in metres, speed of sound. This can be expressed as several values. 344m, 34400cm, 323m, etc. Most of us will listen at room temperature, although the air pressure and humidity will change. If you live someplace that is always dry, or always at higher elevation, it may benefit you to get the precise figure for where you are. If your air pressure and humidity fluctuate wildly during the year, then use the common figure of 343m or 34300cm.

D is in metres, the distance from the mid point of the driver on front of the baffle, around the side to the midpoint of the same driver on the back of the baffle. This is often the width of the baffle, but not always. For example if the driver is off centre the shortest distance will be less than the width of the baffle D. Always measure the shortest distance to get the 1/4 wave figure as the shorter the distance the higher the frequency and the sooner/higher the roll off begins.
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Old 13th May 2013, 10:35 AM   #3
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2) The -3db point of a simple one capacitor high pass filter.

Many cheap boxes come with a midrange/woofer driver and a tweeter and one capacitor, usually 4.7uf, wired in series with the tweeter. This is a high pass filter designed to pass frequencies to the tweeter and keep low frequencies away from it, where they would do damage.

To calculate the xo frequency of this "crossover", which is a modified 1st Order Butterworth minus the coil, you need this:

The DC measured impedance in ohms of the two speakers
The value of the capacitor.
The desired xo frequency point for -3db.

You can use this formula for 1st order BW and ignore the coil value:
2-Way Crossover Designer / Calculator

Or you can use this:
f = 1 / [2πCR]

f is the crossover frequency in hertz where the two curves meet at the -3db point.
π is pi, 3.14159
C is the value of the capacitor in FARADS. A 4.7uf capacitor would be .0000047 farads.
R is the impedance in ohms of the tweeter, not the other driver. Measure using your multimeter. This is the DC measurement and not the impedance curve, but it will suffice.

If you had a tweeter that was 8 ohms and a 10uf capacitor the crossover frequency would be:
f = 1 / [2πCR]
f = 1 / [2 * 3.14159 * .00001 * 8]
f = 1989 hz

Which is rather low for a tweeter xo point.
To raise this number to 2500 or 3000, more appropriate, you would need a smalerl ohms impedance for the tweeter driver and/or a smaller value capacitor.
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Old 13th May 2013, 11:43 AM   #4
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Location: Sheffield
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When wood working with few clamps, superglue can be used to hold pieces together until the PVA sets. Be careful though, once the superglue is set (you literally have seconds of re-arranging when the pieces are together), that's it.

It also means you can keep working on something while glue's drying. Rather handy IMO, especially for things like folded horns where it can be difficult to get clamps in the right place.

Chris
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Old 25th June 2013, 06:19 PM   #5
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-When using less than top shelf drivers, silicone sealer or glue in a few beads between the basket and magnet assembly can anchor any weak structures that may vibrate.

-Electrical tape wound around the magnet and base of the basket (BUT NOT THE BASKET ITSELF) will dampen vibrations and ringing for cheaper drivers. Small pieces of material over each basket ridge also dampen vibrations. Felt on the magnet back face, too.

-Wrapping that same area with dampening material, such as used to soundproof firewalls and engine compartments in autos, also assists in dampening the driver.

-When using Edge software, select 4, 6 or 8 source points to get a smoother, more realistic graph of FR and SQ. Unless you have a 2" driver and it's 50 feet away, you are not going to have a single point source in your listening room. Select 4 to 8 sources to smooth the curve to a realistic FR, not 1.

-Jigsaws can do multiple duty as a router, band saw, and circle cutter.
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