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Bass Stopper Inline Crossover 300 Hz/150 Hz
WOULD LIKE TO KNOW WHICH BASS BLOCKER IS GOOD TO USE ON 6.5 MID RANGE SPEAKERS AND WHICH ONE FOR TWEETERS
I would use the 300hz on the mid, and at least 2.5khz on the tweets.
Date published: 2012-01-12
Will impedance curve changes high pass frequency?
OK, I'm revealing gross ignorance but here goes.... voice coil impedance changes radically with frequency right? So how does this high pass filter work with constantly changing ohm values? So confused.....
Your confusion is warranted. As with a lot of car audio equipment, the functionality of this device is overly simplified. In essence though, it is a simple device that can be understood. Lets look at the device, the speaker, and then how they will interact.Bass Blocker:This 0-300Hz bass blocker with 6db roll off is another way of saying First Order High Pass Filter at 300 Hz. Throwing all the fancy terminology aside, it is just a Capacitor in series with the speaker. Like the speaker, a capacitor has changing impedance called reactance(1). In this configuration(2), Low frequencies encounter high current opposition and High frequencies are passed.Speaker:You are correct that the voice coil impedance changes radically over frequency. A 4 ohm speaker can vary from 2 to 20+ ohms. However this happens a generally predictable way. Its lowest impedance will be at DC. The impedance will spike at its Fs (Free air resonance)(3) another spike can occur with the type of enclosure you put a speaker in but this is beyond the scope of your question. The impedance will then quickly reach another minimum and then start to rise again.Interaction:Without test equipment and/or a published impedance graph(rarely included in car audio) it is impossible to know the exact values of a speaker's impedance over the frequency range. An estimate of can be made though from the published frequency response. For example a speaker rated 40Hz-20kHz the Fs would be close to 40Hz.With first order high pass crossovers, a good rule of thumb is to pick a crossover frequency 2 or more octaves(4) above Fs.Fs=40HzOne Octave = 80HzTwo Octaves = 160HzThree Octaves = 320HzNow lets see what happens with this bass blocker. So at 300Hz the speaker's impedance will be close to 4 ohms the capacitor will be slight so some anntenuation has occured. As the frequency increases the capacitors reactance decreases as to be negatable to the circuit and the speakers impedance slowly increases but functions normally. As the frequency decreases from 300Hz the reactance of the capacitor increases and at first the impedance is around 4 but quickly increases. As the impedance increase around Fs the capacitor is not attenuating the signal as well. Think of it as a leveling off. However below Fs the inductance drops for the speaker and the capacitor can attenuate the signal further.I hope this helps and is not too convoluted. Frequency response with complex impedance can be confusing (probably why everything is generalized). Try googling Crossover Basics for some more insight. Also PE sell some good books that cover passive crossover design.NOTES:(1)Impedance, Reactance, and Resistance. These terms get thrown around in audio and cause a lot of confusion, so shed a little light.-Resistance is the opposition to current flow independent of Frequency. Think of a metal film resistor with the color rings around it. It will do the same thing with a DC Current or AC Current.-Reactance doesn't get used very often and can sometimes cause anxiousness and confusion. Reactance is also the opposition to current flow but only in regards to frequency changes. A series capacitor will have a higher opposition to current at lower frequency as it does to higher frequency.-Impedance is the total opposition to current, so the Resistance and Reactance of a circuit.(2)In respect to the speaker, how the capacitor is connected to the speaker matters. A series capacitor will 'block' the lower frequency because its reactance is higher at those frequencies. If the capacitor where to be connected in parallel, the opposite would happen. So at low frequencies the capacitor would act as a high value resistor and the current would flow through the speaker. At higher frequencies the capacitor would have a much lower opposition to current and act as a sink.(3)Fs is not generally given on car speakers specs. For a close approximation I use the low frequency of the stated frequency response.(4)An octave is a double in frequency. So an octave higher than 100Hz would be 200Hz. To get to higher octaves you keep doubling the frequency. Two octaves higher from 35Hz would be 140Hz (35 doubled is 70 and 70 doubled is 140). Three octaves lower than 10,000 Hz would be 1,250Hz (10,000/2=5000; 5000/2=2500; 2500/2=1250)(Extra)The Capacitance of this Bass Blocker can be calculated from First order equations:High Pass FilterC = 0.1590/(R*f)Low Pass FilterL = (0.1590*R)/fIf f=300Hz, R=4ohmsC = 0.1590/(4*300)C = 0.0001325 FaradsC = 132.5 uFStill the same if you use R=8ohm, f=150Hz.
Date published: 2014-04-04