Measuring Your Subs' T/s Parameters

[RamenNoodles]

This would be neat to know for those of us who have beat on our subs and are completely broken in considering a completely loose suspension will differ greatly compared to one with a stiffer suspension

[Ben Building]

Well, okay then.

The first parameter we have to discuss is the resonant frequency. A subwoofer's cone is acutually suspended in midair by the mechanical suspension of the speaker (the spider and surround). The mass of the cone in conjunction with the relative stiffness of the sub's suspension interacts in a direct and measurable way. Every subwoofer ever made has a frequency that tends to vibrate at. That frequency, its Fs, is called the sub's resonance. Knowing that number is vital for a number of different reasons we'll get into in a minute.

DETERMINING FS

Fs, the resonant frequency, is a number that almost all subs come with. If, however, for some reason you do not have that number, or you want your box to be EXACT, then you have to use another method. You have to look at the impedance curve of the subwoofer.

Let's talk a moment about voice coils. The voice coil of a sub is typically referred to as having a certain impedance (say 2 Ohms). What we have to understand that a voice coil is acutally suspended inside a very strong magnetic field. As we learned in science class, moving magnets past coils of wire affects the flow of electricity through the wires. a process called Faraday induction.

This means that while the wire in the voice coil has a direct-current (DC) resistance (ohms), that voice coil is also an inductor. Since inductors have inductive reactance, which, along with resistance makes a complicated impedance curve in the range of frequencies the sub is to be used for. Since a sub tends to vibrate at a certain frequency, it will also move FARTHER at that freq with the same energetic input. The coil moving farther through the magnetic field inside the motor causes more of an effect on the current trying to pass through the voice coil. As you might have figured, the coil will read its maximum impedance at the subwoofer's Fs.

That is where the Bass Mechanik CD comes in. You need some way of knowing exact frequencies going through your subs during the test. At the end of the Quad Maximus Cd there is are test tones from 20-80hz. These are very useful for measuring parameters.

Is anyone listening?

[devsmazduh]

im listening mr.teacher man lol.

just tryin to keep up.

[Chris from Mi]

definitely listening, wayy more fun to learn than calc 2 and i need a distraction from studying

[bobonit]

Grasshopper is paying attention oh honorable one.

[Ben Building]

Ok, cool. This is waaay easier than Calc2 or I wouldn't be doing it!!

Back to Fs then.

The impedance of a typical subwoofer is represented by a curve on a graph and not a straight line like DC impedance would suggest. Maximum impedance comes at the subs Fs.. In a minute we will go over a test routine that will let you figure the impedance curve and Fs of a subwoofer in FREE-AIR (not inside the box). After the sub gets put inside an enclosure, the Fs rises to a higher number called the Fc (total system resonance) for reasons we'll go into shortly.

SUBWOOFER Q

The parameter Q is a measure of the amount of control the sub's mechanical and electrical parts have over the motion of the cone. Two subs can have the same Fs, but because of a different Q, will sound totally different once inside the box. Like the Fs, the Q value of a sub referrs to Free Air. Once inside the box, Q changes to a larger value called the Qtc (total system damping).

QTS

Qts is composed of two factors, Qes (electrical damping) and Qms (mechanical damping). the Qes is just a measure of the Electrical control the voice coil exerts on cone motion. Qms is the measure of the Mechanical control exerted by the spider and surround material. Qes is usually a smaller number than Qms reflecting the fact that the EMF generated in the voice coil plays a much smaller role in damping than do the mechanical aspects near Fs.

The Qts (total Q in free air) is figured by a simple product over sum formula:

Qts= Qes x Qms/ Qes + Qms.

QTC (Sealed Box)

When you put a sub into an enclosure, its Q factor changes to a higher value called the Qtc (total damping). Enclosures with large Q numbers are overdamped since they don't have as much control over the subs' response as a low-Q enclosure (attention SQ guys). A high Q sealed enclosure will be louder (+6db where Q=2) than a low Q underdamped (<1) enclusure because of the high-Q example's lack of control around resonance. This same lack of control, however, also translates into muddy, ringing bass when Qtc approaches 2. Most of the time, you will want to compromise with a Qtc of around 1.

HOW TO MEASURE IMPEDANCE (Ohms)

As we already discussed, subwoofer impedance changes as its frequency changes due to the inductive nature of its voice coil. We are going to measure those changes in resistance as closely as possible.

All you will need is a DMM, the sub to be measured, and a test tone CD. If your sub has dual voice coils, the better, because you will only need one subwoofer. If you are measuring a SVC sub, then you will need another mule subwoofer to make the tones.

To start, get a peice of paper and number it from top to bottom starting from 10. Then, hook one voice coil of the sub up to the amp (you don't need a lot of power to do this) and the other voice coil up to your DMM. Set your DMM to read DC resistance and write down the DC (0 hertz) resistance, Re, of your sub. If you are measuring a 2 ohm voice coil, for example, it should read anywhere from 1.8 to 2.2 ohms.

First, play about 15-30 minutes of music through your (free-air) sub to loosen its suspension. Next, start your test tone Cd. Measure and write down the resistance of the open coil at each frequency up to 110hz. You will have noticed that the impedance numbers rise to a high point, and then descend as the frequencies go up. That high point, the Rmax, comes at the sub's resonant frequency, Fs. Go back to those freqs and check your numbers again for the Fs.

Now that you have a Fs (resonant frequency), an Re (DC resistance), an Rmax (maximum impedance), and an impedance plot for the speaker, you can go ahead and figure out the Ro, Rx, Qms, Qes, and Qts. The formulas for those are:

Ro= Rmax/Re

for Reference Impedance (Rx)

Rx²= Ro x Re

for Mechanical Damping (Qms)

Qms= (Fs x the square root of Ro)/F2 - F1

for Electrical Damping (Qes)

Qes = Qms/ Ro-1

and for Total Subwoofer Damping (Qts)

Qts = Qms/Ro

Edited May 29, 2009 by Ben Building

[bobonit]

Yea brother

Edited May 29, 2009 by bobonit

[Ben Building]

Wife needs some q.t. so I guess this will have to wait until tomorrow. BB

[Torres]

very interesting thread/info. lookin forward to the update

[matt402]

kool topic. i have been needing to learn more technical stuff.