Speaker Is 4 ohms, However...........

[OMNIFEX]

Wednesday, I built two 8 inch bass cabs for subwoofer duty
on my mom's video entertainment center. (I was going for
12's, but space was limited)

The cabinets are 24 inches high, 11.5 inches wide, and,
24 inches in depth. The box is tuned @ 38.71 Hz, which
gives me a f3 @ 33.28 Hertz.

The impedance curve of the cabinet starts at 5.93 ohms
(100 Hz) and, ends at 22.2 ohms (33.3 ohms)

Note: 5.93 ohms, being the lowest, while 39.1
ohms being the highest (51.3 Hertz)

I'm assuming due to the box's tuning, the speaker's
interaction with the internal chamber, and, the cabinet's
size, the amp is not registering 4 ohms, but the impedance
curve of the box. Correct?

[kelticwizard]

A) Measure the DC resistance of the voice coil. For a 4 ohm speaker, that can vary from 2.5 to 4 ohms, (even though a speaker with a 4 ohm DC resistance is usually considered a 6 ohm speaker).

B) The lowest impedance of a bass reflex speaker is normally the tuning frequency-in your case, 39 Hz. Generally, there it is between 1 and 1.5 ohms over the DCR. What is the impedance there?

[OMNIFEX]

Hey kelticwizard


Thanks for your input!

Originally posted by kelticwizard
A) Measure the DC resistance of the voice coil. For a 4 ohm speaker, that can vary from 2.5 to 4 ohms, (even though a speaker with a 4 ohm DC resistance is usually considered a 6 ohm speaker).

The DC response is 3.43

B) The lowest impedance of a bass reflex speaker is normally the tuning frequency-in your case, 39 Hz. Generally, there it is between 1 and 1.5 ohms over the DCR. What is the impedance there?

We are looking at 8.41 Ohms here.



And the answer is...............................................

[kelticwizard]

A) Hmmm, are the ports stuffed or open?

Some of the numbers you are mentioning don't quite seem to figure.

B) For instance, you say that the box is tuned to 39 Hz, but the F3 is 33 Hz. What was the Qts, Fs, and Vas of the woofer? Or even better, the Thiele-Small specs entirely? Because normally you do not have an F3 that is below the box tuning frequency, although this arrangement is possible if the Qts is above 0.4 and the box is larger than the Vas.

C) Those dimensions of 24" x 10" x 24"-I assume those are outside dimensions? If they are, and you used building material 3/4" thick, then your inside volume is 2.92 cu Ft. That is 83 liters.
What is the dimensions of your port-what diameter and how long? Because I have begun to wonder if you really are tuned to 39 Hz. I am not saying you aren't-but I just want to check the dimensions to make sure.

Just trying to get a picture of the whole setup before trying to figure out why the impedance is the way it is.

[OMNIFEX]

Ok Dude.

Where did you get 39 Hz tuning? It's tuned at 38.71 Hz.

Box Dimensions External

24 inches High

11.5 inches Wide

24 inches Deep

Cabinet Tuning

fs = 38.71

f3 = 33.28

Port Size

Diameter = 2.5 inches

Length = 18 mm

QTY = 1

Driver Parameters

fs = 42.48
QMS = 7.056
QES = 0.41
QTS = 0.387
VAS = 17. 12 Liters
sd = 214 sq. cm
Xmax = 8.458
Re = 3.43 ohms


Hope This Helps!

[kelticwizard]

Incidentally, how are you measuring all this? I see these precise fractions, and I am wondering how it is possible.

For tuning, 2.92 cu ft equals 83 liters and I knocked off 4 liters for speaker and the port. For your 2.5" by 18mm long port I got 43 Hz tuning.

Now, one thing. Did you run a frequency test from say 25 Hz to 60 Hz to see if the box where the impedance is the least? where ever the impedance is the least in that area, that is the tuning frequency. I am wondering if you are reading 8+ohms at what you think is the tuning frequency, when it actually is a little bit off.

Finally, you seem to have a box much larger than the Thiele-Small parameters would call for. Your Vas is 17 liters, your Qts is .387, your Fs is 42.5. Such a box should be around 17 liters, tuned to the Fs. That would be the "classic" reflex. You can make it a little larger and tune it a little lower, of course, nothing wrong with that. But your box seems to be 79 liters, and that is pretty huge for what your speaker calls for. It looks like you could have gotten away with a smaller box.

By the way, are you sure abuot that Vas for the 8 inch? 44 Hz Fs and a 17 liter box-this woofer puts out a lot of bass in a small box, that is for sure.

Also, you gave the length of the vent as 18 mm. That means this is just a cutout in the box with no tube, right? Cause I am assuming your wood or MDF is 18mm thick.

Here is the simulation for your woofer in a 79 liter box, (red) and a 17 liter box, (blue). Red is tuned to 43 Hz, blue to 42 Hz.

[OMNIFEX]

Originally posted by kelticwizard
Incidentally, how are you measuring all this? I see
these precise fractions, and I am wondering how it is
possible.

I'm using Eminence Designer. Not WinISD.

For tuning, 2.92 cu ft equals 83 liters and I knocked off 4 liters for speaker and the port. For your 2.5" by 18mm long port I got 43 Hz tuning.

Now, one thing. Did you run a frequency test from say 25 Hz to 60 Hz to see if the box where the impedance is the least? where ever the impedance is the least in that area, that is the tuning frequency. I am wondering if you are reading 8+ohms at what you think is the tuning frequency, when it actually is a little bit off.

At 60 Hz, the box reads 18.3 ohms
At 25 Hz, the box reads 10.6 ohms

Finally, you seem to have a box much larger than the Thiele-Small parameters would call for. Your Vas is 17 liters, your Qts is .387, your Fs is 42.5. Such a box should be around 17 liters, tuned to the Fs. That would be the "classic" reflex. You can make it a little larger and tune it a little lower, of course, nothing wrong with that. But your box seems to be 79 liters, and that is pretty huge for what your speaker calls for. It looks like you could have gotten away with a smaller box.

Yes. I know I built an oversized box. You see, 8 inch
woofers are inefficent in my book. (I'm acustomed to
15, and 18 inch woofers) So, to increase the SPL made
box larger, thus gaining a huge hump to compensate for
the ineffiency. I have around 4 dB gain at 40Hz. I did this
on purpose to achieve more SPL at around 40 Hz.

By the way, are you sure abuot that Vas for the 8 inch? 44 Hz Fs and a 17 liter box-this woofer puts out a lot of bass in a small box, that is for sure.

Its a Credence Woofer. However, the Magnet says
Kicker. I did a search on google, and, here, and found
out Credence builds speakers for alot of Car Companies.
Picked them up on Ebay.

Actually, it was going in a 19 High, by, 10.5 Wide,
by, 8 inch deep box at first. Plopped it in, and, it didn't
offer enough bass for my liking. So, I just built a box
that would give me the bass I desired.

Also, you gave the length of the vent as 18 mm. That means this is just a cutout in the box with no tube, right? Cause I am assuming your wood or MDF is 18mm thick.

Yep. Just a cutout hole. No tubing. Its for my mom's
entertainment system. She'll never crank it up. Got
Soundcraftsmen 7502 feeding these puppies 275 -
420 watts per channel continuous, depending on
the impedance. The box is 18 mm Baltic Birch.

Here is the simulation for your woofer in a 79 liter box, (red) and a 17 liter box, (blue). Red is tuned to 43 Hz, blue to 42 Hz.

Yeah Dude! I got the same response. I wanted it that
way. Still don't know how you got 43 Hertz.

[kelticwizard]

First, there is something important I want to point out.

A simulation is not a measurement. It's all right to say that you are not equipped to do the actual measurement, ( I don't know if you are or not). I just want to know if you are measuring or simulating.

This is a very important distiction, since this is an unusual box and there is some uncertainty if the Thiele-Small equations work when the box volume is many times the Vas. So please make the distinction clear-it is important.

[kelticwizard]

Okay, that being said, I have an answer as to why Eminence Designer seems to think your 4 ohm woofer is actually an 8 ohm woofer.

It's the overvolume.

If you had done a conventional setup for your particular Qts and made the box volume somewhere around the Vas, your woofer would read like a typical 4 ohm woofer.

But add the huge, extra box volume, and the impedance reading goes right up.

Below is a simulation using BoxModel, a Dos freeware program that still does a good job. Note-do not ever use the Transmission Line portion of the program-even the authors admitted it did not predict anything accurately. As for Closed, Vented and Bandpass though, BoxModel does a good job.

The light blue line is the impedance of the woofer in a 17 liter box tuned to 42 Hz. The red is for an 81 liter box tuned to 42 Hz. both Win ISD and BoxModel say your box with your vent should tune the box to 42 Hz, not 39 Hz. Anyway, the difference is not important.

As you can see, the red line, (81 liter box) is considerably higher in impedance than the blue line, (17 liter box). The response curves are similar to what WinISD shows, which I posted above.

Please note that by the time you go up to 150 Hz, the speaker in the 81 liter box, (red line), is back to being a 4 ohm speaker again.

[kelticwizard]

As stated before, though, we cannot be sure, with this box many times the speaker's Vas, whether these simulations actually work or not.

Do you know how to do an impedance measurement?

If not, there is a shortcut.

Take a 50 Hz tone. If you don't have a tone generator or test CD, you can download a freeware tone generator here:
www.satsignal.net Click Audio Tools

Simply write in 50 Hz for your tone. Use a cable to run the Line Out to an amp-or if your sound card has a powered out, run the sound card to the speaker. Play the 50 Hz tone.

Now, cover the port with a book or something. Then uncover it. cover it again, then uncover it.

If the 50 Hz tone is much louder when the port is open than when it is closed, then the output and the impedance are probably close to what the charts say. If there is little difference if the port is open or closed, then these charts are meaningless.

According to Win ISD, at 50 Hz you should be about 11 dB louder with the port open than with the port closed. This should be very hearable. If it is not hearable, then these charts count for little.


Try it, and let me know how it turns out.