Need spec for KFC-7180

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hello guys, its been a while since I posted here. I have an old pair of Kenwood KFC-7180 7x10'' car spearkers and I am wondering if someone has the Qts, Vas and Fs parameters or where I can find these parameters. I sent an email to Kenwood and they don't know the parameters.

I want to build a pair of floor standing home speaker with them (I want to practice with my new table saw). Maybe I will have to build a "typical" box and play with the interior volume and port diameter/length.

Thanks for your help
diyAudio Moderator Emeritus
Joined 2001

I found the service manual at the following site, but they do not list the Thiele-Small parameters.

Do you have any test equipment? If not, you can download it off the web. Build a 1 cu ft test box with your new table saw, and measure how much Fs rises when placed in the box. Also measure the output at resonance, both in the test box and outside. From these, you can get the approximate Qts and Vas.

Many manufacturers do not pubish the Thiele-Small parameters for car component speakers, unfortunately.

I can give you the formulas to figure this out if you are interested.
Hello, I am new to the speakers design/construction and I don't have any idea on how to test and find the paramters of a speakers with a "typical" box. I used to design and build amplifiers (really different from woodworking).

Can you point me to few links on the net about how to test speakers (finding Thiele-Small parameters)? The only hint that the Kenwood support team gave me was that the speakers is designed for "free air" use.

In fact, this exercise is only to get expericence before building my real pair of loudspeakers: The Ariel :)

Thanks a lot!
diyAudio Moderator Emeritus
Joined 2001

"Free air" means designed to be used without an enclosure. There may not be a satisfactory box for your speaker.

If there is a satisfactory box, you can be pretty sure it will be sealed, since ported boxes require speakers with a much lower Q factor. Some "free air" speakers can be used in a sealed cabinet with satisfactory results, however.

You mentioned that you design and build amplifiers. I was wondering what kind of test equipment you might have. Do you have a meter that can read AC amps? How about an audio tone generator, one that can generate a tone between 20 and 20,000 Hz or so? If not, they have them on the web for free, though I have not tried it out yet.

Even if you do not have a meter that can read AC amps, you can use a simple analog voltmeter. It will be slightly less accurate, but effective for our purposes. And then you need a simple audio noise meter, one that can tell you how loud a sound is. Or just use a microphone and measure it's output thru an any test amp you might have.

What I am trying to do is run the the audio tone generator down thru the bass region-say from 200 Hz down to 20- until you find the unenclosed speaker's resonance frequency. That is the point of highest impedance, (and lowest current). Then, I would run the audio tone generator from 1,000 Hz all the way down to the resonance frequency. While I did this, I would hold the microphone or audio noise meter right up close to the speaker's cone, almost touching it. That is "nearfield" measurement.

I would then measure the nearfield loudness of the speaker at the resonance frequency and compare it to the speaker's loudness thru it's midband point-between 200 Hz and 800 Hz or so. If you are using a microphone instead of an audio noise meter, measure the voltage difference.

If the speaker's sound output at resonance frequency is down somewhat from it's midpoint output, then you can probably build a sealed box for it. If it is the same as the midpoint sound output, then you cannot build a satisfactory box for it.

All this should be done with the speaker unenclosed-just resting on a table with it's cone up and the back of the magnet down.

The book, How to Design, Build and Test Speaker Systems by David Weems also tells you how to measure a speaker. It is in most libraries.

If you decide to run the test, let me know how it comes out. Let me know how far down the speaker's output at resonance is from midpoint output.

To summarize: We are trying to find what the speaker's sound output is at resonance frequency compared to what it is thru the midrange frequencies.

If I have not made myself clear, please let me know.

[Edited by kelticwizard on 11-20-2001 at 10:46 AM]
Hello, I did my homework and following are my results:
F (Hz) V (mV) I (mA) Z L (loudness in mV)
2000 3.8
1500 1.2
1000 6.4
800 4.6
500 280 33.5 1.8
200 176.8 29.6 0.6
100 136.5 20.1 0.2
90 149 17.9
80 184 14.8
75 224 12.4 18.06 0.2
70 287 9.3 30.86 0.2
67 360 8 45.00 0.3
66 392 8.3 47.23 0.3
65 435 9.2 47.28 0.3
63 511 13.2 38.71 0.4
62 533 15.3 34.84 0.4
61 538 16.9 31.83 0.4
60 506 18.8 0.4
59 455 19.5
58 400 19.8
55 259 18.9
53 204 17.9
50 149 16.6 0
20 27.8 6.5
10 12.9 1.0
5 0.7 1.1
Conclusion: I think my resonance frequency is 65Hz and I can build a sealed box for these speakers since at 1000Hz the power output is more than 20 time bigger than at the resonance frequency.

There is one thing that bugs me: these speakers have three ways (3 cone) and I don't know if it can screw up the results.

If my results are good, how can I determine the parameters needed to build my enclosure?

diyAudio Moderator Emeritus
Joined 2001

Wow! You really DID do your homework!

One thing-did you use a resistor in series with the speaker to measure the current? That is fine. But when you measured the speaker's nearfield output, I noticed the mV sent to the speaker varied with frequency. For instance, at 200 Hz you listed 176.8 mV sent to the speaker, while at 61 Hz, you listed 538 mV sent to the speaker. Quite a difference.

We need a constant voltage to sweep frequencies from midrange down thru the bass region to measure the speaker output nearfield.

Let me give you an overall view of what we are trying to do.

A sealed box with a Q of 1 is normally the highest Q that is desirable. Any higher Q and it sounds boomy. Putting the speaker in a sealed box raises both the resonance frequency and the Q. By how much depends on the size of the box and the Thiele Small parameters we are about to measure.

At Q=1, at resonance frequency, the speaker is equal with the midband volume.

At Q=.7, at resonance frequency the speaker is 3 dB down from the midband.

At Q=.5, at resonance frequency, the speaker is 6 dB down from the midband-and you might even be able to build a ported box for the speaker.

Most unenclosed free air speakers have a Q between 1 and .7, though I have seen some with Qs as low as .5. If you start out with a speaker with a Q of 1 or above-yes they go higher-then no satisfactory box can be built, since the box is going to raise that Q value too high.

However, if the unenclosed speaker has a Q lower than 1, then we can probably build a box that will raise the Q no higher-or not much higher-than 1. A sealed box system with a final Q of 1 is a very satisfactory speaker, and many commercial systems are built that way.

If the speaker does in fact have a Q lower than 1, our next step is to build a cheap, quick test box of perhaps 28 liters, (1 cubic foot), and measure:
A) How high the resonance frequency was raised
B) The output of ths speaker at resonance frequency compared to the midband.

The reason I selected 28 liters is that it is probably how big the final box is likely to be if the unenclosed speaker has a Q below 1. Actually, you can make the test box much smaller if you wish-all I need is to find out what happens to the resonance frequency and output at resonance in the test box, and we will know the necessary parameters to build the proper box.

One more thing. The output of the microphone-is it possible to make it a little more accurate? Going from .4 mv to .3 mv doesn't tell me that much. Going from .44 mV to .31 mV is much better-will give much more accurate readings. Or going from 4.4 to 3.1 mV will accomplish the same thing. Could you turn the volume up slightly when measuring?

We can convert the mV of the microphone output to decibels easily once the measurements are made.

PS: When we measure nearfield, you can put the microphone about an eighth of an inch from the main cone-that should prevent the "whizzer" in the middle from interfering.

[Edited by kelticwizard on 11-23-2001 at 02:14 PM]
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.