Hello all,
I measured the impedance using Speaker Workshop using two 2" ports in a 1547 cu. in (25.? liter) box in order to tune the ports to the right length. I got about 3.31 to 3.37 inches (also from WinISD). Am I supposed to get a saddle point at around 62Hz which is the box resonance freq? A couple of projects I have seen have two resonance points (two humps in the impedance graph) but mine has the one resonance point at driver resonance and a small hump below the supposed box resonance freq. I do not get the pronounced saddle point for some reason and so I cannot find the box resonance accurately. I would like to know what I am doing wrong if I am.
Another problem is that I reduced the port lengths to 3" and still dont see an appreciable shift to the right box resonance. I am trying to tune it to 62 Hz but cannot go beyond 58 Hz (if what I am seeing is indeed the saddle point which isnt very clear from the graph) even for 3" port length.
I am using the Vifa P13-WH in an MTM config. I hope my question makes sense. I know it is hard to see what I am saying without a graph.
I would appreciate some help from people who can answer this.
thanks.
I measured the impedance using Speaker Workshop using two 2" ports in a 1547 cu. in (25.? liter) box in order to tune the ports to the right length. I got about 3.31 to 3.37 inches (also from WinISD). Am I supposed to get a saddle point at around 62Hz which is the box resonance freq? A couple of projects I have seen have two resonance points (two humps in the impedance graph) but mine has the one resonance point at driver resonance and a small hump below the supposed box resonance freq. I do not get the pronounced saddle point for some reason and so I cannot find the box resonance accurately. I would like to know what I am doing wrong if I am.
Another problem is that I reduced the port lengths to 3" and still dont see an appreciable shift to the right box resonance. I am trying to tune it to 62 Hz but cannot go beyond 58 Hz (if what I am seeing is indeed the saddle point which isnt very clear from the graph) even for 3" port length.
I am using the Vifa P13-WH in an MTM config. I hope my question makes sense. I know it is hard to see what I am saying without a graph.
I would appreciate some help from people who can answer this.
thanks.
By "saddle point" I suppose you mean a point of low impedance in between two peaks of high impedance.
Yes, that is exactly what you are supposed to get. It depends on the various setups, (called "alignments" in the Thiele-Small world), but those two impedance peaks do not have to be equal in amplitude, and they can vary in distance from the resonance frequency. But that is what you are supposed to get.
You say you are not getting it? Hmmm. You are not putting stuffing in your ports are you? Are the ports inside free and clear of any obstruction, such as stuffing?
PS: I checked your numbers on WinISD. You are not using the Vifa P13WH-10, are you? The P13WH-8 conforms to the numbers you are giving.
If, in a vented box, you come up with an impedance peak at what you think is resonance, and another peak below that, then there is an excellent chance that your box is tuned to the impedance valley between those peaks, not to the frequency you think it is. Why is anybody's guess.
Yes, that is exactly what you are supposed to get. It depends on the various setups, (called "alignments" in the Thiele-Small world), but those two impedance peaks do not have to be equal in amplitude, and they can vary in distance from the resonance frequency. But that is what you are supposed to get.
You say you are not getting it? Hmmm. You are not putting stuffing in your ports are you? Are the ports inside free and clear of any obstruction, such as stuffing?
PS: I checked your numbers on WinISD. You are not using the Vifa P13WH-10, are you? The P13WH-8 conforms to the numbers you are giving.
If, in a vented box, you come up with an impedance peak at what you think is resonance, and another peak below that, then there is an excellent chance that your box is tuned to the impedance valley between those peaks, not to the frequency you think it is. Why is anybody's guess.
kelticwizard said:
No, both ports are clear of any obstruction. They are 2" in dia each. Two Vifa P13WH-00-08 5" mid/bass drivers are being used in an MTM config. I will try and include the graph if possible, from SW to clarify. It seems that there are two peaks, one is very pronounced most probably from the driver resonance and the other is very small around 58 Hz or so but I am not sure what it is.
By the way, I am using lightly filled polyfill as a lining material at the back and sides of the box.
kelticwizard said:
As expected there was only one resonance pt at about 106 Hz. The orig *measured* resonance in free air was about 83 and Vas was 7.11 liters.
This new measurement is a slightly high number however.
With the ports unobstructed, I get the "dip point" around 59.7Hz for the two 3" ports. Whereas I should get it at 62 Hz for a port length of about 3.25" for the two ports.
Prash:
I think I am getting a little lost here. Let me retrace.
If I follow you correctly, you measured the parameters of the unenclosed Vifa P13 and it came out to:
Fs=83
Vas=7.11 liters.
Okay, we will use those numbers instead of the publiched specs.
Your box is 25 liters, am I right?
With the ports closed, your Fc, (resonance frequency of the speaker in a closed box) should be, according to the formula, 94 Hz. Yet you seem to have expected, and gotten, a number of 106 Hz-which is not that far off, by the way. That is only a sixth of an octave.
Now, back to the ports. You want to tune the box to 62 Hz. Right now, it is at 60, (59.7) Hz.
First. let me just say that 60 Hz is close enough to 62 Hz for me. Basically, you might wish to consider stopping right there.
However, I want to just clear up something. How long are the ports when the box is tuned to 60 Hz? Are they 3" long or so, or did you remove the tubes completely and that 60 Hz tuning is for just two 2" diameter holes in the wall of the cabinet?
If your ports are in the 3" to 3.5" long range, I would say you already have approximately what you were aiming for.
I think I am getting a little lost here. Let me retrace.
If I follow you correctly, you measured the parameters of the unenclosed Vifa P13 and it came out to:
Fs=83
Vas=7.11 liters.
Okay, we will use those numbers instead of the publiched specs.
Your box is 25 liters, am I right?
With the ports closed, your Fc, (resonance frequency of the speaker in a closed box) should be, according to the formula, 94 Hz. Yet you seem to have expected, and gotten, a number of 106 Hz-which is not that far off, by the way. That is only a sixth of an octave.
Now, back to the ports. You want to tune the box to 62 Hz. Right now, it is at 60, (59.7) Hz.
First. let me just say that 60 Hz is close enough to 62 Hz for me. Basically, you might wish to consider stopping right there.
However, I want to just clear up something. How long are the ports when the box is tuned to 60 Hz? Are they 3" long or so, or did you remove the tubes completely and that 60 Hz tuning is for just two 2" diameter holes in the wall of the cabinet?
If your ports are in the 3" to 3.5" long range, I would say you already have approximately what you were aiming for.
Incidentally, I downloaded Speaker Workshop but never seem to be able to get it set up. Many people rave about it so it must be good.
However, if you are having trouble reading the impedance graph, I can give you a simple method to measue the points of highest and lowest impedance-not the actual values, just the frequency of highest or lowest impedance.
It requires a multimeter and a downloadable online tone generator. Are you interested? I can give you details if you are.
However, if you are having trouble reading the impedance graph, I can give you a simple method to measue the points of highest and lowest impedance-not the actual values, just the frequency of highest or lowest impedance.
It requires a multimeter and a downloadable online tone generator. Are you interested? I can give you details if you are.
kelticwizard said:
kelticwizard,
Yes those were the values measured using SW. They are
Fs = 83 Hz and Vas = 7.11 liter. So I arrived finally at a box vol of 25.3 liters (considering everything, incl deducting volumes of the driver magnets sticking into the box.)
I want to tune it to 62Hz and even WinISD (and SW) give me a port length of about 3.25 to 3.3" for the two ports. But it seems like I cannot get the freq beyond 60Hz even if I reduce the length of the ports to 3". So basically the ports are both 3" long (not 0.75", the thickness of the MDF cabinet) when the tuning freq is 59.7Hz. I havent removed the ports and measured it yet which would be way off anyway. the question is why i am not getting the measurement that i am supposed to get.
Why are you not getting what you expect?
My guess is either
A) Mismeasurement
B) The box is slightly too large.
Remember that 62 Hz is only 6% of an octave higher than what the box is tuned to now-believe me, nobody would know.
I would either:
A) Try a couple of ports that are 2.5" long, or
B) Stick a couple of bricks or pieces of two by four in the box, (in order to occupy volume), and retest.
My guess is either
A) Mismeasurement
B) The box is slightly too large.
Remember that 62 Hz is only 6% of an octave higher than what the box is tuned to now-believe me, nobody would know.
I would either:
A) Try a couple of ports that are 2.5" long, or
B) Stick a couple of bricks or pieces of two by four in the box, (in order to occupy volume), and retest.
kelticwizard said:
ok, let me try that and a few other things that I might think of.
thanks a lot for your help.
But I am not getting the two distinct peaks which I am told I should get when I try to measure the driver in the vented box, is that fine though? like I mentioned I get the small peak at around 55 and the other distinct resonance one at around 85. I have checked and rechecked the connections and remeasured, I dont believe that is the problem. havent broken in the drivers though and I hope that is not a cardinal sin.
No, not breaking in the drivers is not a cardinal sin, and besides, you should have that "two peak with a valley in the middle" pattern anyway, broken in or not.
In a ported box, your impedance minimum is at the box frequency. That is the way it works.
Unless you have a rubbing voice coil or something, I'm runnng out of options.
By the way, if you do have resonance peaks at 55 Hz and 85 Hz, then the midpoint between them is 68 Hz.
In a ported box, your impedance minimum is at the box frequency. That is the way it works.
Unless you have a rubbing voice coil or something, I'm runnng out of options.
By the way, if you do have resonance peaks at 55 Hz and 85 Hz, then the midpoint between them is 68 Hz.
kelticwizard said:
Hmm ok. that's strange. let me a try a few things and see.
why should there be two peaks anyway? i can understand the one at resonance freq but is the other one produced as a result of the impedance minimum at box freq? ie., a low imp at box freq makes the frequency immediately lower than the box freq look like a peak? if so, what should be the impedance at the frequencies below the box tuning freq? i dont mean the exact impedance value as that is driver specific, but the general pattern.
Prash:
Not sure. The way I figure it is-the ported box does a lot of things opposite to what the closed box does.
For instance, cone excursion goes up and up as frequency gets lower in the closed box. In the ported box, the cone excursion actually lowers as it approacheds the box frequency. So I am not surprised that at resonance, the closed box has an an impedance peak, while the ported box has an impedance minimum.
I have a mental image of the port action acting like a hammer, squashing the impedance peak in the middle and forcing two smaller impedance peaks to erupt on either side of the resonance as a result.
Anyway, that is the way it works.
Not sure. The way I figure it is-the ported box does a lot of things opposite to what the closed box does.
For instance, cone excursion goes up and up as frequency gets lower in the closed box. In the ported box, the cone excursion actually lowers as it approacheds the box frequency. So I am not surprised that at resonance, the closed box has an an impedance peak, while the ported box has an impedance minimum.
I have a mental image of the port action acting like a hammer, squashing the impedance peak in the middle and forcing two smaller impedance peaks to erupt on either side of the resonance as a result.
Anyway, that is the way it works.
My understanding is that the the first peak is the driver res., the second is the port res. The port has its own characteristic frequency lower than the drivers, a function of its length (its basically an organ pipe driven by by the box/driver). The driver and the port (driven by the driver) give you a composite box freq. somewhere between the two, but basically in the middle. FWIW.
Prash:
It has been a while, but I just thought I would drop in with something.
Your box, 25 liters with two 2" diameter ports 3" long, is tuned to 65 Hz, according to WinISD.
I modelled one P13WH to a box half the size-12.5 liters. so the reading will be the same. The box is tuned to 65 Hz, just as your box, with 2 woofers, is.
I used Subwoofer Simulator by our own F4ier, which models any of 7 different box types with your choice of 18 curves. It is free and downloadable.
http://www.geocities.com/f4ier/speaker.htm
I chose to display the frequency response, impedance and cone excursion for one watt of your P13. I used your measured results of 83 Fs and 7.11 liters Vas.
The high frequency response of the real-world Vifa will be better than the graph. The graph only represents a perfect cone, which no speaker has. The impedance and cone excrusion should be awfully similar to reality, though.
As you can see, the impedance peaks are quite well defined with a valley in between. The bottom of the valley is at the tuning frequency-in this case, 65 Hz. This is what your impedance curve should look like.
If it does not, I don't know what to say.
How did your speaker work out, anyway?
It has been a while, but I just thought I would drop in with something.
Your box, 25 liters with two 2" diameter ports 3" long, is tuned to 65 Hz, according to WinISD.
I modelled one P13WH to a box half the size-12.5 liters. so the reading will be the same. The box is tuned to 65 Hz, just as your box, with 2 woofers, is.
I used Subwoofer Simulator by our own F4ier, which models any of 7 different box types with your choice of 18 curves. It is free and downloadable.
http://www.geocities.com/f4ier/speaker.htm
I chose to display the frequency response, impedance and cone excursion for one watt of your P13. I used your measured results of 83 Fs and 7.11 liters Vas.
The high frequency response of the real-world Vifa will be better than the graph. The graph only represents a perfect cone, which no speaker has. The impedance and cone excrusion should be awfully similar to reality, though.
As you can see, the impedance peaks are quite well defined with a valley in between. The bottom of the valley is at the tuning frequency-in this case, 65 Hz. This is what your impedance curve should look like.
If it does not, I don't know what to say.
How did your speaker work out, anyway?
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pjkunz said:
You're right, and we now see the reason for a basreflex construction. The driver output will roll-off below its res frequency, which is at the upper peak. Because of the resonance of the basreflex, below the driver res freq, the output is "propped up" a bit so that the system still has some output below the driver res. The valley between the two has no specific significance, it is really the driver output before it gets augmented by the reflex action. Note that the extra low freq output actually emits from the port, not from the driver!
Jan Didden
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