Hi everyone,
I want to be able to create my own enclosure for some woofers that I am recycling from some old equipment, and I wanted to calculate their response in WinISD, and by inserting the Thiele-Small parameters, I realize that they ask me for Xmax. As they are recycled woofers, I do not have the Thiele-small parameter table provided by the manufacturer, so I had to do all the measurements manually in REW, using a non-amplified system, but this measurement does not allow you to calculate Xmax.
My question is if Xmax can be calculated in some other way, knowing other T/S parameters, or I don't know, in some other way that does not require many very sophisticated measurement tools.
I want to be able to create my own enclosure for some woofers that I am recycling from some old equipment, and I wanted to calculate their response in WinISD, and by inserting the Thiele-Small parameters, I realize that they ask me for Xmax. As they are recycled woofers, I do not have the Thiele-small parameter table provided by the manufacturer, so I had to do all the measurements manually in REW, using a non-amplified system, but this measurement does not allow you to calculate Xmax.
My question is if Xmax can be calculated in some other way, knowing other T/S parameters, or I don't know, in some other way that does not require many very sophisticated measurement tools.
You aren't going to know Vd without a spec sheet though. It isn't something you can easily measure.
Your best bet is to try and shine a light through the spider, in an attempt to glance at the voice coil, and then guess at what the amount of coil overhanging the top plate is.
Your best bet is to try and shine a light through the spider, in an attempt to glance at the voice coil, and then guess at what the amount of coil overhanging the top plate is.
I don't have you either (I understand that Vd is the peak displacement diaphragm volume that the total volume of air in the cone will move, if not, please correct)
How can I get it? Without having Xmax, obviously
How can I get it? Without having Xmax, obviously
The "Top plate" is usually the magnetic gap length.
Voice coil length - magnetic gap length / 2=Xmax.
12mm voice coil -6mm gap=6/2= 3mm Xmax.
Xmax is one way excursion.
If you can see the voice coil above the gap, you can estimate it's total length.
This speaker would have very little Xmax, the coil length/height is not much more than the top plate/magnetic gap/length/height:
STV's post #7 describes a simple measurement system to determine Xmax:
https://www.diyaudio.com/community/threads/xmax.413692/#post-7705290
You could use batteries rather than a DC power supply to do the measurement, though their voltage drops under load, so more measurement....
Keep DC tests very short, and leave some cool down time between each.
Voice coil length - magnetic gap length / 2=Xmax.
12mm voice coil -6mm gap=6/2= 3mm Xmax.
Xmax is one way excursion.
If you can see the voice coil above the gap, you can estimate it's total length.
This speaker would have very little Xmax, the coil length/height is not much more than the top plate/magnetic gap/length/height:
STV's post #7 describes a simple measurement system to determine Xmax:
https://www.diyaudio.com/community/threads/xmax.413692/#post-7705290
You could use batteries rather than a DC power supply to do the measurement, though their voltage drops under load, so more measurement....
Keep DC tests very short, and leave some cool down time between each.
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the volts of direct current to be applied to the woofer, must the maximum watts of the woofer be reached to be able to measure Xmax?
No!
Just be sure not to exceed the max power (thermal power) of the voice coil.
Usually you will reach Xmax with DC current far below the one corresponding to Pmax.
It would be something like this?
That is a small woofer that I have, I just used it for the test, I don't want to measure the Xmax of that woofer.
It is a 1.5 Volt battery.
There I should stick a piece of paper or something at a right angle to the woofer, put a ruler on top, and see how much it moves, right?
That is a small woofer that I have, I just used it for the test, I don't want to measure the Xmax of that woofer.
It is a 1.5 Volt battery.
There I should stick a piece of paper or something at a right angle to the woofer, put a ruler on top, and see how much it moves, right?
Radew,
Yes.
The idea is to chart the movement (excursion) against voltage.
Below Xmax, the range of linear operation, doubling voltage (+6dB) should result in a doubling of excursion.
At the point where excursion no longer tracks the voltage, the linear yellow line, Xmax has been reached.
Distortion will be ~10%, output will not be linear with more voltage applied.
The further the voice coil is driven out of the magnetic gap, the less control the amplifier voltage has on the cone.
Yes.
The idea is to chart the movement (excursion) against voltage.
Below Xmax, the range of linear operation, doubling voltage (+6dB) should result in a doubling of excursion.
At the point where excursion no longer tracks the voltage, the linear yellow line, Xmax has been reached.
Distortion will be ~10%, output will not be linear with more voltage applied.
The further the voice coil is driven out of the magnetic gap, the less control the amplifier voltage has on the cone.
but when I do the test with a 1.5 volt battery, it takes 1mm, then I connect two 1.5 volt batteries in series, and it gives me 2mm, then 3 batteries in series, and it gives me 3mm, that doesn't mean I have Do you have to reach the maximum power of the woofer to be able to find Xmax?
And if I need the maximum power of the woofer, it would be complicated for me, since the woofers I have do not present information about their Pmax. Any recommendations in this case?
For a woofer with a 1 inch voice coil you would need around 8 batteries and leave them connected for quite long before there is could be any danger for the driver. you will also smell it before damaging the voice coil.
Also if you plot the excursion / voltage graph you will reach Xmax long before the thermal limit and you can stop the test.
1 inch voice coils can usually swallow 20 W. That's the common minimum voice coil size for woofers.
Also if you plot the excursion / voltage graph you will reach Xmax long before the thermal limit and you can stop the test.
1 inch voice coils can usually swallow 20 W. That's the common minimum voice coil size for woofers.
In general, I agree, but a "quite long" time for a 4 ohm DCR driver with 12v DC applied could be "fairly short" 😉 .
One side of a Hafler DH-200 amp (200w@8ohms) driving my Tannoy PBM 6.5 speakers had a little DC voltage on one channel that slowly grew from a small fraction of a volt to 5 volts over the course of 20 years.
5 volts into the “100 watt 8ohm” Tannoy PBM 6.5 woofer's DC resistance of around 4 ohms, just 6.25 watts, was enough to cause the woofer's voice coil to burn, as the DC held the voice coil in a fixed position, allowing little cooling as the speaker sat powered, but idle.
I never noticed any smell or distortion, but one day noticed the left woofer was not working..
An alternate to the DC test would be using a 20Hz sine wave signal through an amplifier with a volt meter connected to monitor the voltage as level is increased, while measuring excursion.
A small speaker open air won't produce much SPL at 20Hz (the fundamental frequency), but when you start hearing harmonics buzzing, multiples of the 20 Hz signal at 40Hz (x2), 60 (x3), 80(x4), 100(x5), 120(x6) the speaker probably has reached or is past Xmax, 10% distortion, harmonics -20dB below the fundamental.
Using a RTA (available for free with REW https://www.roomeqwizard.com/) the output may look something like this:
In this example, 20Hz reads 95.3dB, the 3rd harmonic reads 84.6dB, -10.7dB below the fundamental, ~30% distortion, well past Xmax.
Any harmonic above 75.3dB in this example indicates distortion above 10%.
The voice coil won't heat up as fast using sine waves compared to DC, but it still does not take long to heat them (raising the voice coil's impedance..), work fast!
Art
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but in the video they do not explain if I have to use the maximum voltage of the coil, or if I have to use a specific frequency, they do not explain it.
My guess to this would be that the gradual degradation of the amplifiers DC offset would have also caused a gradual breakdown of the winding insulation. To the point where the increased, long term, temperature was enough to cause the insulation/glue to fail after many weeks of exposure but not enough to cause much of a smell.
Whereas in the case of this kind of test we're talking very short term, which would require higher temperatures, and would smell. It's also different when the driver is inside a cabinet with the voice coil trapped inside a box. There's very little way for the smell to get out!
A specific frequency is not required.
A low frequency, like 20Hz, makes hearing the distortion easier- the reason you are concerned with Xmax is the speaker distorts and generates multiples of the drive frequency when driven beyond it.
The excursion can still be plotted as in post #11.
There is no need to "use the maximum voltage of the coil" whether using DC or AC (a sine wave) to test excursion.