Xmax

What is the proper way to use a subwoofers Xmax rating? Some specify 1-way and some are vague. What is the correct way to rate it?

WinISD for example asks for peak
1718002526750.png


If a manufacturer specifies 25mm one-way, do I double this to 50mm to get the peak value?

Thanks and regards
Randy
 

stv

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Joined 2005
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Unfortunately the measurement conditions for Xmax are not standardized. Usually Xmax indicates maximum (more or less) linear excursion in one way ("peak" as stated in winISD).

So for a manufacturer indicating max peak to peak excursion (e.g. sb acoustic do it that way, as far as iI know) you would halve the value before putting it into a simulation tool.
 
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stv

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I hope it's OK to post this here in your thread, @Randy Bassinga:

yesterday I just did a very quick and dirty Xmax measurement of an old sony 6" woofer to check if it is usable at all.
I used a crude DC voltage vs. excursion method, already described here (corresponding distorsion measurement here).

Caution: it is essential not to overload the voice coil with DC!
an oscillating voice coil is being cooled by circulating air, but a steady DC current cannot do this.
So be sure to activate DC for the woofer only for very shor time!
(also heating up the voice coil will raise it's resistance and will influence the measurement - you could of course use constant current method ...).

My woofer has a small 1 inch VC, so max thermal power will be around 30-40 watts.
The woofer has 5,2 Ohm DC resistance, so 12 V equals about 28 W which is already near the limit.
Also, the woofer will reach it's Xmax quite easily with DC, so 12 V was my maximum voltage.

Here is my setup:
IMG_2436.JPG


I attached a paper strip to the dustcap with adhesive tape and used a ruler for measuring the excursion.
The camera is on a tripod for exact same perspective with each shot.
I consecutively set the lab supply to 2 - 3 - 4 - 5 - 6 - 7 - 8 - 10 - 12 V in standby mode, switched it on for only about 1 second and took a picture.

Plotting the corresponding voltage/excursion graph in excel provides a good idea of the woofer's Xmax of about 3 mm:

sony_I-X.png


Just for completeness, this is an old fashioned driver, spec wise, but not that bad:

fs 68.4 Hz
Qms 7.798
Qes 0.837
Lp (1W/1m) 88.48 dB
Vas 11.49 litres
 
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I hope it's OK to post this here in your thread, @Randy Bassinga:

yesterday I just did a very quick and dirty Xmax measurement of an old sony 6" woofer to check if it is usable at all.
I used a crude DC voltage vs. excursion method, already described here (corresponding distorsion measurement here).

Caution: it is essential not to overload the voice coil with DC!
an oscillating voice coil is being cooled by circulating air, but a steady DC current cannot do this.
So be sure to activate DC for the woofer only for very shor time!
(also heating up the voice coil will raise it's resistance and will influence the measurement - you could of course use constant current method ...).

My woofer has a small 1 inch VC, so max thermal power will be around 30-40 watts.
The woofer has 5,2 Ohm DC resistance, so 12 V equals about 28 W which is already near the limit.
Also, the woofer will reach it's Xmax quite easily with DC, so 12 V was my maximum voltage.

Here is my setup:
View attachment 1320337

I attached a paper strip to the dustcap with adhesive tape and used a ruler for measuring the excursion.
The camera is on a tripod for exact same perspective with each shot.
I consecutively set the lab supply to 2 - 3 - 4 - 5 - 6 - 7 - 8 - 10 - 12 V in standby mode, switched it on for only about 1 second and took a picture.

Plotting the corresponding voltage/excursion graph in excel provides a good idea of the woofer's Xmax of about 3 mm:

View attachment 1320338

Just for completeness, this is an old fashioned driver, spec wise, but not that bad:

fs 68.4 Hz
Qms 7.798
Qes 0.837
Lp (1W/1m) 88.48 dB
Vas 11.49 litres
That's some good work there, man. Would a cup on at the end of a bolt ina jig be good for pushing the cone back to take 'almost' limits? Cup to fit over the dust cap. Rotate the bolt one way to push back, the other way to release tension back to zero. Something like this with a freewhieeling cup can be easily printed
 

stv

Member
Joined 2005
Paid Member
pushing the cone back
Not sure if I understand that ...
The important value is Xmax, where the voice coil starts leaving the magnet gap and excursion is no longer linear, thus causing distorsion.
This is why i did the current/excursion test.

The mechanical limit Xmech or Xdamage, where the spider or even the voice coil hits a limiting edge is usually much higher, but its not very relevant for music reproduction.
 
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I hope it's OK to post this here in your thread, @Randy Bassinga:

yesterday I just did a very quick and dirty Xmax measurement of an old sony 6" woofer to check if it is usable at all.
I used a crude DC voltage vs. excursion method, already described here (corresponding distorsion measurement here).

Caution: it is essential not to overload the voice coil with DC!
an oscillating voice coil is being cooled by circulating air, but a steady DC current cannot do this.
So be sure to activate DC for the woofer only for very shor time!
(also heating up the voice coil will raise it's resistance and will influence the measurement - you could of course use constant current method ...).

My woofer has a small 1 inch VC, so max thermal power will be around 30-40 watts.
The woofer has 5,2 Ohm DC resistance, so 12 V equals about 28 W which is already near the limit.
Also, the woofer will reach it's Xmax quite easily with DC, so 12 V was my maximum voltage.

Here is my setup:
View attachment 1320337

I attached a paper strip to the dustcap with adhesive tape and used a ruler for measuring the excursion.
The camera is on a tripod for exact same perspective with each shot.
I consecutively set the lab supply to 2 - 3 - 4 - 5 - 6 - 7 - 8 - 10 - 12 V in standby mode, switched it on for only about 1 second and took a picture.

Plotting the corresponding voltage/excursion graph in excel provides a good idea of the woofer's Xmax of about 3 mm:

View attachment 1320338

Just for completeness, this is an old fashioned driver, spec wise, but not that bad:

fs 68.4 Hz
Qms 7.798
Qes 0.837
Lp (1W/1m) 88.48 dB
Vas 11.49 litres
LOVED your actual measuring it.
State of the art designing as far as I am concerned 👍🏻
 
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25mm Xmax is not "absurd", there are many examples of drivers built for excursions well past that being built that still sound as good (or better) at their rated excursion as drivers with half the Xmax:
Screen Shot 2024-06-10 at 5.20.27 PM.png

Their specified Xmax of 35 and 38mm is one way, peak to peak twice that.
Tall surrounds, large spiders, long voice coils and magnet structures are required.

That said, there are plenty of manufacturers that use "Xmax" (linear excursion) when "Xlim" "Xmech" or "Xdamage" is the actual spec, and others that wrongly use a peak to peak (double Xmax) figure...

STV's DC voltage vs. excursion test would quickly sort the wheat from the chaff ;)

Art
 
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The "Stereo Integrity" woofer mentioned above has abnormally thick 30mm 😲 top plate.
Which translates to very low flux density at the gap.
Which translates to very low sensitivity, sluggish response, wrong Q, the works.

For reference Electro Voice uses 12.5mm, one of the thickest in the market, and that with a huge magnet.
The one shown in the picture is nothing to write home about.
Typical trick used to impress users is to stack 2 - 3 magnets, I have seen 4 in some monstrosities, which is an absolute waste.
Stacking 2 only increases flux at the gap by 10% , maybe 15% tops.
Any more stacking is wasted.

JBL uses 8-10mm plates tops.

Yes, that car subwoofer will have lots of excursion.
Weak excursion which will require kilowatts drive

What you need is higher magnet diameter or surface, nobody does that.

EV typically uses 190mm diameter magnets, JBL 220 mm, guess highest diameter manufactured is 250mm although never met one.
 
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