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kipman725

measuring excursion

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Long time reader, first time caller.  In lots of the reviews the excursion capabilities of the drivers are stated E.G on the 2269H:

 

It is a relatively quiet operating driver with more excursion potential than its looks suggest. It was pushed well past the 19mm Xmax rating and still seemed to be relatively clean at around 50mm peak to peak.

How is this measured? I have some low cost drivers (Pyle plpw15d) with a stated xmax of 8mm, however the other T/S parameters were  not correctly stated by the manufacturer.   They seem to start making bad noises far beyond 16 mm peak to peak, the top plate looks to be about 6-8 mm thick  (hard to get calipers in).  I would like to have a better idea of the actual excursion capabilities for system optimisation as I am using 12 of these drivers but would like to re-design my system to use 16 of these drivers and need to do some thinking about whether its worthwhile and how much amplification I should apply before diminishing returns.

I have attached a picture of one of my current subs, its a quad driver opposed sealed about 250L.  My current plans are to use 16 drivers in dual opposed slot loaded magnet out sealed boxes that are sized so that using the quasi continuous full output of a NU6000 (60Vrms) they can reach 8mm excursion at 20Hz when wired as series parallel 4ohm loads (30Vrms per box).  This change is mainly because the quad boxes are too heavy, I lose output over time due to driver heating and I keep having to un-bend grills.  The main objective of the system is maximum output at 20Hz which for 8mm excursion should be about 120dB/1m/2pi (assuming I could locate the drivers in the same point of physical space). 

 

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Can't really measure it without something like a Klipppel. The only other way I know of if to model the motor and coil and FEA the BL curve (I have a tool to do this, but its a lot of work to set up). Mostly you can get a good aprox based of the coil length and gap height, but its very hard to measure those things on a driver that is not torn apart.  On parts express, they are rated at 8mm so I would expect at 16mm they are jumping out of the gap and are going to have a lot of distortion. You can model that up in a simulation to try and keep em at 10mm or under. They won't take too much power down low, 20Hz might be asking a lot from these guys.... but then again, at $45 or so, they are really hard to beat!

 

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Kyle is right. Xmax is based off of either physical measurements of the coil and gap geometry, a distortion percentage (with a certain signal, at a certain frequency in a certain loading), or measurements of things such as BL, suspension compliance or inductance shifting at various positions of the coil in the gap. There are ways to measure the amount of driver excursion but you need one of these other methods to determine if it is within the limits set for xmax. It's a complicated subject. The "Myths" section on xmax covers some of it.

 

When I make note of excursion behaviors for driver tests. I am not rating the drivers xmax and I'm not measuring the distortion, or BL loss, etc. I am simply making a note of how the driver behaves in free-air, at what point it becomes obviously  distorted or loses it's composure, where it stops responding to power increases with more excursion, how loud the mechanical operational noise becomes, etc. It's a set of general observations about how the driver seems to respond to high excursion in a very basic sense.

What all of this means for your drivers and app is this. Don't worry about the xmax rating of the drivers. What matters to you is at what point the drivers start to sound bad or become in danger of damage. With some careful probing of their limits with measurements and test signals you can determine where they start to lose it and at what voltage that occurs.

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Thanks for the advice, I just wanted to check there wasn't something obvious I was missing... Using the wedge micrometer method (http://www.linkwitzlab.com/faq.htm) on my spare driver I got to about 20mm peak to peak before the noise was objectionable.  However I suspect that this noise was at least in part the amplifier I was using clipping as the only one available to me right now is only 50W.

I have already used 12 of the drivers in anger with 8 of the drivers (quad driver boxes) on one channel of the NU6000 to make a 4 ohm load and 4 of the drivers (dual driver boxes) on the other channel to make an 8 ohm load with bass heavy program material, often hitting the built in limiter, for about 20hrs without driver failure.  So unless I up the amplifier power significantly I'm not too concerned about thermal failure when I change the boxes run them magnet out as the power available per driver is low.

I have included some pictures of the driver (with vent mesh removed), the voice coil former looks to be aluminum and claims to be a "2.5-Inch four-layer dual voice coil".  The ventilation seems to work pretty well with a lot of air exchange when the excursion is pushed.  If I block off the magnet vent the driver makes far more noise.  When I measure the top plate by pixel counting the mechanical diagram I get 13 mm thickness however this diagram is probably not showing this accurately.  The driver is overhung type and the xmax is listed as 8mm voice coil overhang.

A lot of these excursion questions could be answered by properly measuring my existing boxes in the style of the systems on this site, however I don't have access to a suitable outdoor environment with sufficient power.  I could take one to the park but then there would be no power! indoor measurements of these boxes so far have been pretty useless.

 

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system.jpg

PLPW15D-2.pdf

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Fundamentally, SPL is directly proportional to excursion, and running out of xmax means running into distortion. This is something that can be measured with DIY-grade gear

Still gonna take a series of outdoor groundplane measurements, and you won't have a number in millimeters to brag online about, but one can pretty easily determine the practical limits of their gear with a series of stepped-power groundplane sweeps.

Start at a watt, with the mic at a meter, and make a sweep, noting your drive level. Add 3 dB by increasing the drive in REW and make another sweep. Compare the results and look at distortion levels.  When you get to a point where your distortion increases dramatically or you're putting 3 dB of additional power into the driver that's not coming out as more SPL (power compression), you found the limit. 

Make a note of that drive level in REW, then do a little math to figure out the amount of power you can apply and stay within the reasonable operating limits for your gear. Wattage limit = 10^(difference in drive levels)/10). Say you were at -35 dB for the REW attenuator at the 1-watt sweep, and you saw 3 dB of power compression with the attenuator set at -10 dB. This is a difference of 25 dB, so the formula will be 10^(25/10) so 10^2.5, or 316 watts. For higher-SPL systems, you may need to attenuate the mic or position it at 4 meters rather than 1 meter. 

In the case of the sweep set I attached, I don't have distortion data (predates that feature in REW), but you can clearly see the onset of compression in the highest-SPL sweep. We either ran out of driver, amplifier, or both at that point. 

stepped power sweep.jpg

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I am fully in agreement Lilmike that this is the best test as after all xmax is only the way we generate the SPL.  I just lack suitable facilities as I only have a  small outside area at my house (5m x 1m) and get too many reflections/objects resonating that push peaks above the limits of my mic (130dB) at power levels that are below any visible power compression.  So the idea was that by verifying the xmax I would have a better idea of the system capabilities.    I should be able to perform power compression tests by late August as I'm doing an outdoor event that has electrical power and am planning to allow myself a leisurely setup time span to perform such tests.  For context I live in the UK where houses are tiny and all the space is used by someone so only the wealthy/old tend to have large gardens.

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We're not all "landed gentry" here in the states, but I completely understand your situation. 

Still - the set of measurements above? We did those out in the middle of the street in front of my friend's house. Those are actual, calibrated SPLs measured at a meter groundplane. We did get to meet a few of the neighbors during this process. 

I've also considered using a DC source (or even a really slow sine wave) to measure excursion from rest. Step up the voltage applied and check the one-way displacement from rest at each level. For measuring displacement, I've seen people use a pencil or dowel on a measuring tape. When you get to a point where more volts means no more displacement, you found a practical displacement limit. I have no idea if this will even work, and no idea if a typical voice coil motor will put up with this sort of abuse. Seems to me that if the signal is brief enough, heating should be minimal until drive levels get extreme, so it might work for a smaller woofer. One of these days when I have some free time I will give something like this a shot with a driver that I can afford to let the magic smoke out of.

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I also got into a discussion about optical methods of measuring excursion which some people are doing already for driver protection.  The sensors suitable for higher frequencies are quite expensive but at a 60Hz sample rate there is a sharp sensor that looks quite easy to deal with:

http://www.sharp-world.com/products/device/lineup/data/pdf/datasheet/gp2y0a51sk_e.pdf

could output directly into a sound card and use the computer to calculate the peak to peak excursion.

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I managed to perform some measurements using a sharp GP2Y0A51SK0Fdistance measurement sensor and a scope.  The distance sensor samples at ~60Hz asynchronous to the signal gen so I used 16 average mode on the scope (upper trace is distance sensor).  I drove the driver at 5 Hz with a cloud VTX1200.  I have also taped white paper to the cone to get a better reflectivity.  The distance from the sensor can be found using the following equation (from published curve): z = (V/3.76)^(-1/0.814).  From this I found a difference of 1.5 cm between the minimum and maximum value which is ~ xmax (8mm one way).  Obviously easy way to improve the results would be to get the sensor closer to the cone into its more sensitive region.  I also noticed some asymmetry in the clipping when not driven so hard presumably from gravity.

Quite interesting results could be obtained with some PC data logging and a current sensor as it would be possible to plot the displacement Vs coil current and also the back EMF.

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  • Thanks 1

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