Title pretty much says it all. Is there a general rule of thumb as to the ratio of xmax to max mechanical excursion? That is how far past xmax before danger of damage to the driver. I realise it will depend on the driver but wondering if there is a ball park margin.
mike
mike
Hi Mike:
Low frequency driver excursion limits prior to physical damage vary all over the place depending on driver size and design. Personally, I'd forget about a rule of thumb in this area and approach things from a sonic perspective.
If you limit the drive voltage to the low frequency passband to the point just before it starts to sound bad, you'll very comfortably avoid the possibility of damage. This is my rule of thumb for all passbands.
There are several ways to test for this, each have to do with sending a short term tone (sinusoidal) burst to the DUT while observing THD or simply using your ears. The transition into "driver slap" is extremely easy to hear.
The ideal stimulus for this test is Don Keele's 6.5 cycle Gaussian shaped tone bursts, which you can download for free from his web site. You can also generate these tone bursts using CLIO Pocket and CLIO FW. The latter has a fully fleshed out CEA-2010 test routine built into it.
THD levels up to about 25% are inaudible (due to psychoacoustic masking) at sub frequencies and this is the point where I engage hard limiting. Xmax is typically defined as the point where 10% THD is achieved. This is far too strict for subs and companies like B&C have recognized this fact and now spec something called Xvar that indicates the maximum peak to peak excursion of the driver at much higher levels of distortion that more closely approximate the threshold of audibility. Xvar is not just another marketing ploy to be able to publish higher Xmax figures.
With drivers inside bass-horns it is much more difficult to 'hear' distortion. Harmonic distortion of third order or higher disturbing order distortions and also inter modulation distortion are dramatically reduced.
Most of the time you hear it when no sound is coming any more from the horn but then you know you have exceeded the drivers max and probably destroyed it. However if you are a bass-horn junkie (i am) you get a feel for how far you can push it.
I always try to limit the input signal in volts based on the maximum xmax measured in the HR simulations. But when testing new designs most of the time we push it to overdrive for a couple of hours to see how the driver and how the enclosure holds up, I am in a location where I can push it till it explodes, but a recone is eminent though.
Most of the time you hear it when no sound is coming any more from the horn but then you know you have exceeded the drivers max and probably destroyed it. However if you are a bass-horn junkie (i am) you get a feel for how far you can push it.
I always try to limit the input signal in volts based on the maximum xmax measured in the HR simulations. But when testing new designs most of the time we push it to overdrive for a couple of hours to see how the driver and how the enclosure holds up, I am in a location where I can push it till it explodes, but a recone is eminent though.
I'd recommend running the driver free air to find what's what.
To take two examples:
1) Beyma 15P1200Nd. High power PA sub driver. 1200w continuous power rating, 9.5mm rated Xmax, rated for 52mm p/p before damage.
2) JBL GTO1214. Mid-range car sub. 350w continuous rating, 14.5mm Xmax.
The Beyma driver, driven free-air, gets to around 14mm one-way travel before the suspension etc starts audibly locking up. Unfortunately, I've found they can do their 52mm p/p travel, but don't survive for long with that sort of abuse. Very impressive to watch, though.
The JBL driver never quite reaches Xmax. Its probably to stop car audio guys damaging them by putting them in ported boxes and driving below tuning.
Chris
To take two examples:
1) Beyma 15P1200Nd. High power PA sub driver. 1200w continuous power rating, 9.5mm rated Xmax, rated for 52mm p/p before damage.
2) JBL GTO1214. Mid-range car sub. 350w continuous rating, 14.5mm Xmax.
The Beyma driver, driven free-air, gets to around 14mm one-way travel before the suspension etc starts audibly locking up. Unfortunately, I've found they can do their 52mm p/p travel, but don't survive for long with that sort of abuse. Very impressive to watch, though.
The JBL driver never quite reaches Xmax. Its probably to stop car audio guys damaging them by putting them in ported boxes and driving below tuning.
Chris
Good point, Chris
There is no rule of thumb and the first rule of thumb is all those rules are wrong--but more right than wrong. Think of Xmax like the redline of a car engine, some engines can fly by their redlines but others will eat themselves quickly. The same applies to speakers.
Another good example is the Alpine SWS-15, it has an Xmax of 15.1mm but the Xmech is 38mm. I don't think you could bottom the thing as the suspension will tighten up and the voice coil won't have enough BL to push the cone that far. You have to remember that car audio drivers tend to be used by teenagers that play Bass I love you which has 8Hz pulses to watch the drivers "flex". Alpine was smart in designing the sub that way, leaking boxes, wrong porting and every way possible to screw up the build. The GTO1214 goes along the same lines, I have one in a sub for HT use and it can take abuse but no issues with Xmax. It will sound weird at high levels with really deep bass but as long as the voice coils are not overheating, it has worked well for the last 5 years.
Just another bonus for DIY sub building, car subs tend to be much more forgiving of bone head mistakes, leaking boxes and forgeting or screwing up the filters.
If you HAVE to have a "rule of thumb" for Xmax, the best I can offer is decent car audio drivers tend to be much more idiot proof than consumer grade/audiophile subwoofer drivers. Also PA drivers tend to have more idiot proofing in them but are generally not used for below 25Hz by design.
I'm pondering using a SWS-15 in a 17Hz tapped horn (Lilwrecker) since it will be my first horn build. The pure idiot proofing of the driver should allow my traditional screw ups with filters, a leak here or there or any new way I can blow things up.
There is no rule of thumb and the first rule of thumb is all those rules are wrong--but more right than wrong. Think of Xmax like the redline of a car engine, some engines can fly by their redlines but others will eat themselves quickly. The same applies to speakers.
Another good example is the Alpine SWS-15, it has an Xmax of 15.1mm but the Xmech is 38mm. I don't think you could bottom the thing as the suspension will tighten up and the voice coil won't have enough BL to push the cone that far. You have to remember that car audio drivers tend to be used by teenagers that play Bass I love you which has 8Hz pulses to watch the drivers "flex". Alpine was smart in designing the sub that way, leaking boxes, wrong porting and every way possible to screw up the build. The GTO1214 goes along the same lines, I have one in a sub for HT use and it can take abuse but no issues with Xmax. It will sound weird at high levels with really deep bass but as long as the voice coils are not overheating, it has worked well for the last 5 years.
Just another bonus for DIY sub building, car subs tend to be much more forgiving of bone head mistakes, leaking boxes and forgeting or screwing up the filters.
If you HAVE to have a "rule of thumb" for Xmax, the best I can offer is decent car audio drivers tend to be much more idiot proof than consumer grade/audiophile subwoofer drivers. Also PA drivers tend to have more idiot proofing in them but are generally not used for below 25Hz by design.
I'm pondering using a SWS-15 in a 17Hz tapped horn (Lilwrecker) since it will be my first horn build. The pure idiot proofing of the driver should allow my traditional screw ups with filters, a leak here or there or any new way I can blow things up.
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