Is Bassbox flawed? |
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stevie
Registered User Joined: 16 March 2008 Location: Dorchester Status: Offline Points: 425 |
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Posted: 10 November 2009 at 6:02pm |
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I believe there a few here who use Bassbox Pro. I’ve had it for a while and have been increasingly concerned about the accuracy of its power handling figures. I find no mention of this anywhere on the web. I've modeled a JBL 2204 12" driver that I
plan to fit in a 47-litre box. The box is tuned to 45Hz. The software tells me I will get about 112dB at 40Hz at maximum power, which I have entered as 350 watts. OK. Cone displacement at 40Hz and max. power is about 4.5mm - well within the xmax which JBL specifies as 6.85mm. So far, so good. However, the Maximum Electric Input Power curve tells me that the system will only handle 40 to 50 watts at 40Hz. This has to be wrong – or am I missing something? |
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stevie
Registered User Joined: 16 March 2008 Location: Dorchester Status: Offline Points: 425 |
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Sorry about the garbage in my post: I don't seem to be able to edit it out. Here's a jpg of the Bassbox simulation.
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Elliot Thompson
Old Croc Joined: 02 April 2004 Location: United States Status: Offline Points: 5172 |
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Bassbox pro is correct. The electrical side of the driver is not robust enough to manage the amount of wattage you plan to feed it at the given frequencies. The driver has gone into power compression.
You can always contact Harris-tech if you encounter a problem. They are very prompt on their replies. Best Regards, |
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Elliot Thompson
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stevie
Registered User Joined: 16 March 2008 Location: Dorchester Status: Offline Points: 425 |
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I still don't get it. Bassbox tells me that the driver can handle 350 watts at 100Hz but only 50 watts at 40Hz, yet the driver excursion graph shows that the coil is well within the gap at 350 watts. According to the program, xmax is reached at 800 watts input at 40Hz.
The low frequency performance of the JBL 2206, a very similar driver but rated at 600 watts, is identical. Is there some other factor limiting the power handling to 50 watts? I suspect I may be missing something quite fundamental, but I'm blowed if I know what it is. |
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fredos666
Registered User Joined: 11 October 2008 Status: Offline Points: 205 |
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What is impedance of your box at 40hz? If in the 50-100 ohms, you will never be able to put more than 50 watts in your box, you are at resonance and maximum effiency, so little power need to get good spl...Wish that light you up!
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Jake_Fielder
Old Croc Joined: 08 October 2007 Status: Offline Points: 4231 |
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If the cone is moving more and more towards the lower frequencys its impedance will rise, if you are putting 50v (=250w through 8r, very roughly) accross the driver when the impedance is at 50r, the 50v accross it will only equate to 50w, ohms law. V=IR
V/R=I
50v/50r = 1A
VxI=P
50v x 1A = 50w
where as at 8ohm...
50v / 8r = 6.25A
50v x 6.25A = 312W
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stevie
Registered User Joined: 16 March 2008 Location: Dorchester Status: Offline Points: 425 |
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Sorry, Jake and Fredos, but Bassbox shows an impedance of 9 to 10 ohms at 40Hz,
peaking at a mere 16 ohms at 24Hz. Also, if I increase xmax to 15mm (!)
in the driver parameters, electrical power handling goes up to
200watts.
I think maybe I should try another program. |
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opus jody
Young Croc Joined: 06 June 2006 Location: United Kingdom Status: Offline Points: 1246 |
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If you tune the above example higher, until the 'acoustic power' graph shows a really neat knee shape, and if the other graphs (excursion,vent air velocity) still look ok, and impedance graph shows two peaks of roughly the same size, then that's about as good as you'll get it on bass box. that'll keep it (for example) flatter to 50Hz, even if there's less level at 30Hz, and that sounds much better to me.
if you need lower, then you need bigger box, or different drivers. |
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Improvised Hardware Music http://vimeo.com/user9389813/videos
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Elliot Thompson
Old Croc Joined: 02 April 2004 Location: United States Status: Offline Points: 5172 |
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There are two ways a speaker will become damaged. Those two things are Mechanical Damage (There is no reason to explain the obvious) and Thermal Damage. Thermal damage stems from the voice coil becoming too hot and burning up based on too much power. Xmax has nothing to do with Thermal damage and, “Maximum Electric Input Power” means how much power the voice coil can manage before it becomes damaged. Did you know that the JBL 2204H is at least 25 years old and, the wattage is based from an AES measurement of 150 – 500 Hz? Go to JBL’s website, download the spec sheet of the JBL 2204 H and, read it thoroughly. The limiting factor is not Bassbox Pro. It is the JBL 2204H Best Regards, |
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Elliot Thompson
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stevie
Registered User Joined: 16 March 2008 Location: Dorchester Status: Offline Points: 425 |
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Thanks for the reply, Elliot. The first point I would like to make is that Bassbox doesn't know how old the driver is and how its power rating was obtained. It just accepts the figures I give it.
The first screen shot shows the JBL 2206 (I’ve changed the driver to something more current to avoid clouding the issue). I've specified three power ratings for it, changing nothing else. The three curves are for 100W, 600W and 1000W. The curves above 100Hz are what you would expect. However, why does electrical power handling fall below 100Hz? As long as the coil remains within the gap I might even expect power handling to increase because the cone is providing more cooling. According to Bassbox, xmax at 40Hz is reached at 600W, a fact that is not reflected in the 1000W curve. I absolutely agree that electrical power handling has nothing to do with xmax. Now look at the second screen shot. It’s the same driver in the same box. Power handling is as per the manufacturer's spec of 600W. The lower curve shows electrical power handling based on the manufacturer's xmax of just under 8mm. The higher curve shows what happens when I increase the xmax figure to 12mm - electrical power handling doubles. It should do nothing of the sort. Do you see what I’m getting at now? |
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stevie
Registered User Joined: 16 March 2008 Location: Dorchester Status: Offline Points: 425 |
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This is a compact cabinet for bass guitar. I'm going to get some floor lift and will be using some eq to bring the bottom end up a bit, which is why I'm concerned about power handling at 40-50Hz. I would certainly prefer to tune higher, but Bassbox tells me that if I tune the box to 50Hz, the speaker's electrical power handling will drop to 10 watts - even though I am still well under xmax at 350W. I find this very difficult to accept. |
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Elliot Thompson
Old Croc Joined: 02 April 2004 Location: United States Status: Offline Points: 5172 |
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You are trying to get something not even JBL stated it can do. The spec sheet states -5 dB @ 50 Hertz and, -10 dB @ 40 Hz on JBL’s 2204 H graph. If you alter the xmax of course the wattage is going to increase slightly more. However, the driver only offers a 22 mm xmech peak to peak (11 mm one way). It seems you do not want to accept the JBL 2204 H cannot offer a lot of power under 60 Hertz despite what JBL's graphs state. You tuned the box @ 45 Hertz and, have a -3 dB (f3) @ 71.06 This should tell you that the driver cannot deliver a lot of output under 70 Hertz. That type of response is common (Tuning @ the fs of the driver and, having an f3 higher than the fb) when you house a horn-loaded driver in a reflex box. If the JBL 2204 H was designed for a lot of output under 60 Hertz, you would attain a –3 dB @ 45 – 50 Hertz which, would co-inside with a box tuning of 45 Hertz. Best Regards, |
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Elliot Thompson
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