Jeff Wheeler wrote on Tue, 13 April 2010 17:24 |
Evan Kirkendall wrote on Tue, 13 April 2010 18:02 | Jeff, I'm sorry this thread is drifting so far off topic. I'm done with this, and hopefully you've figured out what you need to do.
|
Off-topic discussion is often educational. As long as no one asks what kick drum mic I am using, I will keep reading the thread.
Art Welter wrote on Tue, 13 April 2010 17:11 |
Jeff Wheeler wrote on Tue, 13 April 2010 16:04 | My experience emailing JBL for spec help has been mixed, but I will try that and see what they say.
|
Contacting JBL would be interesting, but I was suggesting you check the excursion yourself, since you have the cabinet, amp, etc. needed to do the test.
|
I don't understand how I could measure Xmax if it is defined as when the driver begins producing more than 10% distortion vs useful output, except if I measured in a particular cabinet with an analyzer capable of measuring distortion vs program material or generated signal.
Incidentally I did briefly look at the 2269 drivers but it looks like they are over-engineered to take a lot of abuse at the expense of efficiency, which is not what I think I need.
|
Jeff,
Distortion testing does not require fancy equipment, though fancy equipment makes it faster.
You can make a real excursion chart , and see what power level is required to exceed Xmax by using the chart below to measure distortion.
You can also just do it “by ear”, if harmonics are quite noticable, you have exceeded Xmax.
It will require an RTA, and a sine wave source, which are available as free software. You will need a flat response test microphone able to handle high SPL with low distortion. You will also want to check your amplifier output to see that you do not exceed the power your speaker can safely handle.
You will need to remove the grill to see the cone excursion clearly. A dot of Whiteout or a silver sharpie makes cone movement easy to track because of persistance of vision, holding a ruler next to the dot will make it easy to measure. You will be seeing peak to peak motion, Xmax is measured as half of that.
Wear hearing protection, the test will get loud.
The test should be done outdoors so room rattles and standing waves do not effect the results, but if the microphone is placed quite close to the center of the cones and port, an indoor test will still give you a reasonable idea.
Below Fb, the box tuning, excursion and distortion will rise dramatically, so it is a good idea to find Fb by rocking the sine wave tone up and down in frequency, the excursion minima will be Fb.
Testing below Fb is interesting,but watch out that Xlim or Xmech is not exceeded, obvious bottoming out of the spider of voice coil, or suspension pushed to the point where the pleats are stretched flat.
You will find that Xmax can be exceeded with very little power below Fb. The excursion test will allow you to set a prudent HP, usually just a few Hz below Fb.
Now advance the level of the sine wave tone into cabinet, raise level until a harmonic is noted at 10% distortion of the fundamental tone, in other words, if 50 Hz measures 100dB SPL, and 100 Hz (the second harmonic) measures 80 dB SPL, you have reached Xmax.
Well, technically you need a little more math, adding up the contributions of all the harmonics, but usually the second harmonic is far enough above the third and fourth that their contribution is minimal above Fb. However, the odd order harmonics, being not musically related, are more objectionable.
This test will reveal much about the character of the sound of your speaker, and how it changes as level is increased. It may suprise you with how much LF distortion you accept as “not too much”.
Since JBL says the PE is 800 watts for the 2268H, to be safe, do not exceed 80 volts for more than a few seconds at a time. If you do approach that level, don’t apply it for much time, and back off if you start to smell anything funny.
If you want to get a feel for the different distortion levels,(they will go up as the level is increased) for a four ohm load, a 2 volt(1 watt), 6.3 v (10 w), 20 v (100 w), 63.3v (1000 w) and 80v (1600 w) .
For an 8 ohm load, those wattage levels are 2.83 v (1) , 9 v (10), 28.3 v (100), 63.3 v (500), 80v (800).
Probably best to use 60 Hz to set your voltage levels, unless you know your volt meter is accurate at all frequencies.
Sine waves really make it clear what a speaker is capable of, and show distortion quite easily.
Art Welter