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Old 12th February 2009, 07:58 PM   #1
pjpoes is offline pjpoes  United States
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Default Tweeter Max Spl

Ok so often we can get a sense of the maximum spl capability of a driver simply by looking at it's displacement ability. Programs like WinISD use these formula's to calculate the max spl abilities of the driver, as well as power handling, etc. Is there any reason that a tweeter is any different, can I calculate the maximum spl of a tweeter in a similar fashion?

In other words, if I want to know, very roughly, how loud a tweeter will be able to play with a given crossover, can I simply treat it like a woofer, enter it into something like winisd (or any number of other programs with similar abilities), and add in the filter to find the max spl it will do with said filter. Again, I know this won't be exact, it's rough, but at least get me in the ball park?

I tried this, and it doesn't blow up, but I have no idea if it's accurate. I took known parameters for my Focal TC120 tweeters and for some Scan Speak models, and placed the tweeter into a very small enclosure, then added a filter, say a B-W 2nd order at 1800hz, apply a certain amount of power, look at the xmax, and see if I'm still in the linear range. When I did this, I came up with 100db's for the Scan-Speak with a 2nd order butter-worth filter at 1800hz and 50watts, putting it right around the xmax limit. I then put in a 6th order filter at 3600hz and found that I couldn't hit the xmax before hitting the power handling limits, but could push it to the xmax at around 2000 watts.

If I wanted to know the max spl, and had it crossed over in such a way that the tweeter couldn't hit it's mechanical limits, how much above the iec rating can you typically expect such a driver to handle for short transient bursts? I've read some articles indicating 2-3 times the iec is common.
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Old 13th February 2009, 07:11 AM   #2
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Default designing

figure about about 90 to 92 db range
look at the frequency response find the area the has a + - 3db or less, less is better but more expensive and difficult
double the Fs of the woofer and look at impedance chart 8 ohm speakers should be used in the 12 to 6 ohm range for amplifier stablilty
plan the crossover hopefully at full octave or 1/2 octaves closest to the point where the spl dips or rises going into second resonance.
you will also have to figure out the impedance of two speakers in parallel if you using a mtm or wtw arrangement and figure in a load resistor into the crossover.
the tweeter may be 4 ohm which will draw more amps so figure on a load resistor will be needed there too.
use the same rules for the woofer
do a spl analysis with speakerworks and other software that is out there and remember to look at the differnt crossover calculators 2nd orders will need to have the tweeter in the real speakers polarity reverse -180 Degree phase shift
use a different resistors in series and parallel to the tweeter to bring the spl down to around the same spl as the woofers just remember that this is just a starting point for you will have to fine tune it later.
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Old 13th February 2009, 07:36 AM   #3
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Default dont go past a 4th order

no the program will not be able to tell you that because those measurement are done with an actual speaker if the manufacturer says the the max spl it is at 95db or - 3db of the highest peak for the actual sound will shift up -+ 3 db with burst for you have to remember that depending on the sound and the reaction of the speaker is fighting it self and the heat and if you exceed the current capablities of the wire of the voice coil you just burnt up that tweeter that is why they usually will put a load resistor in a crossover to help keep the current down and also to match the spl of the woofer and the spl is done at 1 watt and 1 meter. The spl db is used as a measure of effeciency
The spl will vary by distance and location in relation to the speaker.
High db does not always mean that is it the best way to go
1. Are you going to play it that loud all the time
2. The room acoustics will alter the sound
3. The more nth degree crossovers cause more problems
4 uses few components as possible
5 match speakers that compliment in frequency summimg
6 balance the load to the amplifier
7 the best polar dispersion capabilities and the more expensive area
8 the mininum wave defraction as possible
9 minimize cabinet resonance
there are great speakers that can be made but you dont have to spend an arm and a leg to get it
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Old 13th February 2009, 11:14 AM   #4
Ron E is offline Ron E  United States
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You can model the LF rolloff and mechanical excursion characteristics of a tweeter with measured T/S data, but usually Vas is not specified, you can just assume something low (for guess purposes, note that a typical Vas in liters is just the square of the diameter of the speaker in inches) and and put it in a box at least 10x Vas for modeling purposes.

Don Keele (in Audio equipment reviews) used to do burst tests on tweeters and typically the tweeters would handle 1.5kW bursts as long as they weren't displacement limited. Look to a library for some old Audio speaker reviews for context.
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Old 13th February 2009, 05:30 PM   #5
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Agreeing with Ron, yes, you can model things that way if you know all the relevant information. If you are only interested in the excursion limit this works fine. If you are thinking about trade offs between power input and mechanical excursion, then the other things to keep in mind are power compression and driver non-linearities. The coil will heat up as you apply power. This increases the coil resistance causing less current to flow for a given input voltage. This is not modeled in typical box programs, so you will probably get a few dB less output in a steady state scenario than the box program would predict if you are looking at the thermal limit. You will get some compression even on bursts, but not as much. Also, driver suspensions typically stiffen with excursion requiring more force (current) than expected to push them further. This will also change the response of the driver around resonance. Also, BL typically drops as the coil moves away from center, so more current is required to produce the same force as the coil moves away from rest. Once again, if you know enough details, you can model everything, but most of the time it's easier to do some tests with the actual driver. Look at things like where the output stops increasing linearly with the input signal, where distortion starts to get too high (subjective, but maybe 10% 2nd + 3rd harmonics would be a place to start), and where you start to get close to the mechanical limit of the driver (typically the other two things would happen first).
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Old 13th February 2009, 07:48 PM   #6
Pan is offline Pan  Sweden
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Some lesser tweeters don't handle high power burts well but then again, many do.

Dynaudio use to tests their speakers with 1kW bursts.

A Swedish designer use 3kW bursts for dome tweeters and 10kW for bass/mids. One design is a four 8" 2.5 way system that can handle 40000W peak in the midrange.

If the bursts are short enough in time obviously the amount of energy is not very high and the VC won't burn. What you really get a grip on is the mechanical strength of the driver and short transient capability. Some are ripped apart while other well made designes can withstand extreme G-forces.

Thermal behaviour in dome tweeters are improved with ferrofluid in the gap and with non fabric domes as metal for ex.


/Peter
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Old 13th February 2009, 08:59 PM   #7
pjpoes is offline pjpoes  United States
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as I said, I knew it would be rough, I knew it wouldn't take into account thermal compression, non-linearities other than xmax non-linearities, etc. My goal was to just be able to model very roughly about what the spl capability of the tweeter would be with a given amount of power with different crossovers.

Some of this was also to "check" my view that few speakers are capable of linear low compression output in excess of 100db's over a wide range, and thus far, this does seem to be the case.

For instance, if one was to be designing a speaker inwhich the goal was to achieve realistic sound pressure levels, and lets say that means being capable of at least 105db's with little or no compression or mechanical distortions, what would it take to do that? I know my speakers from Dr. Geddes can do that, but what about other more conventional designs. Can a 1" dome tweeter with .4mm of xmax peak to peak produce 100db's at say 1500hz (according to the formula Linkwitz provides, the answer is not even close). Can a 6" midbass produce 50hz at 100db's, and again, I'm finding that while not impossible, is uncommon. This seems to support my contention that a lot of conventional speakers on the market can't achieve realistic levels with suitably low levels of compressions and distortion. The midbass issue was more easy to model and test, but tweeters are harder. Listening to 10khz tones or even broad band tones from 1khz to 20khz is very obnoxious, especially that loud.

In thinking about the amount of reserve power one wants to meet these transients, it's also good to know just how much is safe and reasonable before you chance thermal problems. In other words, while a tweeter will likely blow rather quickly from hitting it's x-damage point, if the crossover is suitably high and of a steep enough order, it's possible that you will reach thermal limits before mechanical limits, and this brings about the issue of short term transient power handling. I knew that Dynaudio and Morel often stated short term peaks in excess of 1kw, but I wasn't sure if this was true of others.
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Old 14th February 2009, 03:41 PM   #8
Ron E is offline Ron E  United States
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IMO, tweeters are not necessarily the design problem when seeking a speaker that can produce 105dB average SPL. With typical Hifi tweeters you would be running them at 30-100W or so, which is out of their comfort range, so to speak - many are more designed for 1-10W input, whatever the specs may say.

So failing the HiFi tweeter route, you go the Pro route and you just gained 15dB of output capability by making that selection and going with a horn loaded driver.

That's not to say you can't make Loud direct radiator systems. It just takes bigger more sensitive drivers and higher crossover points.
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Old 14th February 2009, 04:28 PM   #9
pjpoes is offline pjpoes  United States
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well and if you've followed my thread my new speakers are Gedlee Abbeys, so obviously I did finally figure that out myself. However, when trying to discuss this with others, and explaining why speakers like these are not just best for this, but potentially the only reasonable approach, I need to best understand why other approaches can't or struggle to meet it. I had what I needed to explain the difficulties in something like a 6" driver or two producing 80hz at 105+db's, but I didn't really know how the tweeters faired by comparison.
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Old 14th February 2009, 06:01 PM   #10
Pan is offline Pan  Sweden
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A slight waveguide makes a lot of sense for a hifi style dome tweeter and crossed at 3k makes for a very potent speaker.

One must realize what the typical spectra is of music before deciding on what is needed. Obviously personal listening preferences and habits play a big role as well.

Small midranges is of no use so either one big midbass or several small/medium.

Pictured speaker has about 0.05%THD at mid frequencies at 100dB and no thermal compression. Ouput 130dB SPL and work over 80Hz with separate woofers down below to 16Hz or so. Handle 40kW peaks in midrange and 3kW peaks in tweeter range. Tweeter has about 0.05%THD at 90dB SPL in the low range.



/Peter
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