I've been experimenting, using the JBL D2430K driver with the STX825 horn and am beginning to wonder if a 1000 Hz crossover point is too low—especially if I change the slope from 24 dB/oct. Linkwitz-Riley to 12 dB Butterworth. I know it's not too low for the driver, but perhaps it is for that small horn. All I've been able to determine from JBL's published information is that in the STX-825 speaker system a 1.3 kHz crossover point is used, though the slope is not specified.
Do you have measurement tools?
The crossover JBL specifies in their loudspeakers datasheet is typically an acoustical crossover, and a Linkwitz-Riley one 99% of the time.
The electrical filters and EQ points used to obtain it are at different frequencies and slopes.
Hi dubkarma,Originally Posted by dubkarma
Looking at the JBL STX825 service manual
http://www.jblproservice.com/pdf/STX...ies/STX825.pdf
it seems that it is used 3rd-order hi-pass network for the UHF driver (slope= -18dB/octave) and the crossover point is mentioned to be round 1.3kHz. Applying 2.7uF, 1mH, 12 uF elements, the other elements in the UHF section is used to 'flatten' driver-horn combo FR response.
regards
ivica
So using some
With a little eq it is usable down to 1kHz with excellent results. XO 24LR
The solution to the problem changes the problem.
-And always remember that all of your equipment was made by the lowest bidder
Greetings -
The rule of thumb is that a high frequency horn, or horn tweeter, will be crossed over approximately one octave above the horn cut-off frequency. Consider the horn mouth perimeter dimension as the wavelength of the lowest frequency it can reproduce before reaching a frequency below that which will cause distortion due to standing waves.
When a wavelength is equal to or greater than the circumference of an orifice, a portion of that energy is reflected back to the source causing a standing wave. The same problem will hold true for low frequency horns. All commercial and domestic folded bass horns have distortion problems due to internal standing waves because the wavelength of the lowest frequency they will reproduce is always greater than the horn mouth perimeter. Corner horns can reduce that problem.
This is why you can watch the food cook in a microwave oven, without being cooked yourself. The wavelength of the microwave is greater than the circumference of the individual holes in the door screen. The energy is reflected back into the oven.
For example; the crossover point for the ALTEC 811 horn is 800 cycles, although the horn mouth cut-off frequency is approximately 400 cycles based on the perimeter dimensions. Those dimensions being the height of the horn mouth by the length of the arc, not the horn mouth width.
Perhaps you could provide the actual horn mouth dimensions without the flange. If the horn perimeter is equal to or less than 13 inches, and you're crossing over at 1K, you may have a problem.
HF
These two pages are an extract from the book "Acoustics" from Leo L. Beranek.
Ruediger
Not sure if this helps, but here's an impedance measurement of a 2447 (2452SL diaphragm) on an STX825 horn.
In series with the driver were a 15 ohms sensing resistor and a 56 uF capacitor for blocking out lower frequencies. This capacitor and the fact that I forgot to turn of the highpass filter in the amp while doing the measurement (24dB/oct at 1100 Hz) lead to a terrible SNR at the lower frequencies. Also, the measurement rig is not calibrated, so the measurement is only qualitative.
I'm not adept at reading this image - just thought it might possibly be useful for you to identify the resonances mentioned above. If you want, I can repeat the measurement without all the highpass stuff to get a cleaner and more extended result.
The mouth perimeter of the STX825 horn is about 74 cm/29 in, which is the wavelength of a 465 Hz-wave.
It’s really a case of evaluation of your woofer and compression driver/horn with pink noise varying the crossover point to get the best blend. It’s nearly always best to err on the high side for horn woofer crossover points rather than attempt the lowest crossover point like many audio diy amateurs (American) who think a horn always sounds better crossed over as low as possible. Distortion, pattern control and power handling always go to hell when attempting the lowest crossover point.
I recommend you acquire a measurement kit and consider an active crossover.
I hope this helps
In my experience, those newer JBL very short horns (or Waveguides) + snout-less drivers can be crossed at lower frequency than the older conventional horns with the same mouth size for much less side effect, probably due to extremely short overall horn length. Only +-2dB resonance and no rising distortion seen even at @800Hz when I measured 2450SL + STX/2332 (STX825 and 2332 are the same size, but a bit different shape). In fact, old Electro Voice 800Hz horn with larger mouth size had much higher distortion and resonance at the same frequency.
I guess the reason why JBL crosses STX @ 1.3K is just for max power handling. JBL DMS-1 Studio Monitor with 2450SL + 2332 is officially crossed at 1K with 14" woofer, not 15", and I confirmed that I could go even lower than that without any drawback.
BTW, for home usage, I found that it would be preferred to apply -2dB BBC dip EQ (google it) for those JBL horns. A bit more enjoyable to listen to compared to flat frequency response setting. No harshness, no brutally honestness of typical JBL horn.
Ian, thank you for the interesting link.
I'm also in Long Island, and it's good to know someone in my neighborhood has the same preference as mine.
PS: I rechecked the REW measurement data that I have. I said +-2dB resonance, but it is actually +-1dB 800Hz - 2K after applying 1/12 smoothing and some correction EQ. Not bad at all.
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