Yep finally got to it, building a serious 2x15 unlike my last thread
which was for fun and a little experimentation.
So I have a extra pair of 3015's laying around, so decided to build a 2x15 for these drivers. The total volume and tuning would also be suited also for many other 15" drivers.
There will be a cavity for each driver to minimize
distortion, and dedicated ports for each.
The design also includes symmetrical ports. The driver is centered to the enclosure. So any pressure leaving the rear of the cone will arrive at surrounding wall/corners at approximately the same time. Likewise with all port exits, all of them are symmetrical to the driver. The placement within the cavity of the ports is also spaced the same for all 4 ports.
The combination of symmetrical ports and centered drivers within the cavity will promote all pressures to react the same and minimize offset distortions
Typical baffle offset of drivers and offset or non symmetrical placement of ports is not being used which is common for most loudspeaker design. Symmetrical ports are being used to minimize cone distortion. And symmetrical driver location is being used so all pressures entering the cavity, and leaving the ports will react to the cavity in the same manner.
Again also symmetrical ports to reduce distortion from cone pressure and port pressure when they combine to create the end waveform being produced. Viewed in models of port pressures with cone pressure, non symmetrical port location can create waveform distortion within frequency when port is not fully resonating at resonate frequency. But the port is still beginning to release pressure beginning when cone movement starts to reduce and ports are producing in phase or out of phase pressure from previous half cycle of the waveform. Depends on frequency Fb of reflex cabinet and current symmetry of waveform. Modeling port behavior with a symmetrical waveform without harmonic partials such as a sinewave. Will simply not show the complexity and distortions created by a reflex type transfer function. Again trying to equalize internal pressures and exiting port pressures will reduce distortion created by non symmetrical waveforms.
when negative pressure and positive pressure meet distortion of the waveform occurs and can also create standing waves and more distortion within the port. Again with non symmetrical or harmonic waveforms each half cycle of the waveform is not equal and is more likely for standing waves and distortion to be created within the port. Since each half cycle of the waveform is not equal being non symmetrical, resonance does not occur evenly distortion will be created. The reflex or ported cabinet relies on a positive pressure to move forward from the cone, which in turn fills the enclosure cavity behind the speaker with a negative pressure, this completes on half of the sine wave waveform. Then when it reverses to finish the other half of the waveform. The negative pressure is now leaving the cone and a positive pressure is being produced behind the speaker in the cavity. Since the negative and positive pressures do not combine, the now positive pressure being created inside the cabinet will push out the negative pressure that just filled the cavity from the previous half cycle. This negative pressure will now be pushed out of the ports to combine with the current negative pressure leaving the cone. This is why very little cone movement at resonance occurs since you now have a speaker trying to create a negative or positive pressure within the enclosure, but it is pushing against a pressure of equal force of the opposite polarity. Pretty much reducing the ability of the cone to create a waveform and simply relieving the pressure out of the only place it can escape the port.
So say a longer half cycle of say negative pressure has now filled the cavity, and will continue to fill the ports and possible start to exit the ports to meet the positive waveform now leaving the cone. or will also create standing waves within the tube since the negative pressure is colliding with positive pressure now being pushed out of the ports to combine with the current positive pressure leaving the cone. Remember the only amount of available pressure is from the previous half cycle. So if the other have cycle is larger or smaller, the smaller waveform will leave more of the opposite pressure inside the cavity,or a larger waveform will push out all the previous pressure and start to leak out. And then again since more pressure was introduced gives the likely hood that the next cycle might not be large enough to relieve the now extra pressure being produced. It does not magically disappear the pressure goes somewhere, which is distortion, either outside the enclosure as distortion to the waveform, inside the enclosure where it produces standing waves, or inside the port itself where it produces standing waves.
reduction of the distortion i see happening a few ways, increase the length of the port, so when extra pressure is produced more likely standing waves are produced inside the port instead of pressure exiting and causing distortion at the cone. Also the port could include dampening material at the beginning or end of the port, which will actually turn the extra pressure not occurring from resonance into heat instead of audible pressure waves. similar in practice to transmission lines where the line is stuffed to reduce distortion. So when resonance occurs the port will perform as designed, but when standing waves or out of phase pressure is being introduced to the port the damping material is their to help get rid of it.
Or simple by centering the driver within the enclosure and promoting equal pressure within the enclosure and using multiple ports of longer length which are also symmetrical to the driver. Standing waves are more likely to distributed and dissipated by being evenly divided to multiply long ports before exiting and effecting the cone , instead of just one short port being pounded with standing waves and dissipating pressure. and due to the short length more likely for more out of phase material to exit, and also creating standing waves which would promote any new resonate cycles to be distorted, since the port volume is now being occupied by standing waves and cannot resonate to calculated length.
pressure leaving a port at a farther distance from the source, the speaker cone, can produce distortion of the waveform since the port can conduct before the waveform is starting to be produced. So the cone pressure will collide with the port pressure since the port pressure has left first, depends on frequency and symmetry of waveform being produced, also the opposite can happen cone pressure will leave first then port pressure will leave after. Essentially pushing against the developing wave instead of the wave colliding with a already existing pressure as before. From a single offset port source this pushing or colliding will be more intense and cause more distortion to one side of the waveform since all the pressure is being created at a certain velocity in a certain location.
By using multiple longer lower velocity ports placed closer to speaker cone and in a symmetrical locations , it is more likely for the pressures to combine earlier and evenly and create less distortion. also due to the extra length of all the ports combined and extra port area, port pressure is being released more slower over multiple smaller openings placed closer to the source over the period of the half cycle of the waveform. Not all at once in a centralized area again reducing distortion and allowing the 2 pressures to combine smoothly and be produced from the same source being the speaker cone.
Very similar practice to keeping 2 drivers as close as possible so they react as a single source, likewise keeping speaker cone and port openings as close together as possible so they react as a single source. trying to mend the port and speaker as one unit, since you cant really put a port in the middle of the speaker cone, simple just mount it as close as you can. possible even shape the exit to the shape of the cone itself.
Maybe those old round port openings directly around the driver on the old fender tone ring speakers were on to something.
Above image is for response of single driver
Volume chosen was for good power handling and decent bass response. Each cavity is 3.2 Ft3 (90 L) for a total of 6.4 Ft3 (180 L). No specific alignment is being used, practice for Fb is very similar to a BB4 alignment where cabinet tuning is set at resonant frequency of the driver which is 45 hz for the 3015. Required volume for a actual BB4 is around 2.8 Ft3 (79 L) decided to use a larger volume for more bass response at 3.2/90L which is closer to a SC4 alignment.
But still using a lower BB4 tuning frequency (Fb) of 45hz to match driver resonant frequency.
Actual real world Fb im hoping will fall slightly lower around 43/44 hz.
Power handling is roughly 300 watts per driver below 100hz for
a total of 600watts with 2 drivers. Above 100hz the driver can be driven to its 450watt thermal limit without distortion for a total of 900watts
External dimensions are 24"wide 38" high 16" deep
again i have based the front dimensions according to the
Golden Ratio 1.618. The ratio has been used for many years by famous architects and artists to produce a object that is aesthetically pleasing. Also the dimensions are very similar to a Sunn 200s and will also include a false front edge like the 200s to make the side walls look beefy. And will also use a pretty cloth grill like all classic cabinets should have. The sunn 200s is approximately 1inch higher than what the golden ratio would give you to the width of 24", Golden Ratio for 24" wide would be 38 53/64 ". I decided to round it off to a nice 38" dont need to twist my head on the table saw with a bunch of fractions to calculate. Biggest downfall i would say to the measurement system that i grew up on and was taught. Freakin fractions ....blah