Thank you for the graphs...
Based on my testing - I'm seeing my most efficient scores are from the setups that have a high rise on the right 'peak', with best scores occurring near the top - but not actually at the top... In other words - it appears the higher I can get that right peak (around 60hz) the better the SPL/power ratio.
Typically my tuning seems to be around 46hz, with the peak SPL/ratio happening around 60hz.
Based on my testing - I'm seeing my most efficient scores are from the setups that have a high rise on the right 'peak', with best scores occurring near the top - but not actually at the top... In other words - it appears the higher I can get that right peak (around 60hz) the better the SPL/power ratio.
Typically my tuning seems to be around 46hz, with the peak SPL/ratio happening around 60hz.
Did you enlarged the length of the 7" tube too, to maintain the same tuning frequency?
OK, so it is the measured amplifier power at specific loudspeaker impedance at specific frequency. Than you have to experiment with 6-th order bandpass box and play with both tuning frequencies and both volumes, while monitoring the SPL and impedance at 60Hz. Bandpass boxes usually are large.
This is I'm afraid incorrect. There are only two resonances in the system. The enclosure alone, yes, has a Helmholtz resonance. The moment that enclosure is excited by a driver, the Helmholtz resonance goes away and you get two new resonances. The saddle indicates the resonant frequency of the enclosure, but there is no resonance there. The saddle is the combination of the two mode shapes in the system working together and that creates the driver excursion minimum and port displacement maximum. This, however, is not a box resonance. It is a function of the modal behavior above and below tuning. A ported enclosure has two degrees of freedom (a driver resonance, and an enclosure resonance), so it must have two resonances only.
It is not intuitive, but it is correct. Resonances are eigenvalues of the loudspeaker system model, and a ported box only has two. Look into the electro-acoustic model of a loudspeaker and ported enclosure system and you will see what I mean.
In you opinion - should I be going a different direction to get the desired high spl/lower power combo? (Large box with large port)?
Something important to understand is exactly what the impedance curve means. A ported box has two resonances (not one!), but has one tuning frequency. As you may know, a resonance is when the stiffness and mass are equal in magnitude, yet sum to zero. You have the stiffness of the driver, given as Cms (compliance = 1/stiffness), the moving mass of the driver, the stiffness of the air in the box, and then the mass of air in the port. Think of the port as a large barrel of air that moves as a solid unit. That solid unit is modeled as a mass. This summing to zero business happens twice in a ported box, hence you get two peaks:
Fhigh: This is the higher frequency peak above the tuning frequency (your right peak). This is the stiffness of the driver's suspension and the stiffness of the air in the enclosure balancing the moving mass of the driver. This is pretty similar to the single peak in a sealed enclosure as the port is not really coming to party yet.
Flow: This is the frequency peak below the tuning frequency. This guy is the stiffness of the driver's suspension balancing out the moving mass of the driver and the mass of air in the port. The larger the port, the more mass there is. It's for this reason that not all tunings are created equal: port area does matter along with tuning.
The low point in between, as you already know, is the tuning frequency of the enclosure. While the distance between the peaks depends on the chosen drivers, the point where the minimum occurs is a function only of the box/port combination. It matters not what's driving it.
The peaks are usually at some combination of heights. When the first peak (higher frequency) is equal in magnitude to the lower peak, you get the indication that the box tuning is identical to the Fs of your woofer. When the first peak is lower in magnitude, this means the box tuning is higher than the woofer Fs. When the opposite, that is, the lowest frequency peak is the smallest, the box is tuned below woofer Fs.
When you're at a resonance (NOTE: not at the tuning frequency), all that's in play are the resistive losses in the system. High Qms drivers (low resistive losses) will give you large impedance peaks for this reason. What you're noticing, that is when you extend the port, is that you're adding more and more mass for the woofers to drive. That SPL keeps getting higher and higher as you reach the point where the suspension can no longer fully keep up with its own mass and the port mass. Once the port mass becomes too great, the efficiency falls. You're able to do this iteratively because you have 3 woofers in a small enclosure, and it takes a large amount of port mass to cancel out the high box stiffness with the driver's own stiffness. In other words: your box is incredibly stiff, so you need a lot of port mass to create that second resonance.
To put this another way, by continually extending that port, you're optimizing the system to the tuning frequency that you want. After a certain point, the driver suspension can no longer continue to keep up with the mass of air moving out of the port, and the drivers start to lose control over it. Your efficiency starts to fall at this point.
Ported boxes are fascinating things. I recommend a book, Bullock on Boxes, which gives great insight into ported enclosures. The original Richard Small and Neville Thiele vented box papers are still required reading for every loudspeaker enthusiast, in my mind.
Hopefully this helps some.
OK, but did you enlarge the length of 7" port enough to achieve the same tuning frequency as with 6" port? If the tuning frequency is different, than it is no wonder that you get different impedance.
If the port is longer than the box, use 90 degrees elbow joint.
Honestly no, I'm with Sonce and chris in that a higher order enclosure will likely win the day. That said, a very high Q ported box at vehicle resonance is also a tried and true favorite which is what you're trying. I'm assuming that these ports are all external, so that whenever you're increasing the length, the net volume is not changing?
Your right, I need to compare apples to apples here...
Looking at my notes - with a 65hz tuning on both setups:
6" port, 10" long = 1.83 ohms @ 58hz
7" port, 19.75" long = 4.00 ohms @ 58hz
The ports are ran externally so the enclosure volume remains the same...
I'm trying to rethink my box so I can get the tuning down more around the 60hz area - but based on my testing results - still seeing significant impedance differences between the two (higher power readings with the 7" port and lower scores).
Looking at my notes - with a 65hz tuning on both setups:
6" port, 10" long = 1.83 ohms @ 58hz
7" port, 19.75" long = 4.00 ohms @ 58hz
The ports are ran externally so the enclosure volume remains the same...
I'm trying to rethink my box so I can get the tuning down more around the 60hz area - but based on my testing results - still seeing significant impedance differences between the two (higher power readings with the 7" port and lower scores).
Your explanation is very detailed (especially post #11), very valuable and of course a correct one, but I was using the word "acoustical/box resonance" as an acoustical indication of the port/box resonance (peak of the port output), the same as in the Neville Thiele paper Loudspeakers in Vented Boxes, part 1:This is I'm afraid incorrect. There are only two resonances in the system. The enclosure alone, yes, has a Helmholtz resonance. The moment that enclosure is excited by a driver, the Helmholtz resonance goes away and you get two new resonances. The saddle indicates the resonant frequency of the enclosure, but there is no resonance there. The saddle is the combination of the two mode shapes in the system working together and that creates the driver excursion minimum and port displacement maximum. This, however, is not a box resonance. It is a function of the modal behavior above and below tuning. A ported enclosure has two degrees of freedom (a driver resonance, and an enclosure resonance), so it must have two resonances only.
It is not intuitive, but it is correct. Resonances are eigenvalues of the loudspeaker system model, and a ported box only has two. Look into the electro-acoustic model of a loudspeaker and ported enclosure system and you will see what I mean.
"OMEGAb (fb) is the box resonant frequency, or more exactly, the frequency at which the acoustic mass of the vent resonates with the acoustic capacitance of the box."
I am well aware of the electro-acoustic model of a loudspeaker and I can only repeat your recommendation for enthusiasts to read the good Bullock book and papers of Thiele and Small. A real treasure, indeed.
Seeing that there is a direct correlation between port length (and area) and impedance (e.g. longer port = higher impedance = lower amperage).... How do I do the same spl with lower power?
=)
=)
As others have suggested, a 6th order bandpass "could" achieve that one note boom ! For eg, with just one driver in 2 cu ft, see my screenie. 1.5 cu ft = back box 0.5 cu ft = front box. Both tuned to 58Hz.
I think part of the problem of attempting to get what you want, is trying to stuff too many drivers in one undersized box for them to work effectively.
I think part of the problem of attempting to get what you want, is trying to stuff too many drivers in one undersized box for them to work effectively.
Attachments
I do plan to play with 6th order, and possibly 8th order - but not enough time left before big show this weekend...
I'd also be open to running fewer woofers - but it needs to be louder than what I've currently got. (on less power). In my class, I can run a single 15", pair of 12's, or three 10's.
Currently, my 3-L710's setup and 2-soloX10's setup do almost the same score on 500VA, just trying to gain another half db or so without increasing power.
I've not see a TH do well in a vehicle (for spl purposes)... but don't you 'tune' those to the FS of the woofer? Would be waaaay to low of a note for my purposes. =(
I'd also be open to running fewer woofers - but it needs to be louder than what I've currently got. (on less power). In my class, I can run a single 15", pair of 12's, or three 10's.
Currently, my 3-L710's setup and 2-soloX10's setup do almost the same score on 500VA, just trying to gain another half db or so without increasing power.
I've not see a TH do well in a vehicle (for spl purposes)... but don't you 'tune' those to the FS of the woofer? Would be waaaay to low of a note for my purposes. =(
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.