This question comes from a place of ignorance and oversimplified logic. Feel free to be as condescending as you wish in your answers.
I was involved in a discussion and using my home system as 'typical'. My main speakers are good down to 50hz. Subsequently, I only need my new subs to respond between 35Hz and 50Hz. For argument's sake let's tune the ports to 42.5Hz. If impedance is highest at the port tuning frequency e.g 32 ohms and does not fall below 12 ohms within the specified does this mean the cabinet is 12 ohms regardless of the nominal resistance of the driver?
I was involved in a discussion and using my home system as 'typical'. My main speakers are good down to 50hz. Subsequently, I only need my new subs to respond between 35Hz and 50Hz. For argument's sake let's tune the ports to 42.5Hz. If impedance is highest at the port tuning frequency e.g 32 ohms and does not fall below 12 ohms within the specified does this mean the cabinet is 12 ohms regardless of the nominal resistance of the driver?
impedance has a local minimum at helmholtz resonance frequency, between two impedance peaks.
impedance will probably be much lower below tuning frequency and also above it. however the nominal DC-resistance of driver is usually lower than nominal impedance. i suppose a driver with 12 ohms DC resistance is probably a 16 ohm driver.
Again, you may have to excuse my ignorance but I need to rephrase the question for clarity.
If the signal bandwidth is limited to the 35-50Hz range, and the impedance is measured at, 12 ohms@ 35Hz, 32 Ohms@ 42Hz, and, 12 ohms @ 50Hz, is it correct that nothing beyond this range can be considered? If there is only one 'peak' with the range, the low impedance point between the peaks becomes irrelevant as there is no input at this frequency?
If the signal bandwidth is limited to the 35-50Hz range, and the impedance is measured at, 12 ohms@ 35Hz, 32 Ohms@ 42Hz, and, 12 ohms @ 50Hz, is it correct that nothing beyond this range can be considered? If there is only one 'peak' with the range, the low impedance point between the peaks becomes irrelevant as there is no input at this frequency?
i assume you are speaking about an active crossover.
even with very steep slopes you have signal above and below crossover frequencies. "no input at this frequency" is technically not possible.
passive crossovers will probably increase impedance in the stop-bands. but this depends on the crossover configuration.
why is the impedance important for you?
It's simply good information to have. It provides knowledge in order to perform practical calculations.
e.g. if I wanted to upgrade this system but couldn't afford to do it all in one hit, could I get away with driving another pair of subs off the same amp until I could afford another amp?
Surtsey,
Considering you seem to be trading in small drivers with little excursion capability, you could get considerably more output using multiple subs in series, or series parallel to increase cone area and output using the same amplifier.
That said, you seem to have the impedance response of ported and sealed subs reversed in your examples.
An example is attached below to illustrate, ported on left (Fb 44Hz) and sealed on the right. As STV explained, Fb (box tuning frequency) and the minimum impedance are at the same frequency, surrounded by higher impedance peaks on either side.
In a closed box, the impedance peak (Fc) is always higher than the driver’s free air resonance impedance peak (Fs) the Fs of 37 Hz rises to about 61Hz in the example.
Below Fb, response drops at around 24dB per octave, and your hearing sensitivity also drops off at lower frequency- if you want decent response at 35Hz, the Fb should be near that frequency.
Art
Attachments
Things outside the range can be "considered", but the amplifier will act as if a ~12ohm speaker is connected.
Current is only drawn at frequencies where voltage is present.
Similarly, I don't care if a speaker drops to 0.5ohm at 0.1Hz, because I know my amplifier would never try to reproduce 0.1Hz.
All that said, I still think you should expand your horizons and see what happens below 35Hz.
A steep slope is good enough: -6dB means the voltage has halved, and so has the current. ie, the demand on the amplifier (as a result of restricting the bandwidth) has substantially reduced.
Chris
For my own domestic set-up 35Hz is good enough. I've been down there and there's nothing to see - at least nothing worth the money. I've had a pair of 15" which allegedly responded down to 18Hz - unimpressed. If you asked me to design a PA system I'd adopt a different set of values.
The Narrow Bandwidth Subwoofer is simply a theory (more applicable to PA systems) that I thought of after talking to both you and Weltersys. The 42.5Hz port tuning frequency is arbitrary. I'd assume the designer would tune the port in accordance their requirements.
The "biggest" amp I have ever owned is a Peavey IP 8.5c https://peavey.com/manuals/80301987.pdf If SPL increases through diaphragm area, the ability to drive 6, 18" cabs from a single stereo amp must be a benefit. Peavey claims the amp weighs 45lbs but I remember it being at least 1.5 tonnes, and I'd much rather have to transport 1 than 2.
It's an idea that I'd welcome more experience users to shoot down based on facts or reasonable theories.
Don't get me started . . .
A bandpass subwoofer is antithetical to the theory. Bandpass subwoofers are good for cars, doorstops, and standing on to reach high shelves. The design only exists to save car audio enthusiasts spending money on hi-pass filters.
The basics of this theory are all about efficiency: the amplifier is only required to amplify the relevant frequencies, the driver is only required to respond to the frequencies it receives - the very basics of an active cross-over system.
The bonus of the narrow bandwidth design is "knowing" the impedance of cabinet, as opposed the driver, will never fall below specification. The result is that several cabinets can be connected to a single amp and remain within the amp's 2 or 4 ohm specification. Efficiency is gained through diaphragm area at a minimum.
A thought that highlights the difference between domestic users, car enthusiasts, and a PA system designer is the notion "watts are cheap".
They are not.
Consider your sub to be an electric kettle, not continually involved but required to deliver full power 10 times per day - that **** is costing you $100 per year.
Did you measure the results in any way? Which 15"s? Was 18Hz the -3dB point, -10dB point, or just "this speaker does technically make some noise at 18Hz"?
I think it's worth pointing about that the Peavey amp you've mentioned is pretty weedy by today's standards. Try searching for a Behringer NX6000. That thing is about as good as it gets for power/price ratios. It's also very light - easily lifted with one hand.
I prefer to use my Powersoft T-series amps, which are considerably more expensive, but pack in some excellent DSP, and are 1U.
In terms of running lots of subwoofers, over a narrow bandwidth, off one amplifier:
Yes, it can be done as you describe. If you can find/design a cabinet that maintains a high impedance in the frequency range you're looking for, you could indeed run a large number of speakers on one amplifier.
However, I'd suggest that would be a waste of speakers:
Ported boxes exhibit a cone excursion minimum (ie, maximum excursion-limited output per cone) right where the impedance is low. You could choose to run the ported boxes above the tuning frequency, where impedance is high, or just use a sealed box. Either way, you're choosing not to take advantage of the mechanical help a cabinet can offer. You will, therefore, need more cabinets to keep up.
Let's take a real-world example.
A while ago, I was using sealed boxes for my PA subwoofers. I then moved the same drivers to ported boxes, and gained a lot of output. The simulations suggested 3x sealed drivers would approximately match 1x ported. The drivers are Beyma 15P1200Nd, and I was using four of them.
Let's take that ratio of 3:1, and apply your theory that we could (by limiting bandwidth) run lots of drivers on one amplifier.
If I have 4x ported boxes, then 12x sealed boxes will match in SPL. We'll keep to your theory, and run all 12x sealed boxes off one amplifier.
That's fine, because we've done the maths, limited the bandwidth, and the amplifier is happy.
However, instead of 1x amplifier running 4x ported cabinets and using their full bandwidth, we must now carry 12x sealed cabinets into each venue!
We'd also need to buy 8x additional drivers, which would cost a couple of thousand pounds. And then there's the woodworking, the transport...
The flaw in your plan, then, is revealed: in order to operate a cabinet in its high-impedance bandwidth, SPL is lost, and therefore more drivers will be required to do the job. When those are added in parallel, the high-impedance advantage is lost, and we've had to buy more drivers to do it.
Even if the ported boxes required an extra amp to drive them (they don't), compare the cost of a Behringer NX6000 with another four drivers, cabinets, etc, and then consider which you'd rather carry into a venue.
Chris
PS - Bandpass boxes can sound excellent, but there are many poor implementations out there.
Chris, you provide excellent points, the technical arguments are likely way beyond my pay grade even if I wasn't three sheets to the wind on a Sunday night.
However, my theory relies on very basic principles. By severely limiting the bandwidth amplified the driver can maintain SPL without lowering resistance. What you do with the 'free ohmage' is up to you.
Here's the thing . . . my imaginary 100dB subs can do their job without dropping below 12 ohms. Your 100dB subs drop to 4 ohms whilst addressing frequencies that are 'not their job'.
Here's the main thing . . . my theory was not met with a riposte. You considered what I was saying an argued against it. Ergo, I provided 'food for thought'.
Sidebar: my theory was developed by looking into the reverse issue. A home theatre 2.1 system is specified as having a 4 ohm subwoofer and 4 ohm satellites - pretty standard. However, if the cross-over point is 200hz, what is the true impedance of the satellites?
However, my theory relies on very basic principles. By severely limiting the bandwidth amplified the driver can maintain SPL without lowering resistance. What you do with the 'free ohmage' is up to you.
Here's the thing . . . my imaginary 100dB subs can do their job without dropping below 12 ohms. Your 100dB subs drop to 4 ohms whilst addressing frequencies that are 'not their job'.
Here's the main thing . . . my theory was not met with a riposte. You considered what I was saying an argued against it. Ergo, I provided 'food for thought'.
Sidebar: my theory was developed by looking into the reverse issue. A home theatre 2.1 system is specified as having a 4 ohm subwoofer and 4 ohm satellites - pretty standard. However, if the cross-over point is 200hz, what is the true impedance of the satellites?
A ported cabinet with 100dB 1w/1m half space sensitivity at 35Hz will likely use two 18" drivers, and be in the 20 cubic foot/566 liter size range, a bit larger than you would prefer.
The IEC standard (IEC60268-3) allows any impedance above the rated value, but limits the impedance below. It does not allow the rated impedance to fall below the 80% of the nominal value at any frequency.
If your 4 ohm satellite conforms to that standard, it does not fall below 3.2 ohms at any frequency, though could rise well above that in it's pass band- double (or more) the nominal value would not be unusual.