The losses of a passive radiator should normaly increase with size. So there is no advantage of pairing a small, weak driver with a too large passive radiator.
The 1.5-2.0 rule of thumb is quite OK.
The 1.5-2.0 rule of thumb is quite OK.
Okay that is quite a significant drop, and does pose an issue when updating in VituixCAD...
This has since changed as it was far too limiting for any real application. I'm now looking at a space of ~200x250x500 which would give up to about 14L max internal volume. I've seen the racetrack SB radiators, which look interesting.
There are sometimes limitations if you don't have enough room to share. You want a subwoofer, that adds low frequency to your too small main speakers.
Now you got a limited volume for the sub.
Let me do a simple comparison: Imagine you got a garage you need to use for your car. No parking elsewhere. For some undefined reason, you decide to ditch your trusty Morris Minor (you are British) for a Range Rover. Which is of course stupid, as you are a single household and don't need a huge car for your weekly shopping, but you need it for your ego, maybe. You compare the size of your garage with the dimension of the Range Rover and decide it will fit. That is simulation...
When the rubber meets the road or reality hits, as we might say, you will find out that the Range Rover indeed fit's inside your garage. The only problem is that you can not open a door and exit the car. Maybe you can open the sun roof and squeeze yourself between the car and the ceiling out of your parked SUV. Not really something that anyone likes. It may work, but with pretty limited convenience.
If you want to keep using the garage, limit cost, don't like gymnastics and still want to drive to the shopping mall in a car, stick to the Morris.
Now back to your project. You got a limited volume, but need a certain f3. You can get the f3 from a closed sub, but for a vented (or passive radiator) construction it is just too small. You may use some expensive, specialized drivers to somehow manage the desired f3, but the result will be worse than the simple, proven and logical solution. It is just simple physics, a vented cabinet gives higher SPL for the price of a larger volume than a closed cabinet.
While you can equalize a closed cabinet to the desired f3 using more amp power, a vented system has a steep drop off after reacing f3. Even steeper with a passive radiator, by the way.
In short: The small volume makes a passive radiator the worst solution in your case. For sound and cost.
Now you got a limited volume for the sub.
Let me do a simple comparison: Imagine you got a garage you need to use for your car. No parking elsewhere. For some undefined reason, you decide to ditch your trusty Morris Minor (you are British) for a Range Rover. Which is of course stupid, as you are a single household and don't need a huge car for your weekly shopping, but you need it for your ego, maybe. You compare the size of your garage with the dimension of the Range Rover and decide it will fit. That is simulation...
When the rubber meets the road or reality hits, as we might say, you will find out that the Range Rover indeed fit's inside your garage. The only problem is that you can not open a door and exit the car. Maybe you can open the sun roof and squeeze yourself between the car and the ceiling out of your parked SUV. Not really something that anyone likes. It may work, but with pretty limited convenience.
If you want to keep using the garage, limit cost, don't like gymnastics and still want to drive to the shopping mall in a car, stick to the Morris.
Now back to your project. You got a limited volume, but need a certain f3. You can get the f3 from a closed sub, but for a vented (or passive radiator) construction it is just too small. You may use some expensive, specialized drivers to somehow manage the desired f3, but the result will be worse than the simple, proven and logical solution. It is just simple physics, a vented cabinet gives higher SPL for the price of a larger volume than a closed cabinet.
While you can equalize a closed cabinet to the desired f3 using more amp power, a vented system has a steep drop off after reacing f3. Even steeper with a passive radiator, by the way.
In short: The small volume makes a passive radiator the worst solution in your case. For sound and cost.
Except in this instance it's more like I'm ditching a unicycle (the TV speakers). I can pretty much guarantee any moderately thought out subwoofer will provide improved and lower bass response considering the current speaker "system" is the size (and thickness) of my hand. I'm just looking for a little guidance on how to go about it (or how not to, assuming there's some level of explanation).
Do I need a certain f3? Really I'm just aiming for as low as I can reasonably get given the dimensions I'm working with. I'm not aiming for awe-inspiring home theatre, just a general improvement for a fairly low budget.
The Visaton in a ~8L vented box actually appears to be an option. The datasheet even suggests vented 7.5L so one would assume it's not an awful idea, though that doesn't indicate which coil setup that would utilise, if that makes a difference. I had tried this before but for whatever reason must have moved on to test something else and forgot. I'd imagine that excursion (28Hz and below) could potentially be a problem, but is there anything else I'm missing here?
I'm just trying to save you money and give you a better end result. If you ask me which are the most overpriced drivers on the market, Visaton will be high up if not top of that list. You get a better product for half, sometimes a third of the Visaton price. They just buy them somewhere and mark them up a few hundred percent. They are no producer of anything, just merchants. Most products are very old (some decades), there innovation factor is about zero. They have some fine drivers, but these are much too expensive and you get even better for less with any major driver manufacturer. Maybe you still think you can avoid a dedicated sub amp, but that will not happen if you build a micro sub. All the magic with small designs is in the electronics.
@hodephinitely The guidance that might be provided depends somewhat on the amplification that you are planning to use. Also, it's still unclear what the main speakers are going to be, and it's their low-frequency response in combination with amplification and the requisite low-pass/high-pass filtering that will have a large bearing on the acoustic performance of any subwoofer+mains system.
Can you provide some details/thoughts on the main speakers and amplification that you plan on using? It would be very helpful, as at present we are flying blind, so to speak.
Absolutely! I realise I've been going around in circles quite a bit but the mains would be Mark Audio CHN-50P in a ~1.5L sealed box.
I have an old Denon AVR-1513 but it's pretty basic and dated (no ARC, for example), isn't rated for <6 ohms and would require a secondary sub amplifier. I was thinking to sell that and get a cheap 2.1 amp such as the SMSL A50 Pro which has a powered sub out with level adjustment and a limited number of crossover points, one of which I believe is around 150Hz and seemed like a decent fit.
Edit: as a note, I currently have an SMSL A100 powering a pair of Mark Audio Alpair 5.3s in slot-vented bookshelf cabinets, and have been very happy with them. As a second note, the room is a small living room in a Victorian terraced house.
If you build your own speaker from some drivers, the best thing you can do is a DSP to correct the response. Even the usual full range driver has a very different response from the one in the data sheet, the second you put it in some kind of enclosure, on a baffle or whatever.
If, after a while, you realize the response is somehow strange, you can get a cheap measuring microfone (starting at 20$) and use the DSP to put things right. You will be surprised what you missed without it, but you may have to make your own experience before you understand.
The AVR is nice, you got a remote, FM etc. It has a sub out, using a DSP equiped amp hooked to it will make your sub adjustable to the room and constuction dependent response. Not the worst option and quite budget friendly. Selling it may not be the best idea.
If, after a while, you realize the response is somehow strange, you can get a cheap measuring microfone (starting at 20$) and use the DSP to put things right. You will be surprised what you missed without it, but you may have to make your own experience before you understand.
The AVR is nice, you got a remote, FM etc. It has a sub out, using a DSP equiped amp hooked to it will make your sub adjustable to the room and constuction dependent response. Not the worst option and quite budget friendly. Selling it may not be the best idea.
That's workable and meets your "compact" design requirement with room to spare.
As @Turbowatch2 commented, the Denon AVR is quite nice. It can certainly enable a proof-of-concept investigation. It has 75W per channel @ 0.08% THD available, which is a good power capability. The small 3-inch driver, with Re = 3.3ohms, can't handle anywhere near that sort of power before maxing out, so driving it with a few watts shouldn't pose any major problems for the AVR.
If the AVR has a 150Hz filter setting, that would certainly help protect the CHN-50P drivers from handling excessive high-power low-frequency signals. A simulation of the result when a subwoofer is added is shown below, which shows promise. The CHN-50P 2nd-order response is modelled, to which the AVR's expected 2nd-order Butterworth 150Hz high-pass filter has been added. The subwoofer has a 150Hz 4th-order Linkwitz–Riley low-pass filter included, as this is the type of filter that the AVR would be expected to have added to the subwoofer. As you can see, the bass integration between the subwoofer and the main speakers is quite good.
As a result of the small room size, you probably won't need much power, saving the little 3-inch drivers from too-early destruction.
Thanks for taking the time to run through this! My main issue with the Denon is it's lack of CEC, and therefore control via TV remote. But as you say, I can at least test the speakers with this and one of my other amplifiers for now.