Ask an Acoustic Engineer (me)

[NoDestiny]

Question: I want to find the different in "box size" between no polyfill and a stuffed box. I have a Dayton DATS and ran the sweet for before and after.

What math can I use to determine the airspace for each? Or if not, the supposed gains the subwoofer sees in volume from the polyfill? I've found people mention you only need Qts and Fs, but others that say you need VAS, others say you need Qtc... Would love some input on that.

Attached is the screenshot difference between the same subwoofer and box, but with 4.5lbs of stuffing (as I calculated a very rough 4.5 cubes in my complex enclosure).

[JCsAudio]

Great response to my question Justin and thank you for taking the time to write that. I loved the driving analogy! I’m sure this will help many members here understand the theory. I couldn’t get the DIYMA link to work. I did a bunch of research into regulated vs non regulated amplifiers a long time ago. Not sure if it’s entirely accurate but the consensus was that non regulated power supplies gave amplifiers a slight edge in dynamic power over regulated power supplies but I’m not so sure that would be audible. Just think of the power drop you get when shutting off your engine with the audio playing. Your voltage could go from 14 volts to 12.5 volts yet you likely won’t notice a difference from the reduced power as a result.

[Justin Zazzi]

Great response to my question Justin and thank you for taking the time to write that. I loved the driving analogy! I’m sure this will help many members here understand the theory. I couldn’t get the DIYMA link to work.

I'm happy you were able to read that too. I know you were pretty bummed about not getting an answer earlier!

I fixed the link in the thing. Here it is again, just in case:
www.diymobileaudio.com/threads/how-does-rips-jl-audio-work.89221/page-2#post-1127909

And NoDestiny I'm thinking about your question. We do this at the office too.

[JCsAudio]

Thanks Justin, I just read that article now. Very good information there and now I know exactly how RIPS works. I was kind of bummed about the first question, lol. I was like......AHHHH man, lol. Seriously though it’s awesome that you come on here and share your knowledge like this and I really appreciate it. Reminiscent of that old post from msmith and many from Andy Wehmeyer from back in the day.

[ckirocz28]

How about this for a question?
https://www.caraudiojunkies.com/show...9271#post59271

[Justin Zazzi]

Question: I want to find the different in "box size" between no polyfill and a stuffed box. I have a Dayton DATS and ran the sweet for before and after.

What math can I use to determine the airspace for each? Or if not, the supposed gains the subwoofer sees in volume from the polyfill?

There are many ways to calculate this so you might be able to do it with Vas or Qts in a similar way. I like the method I came up with since it's really simple.

You need three measurements with the DATS system by pressing the "measure free air parameters" button, and record the resonant frequency fs for each:
1) measure woofer in free air without any enclosure
2) measure woofer in sealed box without any stuffing
3) measure woofer in sealed box with stuffing

To find the ratio of how much "bigger" the enclosure became from the stuffing that you added, the formula looks like this:

To find the air volume of the box, I usually measure the woofer with something like DATS and then simulate it in a sealed box. I change the air volume of the simulated box until the resonant frequency of the box Fc matches the resonant frequency when I measure the woofer in the box using DATS.

You can try to calculate it too, however you must make very careful measurement of the woofer when you do since any errors will multiply when you try to calculate the enclosure volume. The formula looks like this:

The units are MKS (meter-kilogram-second) so make sure you convert Sd from square centimeters to square meters, and convert Cms from mm/N to meters/newton and so on. The answer spits out cubic feet. By Fc I mean the resonant frequency of the woofer in the box.

Cool question, I enjoyed working through this one since I can use it at the office more often now : )

[geolemon]

Just wanted to interject with a video link that has an amp technician walking through to explain this technology.

Also wanted to raise an interesting aspect that relates to both subwoofers and amplifiers, that IMO makes the design of these "constant power" amplifiers actually even more remarkable:

I admit amplifier design is not my specialty...

Non-regulated power supplies will allow the rail voltages to increase or decrease as the battery voltage is increased or decreased...
Regulated power supplies will hold the rail voltages constant when the battery voltage changes...

I was going to write a big thing here but Manville Smith of JL Audio wrote a sweet post over at DIYMA about ten years ago.
How does RIPS (JL Audio) work? post #21
Please read Manville's post, it is really well written and it goes into a lot of really good detail.

In short, the output stage is usually made up of transistors that can supply the speaker with voltage up to the rail voltage limits that the power supply has. The speaker will present an impedance (or a load) and then current will flow.

To add complexity to the challenge - speaker's impedance isn't a constant, it's naturally different at different frequencies, and at a speaker's natural resonance can rise surprisingly high - let's say a 4 ohm subwoofer has an Re of 3.5 ohms and an Fs of 24hz, like this completely random sample I found just because it had a plot...
You can see that impedence rise at Fs - up to 100ohms, maybe more:

Want another wrench to throw in the works? That above plot - it completely changes as soon as you put a subwoofer in a box. In a sealed box, it shifts based on the size of your box. In a ported box, the vent has an impact on cone motion which in turn impacts impedance. You could change the height or width of that resonance, or you could add a second high-impedance spike on the plot, again depending on box size and tuning.

The point relative to this "constant power" amp design is - a sub's impedance is all over the place, dynamically, and (from the amp's perspective) unpredictably. That amp is designed to drive one sub in a box as well as another totally different sub in a totally different box, and not impart any false coloration on the frequency response of that sub playing.

Continuing with the car analogy, a given car can drive 65mph on the freeway if the road is flat. The same car will go much slower up a steep hill, even with the gas pedal on the floor. The same car can go much faster than 65mph if you're driving down a steep hill, so fast that it can easily loose control and crash into a fireball and cause the road to be closed and inconvenience literally everyone else in a supremely selfish act of dumbassery.

Relating the car to the amplifier: an amplifier can supply 100watts of power if the impedance of the speaker is a good match (like driving the speed limit on a flat highway). The same amplifier, using the same rail voltage, can supply less power if the speaker impedance is too high (like driving up a steep hill). The same amplifier can supply much more than rated power if the speaker impedance is too low (like driving down a steep hill) and if the impedance is too low then the amplifier can overheat and blow up (like the car going too fast and crashing).

I love the analogies. Definitely driving your amp at too low of a load can cause it to die... I'd call that a car wreck.

It might be fun to take a deeper dive into this "impedance rise" topic, as a lot of people refer to it these days...
I see a lot of SPL competitors these days are actually becoming aware of their impedance plots, and are saying to themselves "Wait up... if my box is tuned to 60hz, and I measured my 1 ohm sub's impedance rises to 16 ohms at 60hz in this box - if my 2000w amp is only making that power at 1 ohm... does that mean my amp is only making... hang on... 2 ohm... 4 ohm... 8 ohm..16 ohm... 125 watts when I'm burping?"
So lately I've seen people wiring to very, very low impedances - 1/4 ohm, I'm sure some people are trying less - so that their amp makes the power their sub can handle at (and ONLY at!!) their burp frequency.

Would be fun to take a deeper dive into the negative consequences - That they've literally created a one-note wonder that would fry it's amp if you fed it anything other than that specific test tone (do people still use "burp buttons"?), and to compound that problem - that they've frequently got to solder across their protection resistors to totally eliminate amp protection (for those amps of this class that have protection) for this to even work at all. But maybe a theoretical dive into the negatives from the speaker perspective?

Constant Power

To get constant power out of an amplifier at 1Ω and 2Ω and 4Ω, you need two main things: 1) enough voltage amplifier power supply so that the power can be realized on the highest impedance load and 2) a current sensing/limiting technique so that the amplifier doesn't overheat or fail when the lowest impedance load is used.

An example amplifier might claim:
100w @ 4Ω
100w @ 2Ω
100w @ 1Ω

To get 100w at 4Ω the rail voltage would have to be

Sam from BareVids is an awesome guy for anyone who wants to understand amplifiers better. I subscribe to his vids, watch them while I'm working - he's a huge distraction, honestly.

In this particular link, he looks inside a Taramps amp that has a "smart" circuit like the JL RIPS amps.
It's important to know that - for the reasons Justin already described - there's limits to this "smart" technology just like there are limits to normal amplifiers that don't have this kind of design.
So these amp ratings would look like BS to me:
100w @ 4Ω
100w @ 2Ω
100w @ 1Ω
...because that's a big range to actually maintain constant power across.

I'd be betting it would really dyno out more like this:
40w @ 8Ω
80w @ 4Ω
100w @ 2Ω
100w @ 1Ω

Check out Sam's dive into this smart technology - he brings up the waveforms on his scope, so you can actually see the technology work, can see the voltages change, and power change.
It sounds like a deep dive, but Sam does a great job explaining it at a layman level. Definitely worth a watch.

https://www.youtube.com/watch?v=DxMGPhNASl4&t=440s

Note that link is actually the 7:25 mark on a Williston Labs review - another guy worth subscribing to IMO - but I subscribe to Sam as well. His channel is "barevids" - don't worry, it's not what that looks like. Nothing NSFW.

[Justin Zazzi]

How about this for a question?
https://www.caraudiojunkies.com/show...9271#post59271

That's a really interesting question! I replied in your thread with a reason why it might not work, and a small tweak to make it work potentially, maybe.

That's freaking fascinating and I'm dying to see if it works!

[Justin Zazzi]

It might be fun to take a deeper dive into this "impedance rise" topic, as a lot of people refer to it these days...
I see a lot of SPL competitors these days are actually becoming aware of their impedance plots, and are saying to themselves "Wait up... if my box is tuned to 60hz, and I measured my 1 ohm sub's impedance rises to 16 ohms at 60hz in this box - if my 2000w amp is only making that power at 1 ohm... does that mean my amp is only making... hang on... 2 ohm... 4 ohm... 8 ohm..16 ohm... 125 watts when I'm burping?"
So lately I've seen people wiring to very, very low impedances - 1/4 ohm, I'm sure some people are trying less - so that their amp makes the power their sub can handle at (and ONLY at!!) their burp frequency.

I've heard people doing that too! But wait, there's more!
When playing a high-output burst the voice coil heats up which changes the impedance of the system, which changes the resonant frequency, and on and on. I met a competitor once who mapped this temperature rise and performance change vs time and then created a burp track that started at some frequency (maybe 52hz) and changed frequency over time to match the change in the performance of the system as it heats up over a very short time. This way the frequency of the tone is always matched to the instantaneous resonant frequency of the system for maximum output not just when it starts, but as the system continues pounding over time. This stuff is crazy!

Check out Sam's dive into this smart technology - he brings up the waveforms on his scope, so you can actually see the technology work, can see the voltages change, and power change.
It sounds like a deep dive, but Sam does a great job explaining it at a layman level. Definitely worth a watch.

https://www.youtube.com/watch?v=DxMGPhNASl4&t=440s

Thank you for the video link, that was nice to see! I like all the practical bits with the scope traces.

[jtrosky]

So.... Just so I understand at a more "basic" level. :-)

- An amplifier with a "regulated" power supply will provide the same amount of power regardless of the speaker impedance.

- An amplifier with an "un-regulated" power supply will provide less power to speakers with higher impedance.

Are those two statements true?

The reason that I ask is because I'm still not clear why my JL XD600/6v2 (which does NOT have a RIPPS power supply) provides 100W RMS at 2 ohms, but 75W RMS at 4 ohms. Why is there such a small difference between the 2 and 4 ohm outputs? Don't most "un-regulated" amps provide twice the amount of power at 2 ohms (compared to 4 ohms)?

While the JL XD line of amps don't have the "RIPPS" power supplies, do they maybe have some "lesser" version of a regulated power supply instead? I honestly don't know if they are regulated or un-regulated.

Thanks - and sorry if this was already explained earlier - maybe I just didn't understand it.