Rythmik FV15HP, how are the measurements even possible?

[lowerFE]

I'm looking at WinISD models and comparing it to data-bass measurements. Now one would expect the actual performance to be less than the modeled performance since that assumes perfect conditions, but no matter how I model, I can't get it to come close to the performance of the FV15HP. 

 

To match the performance with a UM15 in a similarly sized enclosure and tuning I need 3x more power to hit the FV15HP's numbers, and that still doesn't match its upper bass output. 

 

Not only that, in one port mode, where it is tuned to 14Hz instead of 18Hz, it manages to get more 20Hz output? It is almost an octave above tuning, where it shouldn't get as much of a port boost, and the output should be 2dB lower compared to 2 port mode. 

 

Even using the much more efficient TD AE15H+, I get similar results, except I can match the upper bass output.

 

I know amplifiers can burst higher than their rated power, but looking at maximum long term output the deep bass 20-40Hz part is just 1-2dB less than the burst numbers, which does not explain the 4-5dB difference I see from modelling, and I don't think superior drivers could account for this difference since Hoffman's Iron Law determines the low bass efficiency and (therefore) maximum output. 

[chrapladm]

So many variables. I try to get the best transfer function magnitude, I think its called, response. EQ, amplifier and room change so many things. SO getting the best ideal size volume for a "said," driver with capable amplifier and signal shaping should put you close to the Rythmik performance. Some drivers perform better on the low end verse the higher end. BUT as I said before there are so many variables.

 

Things should be similar though if you dont have size constraints or amplifier/EQ constraints because then the only difference is drivers mechanical stroke and of coarse some electrical parameters for those nit picking. Then you have 850cm2 times "X," mm Xmax for your Vd. Vd vs Vd should have most drivers almost the same if they are similar.

 

And i prefer to use HornResp for modeling.

[Steve81]

I suspect one big factor in the top end output is the choice of amplifier. It's not going to be mistaken for a bargain basement BASH unit. I assume that the 600W rating is a fairly conservative continuous rating (say, 1% THD or less), with a lot more in reserve.

 

 

[Ricci]

The Rythmik driver is very good.  The amp is most definitely rated more conservatively than the typical Class D, BASH, etc...Don't forget the motional feedback circuit as well.

 

Another one to consider is the PB13-Ultra. Try to model that one up and hit that performance with an even smaller 13.5" driver.

[lowerFE]

The Rythmik driver is very good.  The amp is most definitely rated more conservatively than the typical Class D, BASH, etc...Don't forget the motional feedback circuit as well.

 

Another one to consider is the PB13-Ultra. Try to model that one up and hit that performance with an even smaller 13.5" driver.

 

No doubt the Rythmik driver is amazing. Does the motional feedback circuit increase efficiency?

 

I'm still baffled since it is not a difference of a few hundred watts, but 3x the power. Plus, shouldn't a theoretical model like WinISD give higher than measured numbers as they assume no distortion, no power compression, no port compression, etc? With such a big discrepancy in the measured result's favor over modeling, it makes me think my understanding in either measurements or modeling is missing something. 

 

The PB13 numbers, while incredible down low, are more "believable" in a sense because the amp is rated for 3600W peak, so it has a ton of extra power to hit the CEA burst numbers. It also has more enclosure volume and port area helping out the low end a bit. The Rythmik baffles me because of the combination of low power, small enclosure volume, and small port area, especially in 1 port mode, where port compression should have severely affected output down low. 

[Bossobass Dave]

I agree. The sub should be re-tested.

 

There's nothing "amazing" about the driver at all and the plate amp is just that... a typical plate amp (POS, IMO).

[Ricci]

Problem #1 is using things like WinISD for much of anything.  Wattage ratings and plugging in things like "600 watts" into a simulator are another big problem. That's not how amplifiers really behave driving speakers.

 

Every sub I've tested from a reputable manufacturer with internal data has been within 1dB or less 20-100hz. Something like 5 companies now and 10+ subs now. Rythmik wasn't one of them but I'd be surprised if they were a crazy one off. There's no way I'm going to waste time re-testing something from 6 years ago just to get basically the same results.

 

The PB13-U cab isn't much bigger internally at all. It may actually have less net air-space. I'd say the amplifier is no more powerful in reality either. And the driver is smaller cone area and less sensitive.

[Steve81]

IIRC, Paul Apollonio tested the exact same unit for Audioholics. Comparing those numbers with Josh's results, they're within a dB at every frequency he tested (20Hz-63Hz).

[Steve81]

           Paul     Josh

20      107.5    108.1

25      113.0    113.1

31.5   117.8    117.0

40      120.0    119.5

50      119.9    119.1

63      119.7    118.8

[lowerFE]

I don't doubt Ricci's measurements. I doubt my own understanding of subwoofer modeling.

 

Could you please explain by "Problem #1 is using things like WinISD for much of anything"

 

Also, for some subs, you list internal volume. Where is that figure from? Is that the volume after subtracting driver/bracing/amp?

[Ricci]

Thanks Steve. I'd forgotten that Paul also measured the FV15HP. That further confirms that the data is good.

 

lowerFE

There has been a lot of discussion over the years on modeling subs and some of it quite recent. Some here, some at AVS and quite a bit at DIYAudio, etc...This stuff goes back many many years. This is mostly DIY section stuff. The very short version is that todays long excursion drivers with very massive voice coils typically have a lot of complex inductance related effects unless there is extensive and expensive work done in the motor system to combat it. Rarely is the extra complexity and expense taken during driver design to really combat the effects of a moving coil in an asymmetrical motor. What this causes is changes in the drivers behavior, response shape, higher distortion, energy storage, sensitivity, etc...The general consensus is that it doesn't matter in the bass frequencies much. To an extent that is true. It does cause issues with modeling the system with a basic TSP simulator such as WinISD though. Basically the issue is that an extra set of parameters is needed that describe the inductance behavior and the impedance curve of the driver. there are a number of different ways to quantify it, some have 3,4 or even 5 parts (parameters) for that aspect of the model. If this data is not used the simulation will not match the actual performance of the driver. The problem is that no manufacturers provide this data and virtually none of the free or even cheap simulators can use the data. If you had a software that could accept this data you would have to measure it yourself for the drivers you wanted to model. You can see what I mean by modeling a Fi driver I've tested here and then comparing with the measured results.

 

Back to the point that this stuff does not matter for bass drivers...It kind of depends. For simpler systems such as sealed and IB it's not a big deal. A bit of signal shaping to get the response where you want it and enough displacement and power to do the job and you are pretty much good to go. Vented systems are a bit more complex and the interaction between driver and cabinet is a bit more complex so the results can be thrown off a bit more and become a bit more problematic. Even more complex systems such as bass horns are where a real problem with modeling is. If your driver does not behave as the model is predicting it can turn what would be a solid performer (on paper) into a complete turd. This is because the interaction of the driver with the cabinet loading is much more nuanced and driver dependent. Moral of the story is if you are planning a horn or vented system it is worthwhile to measure the drivers you are using to understand how they actually behave. Then using the known measurements make the simulation match the measurements. Then you will be getting somewhere. That's how I've been doing it for many years now and I've talked about it a number of times. There are a couple of guys who are also working on a write up about it and I hear that Dave is getting something to help out built into Horn Response. This will still require accurate measurements of the driver to be modeled though as manufacturers do not provide the parameters needed. A big part of the secret to an accurate model is the impedance curve. Get that to match and you are in business. Plugging in the manufacturer provided basic TSP specs off of the website into WinISD is likely to be WAY off.

 

There is also another issue that hasn't been discussed nearly as much but is also important and that is sensitivity. My measurements and other ones from reputable and competent companies (Danley, JBL, etc...) consistently show 1-3dB higher voltage sensitivity than what an otherwise accurate simulation of that system would show. I believe this is due to directivity. Bass is often described as omnidirectional but this is not entirely true.

 

Amplifiers and simulations...Learn about the impedance curve and how it changes with frequency. Amplifiers are rated in power but in reality modern amps are constant voltage devices. Current and therefore power vary radically depending on the signal and frequency content. Amplifiers do have a limit on the amount of power that they can generate of course but that is only because they will have both a voltage limit and a current limit. If the impedance is high the amplifier will likely hit its voltage limit first and despite really high voltage the actual power can be quite low. Your "8ohm" driver may have a maximum impedance of 80 ohms. 200volts into 80 ohms is only 500w. Most amplifiers cannot produce 200 volts unclipped BTW. If impedance is low it will be current that is the limitation. If you are using a "2ohm" driver and the actual impedance drops to 1.5ohms minimum, just 86 volts requires over 57A from the amplifier and results in about 5000w into that impedance. There will be a range where this overlaps as well.

[lowerFE]

^Whoa, you just opened up a whole new world for me, no way I can resist the temptation of not going in! =) Thanks for that long very short introduction. Where can I read more about the long version of this?

[Bossobass Dave]

There is no impedance curve for the Rythmik, but we know the amplifier's limit within a dB.

 

At 40 Hz, the Rythmik gives more output in dBSPL with 500 watts than the sealed DIY MTX-9515 with 7700 watts, That's 12dB more amplifier yielding 1dB less output. Even if you give the Rythmik amp a 3dB benefit of the burst doubt, there is still a huge disparity in power with less output. And, the Rythmik driver isn't amazing compared to the MTX driver.

 

Of course, the one mistake I made was to blindly pull another commercial 1x15" ported sub for comparison and it happened to be that wretched PSA sub.

 

The results are probably close but you guys talking about a dB or two like it doesn't matter cracks me up a bit. The people who buy these subs split a decibel like it's the last donut on Homer's desk at the power plant.

[Steve81]

At 40 Hz, the Rythmik gives more output in dBSPL with 500 watts than the sealed DIY MTX-9515 with 7700 watts, That's 12dB more amplifier yielding 1dB less output. Even if you give the Rythmik amp a 3dB benefit of the burst doubt, there is still a huge disparity in power with less output. And, the Rythmik driver isn't amazing compared to the MTX driver.

 

To be fair, the MTX was running with the big handicap of a 1.25 cu ft box. Measured sensitivity was 82dB with 2.83V at 1m at 40Hz, which is quite low.

[Shredhead]

Amplifiers and simulations...Learn about the impedance curve and how it changes with frequency. Amplifiers are rated in power but in reality modern amps are constant voltage devices. Current and therefore power vary radically depending on the signal and frequency content.

 

Here is an illustration of that point using a test that I recently did. 

 

Here are 4 impedance curves for 4 drivers of the same make.  The scale of resistance in ohms is on the right hand side.

 

This is the impedance curve of the 4 drivers in series with the peak of the curve at around 25.6Hz.

 

I picked 2 frequencies to measure current and voltage.  25.6Hz where the impedance was 59.2ohms and 50Hz where it was 11.2ohms.  Voltage stayed around 6 while current and power fluctuated. 

 

I did the same test using 3 different amps.  A class A/B, a class H, and a class D.  Results were identical. 

[Bossobass Dave]

To be fair, the MTX was running with the big handicap of a 1.25 cu ft box. Measured sensitivity was 82dB with 2.83V at 1m at 40Hz, which is quite low.

 

 

You can move the example to 80 Hz where the sens of the MTX is 89dB and where the small box is a 1-2dB advantage. With same power input, the Rythmik gives +12dB more output at 80 Hz.

[lowerFE]

You can move the example to 80 Hz where the sens of the MTX is 89dB and where the small box is a 1-2dB advantage. With same power input, the Rythmik gives +12dB more output at 80 Hz.

 

A lot of high powered drivers have trouble up high because of inductance. You can see from the output compression graphs that it is compressing starting from 110dB. And to the earlier point on 40Hz, the Rythmik has 4x more enclosure volume, which translates into 6dB more sensitivity. The port contributes ~2dB of output, and the amp should easily burst double its rating and get another 3dB, so it explains around 11dB of the difference. 

[Steve81]

You can move the example to 80 Hz where the sens of the MTX is 89dB and where the small box is a 1-2dB advantage. With same power input, the Rythmik gives +12dB more output at 80 Hz.

 

Ignore the burst numbers a minute and look at the 115dB compression sweeps. The MTX requires a 144V input, which equates to ~2.6kW, while we can guess this sweep requires everything the Rythmik's 600W amp has to give. Now looking at 80Hz, this means the MTX requires ~6dB more power to deliver ~0.5dB more output than the Rythmik. That's significant, but that apparent gap is reduced when you look at the actual compression at 80Hz: 3.6dB for the MTX, and practically zilch for the Rythmik.

[Bossobass Dave]

A lot of high powered drivers have trouble up high because of inductance. You can see from the output compression graphs that it is compressing starting from 110dB. And to the earlier point on 40Hz, the Rythmik has 4x more enclosure volume, which translates into 6dB more sensitivity. The port contributes ~2dB of output, and the amp should easily burst double its rating and get another 3dB, so it explains around 11dB of the difference. 

 

You can explain anything with conjecture. Maybe the mic placement and baffle size account for something as well. I simply doubt the Rythmik results as accurate to within 1dB and you accept them as accurate to within 1dB. We can leave it at that rather than guess...

[lowerFE]

You can explain anything with conjecture. Maybe the mic placement and baffle size account for something as well. I simply doubt the Rythmik results as accurate to within 1dB and you accept them as accurate to within 1dB. We can leave it at that rather than guess...

 

Bossobass Dave, you have a ton of experience, way more than many of us here, especially with 15'' woofers. How do the performance of the dozens of 15'' woofers you've tested over the years compare to the measured result of the Rythmik?

[Steve81]

I simply doubt the Rythmik results as accurate to within 1dB and you accept them as accurate to within 1dB. We can leave it at that rather than guess...

 

There are always variances in the results due to a number of factors (environmental, equipment, human error, etc.). Given that Josh's results were within a dB of Paul @ AH's with the same sub, it would seem to reduce the possibility of human error or serious equipment issues. Of course, there are more nefarious possibilities I suppose (i.e. the unit could have been a ringer).