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Author Topic: 21" subwoofers - opinions  (Read 1843 times)

Ivan Beaver

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Re: 21" subwoofers - opinions
« Reply #20 on: February 23, 2017, 07:32:13 am »

I'll also look around for response graphs. The on paper specs put +/- 3db at 30-105hz, fwiw.

The response graph will tell you more than "simple numbers".

I have seen some spec sheets in which the "claimed -3dB point", is actually 9 to 20dB down from the rated sensitivity.

The -3dB MUST be tied to something.

There is no such thing as "-3dB",  It MUST be 3dB down from "something".

Knowing what that "something" is, is VERY important.

Also be wary of just a processed graph.

If they are doing a boost at the low freq (VERY popular these days to get lower numbers), REMEMBER that whatever boost they are applying, MUST be subtracted from the maximum output.

So if there is a 10dB boost down low, and the claimed max output is 130dB, then at the boosted freq, the max output will be 120dB.

This is NOT what you want or expect.

Hence the reason to dig a little deeper to see the ACTUAL performance.

Simple numbers give all sorts of wrong answer.

I have no idea about the product in the post-so have no opinion how accurate it may be.

But just stating things to look for when comparing "simple numbers"

Any car can easily get 100 miles per gallon.  As long as it is coasting down a hill.  But you will probably not always drive down a hill.   Same sort of thing.

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John Rutirasiri

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Re: 21" subwoofers - opinions
« Reply #21 on: February 24, 2017, 11:51:05 pm »

FWIW, I have a pair of old K-Array KL21ma powered subs.  21" Faital Pro neo driver powered by 1500W RMS Powersoft plate amp.
IMO the much cheaper JBL PRX718XLF whips its butt when it comes to output and low-end.

Driver size is not everything.

John R.

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John Ferreira

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Re: 21" subwoofers - opinions
« Reply #22 on: February 25, 2017, 07:06:19 am »

Cones being fast/slow is a myth and needs to be sorted IMO.
The motor in any modern driver is more than capable of keeping control over the cone and accelerating it as fast as needed.

Here are two drivers that use pretty much the same motor, one 18" and one 21":
http://bcspeakers.com/products/lf-driver/21-0/4/21sw115?impedence=4
http://bcspeakers.com/products/lf-driver/18-0/4/18sw115-4?impedence=4

The difference we are talking about is more like this:
One has a moving mass of 304g, and the other has a moving mass of 335g. That's about 10% difference.
Subwoofers are like having a car being asked to accelerate to 30mph in about 30 seconds, and then smoothly decelerate. That car will not care if it has a couple of passengers that add 10% to the total weight. It's still well within the capability of the engine.
Low frequencies do not require a fast-moving cone. If the cone is moving quickly, it's producing treble and your crossover is set incorrectly.

High-frequency drivers need to be small and light. Here's an example.
A 21" driver moving 10mm one-way at 50Hz has a peak velocity (as it passes through the zero position) of just over 3m/s. The acceleration is around 1000m/s/s, so the voicecoil is exerting just over 300N of force (about 30kg, 66lb). That will result in an SPL of a smidge over 121dB at 1m, groundplane. That's around 1KW power input.
That same cone producing 121dB at 5kHz would be moving 0.01mm, with peak accelerations of 15000m/s/s, requiring 4500N of force (992lb). To produce that force, you'll need to put around 60KW in there and hope it survives.

A 4.5" driver producing 121dB at 5kHz needs to move 0.07mm one-way, requiring a larger acceleration of 77,000m/s/s. The moving mass is about 3g, though, so you only need 231N of force to get it there. That particular driver would need around 5KW input power to manage that, which it won't survive. Dropping the moving mass further (thin titanium diaphragms) and bolting a horn on the front would improve efficiency and reduce the power requirements, though.


Edit - couple of people have posted while I typed that.
Free-air resonance is not more important than the size of the driver. Resonance (Fs), among a bunch of other parameters, will help determine the shape of the response of the driver in a given cabinet. The size and linear excursion, and thermal power handling of the driver will tell you how loud it can go.

Chris

Chris that's not what I said, and by omitting the first three words of my sentence, you are taking my statement out of context.
 I said (and did not edit) "For sub frequencies..." meaning relative to, and did not say that it is just "more important".

I also did mention low wattage handling for the mentioned  example size.
As an example, you can have a perfect 1" subwoofer on a headphone. Obviously many other TS parameters are relevant, as I also mentioned in the same post.

The reason I mentioned that particular example, is because, as you probably know, too many people get hung up on the  12" or 15" versus, for example, a 21", and automatically think bigger is better.

Of course there are many other factors that dictate which subwoofer is better suited for a determined enclosure, a determined SPL, and a determined frequency range.



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John Ferreira

Instead of "sweet sound" or "warm sound", I prefer to use the terms "more or less high, or mid, or low frequency"

None of my equipment has the "sweet" or "warm" button.
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Chris Grimshaw

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Re: 21" subwoofers - opinions
« Reply #23 on: February 25, 2017, 08:11:29 am »

Chris that's not what I said, and by omitting the first three words of my sentence, you are taking my statement out of context.
 I said (and did not edit) "For sub frequencies..." meaning relative to, and did not say that it is just "more important".


Sorry John, looks like I misinterpretted you. Thanks for clearing that up.
I find the amount of misinformation around with speakers in general quite frustrating, which is probably why I jumped on your post.

Chris
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John Ferreira

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21" subwoofers - opinions
« Reply #24 on: February 25, 2017, 01:41:22 pm »

Sorry John, looks like I misinterpretted you. Thanks for clearing that up.
I find the amount of misinformation around with speakers in general quite frustrating, which is probably why I jumped on your post.

Chris

No problem Chris.

Your analogy about the accelerating car to 30mph and the extra 2 passengers relate to two parameters (there are a few more of course) in the power amp itself:

One is Slew rate (how fast an amp reaches full power, and usually 100V/microsecond would be typical of higher end amps)

The other is Damping factor, (the ability to stop the speaker cone, or the "breaks" and "torque" in your car analogy, before you put it in reverse and accelerate negatively, a typical example being 800 to 1 at 400Hz).

EDIT: Chris your first link does not work.
« Last Edit: February 25, 2017, 01:43:36 pm by John Ferreira »
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John Ferreira

Instead of "sweet sound" or "warm sound", I prefer to use the terms "more or less high, or mid, or low frequency"

None of my equipment has the "sweet" or "warm" button.
.

Ivan Beaver

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Re: 21" subwoofers - opinions
« Reply #25 on: February 25, 2017, 02:02:50 pm »



One is Slew rate (how fast an amp reaches full power, and usually 100V/microsecond would be typical of higher end amps)

The other is Damping factor, (the ability to stop the speaker cone, or the "breaks" and "torque" in your car analogy, before you put it in reverse and accelerate negatively, a typical example being 800 to 1 at 400Hz).

What I find funny is that one line of amps that has been considered to be "great" has one of the worse slew rates.

The old Crown MA series had a "terrible" slew rate, as compared to others.

Damping factor in amps is also highly over rated.

The "system" has a MUCH larger effect than the amp itself.

Years ago (late 90s) I did some testing (just listening-no measurements) and we had a decent variety of amps by different manufacturers.

The amp that had the highest damping factor (by a good bit) had the "loosest" sound.

The one that had the lowest damping factor stood right up with the highly regarded Crown MA2400.  This amp was also the next to the cheapest one in the test.

Historically, the amps with the best damping factor had the most "controlled" sound-so that was "assumed" to be, because of the damping factor.

The speaker wire and connectors is much bigger factor.

Often the "simple numbers" will result inaccurate assumptions.



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Ivan Beaver
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John Ferreira

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21" subwoofers - opinions
« Reply #26 on: February 25, 2017, 02:50:43 pm »

What I find funny is that one line of amps that has been considered to be "great" has one of the worse slew rates.

The old Crown MA series had a "terrible" slew rate, as compared to others.

Damping factor in amps is also highly over rated.

The "system" has a MUCH larger effect than the amp itself.

Years ago (late 90s) I did some testing (just listening-no measurements) and we had a decent variety of amps by different manufacturers.

The amp that had the highest damping factor (by a good bit) had the "loosest" sound.

The one that had the lowest damping factor stood right up with the highly regarded Crown MA2400.  This amp was also the next to the cheapest one in the test.

Historically, the amps with the best damping factor had the most "controlled" sound-so that was "assumed" to be, because of the damping factor.

The speaker wire and connectors is much bigger factor.

Often the "simple numbers" will result inaccurate assumptions.

Ivan the Crown MA 2400 had a slew rate of 75V/microsecond, which in the early 90s was not so bad for the price point.  Also the damping factor was a lot higher than, for example, a Bryson 4B (400:1 at 400Hz for the Bryson)

The Slew rate would probably impact latency more than anything else.

However, the higher damping factor made the bass frequencies sound better.

EDIT: The MA 2400 I owned also had PIP cards in the back of the amp, that you could replace and affect the its response. I replaced mine, (PIP) and compared to other amps I tried at the time, was like turning off "reverb" and increase a dry low frequency punch. In 1995 I found that amp sounded way better on Apogee 18" subwoofers, than most other amps I had tried.
« Last Edit: February 25, 2017, 03:04:40 pm by John Ferreira »
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John Ferreira

Instead of "sweet sound" or "warm sound", I prefer to use the terms "more or less high, or mid, or low frequency"

None of my equipment has the "sweet" or "warm" button.
.

Ivan Beaver

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Re: 21" subwoofers - opinions
« Reply #27 on: February 25, 2017, 03:14:19 pm »

Ivan the Crown MA 2400 had a slew rate of 75V/microsecond,
Where are you getting that number from?

The spec sheet says 13v/microsecond.

That is what I was going by
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Ivan Beaver
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Ivan Beaver

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Re: 21" subwoofers - opinions
« Reply #28 on: February 25, 2017, 03:17:57 pm »


However, the higher damping factor made the bass frequencies sound better.


But as I said in my test, amps that have a much lower damping factor can sound as "tight" and amps with a much higher damping factor can sound looser.

Damping factor is greatly over rated-at least in terms of amplifiers.

It is basically a measurement of the output impedance of the amp relative to the load impedance.

The cable makes a much larger difference in the overall system than the amp itself.
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Ivan Beaver
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John Ferreira

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21" subwoofers - opinions
« Reply #29 on: February 25, 2017, 11:55:08 pm »

But as I said in my test, amps that have a much lower damping factor can sound as "tight" and amps with a much higher damping factor can sound looser.

Damping factor is greatly over rated-at least in terms of amplifiers.

It is basically a measurement of the output impedance of the amp relative to the load impedance.

The cable makes a much larger difference in the overall system than the amp itself.

Ivan you are correct, at least on line they are rated at more than 30V/microsecond , and some models at 13V/microsecond. I had all my gear stolen in 1997, and I could swear that is what the manual said. I suspect dementia.

But to use your words, "it depends" :()

I don't know how you tested the amps, but remember that when you bridge an amp, damping factor goes to 1/2 value, and also at what frequency was the number obtained.
Damping factor values at 400Hz should not be compared to a 40Hz measurements, where the cone movement has a much lower inertia, and  it is so much easier to apply the "breaks"  (damping factor) before the cone reverses direction. But then again, you already know all that.
« Last Edit: February 26, 2017, 04:00:07 am by John Ferreira »
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John Ferreira

Instead of "sweet sound" or "warm sound", I prefer to use the terms "more or less high, or mid, or low frequency"

None of my equipment has the "sweet" or "warm" button.
.
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