Debunking the Myth that a Lighter Diaphragm Enables a "Faster" Speaker Driver

[NTK]

There is not such a thing as a "faster" driver, and there never will be.

Everyone with an engineering background knows control theory and mass-spring-damper constructions.
(or they should it's part of any curriculum starting mostly from the 2nd year or so)
More mass simply means that something takes more force to accelerate at the same amount.
Or in other words, with the same force, things will accelerate less.
Things resist when they are heavy (you know, Newton )

For loudspeakers and 2nd (and higher) order mass-spring-damper systems, this results in that the frequency response collapses.

In practice the inductance is more dominant with most subwoofers, so this is what most people see.
Unless a proper amount of demodulation rings is being used.

On the lower frequencies below Fs, mass is a constant and the compliance is the dominant factor.

There is something else called a Q-factor, which doesn't have much to the with the mass perse.
Depending if it is under-damped, criticality damped or overly-damped, this automatically translates to an hump in the frequency response.
Or like max flat frequency response etc

As the Purifi blog post (link: https://purifi-audio.com/2019/12/10/a-fast-driver-needs-a-light-cone-or-does-it/) pointed out, the sound wave generated by the speaker driver is NOT the result of a positional change (displacement) of the diaphragm, NOR its velocity, but its acceleration. This is the primary misconception.

The effect of a lower mass diaphragm is, as @dshreter pointed out earlier, is higher sensitivity (efficiency). If we define the "speaker speed" as how quickly a sound wave shows up, then it is determined by how quickly we can apply a force change to the diaphragm. And how fast we can change the force is largely* independent of the moving mass of the diaphragm.

*Note: By largely I mean this can depend on the voice coil design, which will affect mass, but is a secondary or less effect.

 

[rdenney]

Seems to me a lot of talking past one another.

The point seems to me that the mass of the cone is a small factor compared to the resistance of the air that it is moving, and also that speed directly derives from frequency--bandwidth--so the desired frequency is what dictates the speeds and accelerations at a given SPL. Why do subwoofers have big magnets? Not to make them faster, because they don't need to move fast. But absolutely to make them move against a non-resonant air mass with more excursion and force, to create the desired SPL.

This comes up in my tuba world frequently enough. Tuba players talk of heavy versus light instruments. To my thinking, the only effect that has is on how much the instrument itself can be felt to vibrate, which is an important feedback to some players. (Higher instruments create sibilance under certain circumstances that is affected by the vibration of the brass, but that isn't significant with the tuba.) But it's not the vibration of the instrument that makes sound out front--it's the vibration of the air within the instrument. The instrument is just a big impedance-matching device, filtering out the noise of the buzz into the desired range of fundamental and overtones and amplifying it through resonance to create the characteristic sound radiating widely and efficiently. So, the focus of effort for instrument designers is on providing the right shape for the air vibration to be filtered and amplified. Those that focus on the composition of the brass (and some do) are making unevidenced claims.

Example: One famous instrument (used by the Chicago Symphony) has attracted all sorts of unscientific claims about what makes it special (beyond the fact that it has only been used by very special performers). The brass composition is one such. But the instrument itself is 90 years old and has been overhauled, sanded, buffed, beat up, nearly destroyed in shipping, overhauled again, trimmed to accommodate higher pitch standards, shined up, allowed to get dull, polished (it's polished now, but started life with a satin finish), and on and on. It is noticeably lighter now than when it was new. And it still sounds special. No evidence supports the notion that age hardening, a slight variation in the zinc/copper alloy, or whatever has any real effect. But a dent the size of a pencil eraser in the mouthpipe tube can ruin the way the instrument plays. But the instrument absolutely depends on its size and specific shape to be what it is. It is a large instrument and its size sets a lot of air to vibrating, while its shape provides a wide dispersion of that vibrating air compared to other designs. It is said that replicas are not the same, and they aren't--some are better, but lack the world-class performer.

The point, as I understand it, is that speed is not needed to make a loud woofer, simply because speed is wholly dependent on frequency and woofers don't play high. But power is needed to vibrate a lot of air with sufficient amplitude to create low frequencies with high SPL.

Rick "knowing of a tuba made by carving a giant cheese" Denney

 

[OWC]

I would highly recommended reading books like Loudspeaker Handbook (Eargle).
There is a better, updated and newer book that just came out, which I forgot the name of.
I can look it up, if people are interested.

They exactly describe the same as I just did.

And yes, a lower mass will improve efficiency somewhat.
But that has nothing to do with something being faster or slower.

I don't want to sound to be rude in any way, but I don't really care what some company writes on their blogs.

Their is plenty of good, scientific and objective literature out there.

 

[NTK]

It is a very nice analysis of the relationship between excursion, frequency and acceleration, but is it related to the discussion of relationship between speed and moving mass?

In this sentence I assume you are saying using a single driver to play those two frequencies, so in the end you are just comparing the relationship between excursion, frequency and acceleration, rather than two drivers with different moving mass and what will happen.

What I am trying to point out is that the bandwidth of a speaker driver is not primarily limited by its moving mass. And bandwidth is "speed".

 

[OWC]

Seems to me a lot of talking past one another.

The point seems to me that the mass of the cone is a small factor compared to the resistance of the air that it is moving, and also that speed directly derives from frequency--bandwidth--so the desired frequency is what dictates the speeds and accelerations at a given SPL. Why do subwoofers have big magnets? Not to make them faster, because they don't need to move fast. But absolutely to make them move against a non-resonant air mass with more excursion and force, to create the desired SPL.

Just calculate the resistance of the air.
You will see that this air resistance is actually a small factor.

Subwoofers need an higher mass to get a lower Fs.
It's nothing more than just a simple RCL oscillator, (R is mechanical resistance, C is compliance, L is mass)
But with an higher mass the efficiency drops as well (also the frequency response drops with it)
So to compensate for, you need more force = bigger magnet

There is a little bit of wiggle room with optimizing the motor and magnet structure.

Bigger magnets are also needed to bring the drivers Qts down, so they fit in a smaller enclosure.
It's mostly this combination, Fs and low Qt, that magnets are so big.

20-30 years back they had woofers with low Fs, but they also needed huge boxes.

 

[OWC]

What I am trying to point out is that the bandwidth of a speaker driver is not primarily limited by its moving mass. And bandwidth is "speed".

That is correct, bandwidth = speed.
Well, acceleration, but I guess one could do some math tricks to make it speed lol

 

[NTK]

I don't want to sound to be rude in any way, but I don't really care what some company writes on their blogs.

Good information is good information.

 

[OWC]

Good information is good information.

That is correct, but

1 - Most companies have a double agenda with the information they put out there
2 - Seen Purify's track record of certain statements they made against objective scientific comparison methods, I take many things from them with a grain of salt (or two)

 

[AnalogSteph]

That is correct, but

1 - Most companies have a double agenda with the information they put out there
2 - Seen Purify's track record of certain statements they made against objective scientific comparison methods, I take many things from them with a grain of salt (or two)

Huh? What kind of statements are you referring to? These guys are about as focused on objective performance as they come, I mean the company is founded and run by a bunch of engineers... European engineers no less. Their speaker drivers have more published performance data than just about anyone else's.

 

[Raindog123]

To avoid retyping:

Electrostatic speakers? | Page 25 | Audio Science Review (ASR) Forum

Electrostatic speakers? | Page 26 | Audio Science Review (ASR) Forum

 

[OWC]

Huh? What kind of statements are you referring to? These guys are about as focused on objective performance as they come, I mean the company is founded and run by a bunch of engineers... European engineers no less. Their speaker drivers have more published performance data than just about anyone else's.

Since when are engineers by default (100%) objective?

The word engineer only describes the work somebody does.
There are engineers, engineering complexity bogus products, quantum stickers for example.
There are engineers developing solar f*'in roadways that are never gonna work efficiently.

Besides, what Purify is doing, is absolutely not new at all.
In fact the majority of the techniques they are using have been known and used for MANY years by others.
This is exactly what I meant with a double agenda.
Btw, as an European myself, I don't really see what that even is an argument at all?

But to give you one example from a quote in the last edition of Voice Coil Magazine. (see attachment, copyrights Voice Coil magazine)

Sorry, but I find these kind of words HIGHLY disrespectful towards all the scientists as well as engineers who put in all the time, effort and money in research to very clearly show that ears (brains) can't be trusted at all without a double blind test and such.
I can guarantee you, I am certainly not the only person thinking this way.

Importance here is not this specific point, but the fact that they just wave everything away like it is not important.

Anyway, very interesting that apparently people blindly "trust" just some engineers.
I thought this forum was about the science and objectiveness of things?

 

[rdenney]

...In fact the majority of the techniques they are using have been known and used for MANY years by others.
This is exactly what I meant with a double agenda.
...

So, what's your double agenda?

Rick "who followed the link in your sig" Denney

 

[OWC]

So, what's your double agenda?

Rick "who followed the link in your sig" Denney

My double agenda is written in my sig as well as my page.
Trying to convince people that there is no "perfect", everything depends on context.
If people can show with multiple research sources otherwise, I am more than happy to change my vision on things.
(Which I have multiple times in the past)

 

[Dmitrij_S]

It is hardly surprising that adding weight affects LF response and not response at 1kHz as moving mass is what matters with LF. If you wanted to affect response of that same driver at high frequencies you should leave the mass as it is and enlarge or reduce the diameter...

Sorry I couldn't help but comment on this old thread, since this is not the first time I have met these pictures from Adire Audio. These measurements and conclusions by Adire Audio guys are quite absurd because the added mass and the cone stiffness make up an oscillating system whose tuning frequency will be dependent on placement and fixing strength of added mass. So at higher higher frequencies added mass and the cone not move as a single moving mass.

 

[NTK]

Sorry I couldn't help but comment on this old thread, since this is not the first time I have met these pictures from Adire Audio. These measurements and conclusions by Adire Audio guys are quite absurd because the added mass and the cone stiffness make up an oscillating system whose tuning frequency will be dependent on placement and fixing strength of added mass. So at higher higher frequencies added mass and the cone not move as a single moving mass.

Attaching an additional mass to the dust cap of a woofer is a standard measurement method to determine Vas. (Obviously this method is applicable only to woofers, and was what Adire did.)


Source:

Woofer Tester 2

www.ht-audio.com

 

[thorvat]

Sorry I couldn't help but comment on this old thread, since this is not the first time I have met these pictures from Adire Audio. These measurements and conclusions by Adire Audio guys are quite absurd because the added mass and the cone stiffness make up an oscillating system whose tuning frequency will be dependent on placement and fixing strength of added mass. So at higher higher frequencies added mass and the cone not move as a single moving mass.

I fully agree. I guess a better way to add mass would be to spray the entire membrane with some thick polymer instead of attaching mass to the cone.

 

[Dmitrij_S]

Attaching an additional mass to the dust cap of a woofer is a standard measurement method to determine Vas. (Obviously this method is applicable only to woofers, and was what Adire did.)
View attachment 171207

Source:

Woofer Tester 2

www.ht-audio.com

Added mass method is perfect at low frequencies , when the cone approximately moves as rigid piston and resonance of the "cone stiffness- added mass" system is considerably higher than resonance frequency of "cone mass-suspention stiffnes" . In the mid and high audio frequency range distribution of the added mass and fixture strength over the cone become especially important. Measurements made by guys from Adire audio are ok at low frequencies, but they are incorrect at mid and high frequencies.

 

[BoredErica]

This is slightly off topic, but it makes me wonder: If a lighter diaphragm doesn't make for a 'faster' sound, what are planar magnetic and electrostatic technologies even good for? I think in headphones, planar magnetic ones tend to/can have lower distortion (though even with heavy EQ to match Harman Target, a good regular non-planar headphone can probably reach inaudible levels of distortions?).

Then are electrostats just a scam?

 

[KSTR]

This is slightly off topic, but it makes me wonder: If a lighter diaphragm doesn't make for a 'faster' sound, what are planar magnetic and electrostatic technologies even good for?

It's just an alternative construction at hand which tends to have good high frequency properties.

 

[restorer-john]

I can guarantee you, I am certainly not the only person thinking this way.

You aren't.