Driver details

[Twisted85]

Hello,

I've been into speaker building for some years now and can make my own speaker designs from manufacturers driver specs, but as I don't know anything about making the drivers, and even though I've occasionally read about it, it's never been that clear to me how different things relate to each other in making of the driver. I understand it's always about a compromise between things that are considered more important than others, depending of the intended usage.
So I would like to ask very basic questions about conventional bass/midrange drivers, I think I know answers to some of them, but I would like to know for sure.

The thing that wonders me most is the stiffness of the cone surrounding/spider.
If the stiffness is either increased or lowered, what happens to: Vas, Qms (and Qts) or Fs?

When the cone/moving mass is increased, the Fs is lowered, that's easy.

How about the driver effiency? If the Fs is lowered, the effiency goes down? If Sd is increased, the effiency goes up? Which kind of compromises manufacturers needs to make in the motor/magnet design when talking about effiency?

How about the Xmax value. Is there any drawbacks of making a woofer with a big Xmax besides most probably higher construction costs?

I'm also interested how drivers have evolded through the last decades. Few decades ago spekers had much bigger boxes and the Xmax of the drivers was very low. Today it seems that even subwoofers are put into quite small boxes and have huge Xmax values. I would very much like to know which things have lead the woofer design this way and what are the benefits and drawbacks.

Hopefully someone can clear these things a bit for me, I would be very pleased. I will probably have more questions when I undestand more of how different things relate to each other

-Ari

[kenpeter]

Originally Posted by Twisted85

The thing that wonders me most is the stiffness of the cone surrounding/spider.
If the stiffness is either increased or lowered, what happens to: Vas, Qms (and Qts) or Fs?

VAS I think means, "Volume Acoustic Suspension"???
Whatever words those letters actually stand for, that is literally what
it does in action. A volume of trapped air that would have the same
exact springiness as the driver's spider.

If you put a driver in a sealed box the same size as VAS, you have
two identical springs in parallel (one spider, one air). Fs, Qms, and Qts
are all doubled. Somebody correct me if thats not the correct math?

If you now look at this as-if a sealed back loudspeaker. And only
concern yourself how it seems to behave as seen from the front?
Its going to appear that VAS was cut in half (cause the spring is
now twice as tight as before you added the trapped air in back...)

[jerome69]

Originally Posted by Twisted85

Hello,
I've been into speaker building for some years now and can make my own speaker designs from manufacturers driver specs, but as I don't know anything about making the drivers, and even though I've occasionally read about it, it's never been that clear to me how different things relate to each other in making of the driver. I understand it's always about a compromise between things that are considered more important than others, depending of the intended usage.
So I would like to ask very basic questions about conventional bass/midrange drivers, I think I know answers to some of them, but I would like to know for sure.

The thing that wonders me most is the stiffness of the cone surrounding/spider.

If the stiffness is either increased or lowered, what happens to: Vas, Qms (and Qts) or Fs?

Hi

Don't confuse stiffness of the cone and stiffness of the suspension (surround/spider). It is not the same physic phenomena.

You forget Cms
Cms
Measured in metres per newton (m/N). Describes the compliance (ie, the inverse of stiffness) of the suspension. The more compliant a suspension system is, the lower its stiffness, so the higher the Vas will be. Cms is inversely related to Vas and thus has the same tolerance ranges.

Originally Posted by Twisted85

When the cone/moving mass is increased, the Fs is lowered, that's easy.

Yes

Originally Posted by Twisted85

How about the driver effiency? If the Fs is lowered, the effiency goes down?

Yes if you keep equal the other parameter like Sd. The bandwidth of the driver is lower too.

Originally Posted by Twisted85

If Sd is increased, the effiency goes up? Which kind of compromises manufacturers needs to make in the motor/magnet design when talking about effiency?

If Sd increase for one driver, the driver efficiency is goes up but the driver will beam sooner. It's a lot of compromise !
At the point of view of the driver builder, I don't think the efficiency have a direct relationship with the motor. The cone/moving mass have this direct relationship. The motor is optimized for the use of the driver. A midrange is in a close box. A woofer is in a bass reflex box etc.

Originally Posted by Twisted85

How about the Xmax value. Is there any drawbacks of making a woofer with a big Xmax besides most probably higher construction costs?

I don't know, i like big xmax driver because I play music loud and I am not still deaf. See zaph comments

Originally Posted by Twisted85

I'm also interested how drivers have evolded through the last decades. Few decades ago spekers had much bigger boxes and the Xmax of the drivers was very low. Today it seems that even subwoofers are put into quite small boxes and have huge Xmax values. I would very much like to know which things have lead the woofer design this way and what are the benefits and drawbacks.

WAF drawback is the key... I also like small speaker because they are "faster" than big. I understand why SEAS/SCANSPEAK don't make speaker bigger than ten inch

Originally Posted by Twisted85

Hopefully someone can clear these things a bit for me, I would be very pleased. I will probably have more questions when I undestand more of how different things relate to each other

-Ari

I recommend you to read these papers, I think you will have yours answers :
Thiele/Small - Wikipedia
Klippel Publications

I think you should ask : what I want to do with a driver ? How I want to use it ? Is this driver verifies my specification ? I think it's a better way to get the best of a chosen driver. And you can see some manufacturers make their own driver.

Hope this helps
Rodolphe.

[chris661]

Originally Posted by kenpeter

If you put a driver in a sealed box the same size as VAS, you have
two identical springs in parallel (one spider, one air). Fs, Qms, and Qts
are all doubled. Somebody correct me if thats not the correct math?

I'd advise a trip to winISD for this. With my test woofer (8" DVC thingy), a sealed box at Vas (56L) gives a Q of 0.5, system resonance of 49Hz, when the driver resonance is 33Hz.

When you're talking about motors, another thing to keep in mind is power handling - a 4" coil with lots of vents will cool off quicker that a 1" coil with no vents.

Cone area, bass extension and efficiency (or max SPL) are all linked - if you get more bass extension from the same cone area, efficiency goes down, if you want more efficiency, you sacrifice either extension or size (ie, make it bigger) - this is another thing manufacturers have to consider. PA stuff won't go as low as HiFi stuff (generally), but it goes far louder for the same cone area. Car audio stuff has decent bass extension from a smallish speaker, but the efficiency isn't brilliant. Most manufacturers sort this with more power handling, bigger Xmax etc.

Chris

[Twisted85]

Originally Posted by jerome69

Hi

Don't confuse stiffness of the cone and stiffness of the suspension (surround/spider). It is not the same physic phenomena.

Of course. From what I've read, the only thing I really know about softer and harder (like aluminium) cone materials is that harder cones have high break up distortion. (above the actual usable frequency range)

You forget Cms
Cms
Measured in metres per newton (m/N). Describes the compliance (ie, the inverse of stiffness) of the suspension. The more compliant a suspension system is, the lower its stiffness, so the higher the Vas will be. Cms is inversely related to Vas and thus has the same tolerance ranges.

Yes I left Cms out as I've understood Vas represents the same thing.
So:
The stiffer the suspension, the smaller the Vas value is.
And when the suspension gets stiffer, Fs gets higher, right?

So as people nowadays want 10-12" subwoofers in very small boxes and also want very low bass the solution is:
Stiff suspension, the vas goes down and small box can be used. To compensate Fs rising, the cone mass needs to be increased. All this and quite small Sd results in bad effiency. Huge motor with very big power handling is needed and so is lots of power. To produce the lower frequencys, the not so big cone needs to be moving a lot, so also very big Xmax is needed. The exact opposite to PA driver design. Am I correct with this?

The Qms/Qes is still left out from this. The big motor is assumably also needed to lower the Qes and Qts so that Qtc won't get too high in a small box. But how was Qms affected with the changes to suspension and Mms?

A year ago my friend wanted good subwoofers for his new home theater, so I made him two 15" subwoofers in 140liter boxes each and with 22Hz tuning. Very high output and clean bass is produced even when the cones aren't actually moving that much. (And no, he doesn't live alone, but he's fortune to have a large separate room for his HT hobby and don't need to concern about WAF )

I've seen graphs about how Bl changes when the cone moves even within in the declared "linear xmax" range and the changes to Bl are drastic. This has huge changes to other values and I'm very sure this has very bad audible results. All this have lately made me thinking of my next own speaker models and I'm quite sure they won't be tiny 2-ways of effiency of 84dB/1W

[jerome69]

Ok I think you are right Ari. See this latest SEAS alu driver L26ROY

But I don't take care about efficiency, solid state amplifier can make a lot of power. For me efficiency is not a problem, my prefered loudspeaker has 82dB/2.83V, I 'am not very objective ?

When you make a speaker you cannot ignore the electronics. To deliver very clean bass, the amplifier should control the speaker, this is a very difficult task for a lot of power amplifiers.It should be easier to make clean bass when you have a lot of efficiency but I am not sure. I also test big driver and my conclusion was the same.

In my opinion, the problem of bass is not only a question of driver/speaker but also a problem of power amplifiers and rooms. Best bass needs large rooms.

[Twisted85]

Originally Posted by jerome69

But I don't take care about efficiency, solid state amplifier can make a lot of power. For me efficiency is not a problem, my prefered loudspeaker has 82dB/2.83V, I 'am not very objective ?

I think I will try to achieve a bit higher effiency than average and definitely try to achieve low cone movement by using more cone area and not to drive them too low. For example, in my current 2-way system the Peerless 810921 HDS tweeter has a highpass somewhere around 1,8KHz and when playing at higher volumes I'm more than sure that the low XO point causes quite a lot of distortion and stresses the tweeter. This is a problem with 2-way system and I think I will try different approach next time.

In my opinion, the problem of bass is not only a question of driver/speaker but also a problem of power amplifiers and rooms. Best bass needs large rooms.

Of course the listening room has a HUGE effect.

Here is two in-room measurements that I've made.

The green one is done in a very sturdy mid-sized room and the subwoofer was REL Storm III. The red one is the two 15" drivers in my friend's HT that I mentioned earlier. These two subwoofers are not of course comparable as they are totally different, but the measurements shows what the room does to the bass response. Because the red measurement is done in a bigger room with much lighter wall structure and the room modes doesn't ruin the bass response. And I can tell you, the subwoofers in my friends HT sound AMAZING!
(the SPL values in the graphs are not calibrated any way)

-Ari

[jerome69]

Originally Posted by Twisted85

I think I will try to achieve a bit higher effiency than average and definitely try to achieve low cone movement by using more cone area and not to drive them too low. For example, in my current 2-way system the Peerless 810921 HDS tweeter has a highpass somewhere around 1,8KHz and when playing at higher volumes I'm more than sure that the low XO point causes quite a lot of distortion and stresses the tweeter. This is a problem with 2-way system and I think I will try different approach next time.

Nice project, 1.8KHz is Ok for the HDS 0.5mm xmax but you should use a high order high pass 3/4 order. For example I used a seas T25001 0.25mm xmax, 4 order electrical at 1.8KHz and it can play very loud.

Originally Posted by Twisted85

Of course the listening room has a HUGE effect.

Here is two in-room measurements that I've made.

The green one is done in a very sturdy mid-sized room and the subwoofer was REL Storm III. The red one is the two 15" drivers in my friend's HT that I mentioned earlier. These two subwoofers are not of course comparable as they are totally different, but the measurements shows what the room does to the bass response. Because the red measurement is done in a bigger room with much lighter wall structure and the room modes doesn't ruin the bass response. And I can tell you, the subwoofers in my friends HT sound AMAZING!
(the SPL values in the graphs are not calibrated any way)

-Ari

Very impressive the difference