Sub efficiency question (1 Viewer)

[Jason Padrick]

Hi,

I've been looking at these drivers lately, like the Stryke AVs, the DPL12, and others, and they all have fairly low efficiency, like less than 86db/W. Why is that? What is technically going on there?

P.S. Yes I'm noob, but I am curious.

 

[ThomasW]

There's a little thing called Hoffman's Iron law.

"The Iron Law states that the efficiency of a woofer system is directly proportional to its cabinet volume and the cube of its cutoff frequency (the lowest frequency it can usefully reproduce)."

So want a small box that plays both low and loud? Then the price of admission is low efficiency

 

[Jason Padrick]

So many of the newer high excursion drivers have a low efficiency because they are designed to work in small boxes? Is that what you're saying?

So woofer systems that play "both low and loud" require a larger box, assuming a sealed system?

 

[Chris Tsutsui]

That Hoffmans law is interesting,

Would that explain why some high efficiency drivers like some 18s, may be able to play very loud with 1 watt, but they also have little bass given a fairly big box?

And how low efficiency drivers can work with small boxes plus are able to play low.

 

[ThomasW]

Here's the law stated again, this time with an example of how it works.

"Hoffman's Iron Law, described by Henry Kloss in the mid-1950s and later turned into an exact mathematical formula by engineers Thiele and Small, governs the behavior of woofers. Essentially, it says that a woofer's efficiency is proportional to the volume of its cabinet and the cube of the lowest frequency it can produce before losing relative level (aka the cutoff frequency). Take, for example, a woofer whose response is flat down to 40 hertz in a 2-cubic-foot enclosure. To make its response flat down to 20 Hz, you must either increase the cabinet volume by eight times (to 16 cubic feet) or use eight times the amount of amplifier power to achieve the same listening volume."

So basically in the design of a sub one can vary any two of three performance characteristics, the third will be fixed, hence the name "iron" law

1)size of box
2)how low the system plays
3)output efficiency

 

[Jack Gilvey]

Comparing or judging subwoofer drivers by their "efficiency" rating is almost meaningless. Compare the low bass output of a DPL12 in 3ft^3 to a "higher-efficiency" driver with the same input.

 

[Jason Padrick]

I would guess that the low bass output of a DPL12 would be higher than a higher efficiency driver because of its lower resonance. I haven't modeled it at all yet though.

This helps a bit, but what about the efficiency as rated not inside a box?

 

[Jack Gilvey]

This helps a bit, but what about the efficiency as rated not inside a box?

What about it? Which aspect of performance would be more important in your planned use, inside or outside a box?

 

[Jason Padrick]

Well, performance inside a box is much more useful.

I was interested in what makes a high efficiency driver versus low efficiency. I know that adding mass to a cone makes the resonance lower as well as decreases efficiency. A lower mass cone has a higher efficiency but less extension. What other factors contribute to efficiency?

 

[ThomasW]

Why are you so fixated on efficiency?

50 yrs ago when amplifier power was extremely expensive high efficiency drivers were necessary. In the modern era watts are cheap, real cheap; and low efficiency drivers present no problems what so ever.

If you really want to learn about driver design, a DIY internet audio forum is a poor substitute for going to the library and doing some research. Or you could spend $30 and buy Vance Dickason's "The Loudspeaker Design Cookbook"

 

[Jason Padrick]

Actually I have an older version of it (2nd or 4th) at home. I think I need to dig it out and read it cover to cover again.
Same with Cohen's (sp?) book.

I was just thinking that efficiencies below 87dB seemed a little odd, and wanted to know why that is. Apparently, from the above posts, they're meant to run in smaller boxes and you trade eff. for it.

The higher the efficiency for a given speaker and amp, the higher the max SPL will be, that's another reason why I ask.

 

[Brett DiMichele]

Tom,

"Watts are cheap" ? I guess for subs they are.. But man I
priced those Ayre amps that Jon seems to have an likin for..

Woooo boy.. Too rich for this poor man

I agree with Tom and Jack efficiency is one of my least
concerns when looking at a low frequency driver.

 

[ThomasW]

The higher the efficiency for a given speaker and amp, the higher the max SPL will be, that's another reason why I ask.

Sorry but that's not exactly correct. Max SPL is determined by the input power, size/design of the box, the frequencies being reproduced, the excursion limits of the driver and the amount of room gain.

 

[CarlDais]

Thomas & anyone else...
asked another way what is it about,
lets say Adire's Malestrom driver which is a higher
sensitivity dirver--Without getting too technical-- how does motor design impact this aspect i.e., BL curve variance? If all things being equal(as if that will ever occur)does a well engineered driver-meaning motor, cone surround et.al, with higher sensitivity have some advantages? A few years ago I listened to a nice DYI speaker system... the designer seemed to be happy to be able to use some well engineered Focal Drivers which also had better sensitivty numbers than what he had used in the past. He also spoke well of PHL and some Cabbasse drivers. There is a guy near by who has McAuley (sp?) 18" drivers as his LFE channel...are these also medium sensitivity drivers?