Why are speakers so inefficient? (1 Viewer)

[Matthew Will]

Why are speakers so inefficient? I can't think of many things on earth only have .5% "efficiency" Whats the deal? Is it the mass of the driver? The resistance of the surround system? I dont think it would have much to do with the magnets because the coil is either getting pushed or pulled, not both. Is it the coil design that is inefficient at turning the electricity into magnetic power? Is it the resistive forces of the air? What about infinite baffle systems then that have the least resistence? Too many varibles? All of the above? But still...only .5% efficiency? And I'm sure its a hell of a lot lower for a lot of speakers. Matt

 

[Matthew Will]

Didn't think I'd have to bump this question. Any takers? Matt

 

[ChrisBee]

It all started with AR (Acoustic Research) building what they described as an "infinite baffle". It wasn't an infinite baffle at all but an acoustic suspension closed box.

Because the speaker cone had to work hard against a small mass of air in the airtight box behind it it needed more power to compress that air.

Before that point speakers were usually reflex ported or acoustic resistance. Often open backed with slots or perforated board.

I can still remember listening (over a shoulder) to a demo of the first infinite baffle B&W demo in a local snooty music & hifi shop. The B&Ws were standing on the plush carpet. No speaker stands back then and the listeners weren't even offered a seat.

Importantly though, the infinite baffle eventually lead to the AV subwoofer driver. The infinite baffle needed far more cone movement than the previous reflex drive units. Which hardly moved a whisker to produce deep bass.

This lead to roll surrounds and bigger & better spiders. Long voice-coils and bigger magnets to keep things under control.

Efficiency fell through the floor back then and probably hasn't ever recovered with the infinite baffle designs. Amplification also followed the infinite baffle's need for more oomph to get those heavy floppy cones moving and stopping.

Cheap watts followed. Enter AV riding on the back of all those endless small "improvements"!

ChrisBee

 

[Matthew Will]

I'm not asking specifically about infinite baffle setups and I'm not quite sure I understand why you say the efficiency dropped through the floor. What were the efficiency levels before it dropped? I can't imagine the capabilities of a speaker if it even had 1% efficiency.

Something just hit me though. Are speakers really meant to be efficient? If the speaker was completely efficient and converting electricity into mechanical energy to move the air then all of the electricity would become trapped in the coil and used instantly. We don't want that happenening because then the coil would melt right? I'm thinking the only truelly efficient speaker would be one that could move the correct length of a frequencies wavelength. So for instance if frequency = (speed of sound )/(wavelength) then speed of sound/hz = wavelength. A 70 Hz note has a wavelength of 10.95 feet. So the speaker would need to move roughly 11 feet from peak to peak for a 70 hz note to be completely efficient. This is just my theory. WHATS THE REAL REASON FOR ALL THE INEFFICIENCY!?! I AM NOT SATISIFIED! Matt

 

[ChrisBee]

You want efficiency in real terms you build a long concrete flared horn. There are a number of different geometric forms that use different mouth openings, with and without compression chambers. Each type offers variations in effiency or distortion. Exponential, hyperbolic and tractrix come immediately to mind. A couple of watts and a millimtre of cone travel will blow your eardrums out of your nose as a fine spray! That is efficient air movement.

The need for any speaker enclosure at all is down to the bass. You don't really need a box for midrange or treble. Some way must be found to seperate the air movements at the front of the cone from cancelling those at the back.

The cone moves and produces the sound. The method of driving the air around it is what determines efficiency. A reflex port extends the bass by accelerating puffs of air in the port which communicates with the room air and pressurises & rarifies it alternately.

A horn needs to be massive to be efficient in the deep bass. A free flapping speaker in a simple baffle needs to be huge or have multiple drive units to move enough air.

It's always swings and roundabouts. No free lunches.

An infinite baffle speaker cone is fighting the back compression of the box behind it. The harder it hits back against the air cushion the harder it has to work. The smaller the air cushion (box size) and the larger the cone (compared with box size) the worse it all becomes.

One advantage of an infinite baffle is the slower bass roll-off compared with most other speaker enclosere types. To achieve this it needs to move a lot of air. It uses longer cone travel to achieve this. With higher power required to start and stop it's movements. It is also less likely to damage itself when driven below the box/cone resonance compared with reflex enclosures. It suffers from assymtric distortion because the front of the cone sees fresh air while the back sees only the small air cushion.

The designer makes his choice and is then hard up against the physics of his choice. Electrostatics, panels etc all offer alternatives at prices/efficiencies/sizes to baffle the buyer. The speaker doesn't even need a cone to move air. Generally bigger is always more efficient.

Now what was the question again?

ChrisBee

 

[Chris Quinn]

Infinite baffle and acoustic suspension are NOT the same and are about as opposite as you can get.

 

[Chu Gai]

To some extent, you're making more of this than there is.
The efficiency of a speaker is determined by the driver and is a function of the mass of cone, the radiating surface, the resistance within the voice coil, etc. A direct radiator speaker with 100% efficiency would have an SPL level of approximately 109 dB at one meter from the center of a source that's generating 1 watt of power.

See the following link for more information and fun equations.

Further comments regarding speaker efficiencies are provided by Dunlavy here.

 

[John F. Palacio]

Every time you convert one type of energy to another the losses are large.

In the case of speakers you are converting electrical energy into mechanical energy. More accurately you are converting electrical into magnetic into mechanical.

 

[Cees Alons]

Even the best exponential horn system has an efficiency of less than 10%. A piano playing at full loudness emits the equivalent of 0.5 - 1 Watt!

The low efficiency is mainly caused by the problematic transition of energy between the cone and the air. Because air has so little mass, its waves have long movements and little pressure. A speaker cone tries to move that air with - unfortunately - little movement and much pressure. If you could design a speaker with moving air in the first place, it would be more efficient (believe me: they've searched for it).

Exponential horns have the best characteristics (path) to go from little surface/big pressure to big surface/less pressure. That's why they have the best efficiency.


Cees