A question for the experts

[Vaughan]

In my pursuit to improve my knowledge on audio, I ask more questions on the physical operation of cone speakers and the "coupling" that takes place between it and the air.

As I understand it, cone speakers are extremely inefficient and a small percentage of electrical power is converted to mechanical energy. The reasons for this, again, according to my knowledge, are many. One, because the magnetic field and electrical fields are not 100 % efficient, there are eddy currents that reduce overall efficiency.

The suspension has resistance properties, the voice coil has power handling limits and too much input power can result in thermal compression. We also have the enclosure which dictates to a large degree how much overall resistance (or lack thereof) there is going to be within the system.

But the transfer of energy from cone to air is highly inefficent because the cone has mass and it needs to be large to excite a given amount of air. The air also has resistance to motion. Large cones are needed to excite large amounts of air.

Larger drivers can move more air easier because the effective piston area is larger and can "punch" a greater amount of air out in the time required compared to smaller drivers.

I have read that speakers have a poor impedence match to air. I'm not sure what that means. In fact, this is my primary question that I'm hoping someone can answer for me.

If someone more knowledgeable (and I'm sure there are here ) can explain to me how cone speakers have a poor impedence match to the air (and if the reasons I cited have a large influence on it) then I would appreciate it.

Thanks !

--Regards,

[Vaughan]

Anyone ?

--Regards,

[SY]

If you want a rigorous explanation, Beranek's "Acoustics" would be the place to look. But you do have the basic point, the big player in the inefficiency of direct radiators is the impedance mismatch between air and the cone. This is the fundamental motivation behind horns- they act as an acoustical transformer, converting small area/high velocity to large area/low velocity.

[Vaughan]

SY, but why do cone speakers have this impedence mismatch to the air ? Is there one underlying cause or is there a combination of many different factors that I alluded to in my initial post ?

Because I'm not sure.

This question was brought up because I honestly never thought about it before and I think it's fascinating to talk about. You recommend Beranek ? Would there be empirical and practical explanations there ?

As I'm sure you know about me, I'm not one for math. Thanks.

--Regards,

[pinkmouse]

To get basic about it, the air is soft and squishy and the cone is hard and rigid.

If you try and push a pillow with a brick, the first thing the brick does is squash into the pillow, possibly quite a way in before the pillow starts moving. When you stop pushing the pillow doesn't stay squished if the brick is removed, it slowly puffs back up. This is a rough idea of how it happens. Does it help?

[SY]

Beranek is pretty mathematical, but so is the concept of impedance.

Let me try to give an intuitive example. The impedance of a medium (like air) is the product of the density and speed of sound. It's easy to see that water will have a MUCH higher acoustical impedance than air since the density is higher and sound waves travel faster. OK, now let's grab a stick and jiggle it around in the air. Look at the output of a test mike. Doesn't make much sound, does it? Take the same stick, dunk it in the bathtub and jiggle it with the same energy (the displacement will be smaller because of the higher density of the water). Now despite the reduced displacement, suddenly there are some pretty good size acoustic waves in the water, and if you have a test mike in the water, you'll see a considerably greater output. The water is a better impedance match to the stick.

[Vaughan]

Okay. So the energy transfer from cone to air is poor because the amount of air excited is low due to the relatively small radiation produced by the woofer.

This is why big cones are needed to reproduce long wavelengths at low distortion. The transfer of energy from cone to air is more efficient because the effective piston area is greater and can excite a larger amount of air.

Am I heading in the right direction ? BTW, thank you both for responding. Pink mouse, would you be able to come up with another analogy (or two) ?

You have always been good at coming up with analogies. LOL.

--Regards,

[Vaughan]

SY, do you have a copy of Beranek ? If I buy the book would it help me understand the situation in more practical terms or is it purely mathematical ?

Thanks.

--Regards,

[pinkmouse]

Originally posted by Vaughan
Okay. So the energy transfer from cone to air is poor because the amount of air excited is low due to the relatively small radiation produced by the woofer.

Not quite. It's how the cone transfers its own energy to the air, not how much total energy it has. Think of percentages. A driver may transfer 5% of it's energy into the air, whatever power you feed it, so for 10W in you get 1/2W out, you put 100W in you get 5W out. That 5% reflects the impedance mismatch between the hard cone and the squishy air.

Pink mouse, would you be able to come up with another analogy (or two) ?

I'll try...


OK, let's take SY's bathtub. Stand on a chair and jump in. What happens? Big splash, water flies everywhere, your significant other tells you off and you have to take them out to dinner to apologise. OK, same bathtub, but empty. Once again you jump in. What happens this time? Nothing.

What was the difference? In the first example, the water is about the same density as you, so when you hit it, a lot of your energy is coupled to the water and it splashes out. This would be a good impedance match.

In the second, the air is much less dense, and so you merely push the little bit of air around you out of the way and hardly any energy is transferred. This is a poor impedance match. Note that the energy input into the system in both situations is the same, but the results, (and the clean up bill), are totally different.

Any better?

[Vaughan]

Not quite. It's how the cone transfers its own energy to the air, not how much total energy it has. Think of percentages. A driver may transfer 5% of it's energy into the air, whatever power you feed it, so for 10W in you get 1/2W out, you put 100W in you get 5W out.

Is this at all influenced by eddy currents, voice coil and permanent magnet nonlinearities, heat build up in the coil ?

OK, let's take SY's bathtub. Stand on a chair and jump in. What happens? Big splash, water flies everywhere, your significant other tells you off and you have to take them out to dinner to apologise. OK, same bathtub, but empty. Once again you jump in. What happens this time? Nothing.

What was the difference? In the first example, the water is about the same density as you, so when you hit it, a lot of your energy is coupled to the water and it splashes out. This would be a good impedance match.

In the second, the air is much less dense, and so you merely push the little bit of air around you out of the way and hardly any energy is transferred. This is a poor impedance match. Note that the energy input into the system in both situations is the same, but the results, (and the clean up bill), are totally different.

Any better?

Much better. The air has very low density and the cone's mass makes it difficult to transfer energy and to couple this energy to the low density air.

*Meow*.

But if the cone was massless, would this at all change things ?

--Regards,