eli5: How a simple cone can amplify the sound?


That wouldn’t require some way to add energy to the sound?

In: 10

Yes, if *amplification* was what was happening, but they don’t, they *focus* the sound.

I assume you’re referring a cone like those [old-timey cones](https://www.gettyimages.in/detail/photo/female-cheerleader-using-megaphone-royalty-free-image/YSP_073?adppopup=true) cheerleader’s had at football games and not a modern speaker system.

Basically, if you imagine shouting, that sound energy spreads in a sphere around your mouth. Some goes back behind your head, some to the sides, and some forward. But if you shouted into an old-timey cone more of that sound energy is focused, and projected forward. It’s not something for nothing, it would be quieter behind and to the sides and *that* energy is redirected forwards to the audience.

You do something similar with light in [this](https://www.etsy.com/listing/1174475602/antique-brass-wall-candle-holder-sconce) you reflect the light outwards and it appears brighter

There are two things that a cone can do. One is simple – it directs the sound so that it all goes the right direction. There is no extra energy, just less waste.

A “cone” could refer to either the speaker cone or horn shape. The two work very differently but they both do something similar – they improve the acoustic coupling of a sound source. Imagine a spring shaking forward and back. This thing will vibrate a bit of air, making a bit of sound, but it’s tiny. It isn’t making much noise because it interacts with very little air. Even if you keep twanging that spring, it won’t make more sound. Adding some paper will make almost all of the energy become sound because it catches the air. You will have to twang the spring more often because it uses more energy.

Without the paper, the energy is “trapped” in the spring. With the paper it can move into the air much easier.

Likewise a speaker with a cone *will* require more energy than a speaker without, but it will also be able to produce more noise.

If we’re talking about loudspeakers:

Imagine you’re running at high speed into a group of people. They see you and they move slightly to get out of your way.

Now imagine the people are constrained by the walls of a hallway. They can’t get out of the way. You crash into them, transferring the full energy of your speed.

A speaker cone (horn) prevents the air from getting out of the way of the speaker’s moving membrane.

/u/TheJeeronian’s comment provides a more accurate and detailed answer.

It doesn’t amplify sound, it reduces losses. Sound can travel in all directions from its source. Most directions are not towards the intended audience, and so the energy propagating in those directions is wasted. A cone allows the vast majority of the sound to travel only in one direction so nearly all of the energy from the sound’s source goes towards the intended audience.

The intended audience *does* receive more energy, but the source does not use/produce more energy. The energy is focused mostly in one direction.

I assume you are talking about a megaphone cone you shout through to make it louder.

Imagine you are on rollerskates and the difference of pushing against a brick wall or a large soft mattress to accelerate. The brick wall allows you to use more force and get a higher velocity.

The cone does something similar with the air from your lungs. As the cross section of the cone is quite small close to your mouth the air is a bit “stiffer” and gives the muscles in your lungs more to push against. You do use more energy than in open air and that is possible because of the shape.

In physics this is called “impedance matching” and a cone is a way to do this with sound waves.

There is also an added bonus of adding directivity to the sound which concentrates the energy on a smaller area.