Unraveling the Science of Sub-Woofers

[ryan gonzalez]

can anyone explain the science behind a car sub woofer? how do they create air pressure? how does generate enough energy to make a girls hair stand on end or set off a car alarm? what's going on during all that and what's the sub doing to make it happen?

 

[Simon Bridge]

Welcome to PF;
A car subwoofer is still a magnet speaker last I looked.
http://www.explainthatstuff.com/loudspeakers.html
http://www.physics.org/article-questions.asp?id=54
... they all work the same way.

 

[Drakkith]

The general principles behind a subwoofer can be found in the wikipedia article.

how do they create air pressure?

The same way that all modern speakers do. From wiki again: https://en.wikipedia.org/wiki/Loudspeaker

The most widely used type of speaker today is the dynamic speaker, invented in 1925 by Edward W. Kellogg and Chester W. Rice. The dynamic speaker operates on the same basic principle as a dynamic microphone, but in reverse, to produce sound from an electrical signal. When an alternating current electrical audio signal is applied to its voice coil, a coil of wire suspended in a circular gap between the poles of a permanent magnet, the coil is forced to move rapidly back and forth due to Faraday's law of induction, which causes a diaphragm (usually conically shaped) attached to the coil to move back and forth, pushing on the air to create sound waves. Besides this most common method, there are several alternative technologies that can be used to convert an electrical signal into sound. The sound source (e.g., a sound recording or a microphone) must be amplified with an amplifier before the signal is sent to the speaker.

That's pretty much the gist of it. The details are much more in depth, but that's the basic working principles behind how all speakers work, including subwoofers. The key differences between a subwoofer and other speakers is that the diaphragm and voice coil are much larger in a sub woofer than other types of speakers, as a sub woofer has to move a much larger volume of air every cycle.

 

[ryan gonzalez]

Thank you for the reply. What kind of science is at play? When they hit and it's loud enough to blow out a lighter with ease what's going on? What are they doing that's making that happen?

 

[nasu]

They turn the volume way to high for someone driving in a populated area. This is what they are doing.

 

[russ_watters]

The key with a subwoofer, as you might think, is the frequency...or better yet, the wavelength. When the wavelength is short (milimeters?), the sound has trouble interacting with objects. When it is long (meters?), it looks like a variable wind.

Consider a 20hz sound: divide that into the speed of sound and you get a wavelength of 18 meters. That's a slug of air moving forward then backwards 9 meters from where it started, 20 times a second!

 

[ryan gonzalez]

The key with a subwoofer, as you might think, is the frequency...or better yet, the wavelength. When the wavelength is short (milimeters?), the sound has trouble interacting with objects. When it is long (meters?), it looks like a variable wind.

Consider a 20hz sound: divide that into the speed of sound and you get a wavelength of 18 meters. That's a slug of air moving forward then backwards 9 meters from where it started, 20 times a second!

I like this. This helps. How would you describe it? Fluid motion?

 

[nasu]

I thought the displacement of the air is related to the amplitude of the sound.
You can have a 18 m wavelength wave in which the displacement is just microns.
The long wavelength is probably related to the effects described (the low frequency sounds are known to be damaging to the body) but it depends of level, as for any sound.

A sound in air, as described above (20 Hz, with displacement of 9 m) will correspond to a level over 200 dB. (at 150-160 dB they say it may produce eardrum rupture).
A sound at the same frequency but at a reasonable level of 60 dB will correspond to displacement of the order of microns.

 

[russ_watters]

Yeah, you might be right; it is still an 18m slug of air, but it may only move a few milimeters. The principle still applies though: for a short wavelength/small frequency, the wave is gone before the object has a chance to move.

 

[nasu]

What air slug? The air does not have to come in portions equal to the wavelength in order to have a sound in it.
How about breaking a glass with high pitch sound? Does the sound reach the glass before it (the glass) has a chance to move? The frequency is usually of the order of kHz. The wavelength is of the order of cm. The source is usualy at some tens of cm for the glass so it travels many wavelengths before reaching the glass and moving it.

There is a relationship between displacement and frequency of the sound but it is not that the displacement is equal to half the wavelength.
If you look at the formula relating intensity of sound to displacement
$I=\frac{\rho \omega^2 c u^2}{2}$
(u is the displacement amplitude, c is the speed of sound, omega the frequency and rho the density)
you can see that if you want to reach the same level of intensity as for a higher frequency sound you need higher displacements.
So the membrane of the speaker has much higher amplitude, this producing displacements in the order of
centimeters maybe. (At 20 Hz, a displacement of the order of 1 cm will result of levels around 140-150 dB - already painful).

 

[russ_watters]

The "air slug" is a visualization aid. Since the wave is moving, the air in the "slug" is changing, but you can still draw a box around it. It's like the compressed part of a slinky.

Resonance in a glass is a somewhat different issue from what we are discussing. The glass vibrates at such a high frequency, you can't see it. With low frequencies, the long wavelength allows the moving air to interact with the whole object. Ie, if the wavelength is twice as long as the glass is wide (say, 10 cm) or larger, the whole glass is being pushed in the same direction as the wave passes, so the whole glass moves.

 

[russ_watters]

Actually, to make the "slug" more literal, make it a packet of air much smaller than the wave (so it acts like a single particle). As the wave passes, the packet moves back and forth.

 

[nasu]

Resonance in a glass is a somewhat different issue from what we are discussing. The glass vibrates at such a high frequency, you can't see it. With low frequencies, the long wavelength allows the moving air to interact with the whole object. Ie, if the wavelength is twice as long as the glass is wide (say, 10 cm) or larger, the whole glass is being pushed in the same direction as the wave passes, so the whole glass moves.

It was just a counter-example to your statements:
"When the wavelength is short (milimeters?), the sound has trouble interacting with objects."
and
"The principle still applies though: for a short wavelength/small frequency, the wave is gone before the object has a chance to move."

And for the last part, yes, I see how the wavelength being comparable with the size of the object has an effect on the outcome.
This is when resonance happens, at wavelengths comparable with the size of the object. At least for fundamental modes.
But the amplitude of the motion of the air (and so of the object) is not given by the wavelength, at least not directly (as half the wavelength or so), as it may be understood from a previous post.

 

[OmCheeto]

can anyone explain the science behind a car sub woofer? how do they create air pressure? how does generate enough energy to make a girls hair stand on end or set off a car alarm? what's going on during all that and what's the sub doing to make it happen?

I think, in regards to "sub-woofers", it has two factors:

1. Equal Loudness Contour [wiki]
2. Sound Absorption Coefficient [some random website]