How a Speaker Works (1 Viewer)

[Bill B Conner]

Some friends and I are debating where a sound wave developes: at the cone, or away from the cone.

Away from the Cone:
Now I was always under the impression that before you hear the tone the wave has to develope. An 18" subwoofer needs more space for a wave to develope then a 10", etc. I could be wrong, but that's always what I thought. Once the tone is made, its all the same shape in relation but a cone that is twice the size of a another cone makes an actual wave that is twice as tall but the same shape (making the same sound).


At the Cone:
The other theory is that ""when sound emanates from a loudspeaker (or a trumpet or the fat lady singing) the sound IS THE SOUND as it leaves the source!" He states that if you put your ear right up to an 18" subwoofer that is making a 30Hz tone, you will hear it and that "A sound does not need the wave to DEVELOP!"


We can't seem to agree, so I thought I would involve a third party. I see his logic, but I was always under the impression that the it was away from the cone, not at the cone.

Anyways, what's the real story?

Thanks a lot in advance,
-Bill

 

[Jason Wilcox]

soundwaves are longitudinal not transverse....

so it would be ok to represent them by a wavy line....(the bigger the difference in peaks and dips the louder it would be...amplitude)...but in actuality...they are just compressions and rarefactions (less dense areas) of molecules...

if the soundwave had to develop depending on its wavelength...we wouldn't be hearing bass from headphones

i probably messed up somewhere...

 

[Ted Lee]

hmm...interesting question.

i have no clue, but i always thought the speaker driver simply vibrated the air molecules? sound isn't really being "pushed out" from the speaker?

of course, i'm talking out of my butt here...

 

[Bill B Conner]

But in headphones the driver is small enough where it developes.

I'm talking out of mine as well.

-Noesis

 

[Jason Wilcox]

the vibrating air molecules are the soundwaves...

 

[Jason Wilcox]

the wavelength of a 30hz soundwave is the same regardless of what produces it...

though..the wavelength varies depending on the medium i think (water,air,steel)

 

[Doug BW]

The vibrating air molecules are what carries the sound wave.

The speaker cone moves forward, pushing air molecules in front of it closer together, creating a local high pressure area. Those air molecules push on nearby air molecules causing those molecules to compress. The high pressure area moves away from the speaker cone in the shape of a sphere, like an expanding bubble, moving about one foot in a thousandth of a second.

As the speaker cone moves backward, it creates a partial vacuum between it and nearby air molecules, i.e., a low pressure area. Nearby air molecules are "sucked" into this low pressure area (actually, there's no such thing as a sucking force, but close enough). The low pressure area propagates out from the speaker cone just like the high pressure area.

If an alternating high pressure/low pressure area happens to pass by an ear, it moves the ear drum and stimulates some nerves, which the brain interprets as sound. The ear and the brain do not care about the existence of a "wave", and do not care what happens to the wave after it passes the ear (unless it happens to bounce back off a wall and pass the ear again).

That's my explanation, and I'm sticking to it.

 

[Bill B Conner]

But does an 18" cone make a taller wave (but with still the same wavelength) then a 10", etc. ?

 

[Doug BW]

Neither. A sound wave is an expanding sphere.

Think of someone blowing a bubble gum bubble.

With a 10 inch woofer diameter, the beginning sphere is about 10 inches in diameter. With an 18 inch woofer, the initial sphere is about 18 inches in diameter.

Sound is NOT something moving up and down....it doesn't have "height" in that sense. It is air pressure that's increasing and decreasing. The difference between the biggest and smallest air pressure determines how loud the sound is. The amount of time it takes to go from the biggest air pressure to the smallest and back to the biggest again determines the frequency (or pitch).

 

[Bill B Conner]

Then I guess the best way to ask my question based on that is:

Car audio. When some guy has a 18" subwoofer in his car, why doesn't the bass get heard till it leaves the vehical? Is that not do to the waving having a need to develope inside the cab, which its not able to do? Or is it due to another factor?

Yet at the same time, a little 10" sub is heard fine.

Thanks!
-Bill

 

[Michael R Price]

The sound originates at the cone. The wavelength does not have to "unfold" or anything. And I don't see a reason why an 18" subwoofer would be heard better outside the car (except maybe that it produced more of the lower frequencies that would escape).

 

[Doug BW]

Actually, I've never heard of a case where you could hear a sound outside the room/enclosure where the sound is being made, but not inside.

Perhaps if you played a subsonic tone (under 20 hz) you'd hear nothing in the car, but due to the shaking, some piece of the car is rattling and can be heard outside.

Or, if the car was a convertable, the woofer was in the back and the car was traveling faster than mach 1...

In any case, sound waves absolutely do not need room to "develop" in order to be heard. If they did, you wouldn't be able to hear much below about 10,000 hz with headphones.

 

[Chris Tsutsui]

Oh, I think I know what Bill is trying to ask.

Lets say you have an 18" woofer playing 50hz at 100db. Then you have a 1" tweeter play 50hz at 100db (that would be cool looking). Both create the same frequency, but one starts from a 1" blow hole compared to the big 18" moving plate.

Both the 18" woofer and the 1" tweeter need to move the same amount of air and at the same rate to achieve 100db @ 1 meter. However, once you move the microphone closer to the sources, the reading on the mics will start to increase and differ. This is because the 1" blow hole can concentrate more SPL into the microphone so the mic next to the 1" bass tweeter will read off the charts compared to the mic next to the 18" woofer.

This is similar to why mics placed near ports can reach higher SPLs than a mic placed next to the woofer.

So in this case, the smaller driver develops the sound wave from a smaller source. With a smaller souce, a mic (or person) will be able to hear it louder since they can put their ear closer to the source.

BTW, I'd say sound comes from the cone as you can place your ear drum as close to the driver as possible and hear something. You would hear something even if the driver was touching your ear drum because I think the ear drum is simply a passive radiator.

I've never heard of a case where you could hear a sound outside the room/enclosure where the sound is being made, but not inside.

What if you were inside a cube shaped room with an IB setup and your head was positioned smack dab in the center while playing the 1st order resonance frequency. hehe