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Old 28th November 2006, 06:40 PM   #1
Theli is offline Theli  Canada
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Default basic question about sound.

How does a driver ( ie 12" sub ), create a 20 ft wavelength (50 hz)?

For some reason I can't get my head around how a driver moving in and out 1" creates the low tones/long wavelengths.

I searched for an answer, but could not find one.
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Old 28th November 2006, 06:46 PM   #2
lineup is offline lineup  Sweden
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if you throw a little stone into water

how can those small circles
can become so very big circles

from a very little stone
__________________
lineup
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Old 28th November 2006, 06:56 PM   #3
imjatse is offline imjatse  United States
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Some good info here

The above link has some good info, I think it was designed for kids, but still good.
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Old 1st December 2006, 07:10 AM   #4
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Quote:
How does a driver ( ie 12" sub ), create a 20 ft wavelength (50 hz)?

For some reason I can't get my head around how a driver moving in and out 1" creates the low tones/long wavelengths.
50hz means the cones move p-p 50 times in one second, 1/50 is how long each p-p takes to occur. So from rest position the cone moves one inch in 1/50 of a second so its velocity is 1"/(1/50sec) = 50 in/sec.

Sound moves at ~1125 ft/sec --->13500 in/sec

(13500 in/sec)/270in = (50 in/sec)/1 in (Speed/wavelength alway equals the frequency/displacement)

The cone is transferring its low speed and low displacement energy to air which "communicates" the pressure at the speed of sound and therefore creates longer waves than the source is providing.
Another way to word it is it is all time dependant, the speaker and the air both oscillate at the same frequency but the air moves faster and therefore cover more "ground".

Hopefully that was written well enough to make sense, if not I'll try again later.
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Old 1st December 2006, 12:33 PM   #5
clem_o is offline clem_o  Philippines
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Default Re: basic question about sound.

Quote:
Originally posted by Theli

For some reason I can't get my head around how a driver moving in and out 1" creates the low tones/long wavelengths.


The SPEED at which the driver moves in and out gives the TONE or frequency of the sound.

The AMOUNT by which the driver moves in and out gives the loudness, or amplitude, of the sound.

So, by saying the driver moves 1" in and out - this only gives an idea of the amplitude or how loud the sound "is".

Hope this helps!


Cheers
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Old 1st December 2006, 12:48 PM   #6
clem_o is offline clem_o  Philippines
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Wavelength:

Think of it this way: imagine a little red dot that travels horizontally at a speed of 1120 feet/sec.

As the red dot moves (fast!), it also goes UP and DOWN, at a rate of 50 times per second.

Question: How far apart are the PEAKS in position of the red dot?

Answer: The dot moves up and down 50 times in a second - so the amount of time between PEAKS in position is 1/50 of a second - if the dot is traveling at 1120 feet per second, it would have traveled

1120/50 = 22.4 feet

There's your wavelength (50Hz).

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Old 1st December 2006, 06:10 PM   #7
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Default Re: Re: basic question about sound.

Quote:
Originally posted by clem_o

The SPEED at which the driver moves in and out gives the TONE or frequency of the sound.

The AMOUNT by which the driver moves in and out gives the loudness, or amplitude, of the sound.

So, by saying the driver moves 1" in and out - this only gives an idea of the amplitude or how loud the sound "is".

Hope this helps!
No that doesnt help, SPEED is the change in position per unit time, two identical woofers playing the same frequency while one has p-p exc. of 1/2" and the other 1", the one moving 1" has higher speed. Frequency is cyclic and independant speed, and amplitude.

The AMOUNT the driver moves and the resulting loudness are functions of frequency, if two drivers are moving 1"p-p but one is playing 20hz and the other is playing 40hz, the one playing the 40hz tone will be 4 times louder.

His question is about the nature of sound and how a cone moving some small distance(ex. 1") can cause long waves(ex. 20') in air. And Im not sure if the red dot thing was an analogy, but it is generally confusing to reference vertical sine waves when discussing sound especially when the above link illustrates exactly how the air particles move.
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Old 1st December 2006, 06:26 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

Sound propogates at ~ 330 metres per second.

Frequency = number of times per second.

Wavelength = 330 metres per second / number of times per second.

/sreten.
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Old 1st December 2006, 06:32 PM   #9
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Default Re: Re: Re: basic question about sound.

Quote:
Originally posted by nunayafb


No that doesnt help, SPEED is the change in position per unit time, two identical woofers playing the same frequency while one has p-p exc. of 1/2" and the other 1", the one moving 1" has higher speed. Frequency is cyclic and independant speed, and amplitude.

The AMOUNT the driver moves and the resulting loudness are functions of frequency, if two drivers are moving 1"p-p but one is playing 20hz and the other is playing 40hz, the one playing the 40hz tone will be 4 times louder.

His question is about the nature of sound and how a cone moving some small distance(ex. 1") can cause long waves(ex. 20') in air. And Im not sure if the red dot thing was an analogy, but it is generally confusing to reference vertical sine waves when discussing sound especially when the above link illustrates exactly how the air particles move.
Excursion is a function of both SPL and frequency.
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Old 1st December 2006, 06:35 PM   #10
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Default Re: Re: Re: Re: basic question about sound.

Quote:
Originally posted by 454Casull

Excursion is a function of both SPL and frequency.

Hence the statement "the one playing the 40hz tone will be 4 times louder."

And the statement The AMOUNT the driver moves and the resulting loudness...
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