[miloxo]

So, im always wondering about things, and today I was wondering how fast drivers really are. Not in terms of how it sound, but actual speed in kp/h

All I know is that sound travels at a speed of ~1000 kp/h, but I doubt that the drivers really go that fast

So, how fast are drivers?

 

[Steve Eddy]

The speed of the driver would depend on the frequency it was reproducing and what the cone's excursion is. For a given excursion, the higher the frequency, the higher the speed. For a given frequency, the greater the excursion, the higher the speed.

Give me a bit and I'll see if I can work out the math.

se

 

[Jeff Guidry]

As a layman I wouldn't know, but wouldn't the speed of the cone solely be determined by the frequency of sound it was trying to reproduce? Amplitude would seem to dictate the distance of excursion, frequency would determine its speed. Yes?

 

[Steve Eddy]

Quote:

Originally Posted by Jeff Guidry /img/forum/go_quote.gif As a layman I wouldn't know, but wouldn't the speed of the cone solely be determined by the frequency of sound it was trying to reproduce? Amplitude would seem to dictate the distance of excursion, frequency would determine its speed. Yes?

Excursion also determines the speed for a given frequency.

A given frequency has a period of a given amount of time (1/f). So the excursion has to take place within that amount of time. Therefore, the greater the excursion, the greater the speed.

Think of driving.

If you have only x amount of time to drive a given distance, then you'll have to drive faster as the distance you have to drive in that given amount of time increases, i.e. you have to drive faster in order to drive ten miles in x amount of time than you would if you only had to drive five miles in x amount of time.

se

 

[Jeff Guidry]

Of course I see now thanks. However, if a cone must move faster at a 1w signal at a given frequency than a 0.1w signal at the same frequency, wouldn't that have an effect on the frequency being produced? It would seem that because a high amplitude signal moves cones faster it would cause the resulting sound to be of higher frequency, and that would have to be compensated for in some way.

Again, I am just an average guy with no physics background so I have no idea what I am talking about, just trying to wrap my head around this.

 

[Steve Eddy]

Quote:

Originally Posted by Jeff Guidry /img/forum/go_quote.gif Of course I see now thanks. However, if a cone must move faster at a 1w signal at a given frequency than a 0.1w signal at the same frequency, wouldn't that have an effect on the frequency being produced? It would seem that because a high amplitude signal moves cones faster it would cause the resulting sound to be of higher frequency, and that would have to be compensated for in some way.

The frequency is dependent on the periodicity of the cone's movement, not the speed at which it moves.

In other words, the time it takes for the cone to complete one cycle.

For example, one cycle of 1,000 Hz is 1/1,000 or 0.001 seconds.

So as long as the cone is completing one cycle every 0.001 second, it will be outputting 1,000 Hz, regardless of what its excursion is and subsequently what its speed is.

However...

Speed CAN alter frequency in one respect. And that's Doppler shifting.

Say you've got the speaker reproducing 100 Hz and 1,000 Hz. Of course there will be more cycles of 1,000 Hz occurring during that 100 Hz period. And because the same cone that's reproducing the 100 Hz is reproducing 1,000 Hz, the 1,000 Hz signal can become Doppler shifted.

As the cone is moving toward you, it will compress the 1,000 Hz waves causing them to be higher in frequency. As the cone is moving away from you, it will stretch out the 1,000 Hz waves causing them to be lower in frequency.

This is the same effect that causes a car's horn to sound higher in pitch while the car is heading toward you and lower in pitch as it passes you and moves away from you.

Quote:

Again, I am just an average guy with no physics background so I have no idea what I am talking about, just trying to wrap my head around this.

No problem!

I once had to wrap my head around the same thing too (not like anyone comes out of the womb knowing this stuff). So I asked questions, same as you are.

se

 

[b0dhi]

Has anyone ever measured the actual effect this has? Apart from causing frequency shifts it would also cause dynamic phase shifts due to the changing distance from the ear the 1000hz tone would be at at the maximum excursion of 100hz carrier versus minimum excursion of it.

Also, if the carrier waves are out of phase between L/R while the 1000hz waves aren't supposed to be, this could cause phase distortion and might result in an inter-aural time delay in the L/R 1000hz waves that might exceed the average detectable limit of 10uS.

 

[Uncle Erik]

Doesn't the mass of the driver also play a role in all this? I'd expect an electrostat to move faster than a dynamic.

 

[Joelby]

Anyone else's mind blown when trying to comprehend how a single driver is delivering multiple frequencies with precision and clarity?

 

[moogoob]

Quote:

Originally Posted by Joelby /img/forum/go_quote.gif Anyone else's mind blown when trying to comprehend how a single driver is delivering multiple frequencies with precision and clarity?

Not really. It's just a rapidly fluctuating magnetic field in the proximity of a magnet- think about it like this and it sort of makes sense.

 

[DanielCox]

Quote:

Originally Posted by Uncle Erik /img/forum/go_quote.gif Doesn't the mass of the driver also play a role in all this? I'd expect an electrostat to move faster than a dynamic.

Yes and no. They'll move at the same speed given the same frequency and volume.
An electrostat is easier to accelerate though.

 

[Jeff Guidry]

Quote:

Originally Posted by Koyaan I. Sqatsi /img/forum/go_quote.gif So as long as the cone is completing one cycle every 0.001 second, it will be outputting 1,000 Hz, regardless of what its excursion is and subsequently what its speed is.

I see, so it doesn't matter how fast the cone is moving or how long it's excursion is, the length of time to complete the cycle is what determines the frequency of the sound.

Quote:

Originally Posted by Koyaan I. Sqatsi This is the same effect that causes a car's horn to sound higher in pitch while the car is heading toward you and lower in pitch as it passes you and moves away from you.

...which gives the wonderfully swirly sound of the Leslie speaker its signature tone!

+ YouTube Video​

ERROR: If you can see this, then YouTube is down or you don't have Flash installed.

 

[Steve Eddy]

Quote:

Originally Posted by Jeff Guidry /img/forum/go_quote.gif I see, so it doesn't matter how fast the cone is moving or how long it's excursion is, the length of time to complete the cycle is what determines the frequency of the sound.

Right.

Quote:

...which gives the wonderfully swirly sound of the Leslie speaker its signature tone!

You got it!

Thanks for the video!

se

 

[b0dhi]

Quote:

Originally Posted by b0dhi /img/forum/go_quote.gif Apart from causing frequency shifts it would also cause dynamic phase shifts...

For anyone else also curious about this, turns out that an excursion of 1mm at 100hz would result in a dynamic phase distortion of ~3uS in the high frequency tone. This is below the average detectable limit but not by much.

 

[JaZZ]

A membrane vibrating with 1 kHz and an excursion of 1 mm (±0.5 mm) has an average speed of 1000 x 2 mm = 2 m/s (triangle waves). I can't calculate the sinc function, but the estimated maximum speed of a 1 kHz sine wave will be about 4 m/s (= 14.4 km/h) at the zero crossings in this case. But note that an excursion of ±0.5 mm at 1 kHz means a rather extreme volume level for a headphone driver.
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