Average Back EMF Induced in Coil in Loudspeaker

In summary, the conversation discusses the calculation of the average back emf induced in a loudspeaker coil with an inductance of 112 microH. The current must oscillate between peak values of + and - 4.4 A in half a period to produce a sound of frequency 40kHz. The formula for inductive reactance is used to find the rms value of current and then the voltage is calculated using the formula for back emf. The half period information is important as it determines the rate of change of current, which is necessary in the calculation of back emf.
  • #1
equinox2012
2
0
Hi,
I would appreciate any help with the following:

The coil in a loudspeaker has an inductance of L = 112 microH. To produce a sound of frequency 40kHz, the current must oscillate between peak values of + and - 4.4 A in half a period. What average back emf is induced in the coil during this variation?

My main confusion is, why do we need to know the half period thing...I have no idea how to incorporate it into the solution.

Here's what I have now:

Xl = inductive reactance = 2 pi f L = 2 pi 40 x 10^3 x 112 x 10^-6 = 28.15 Ohm.

Then I rms = I/sqrt 2 = 4.4/sqrt 2 = 3.11 A. Am I correct in looking at rms values here?

Then V rms = I rms x Xl = 3.11 x 28.15 = 87.5 V.

Is that all? Is this V rms the average induced back emf they are asking for? Or am I wrong? I'm really confused about this one.

Any help would be much appreciate! Thanks!
 
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  • #2
equinox2012 said:
Hi,
I would appreciate any help with the following:

The coil in a loudspeaker has an inductance of L = 112 microH. To produce a sound of frequency 40kHz, the current must oscillate between peak values of + and - 4.4 A in half a period. What average back emf is induced in the coil during this variation?

My main confusion is, why do we need to know the half period thing...I have no idea how to incorporate it into the solution.

Here's what I have now:

Xl = inductive reactance = 2 pi f L = 2 pi 40 x 10^3 x 112 x 10^-6 = 28.15 Ohm.

Then I rms = I/sqrt 2 = 4.4/sqrt 2 = 3.11 A. Am I correct in looking at rms values here?

Then V rms = I rms x Xl = 3.11 x 28.15 = 87.5 V.

Is that all? Is this V rms the average induced back emf they are asking for? Or am I wrong? I'm really confused about this one.

Any help would be much appreciate! Thanks!

Hi equinox2012, Welcome to Physics Forums.

The back-emf created by an inductor depends upon the rate of change of the current flowing through it. Thus for an inductance L,
$$V = L \frac{dI}{dt}$$
That's why the information about the change in current over a particular period of time was given.
 
  • #3
Thank you for the help! ;)
 

FAQ: Average Back EMF Induced in Coil in Loudspeaker

1. What is back EMF in a loudspeaker coil?

Back EMF, or back electromotive force, is the voltage that is induced in a coil when it is moving through a magnetic field. In a loudspeaker, the coil is attached to a diaphragm and moves back and forth as the audio signal is passed through it. This motion through the magnetic field of the speaker's permanent magnet generates a back EMF.

2. How is back EMF measured in a loudspeaker coil?

Back EMF is typically measured in volts (V) and is directly proportional to the rate of change of the magnetic field. This means that the faster the coil moves, the higher the back EMF will be. It can be measured using a multimeter or by connecting a voltage sensor to the coil.

3. What is the importance of back EMF in a loudspeaker?

Back EMF is an important factor in loudspeaker design as it affects the performance and efficiency of the speaker. If the back EMF is too high, it can cause distortion in the audio signal and affect the overall sound quality. On the other hand, if the back EMF is too low, it can result in a weaker audio output.

4. How does the number of turns in a coil affect the back EMF in a loudspeaker?

The number of turns in a coil directly affects the strength of the magnetic field and, therefore, the back EMF induced in the coil. A higher number of turns will result in a stronger magnetic field and a higher back EMF, while a lower number of turns will result in a weaker magnetic field and a lower back EMF.

5. How can back EMF be reduced in a loudspeaker?

Back EMF can be reduced in a loudspeaker by using a lower number of turns in the coil, decreasing the strength of the permanent magnet, or by using materials with higher electrical resistance. However, reducing the back EMF too much can also affect the efficiency and performance of the loudspeaker, so it is important to find a balance between reducing back EMF and maintaining good audio quality.

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