Filters, plz help!

[elec123]

Hi,

I need some help wit some calculations on filters....

For a woofer, simple L R circuit in series (taking the speaker as R). If the frequency transfer function:

H=(V across R) / (V across L and R)

So i get:

H=IR/[I*(R+JwL)] Where Z=R+JwL

R=8 Ohm L=2.5mH but am not given w so how do I work it out?!!

Also, How do I find the break (3dB) frequency of the transfer function??

Any help would be great cheers!!

 

[David Bridgen]

Deleted by author.

 

[Roff]

You use the transfer function H=R/(R+j*w*L) to calculate the gain at any radian frequency w.

w=2*pi*f, where f=frequency in Hz.

The corner is at |H(3dB)| = |H|/2 (where |H| means 'the magnitude of H'), which occurs when w*L=R.

So, let's say R=8, and you want the 3dB corner to be at 1kHz:

solving for L, L=R/w, L=R/(2*pi*f)

L=8/(2*3.14*1000)

L=1.27mH