Sine Curve - Decreasing Amplitude?

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Ok, here's what I need help with.

I have a Sine curve (-0.5sin(2pi/3 * x)) and I need to learn how to decrease the amplitude, so when x=345, the amplitude is zero.

Can anybody help me out? I'm stumped.....

it should be something like this:
pow(1.01,-x)*sin((2.0*3.14)*(x)),
it's not exactly what you wanted, but maybe you could follow the logic...

Although tomamil's answer is probably the sort of thing you are looking for, bear in mind that it is no longer a sine curve, and an exponential decrease will never attain zero.

If your genuine sine curve is:

y=A*sine(B*x + C)

to ensure this is zero at x=345, either make B*345+C=k*pi or A=0.

Alternatively, if you do not require a sine curve, you could go for many different envelopes... making "A" be f(x), such as:
f(x) = (1-x/345) (linear decreasing envelope)
f(x) = (1-x/345)^2 (quadratic envelope)

the sin function gives 0 for angles of 0deg or 180deg. So for the amplitude to equal 0, you need your (2pi/3 * x) to give 0 or 180 (in degrees). So resolve for:

2pi/3 * x = 0

or

2pi/3 * x = 180

(may i ask why you have that negative sign for your amplitude factor ? this induce a 180degree phase shift....)

345 = 115/3

==> -0.5 sin(2/3*pi*345) == -0.5 sin (2*pi*115)

since sin(2*n*pi) = 0

-0.5 sin (2/3*pi*x) = 0 for all x = 3,6,9,12,.... (multiples of three)

I sort of follow what your doing, I'm not just sure how to go about it. I'm not entirely sure if you get what I'm after either. I need it to stay a genuine Sine curve, have an initial amplitude of -1 and gradually decrease until it reaches 0 at x=345. Sort of like this (Excuse the crudity of the sketch, Yay MS Paint):



Would it be possible to do this without changing the period?

now i see what you meant.... multiply the whole thing by (345-x) and then re-divise by 345 to keep the unity (or the peaks will get out of the screen). So something like:


y = ( (345 - x) / 345 ) * ( -0.5sin(2pi/3 * x) )

You sir, are a God. Thanks very much.

In return - Cookie:

I didn't help, but can I have a cookie too?

miam ! thank you i'm glad i helped. Typical Asperger here: your visual graph talked to me, and sine waves are.... yes, one of my obessions