need sound fading algorithm please

Hi,

I have written a small c++ audio library that supports basic 
functionality including linear fading in and out of gain. I would like 
to implement logarithmic fading instead of linear but I do not know how 
to go about it. Here is the update fade function which gets called on 
every frame, can anyone suggest some code I can use to replace my linear 
solution?, thanks


void Sound::Fade::Update( void )
{
     const double elapsedFadeTime( GetTickCount() - m_startTime );

     if( elapsedFadeTime > m_duration )
     {
         m_duration = 0;
         m_currentGain = m_endGain;
     }
     else
     {
         m_currentGain = static_cast<float>( m_beginGain + ( ( 
elapsedFadeTime / m_duration ) * ( m_endGain - m_beginGain ) ) );
    }
}