Regarding loudness based gain adjustment in gammatone filter

While studying the gammatone filter I came across the following code on mathworks website. I want to know the formula for gain & gt functions. Which literature can be referred to understand these two lines in the code?

% Generating gammatone impulse responses with middle-ear gain normalization

gt = zeros(numChan,gL);  % Initialization
tmp_t = [1:gL]/fs;
for i = 1:numChan
    gain = 10^((loudness(cf(i))-60)/20)/3*(2*pi*b(i)/fs).^4;    % loudness-based gain adjustments
    gt(i,:) = gain*fs^3*tmp_t.^(filterOrder-1).*exp(-2*pi*b(i)*tmp_t).*cos(2*pi*cf(i)*tmp_t+phase(i));
end