BARK BAND SPECTRUM surf(...

[Sulake]

Hi,
i'm trying to plot an sound (

http://www.jarkonkotisivu.org/temp/sound.mat)

and the goal is to get this (

http://www.jarkonkotisivu.org/temp/spectrum.JPG)

but i get an error:
Warning: Z must not be a scalar or vector, not rendering surface.

Here is the problematic program:

[b]
function bark(ID,nom,periode,imin,unite)[/b]
nb=size(ID);
t=1:1:nb(2);
f=1:1:nb(1);
f=(f-1)+imin;
vect_t=(t-1)*periode;
surf(vect_t,f,max(ID,0));
shading interp;
rotate3d;
xlabel('Time (s)');
ylabel('Bark Band');
aff=sprintf('%s',unite);
zlabel(aff);
view(0,90);
title(nom);

------------ And the variables are:
ID=identifier of the variable: dBID+n°ID dBFA32
nom=name of the variable : ID(comment) (string)
periode=period (time step)
imin=index of the min band (1 to 24)
unite=unit (string)

Any idea why max(ID,0) is an scalar or vector?