Frequency response of enclosed space at low frequency(mode dominant range) using transient acoustics

Hello.

I want to check effect of mode by time domain simulation.

I made rectangular enclosed space and make monopole source.

I used multi-sine function(sum of single sine at each frequency) to excite whole frequency range as reference and measured pressure at edge of enclosed spaces.

Theoretically at point of edge, all of mode can be measured.

I expect frequency response function that have peaks only in natural frequency of each mode.

But, I cannot find distinct peak at natural frequency.

Is there any problem using transient acoustics to check about mode of enclosed spaces?

I am waiting your answers