However, before choosing which of the two applications is appropriate, we first have to determine whether our problem concerns broad-band or narrow-band noise: that is, whether we will require the "TRANSPARENCY" or the "TUNED RESONANCE" approach.

Frequency in cycles per second

Figure 4. Piano keyboard and musical staff, showing the relations to the frequency spectrum.

For this purpose, we need some kind of frequency analysis, whether measured or estimated, to tell us how the energy of the noise is distributed among the various frequencies.

We can use the analogy of the piano keyboard, here, to represent the range of frequencies of interest: the high pitches lie toward the right, the low pitches to the left.

Figure 5. Making a sound spectrum, with the sound energy concentrated around 250 Hz ("middle C").

If we play a single key (or three or four adjacent keys near middle-C, the sound energy will be concentrated around the frequency 250Hz (cycles per second).

Figure 6. Making a "broad-band" noise with wide, flat spectrum.

If we use forearms and elbows to play as many adjacent keys as we can, the resulting "noise" will be distributed over a broad band of frequencies.

A suitable frequency analysis would distinguish clearly between these two conditions, and would guide us to the appropriate choice of design procedure, when we seek to attenuate the noise using perforated metals in an accoustical treatment.