Matching the Sound Absorption to the Frequencies Where the Noise Problem Lies
Figure 2. In refurbishing the Rotunda at the Rotunda at the University of Virginia, there was a conflict between the architect's wish to preserve the original appearance of Thomas Jefferson's handsome plaster dome and the need for acoustical treatment to quiet the room. The original plaster was replaced with curved, finely perforated sheet metal behind which sound-absorbing blankets were hidden, with a resulting appearance indistinguishable from that of plaster.
Figure 3. Inlet and exhaust ducts of jet engine, lined with sound absorptive treatment that is faced with perforated metal.
Before beginning to design noise control measures using perforated materials, you must decide what kind of noise problem you have.
As suggested under applications 1 and 2, above, perforated metals can be used in two completely different ways in acoustical applications.
In the first application, we want the sheet to be as transparent as possible to sound of all frequencies. This would be the choice if we want to absorb noise that contains energy in a broad range of frequencies, or if we want the sound of an orchestra in a concert hall to pass freely through a false, decorative, perforated surface in order to reach specially designed acoustical treatment behind the sheet.
If on the other hand, we wish to absorb sound in a relatively narrow band of frequencies, we use the perforated sheet as an integral part of a tuned Resonant Sound Absorber. A common application for this kind of treatment is in the inlet of a jet engine.
The design procedures for these two applications are quite different. They are described in Sections III and IV.
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 Analysis
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.
Figure 7. We can analyze sounds, showing how the energy is distributed over different frequency bands, by means of a Sound Level Meter (sLM).
Such an analysis is made by means of a Sound Level Meter.
This is a piece of hand-held equipment containing:
- a microphone (to convert the sound wave into an electrical signal);
- an amplifier (to increase the strength of the signal);
- a set of filters (to select different ranges of frequencies for measurement); and
- a meter (or digital read-out device) to indicate the sound pressure level being measured.
If all the filters are switched out, the meter reads the total energy of the noise at all frequencies. If only one of the filters is switched in, the meter responds only to the energy in the band of frequencies passed by the filter.
Returning to our piano example above, where only a few adjacent notes around middle-C were played, if we measure the sound level with the filters successively switched from low to high, we would get a strong meter reading only with the filter for the frequency band centered around 250 Hz; all the other readings would be much lower (corresponding to the ambient room noise).
This would tell us that, if we wish to attenuate this noise, we should use a Tuned Resonant Absorber.
Figure 8. The SLM readings are plotted at the standard octave-band frequencies, in order to exhibit the narrow spectrum from four adjacent piano notes, as in Fig. 5.
