A value of TI upwards of 10,000 should be the goal in choosing a perforated sheet intended for an acoustically transparent facing material. This would lead to an attenuation no greater than one decibel (dB) at a frequency of 10 kHz, and at lower frequencies, the attenuation decreases rapidly: in other words the sheet is essentially acoustically transparent over the entire frequency range of importance.

However, it is not essential to insist on very high Transparency Index. For relatively high values of TI, the transparency is not spoiled very rapidly with decreasing values of TI: with TI as low as 5000, the attenuation is only 1.5 dB, and with TI = 2000, the loss is only 2.5 dB.

Therefore, there is no harm in shopping around among the readily available perforated materials to find one whose TI lies between, say 2000 and 20,000. Any value within this range will yield acceptably high sound transparency for most sound absorption applications.

NOTE: The value of TI increases as the hole size and the number of holes per sq in increases and as the thickness of the sheet and the distance between holes decreases. For values of TI less than 2000, the sound transparency diminishes rapidly, and the perforated metal blocks the passage of sound.

One can also see from the formula that TI generally increases with increasing percent open area P, but NOT if this is achieved with larger holes and an increase of the distance (a) between holes.