Page 56 - Master Handbook of Acoustics

The SPL of the reflection at the seat is:

(C) The reflection will arrive after the direct sound at the seat after:

A free field is also assumed here. The 30.7-msec reflection might be called an incipient echo.

Example 7 Combining Decibels

A studio has a heating, ventilating, and air-conditioning (HVAC) system, as well as a floor-standing
fan. If both the HVAC and fan are turned off, a very low noise level prevails, low enough to be
neglected in the calculation. If the HVAC alone is operating, the sound-pressure level at a given
position is 55 dB. If the fan alone is operating, the sound-pressure level is 60 dB. What will be the

sound-pressure level if both are operating at the same time? The answer, most certainly, is not 55 +
60 = 115 dB. Rather,

We see that the 55-dB sound level only slightly increases the overall sound-pressure level
established by the 60-dB level.

In another example, if the combined level of two noise sources is 80 dB and the level with one of
the sources turned off is 75 dB, what is the level of the remaining source?

In other words, combining the 78.3-dB level with the 75-dB level gives a combined level of 80
dB. Both the HVAC and the fan are assumed to yield a wideband noise signal. It is important to

remember that different sound levels can only be compared when the sounds have the same or
similar spectral properties.

Measuring Sound-Pressure Level

A sound-level meter is designed to give readings of sound-pressure level (SPL). Sound pressure in

decibels is referenced to the standard reference level, 20 μPa. The human hearing response is not flat