Four Audio Myths
Jun 6, 2011 2:51 PM, By Bob McCarthy
Misconceptions you need to know.
2. Turn it up so we can excite the room
As a specialist in the measurement of sound systems in rooms, I am often asked how loud I run the speakers in order to do tuning. Folks want to be sure to “excite the room.” So how much sound does it take to excite the room? Does a room of glass and plaster get excited more easily than one full of fiberglass? Imagine this excerpt from the conversation between two bricks on the wall: “It takes about 50dB SPL to wake me up and remind me to reflect the sound. Less than that, I absorb it.” As we know, rooms are inanimate objects. Walls have an absorption coefficient and reflect a fixed proportion of incident energy back into the room. So where does such an idea come from?
We characterize a room’s acoustic excitability with metrics such as reverb time (RT) and others. The standard value for reverb time (RT60) is the amount of time it takes to decay 60dB. For example, if it takes 2 seconds to fall from 100dB SPL after the sound is stopped to 40dB SPL, we have an RT60 value of 2.0 seconds—as illustrated in the opening image. The RT60 of a room is constant over level and requires some change in room volume or amount of absorption to assume a new value.
The root behind this myth is that we perceive more of the reflections when louder sounds are pumped into the room and conversely less of the room when the input level is decreased. This is because we hear sound until it reaches the noise floor, rather than ceasing to listen after 60dB of decay (like the RT value does). If the noise floor in our example hall happens to be 40dB SPL (60dB below the input level), then we would experience the same 2.0 seconds of decay found in the RT test. If the noise levels are higher, the experienced decay time is shortened even though the acoustic properties of the room remain static. The experienced sound is drier. Conversely, a louder sound is perceived as having a longer decay, and we are able to localize more of the room’s surfaces because we have extended the time above the noise.
How can we disprove this theory? Take a frequency response reading of your sound system with a high-resolution audio analyzer. Turn it up 10dB and compare. Rinse and repeat. You will see the same response as long as you are above the noise and below the limits of the speakers.
It is important to know the difference between fact and fiction here because understanding the actual mechanism at play may lead to better decisions in the field. For example, we can add some electronic reverb to quiet songs and leave it off for loud ones, keeping the same perception of reverb for our listeners.
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