Sound info
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`The closer a receptor is to a noise source the louder the noise seems; for every doubling of distance
`from a source the intensity drops by about 6 dB over land and about 5 dB over water. Conversely, if you
`move half the distance closer, the sound intensity would increase 6dB. Practically, if you are 6 ft. from a
`noise source and move to 8 ft. (or 4 ft.) the sound intensity would decrease (increase) by 2 dB which
`would generally not be perceptible. At 9 ft. (or 3 ft.) the sound intensity would change by 3 dB which
`would just be barely noticeable or perceptible.
`
`Adding of equal strong non-coherent sound sources
`
`Level increase Δ L for
`n equal loud sound
`sources
`Number
`of n equal loud
`sound sources
`1
`2
`3
`4
`5
`6
`7
`8
`9
`10
`12
`16
`20
`
`Level
`increase
`Δ L in dB
`0
`3.0
`4.8
`6.0
`7.0
`7.8
`8.5
`9.0
`9.5
`10.0
`10.8
`12.0
`13.0
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`Perception Vs. Reality: What Our Ears Hear
`
`by Acoustics By Design on December 12, 2008
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`It seems like everyone is talking about decibels these days: “5 dB of this” and “3 dB of that.”
`As acoustical consultants, we hear people using these metrics and often wonder to ourselves,
`“Do they really know what a decibel is?” Occasionally, someone is brave enough to ask the
`more important question, “Just how much is a decibel?” The answer to that question is, of
`course: well, it depends. Are we talking about physical sound levels or perceived sound
`levels? There is a big difference!
`Let’s lay some ground work. A decibel is a ratio of how much acoustical energy we hear
`compared to some “reference level”. For humans, the reference level is the “threshold of
`hearing”. This means that we always talk about decibels in the positive sense, such as 50dB or
`60dB; we don’t say that some noise is -20dB. To further confuse matters, humans hear
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`It is “weighted” because our ears do not treat all frequencies the same.
`“weighted” noise levels.
`We “hear” some frequencies as being louder than others; our ears do not have a very “flat”
`response. To accommodate this, there are several different algorithms of decibel weightings
`that we call “scales.” Most commonly, you hear about the dBA scale (this is the one that applies
`to humans) and the dBC scale (but that’s for another blog). Just remember that the most
`commonly used scale is dBA, and that’s what most people are referring to when they talk about
`decibels (whether they know it or not). At any rate, the dBA scale gives us a good starting point
`from which to measure sound.
`It yields a single-numbered result and gives us a good idea of
`the overall sound level (when, in reality, the sound is at many different noise levels all across
`the frequency spectrum).
`
`Reality
`Physics tells us that for every doubling of acoustical energy, there is a 3dB increase.
`Conversely, a 3dB decrease means the sound is cut in half. So, 3 is the magic number right?
`Well, not so fast. This is where we see a conflict between scientific calculations
`and perceived sound levels. “Perceived” sound levels report how our ears and brain interpret
`the sound.
`In other words, perception answers the question of “Whatsounds ‘twice as loud’?”
`
`Perception
`Sound studies tell us time and again that a 3dBA increase in sound level is barely noticeable to
`the human ear. In fact, you have to raise a sound level by 5dBA before most listeners report a
`noticeable or significant change. Further, it takes a 10dBA increase before the average
`listener hears “double the sound.” That’s a far cry from 3dB.
`So is perception the ultimate reality? What really constitutes twice the sound or half the sound?
`And why can’t our ears hear what is proven to be scientific fact? The answer to each of these
`questions is, of course: well, it depends!
`
`Acoustics By Design
`Acoustics By Design consultants provide acoustical consulting services to architects, engineers,
`facility directors, municipalities, and building owners. Our team includes acoustical consultants,
`acoustical engineers, noise consultants, and vibration consultants. Our firm also includes an
`integrated team of audio-visual consultants who design audio, video, theatrical lighting, and
`technical systems and integrate them with the native acoustical environment.
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