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Perception Vs. Reality: What Our Ears Hear

by Acoustics By Design on December 12, 2008

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 futher confuse matters, humans hear “weighted” noise levels. It is “weighted” because our ears do not treat all frequencies the same. 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 “What sounds ‘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

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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Dan December 12, 2008 at 3:21 pm

Great article. I’m curious on your perspective of the relationship between decibels versus frequency after this.

Brian Atkinson December 17, 2008 at 5:09 pm

Thanks for the comment. The weighting of the scales is meant to compensate for the way human ears and our brain hear the sound. Decibels and frequency are, of course, two completely different things. Decibels refer to how “loud” something is; Frequency refers to the “tone” of a sound. If you look at the maximum sensitivity of human hearing, this occurs at about 1,000 Hz. This is the same frequency that contains most of the content of human speech. Above and below this frequency range, our ears hear at lower sensitivities. So, for example, if someone handed you a pair of headphones and asked you to listen to three frequencies to determine which was the loudest, and if they played a fifty decibel tone at 100 Hertz, a fifty decibel tone at 1 Kilohertz, and another fifty decibels at 16 Kilohertz, the 1 KHz tone would be perceived as significantly louder than the other two. In fact, all three have the same amount of acoustical energy. Hope that helps!

Douglas February 1, 2009 at 10:35 pm

I am trying to understand the complete correlation between Decibel and KHz.

What is -36db in KHz?

Thanks,
Douglas

Brian Atkinson February 4, 2009 at 3:19 pm

Thanks Douglas. Again, we’re just scratching the surface here, summarizing much larger principals of sound, but let me try to clarify my response from above. Think of Decibels and Hertz as scales that measure two completely different things. Decibels measure the volume or loudness of a sound, from quiet to loud, and Hertz tell us the pitch of the tone, from a low bass to a high treble. Something with a higher pitch/tone/frequency/Hertz (say for example, a soprano singer) can sing at any volume or decibel. In the same way, a bass singer will sing in a lower frequency range but can sing at any volume level, from quiet to loud (decibels). Measuring 36 dB tells you the volume level, but it doesn’t tell you the pitch (or Hertz) of that sound, so there isn’t a direct correlation.

Here’s a real life example: At ABD, we frequently get calls from industrial facilities who are trying to figure out if they are violating the OSHA standards for noise in the workplace. In many cases they have purchased a cheap noise meter and have found the volume level to be breaking the OSHA standards (ie: above the allowed level). The problem is, how do you diagnose and solve the noise problem? This is where frequency comes in (again, measured in Hertz). Our noise meters are professionally calibrated and can perform detailed frequency mapping. So, when we make noise measurements, we can look at it and tell you where the problems are coming from across the frequency spectrum. From there we can engineer noise control solutions that target the offending frequency range.

Adam Kagan September 24, 2009 at 12:46 pm

Hi wonderful website. Beautifully designed and great expertise.

I was hoping you could tell me what a good “talking volume” is in decibels or Dba. WHat is considered too quiet, what is normal and what is ‘loud/confident’ and what is too loud?

Would REALLY appreciate an email!

Thanks so much in advance,
Adam

Thanks,
Adam

Brian Atkinson January 26, 2010 at 9:01 am

We used a local B2B marketing firm called Proteus B2B. http://www.proteusb2b.com/. They do great work! Highly recommended.

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