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Exclamation Decibel Hell
by James Oakwood 13-10-2009, 17:39 PM

As musicians, we are dealing with sound. Sound comes from musical instruments and is either naturally produced (such as the voice or violin) or produced with the use of amplifiers (guitars, keys etc.)

If someone is singing, the sound energy is sent in all directions from the singers voice and a tiny part of it hits our ears and we hear singing. If the singer were twice the distance away we would “receive” one-quarter of the vocal energy and the song would appear, to our ears, quieter.

Why one-quarter? Sound is sent in all directions from the singer’s voice rather like light from the sun emits in all directions. That energy gets thinned out over distance just like when you inflate a balloon – the balloon rubber gets thinner as it inflates – if the diameter of the balloon doubles, the surface area of the balloon quadruples and the thickness of the rubber must reduce to one quarter.

If the singer was 10 metres distant and they moved to 3 metres distant we would perceive the loudness of the song as doubling. Why double?

At 10m, the surface area of the balloon is approximately ten times that at 3 metres. This means at 3 metres the thickness of the balloon is ten times that at 10 metres. Returning to sound energy, the energy received by the ear at 3 metres is 10 times that at 10 metres BUT…. the ear has a logarithmic response and this means that ten times the energy results in twice the loudness.

A doubling in loudness is called a Bel after Alexander Graham Bell (the guy who invented stuff) and as shown above, the Bel is also sound energy (or power) increasing ten times. A simple example: -

A little transistor radio (as they used to be called) may have an output power of 1 watt. Not much compared to the massive speaker stacks at gigs but quite adequate if you’re listening to the radio at 3 metres distant. Now if you placed the radio 10 metres away, to get the same loudness you’d have to increase the power output to 10 watts. Now if you moved the radio to 30m distant it would have to produce 100 watts to get the same perceived loudness. At 100 metres you’d need a monster 1000-watt stack to get the same loudness.

One-tenth of a Bel is called a decibel and fundamentally relates to sound pressure energies received by our ears. Experiments have established what is known as the “Threshold of Hearing”. This is the accepted scientific sound pressure level that a human can just about perceive. It aint very loud of course – any quieter and it would not be heard BUT this is the standard by which all other sound pressure levels are based on.

In real numbers it is 20 micropascals at 1000Hz. What does this mean? You don’t need to worry about this number at all but if you are interested a “pascal” is the unit of pressure (or force per sq metre), micro means one-millionth and 1000Hz is about the frequency (or pitch) where are ears are most sensitive.

A sound pressure level of zero decibels is the pressure stated above i.e. the threshold of hearing. A 10dB SPL (sound pressure level) is twice as loud as the threshold of hearing. 20dB SPL is 4 x the threshold and becoming noticeable in a quiet room. Normal speech is about 40 dB SPL and yer TV set might be set up for 60dB SPL. Road traffic is about 80dB SPL and if you kept increasing the SPL to 130dB this is regarded as the threshold of pain.

So, decibels are a relative measurement of sound (or energy or power). In electronics the decibel is used as a relative measurement of voltages as well as power.

A decibel is never an absolute definition of anything – it always relates to some calibrated (or agreed) physical, real world, quantifiable measurement.

So what does 0dB mean when we talk about wav files or mp3 files? 0dB relates to the digital limit imposed on such files. We can’t have more than 0dB in digital files. It is called “FULL SCALE” and stricly speaking when we refer to 0dB we should write 0dB FS where “FS” means full scale of course.

This means that all the signal levels from musical instruments embedded digitally into our mix have values somewhat less than 0dB. We have to use negative numbers now and what a bloody pain that is for most of us. Instead of a threshold of hearing we have a threshold of mental pain – the pain of using negative numbers. –10dB is twice as loud as –20dB and –20dB is twice as loud as –30dB etc. etc..

Hope this helps.
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Old 14-10-2009, 00:42 AM

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Thanks Andy,for spending the time to type the information.It was a very interesting read,as my dad used to say "always learn something new every day son" ive always seen the word decibel and never give it a thought of its origins.
Cheers Andy.
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Old 14-10-2009, 11:13 AM

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Nice article Andy, thanks for the taking the time to do it. Interesting read, learning new stuff from each of your posts these days!
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Old 15-10-2009, 15:41 PM

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Very interesting article and very informative. I have just been reading an article about how listeneing to very loud music over long periods could cause the condition "TINNITIS" think I have spelt that wrong!! anyhow....its ringing and buzzing sounds in the ear...sure you have all heard of this. I have noticed recently that I have been turning my headphones up higher to hear things and wonder if this can cause gradual deafness...leading to Tinnitis ?
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Old 19-10-2009, 15:11 PM
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My chucketh in of stuff to help or hinder understanding of this complex subject

My understanding of the ear is that it contains millions of little hairs with groups of hairs sensitive to different frequencies. The hairs vibrate which in turn sends an electrical signal to the brain to processed. Over exposure to certain frequencies or extreme volumes of sound can cause damage to these hairs which is irreversible. These same hairs are simply damaged over time hence why over our lifetime we our audible range is reduced.

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Old 27-10-2009, 16:53 PM

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So one decibel is on tenth of a bell? I always guessed that it meant deci[ten]bel[bell] but they would have to be tiny bells to only be 1db each.

So would it be fair to assume that and airplane taking off is as loud as 14bells [140db]?
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Old 28-10-2009, 15:16 PM

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Heres some good info on this:




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Old 05-03-2013, 23:57 PM
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This is very helpful Thank you.
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