Octaves and Decibels

By Maeve Maddox

A reader asks:

Can you inform on the usage of . . . octave and decible?  I am of the opinion, that octave deals with human voice and decible deals with noise. 

Both words, octave and decibel are terms of measurement. Octave derives from the Latin word for “eight” and decibel incorporates the Latin word for “ten.” The words can be used in various contexts. I’ll just address the uses suggested by the question.

The words bel and decibel are units of measurement of sound intensity. A bel is

A unit, equivalent to ten decibels used in the comparison of two levels of power in an electrical communication circuit.

A decibel (db) is one tenth of a bel.

“Bel” is a shortening of the name of inventor Alexander Graham Bell (1847-1922). A bel is

A unit, equivalent to ten decibels, used in the comparison of two levels of power in an electrical communication circuit.

An octave in the musical sense is an interval embracing eight notes of the diatonic scale. Think of the Do-re-me song in Sound of Music.

Doe- a deer, a female deer
Ray- a drop of golden sun
Me- a name i call myself
Far- a long long way to run
Sew- a needle pulling thread
La- a note to follow so
Tea- a drink with jam and bread
That will bring us back to do oh oh oh

The normal speaking range of the human voice is about 20-50 decibels. Sounds that go above that range become annoying, for example a vacuum cleaner (70 db). Noise becomes painful at 120 db. Sounds above 132 db lead to permanent hearing damage and eardrum rupture.

In answer to the question, decibel refers to sound, pleasant or unpleasant, whether it originates in the vocal cords or elsewhere.

NOTE: Although both the OED and Merriam-Webster give db as the abbreviation for decibel, I’ve been informed by a technical writer that the standard abbreviation is dB.
Sources:
Gale Encyclopedia of Medicine.
Mosby’s Medical Dictionary,
Oxford English Dictionary
Online Etymology Dictionary

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11 Responses to “Octaves and Decibels”

  • Miriam Robbins Midkiff

    In other words, an octave measures pitch (how high or how low the pitch is). Think of a piano; every eight consecutive white keys (with the interspersing black keys) make up one octave. High octaves are on the right of the key board, while low octaves are on the left. High-pitched sounds are made by very fast vibrations, while low-pitched sounds are made by slower vibrations.

    A decibel measures volume (how loud or how soft a sound is). Think of a chainsaw (high decibels) versus a whisper (low decibels). High decibels are very powerful, while low decibels are much weaker.

  • Giorgio Sironi

    Actually they are two logarithmic scales. 40 dB is ten times the power of 30 dB and 50 dB is one hundred times 30 dB.
    For octaves, the La of an octave doubles the frequency of the previous octave (220 Hz, 440Hz, 880 Hz, etc.)

  • Julie

    Hi!

    For once I have to say I found that this post less than clear.

    Wouldn’t it be much easier to simply say that “decibel ” has to do with volume, it is a measure unit of the sound volume, i.e. noise?

    While “octave” has to do with the whole sound spectrum / range? e.g. a high octave note (a T) can have a very small decibel if it’s murmured whereas a low octave note like a do can have a very high decibel if shouted.

    Oh I just see someone clarified the post just above. Sorry for the repetition!

  • JC

    It’s also another log function – each ten decibels is twice as loud (as perceived) and each octave is exactly twice the frequency (in Hertz, or cycles per second).

  • Precise Edit

    Decibel is a measure of the amplitude of the sound wave; greater amplitude = greater volume. Each bel doubles the amplitude, or volume.

    Octave is a measure of the frequency of the sound wave; greater frequency = higher pitch. (As Maeve noted, an octave specifically refers to the diatonic scale, which has 8 (hence OCTave) tones. Each octave doubles the frequency.)

  • Steve

    Now I’m quite scientifically minded but the explanation of a bel or decibel left a lot to be desired given I don’t know much about how sound is measured. I was left wanting and frustrated at the lack of explanation to the question asked.

  • Steve Parmenter

    I think you missed the underlying difference between decibel and octave.

    Octave describes a relationship between two notes of different pitch, or frequency. Specifically, of a pair of tones which are an octave apart, the higher has twice the frequency of the lower. “Octave” is not a unit of measure, although frequency is. (The division of an octave into 8 notes or tones is a part of culture not dictated by any physical property).

    Decibel is a unit of measure which applies to the amplitude (loudness) of sound, independent of its pitch. Note (pun intended) that a sound of any given pitch can be either loud or soft (lots of decibels or few), and a sound at any specific decibel level can be high, low, or multi-pitched.

    Lovers of music and English will appreciate the distinction.

  • Peter

    Decibel is a unit of measure which applies to the amplitude (loudness) of sound, independent of its pitch.

    Actually, no. First, the decibel isn’t an absolute measure; like an “octave”, it only measures the difference between two levels (in the case of acoustics, of pressure) — when you say a vacuum cleaner is “70dB”, it’s lazy language, understood to mean 70dB louder than some mutually-understood reference – usually, but not always, 20 micropascals. Second, it often/usually isn’t independent of pitch: some model of the human ear’s frequency response is applied to try account for the way we perceive loudness relative to frequency (you’ll see units like “dBA”, and occasionally other letters; “A” is the name of a particular common frequency weighting…most of the time when people talk about “decibels” in sound, they really mean dBA)

  • Philip Dragonetti

    Decibel is just a convenient way to express
    VERY HUGE number ratios and VERY SMALL number ratios.
    For instance—instead of saying that something is 100 Billion times larger than something else—we just say it is 140 deciBels is, (dB) larger.

    Each 10dB decibels) corresponds to multiplying by 10 ( or dividing by 10).

    And if something is 100Billion times smaller then we say it is
    -`140dB ( ie, minus 140dB).

    The formula is 10 LOG of the ratio.
    If the ratio is 100Billion = 100,000,000,000 then the LOG of 100,000,000,000 is 14.
    then you multiply the 14 by 10 to get deciBels.

    (Note: LOG means logarithm. On your calculator, to get the ratio 1000 in dbs, press the LOG button then enter 1000 and then press ENTER. Then multiply that answer by 10 to get dbs)

    It is easier to say 140 dB than to say 10,000,000,000.

    Now OCTAVE:
    As far as octave is concerned. An octave is a frequency ratio of 2 to 1. Middle C is 256 cycles per second. The C above that ( 7 notes higher) there are 8 notes to a musical scale, CDEFGABC`.
    C` has two times the frequency than C. Frequency means how many vibrations per second.

    for instance—the first 2 notes of the song “Somewhere over the rainbow” are an octave. Sing it. Some-where = C, C` or 256 vibrations per second and 512 vibrations per second. 9assuming you start on C)

    In engineering we also use the term “octave” to mean that a frequency is twice as high as another frequency. 105 on the AM dial is on octave hight than 55 on the AM dial.

    I hope this cleared it up a bit. I am a musicialn and an engineer
    —so I deal with dBs and octaves every day. 🙂

  • Peter

    Middle C is 256 cycles per second.

    Hmm…that’s a low value; there has been a fair amount of variation, but the usual standard value nowadays is 261Hz and some change (so the A above middle C is 440Hz).

  • Dar McWheeler

    As a composer and author I feel I should chime in.

    A decibel is a scientific reference as to how loud something is. Unless you are doing a tech paper, it’s a non-starter for most sound-loudness descriptions because it’s tech nature doesn’t stick the landing.

    “That was the loudest show I’ve ever heard!” This is fine for most writting.
    vs
    “That show was way over 100 decibels!” No one knows what this means, therefore, it will either cause them to igore it, lessening the impact of your writting, or you will have made them feel stupid and they’ll move along and forget you.

    Octave, on the other hand, is a description of the +/-8 state of something, but also tell the reader that thing being described is much higher, or lower, than the thing it’s being compared to.

    For example, “This car clearly sings an octave higher than it’s competitors.”, meaning the motor really screams. Or maybe, “His impulses reside in the lowest octave of human desires.”, meaning he is a degenerate.

    Cheers

    Dar

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