| It is often necessary to estimate how much a sound level changes. Our ear interprets sound level changes as changes in loudness. The decibel is a very convenient unit for measuring signal levels in electronic circuits or even sound pressure levels in air. However, changes in the loudness of sounds as perceived by our ears do not conform exactly to corresponding changes in sound level. |
Definitions
Sound Level: a physical quantity (measured with instruments)
Loudness: a psycho-physical sensation perceived by the human ear/brain mechanism
Decibel: one-tenth of a bel, which is the logarithm of the ratio of any two power-like quantities
Logarithm: (common log to base 10) of a number is the exponent of 10 that yields that number
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| Review of Logarithms
100 = 10 2 log 100 = 2
288 = 10 2.4594 log 288 = 2.4594*
1000 = 10 3 log 1000 = 3
*from calculator
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| For example, let’s listen to a 1000-Hz tone at a constant level: |
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| When these tones are reproduced on a loudspeaker, you will notice that changes in head position change the loudness of the sound due to room effects. For this reason, keep your head in one position during each test. Of course, if you are listening on headphones, room acoustics have no effect. |
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| A change of level of 10 dB sounds like this: |
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| A change of 10 dB is often considered to be a doubling of loudness, or cutting loudness in half. | |
| This is a 5-dB change of level: |
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| It may be difficult at first to detect a change of 2 dB. Try it. Changes will be made at the same five-second intervals: |
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| A change of 2 dB is easier to detect in louder sounds. Let’s repeat the 2-dB change at a 10 dB higher level: |
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| So, we see that the minimum detectable level change depends on the loudness of the 1000-Hz tone. That’s not all. The loudness change also depends on frequency. Here is a repeat of the 10-dB change in level at 1000 Hz: |
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| This is a 10-dB change in level at 100 Hz:
A 10-dB change in level is less noticeable at 100 Hz but very prominent at 1000 Hz. The minimum discernible level change depends both on the frequency and the level of the sound. |
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| Now, music and speech cover a wide range of frequencies. A change in average level of 10 dB sounds like this in speech: |
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| And now a 10-dB change in music: |
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| A change in level of 5 dB sounds like this in speech: |
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| Now, a 5-dB change in music: |
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| Here are changes in level of only 2 dB:
We can conclude that for ordinary speech and music signals, a change in level of 2 dB is about the smallest detectable by ear. We have also learned what level changes of 5 and 10 dB sound like. |
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| Let’s test this new ability. The following speech sounds will begin and end at a standard reference level, but in between there will be three other levels. Estimate the change in dB up or down from the reference level: |
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| The first change was 10 dB down; then no change from the reference level; and finally, 10 dB up. How do your level judgments compare? | |
| Let’s repeat the speech sound for three different changes, again opening and closing with the reference level, with three unknowns in between.
This is more difficult. The changes were 4 dB up, 8 dB up, and 12 dB up. How do your judgments compare? |
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| Now, let’s exercise our judgment with music. Again, we will open and close with the reference level and present three level changes in between: |
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| These changes were: first, 5 dB below reference; second, 5 dB above reference; and third, 10 dB below reference. | |
| Here is another exercise in music using the same opening and closing reference levels:
The three changes this time were: 5 dB down, no change, and 5 dB up.
This is the end of this section, but go back over these exercises for further practice. |
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