Hearing Science - The Decibel

Define Decibel:

Primary unit of measure for sound; logarithmic scale used to describe the ratio between two quantities.

In acoustics, this is referred with sound intensity, sound power, or sound signal strength.

Define Absolute Difference:

The difference between two values, typically determined through subtraction. Does not apply to acoustics. LINEAR

Provide an example of absolute difference:

Sally bench pressing 115 lbs when she typically benches 95 lbs. {frequency in experimental group minus the frequency in the control group} (taking two numbers and subtracting them from one another)

Define Relative Difference: 

Exponential and logarithmic difference between two values typically determined through multiplication.

Suggests it’s non-linear (if I make a change in one, I am not making the same change in the other).

Provide an example of relative difference:

An alien head is three times larger than a human head.

Why are relative differences important in hearing science?

The human auditory system perceives changes in sound based on proportions, not absolute values. Imperfect and nonlinear, needed to detect the smallest difference.

How is the human response to physical changes in frequency or intensity described?

Human perception of changes to sound is imperfect and nonlinear.

If I make changes in sound such as 1 db, as a human I am not going to perceive that sound and it will sound the same. In order to perceive the change in sound, I need a greater change in decibels.

Volume controls consider that human perception is _________, that’s why they have decibel changes assigned to each of those numbers.

Nonlinear

What is the relative scale for frequency?

The octave scale.

Define the octave scale and how it’s relative in nature:

Logarithmic scale to describe doubling of frequency (relative scale because the double of frequency is not doubled equally). (500 Hz —> 1000 octaves; 1000 Hz —> 2000 octaves = 1000 Hz difference – octave scale)

Changes are not 1-to-1. Same as decibel, needs greater change in frequency in order to perceive the change in sound.

What was the decibel originally used to describe?

(Named after Alexander Grand Bell) Audio level in telephone circuits and to measure transmission efficiency (measured in bels).

It’s 1/10th of a bel to allow for a smaller, more compressed range to describe human perception of power loss.

With regards to hearing, the decibel describes the relative difference between what?

Absolute intensity difference and the human perception of them.

When a value is expressed in dB, it is considered a _________.

Logarithm

Power level refers to…

(how much power) magnitudes measured in dB.

What are the 2 most important level-scales used in hearing science?

Sound pressure level and sound intensity level (both refer to the power of the sound), also power level scales telling you how much power was involved in that sound.

What first needs to be established in order to determine the power level of a sound?

The reference level or a natural zero point.

Define Reference Level:

The zero point (no difference) of a relative scale; the value used as a reference magnitude in a ratio scale.

Does 0 dB (or zero level) does mean the absence of sound? Why or why not? What does it really mean?

NO! The zero point is just comparing two sounds and there’s no difference between them. That doesn’t mean that nothing happens because you have units of measure to compare. It’s the threshold of human hearing.

What do we perceive Sound Intensity Level as?

(or dB IL) We perceive it as the literal volume of the sound. It’s the logarithmic measure of the amount of sound power that travels through a specific area.

What do we perceive Sound Pressure Level as?

(dB SPL) We perceive it as feeling it after the sound. It’s the logarithmic measure of the magnitude of change in the local atmospheric pressure caused by the vibration of a sound source.

What happens to the sound pressure level when sound pressure doubles? 

The sound pressure level increases by 6 dB

Ex: 30 dB SPL + 30 dB SPL = 36 dB SPL

What happens to the sound intensity level when sound intensity doubles? 

The sound intensity level increases by 3 dB (not 6 dB because you can feel the sound difference before you hear it)

Ex: 55 dBIL + 55 dBIL = 58 dB IL

Sound intensity levels and sound pressure levels are different names of the ____ logarithmic scale of intensity. 

Same

EX:

45 dBSPL and 90 dBIL are two different sounds, they will not increase the same.

45 dBSPL = 45 dBIL

   SPL : 45 dBSPL + 45 dBSPL = 51 dbSPL = 51 dBIL

   IL : 45 dBIL + 45 dBIL = 48 dBIL = 48 dbSPL

90 dBIL

   SPL : 90 dBSPL + 90 dBSPL = 96 dBSPL = 96 dBIL

   IL : 90 dBIL + 90 dBIL = 93 dBIL

SPL and IL being the same only applies when looking at the change in sound level for the ____ or ______ sound

Same ; Single

If we double the sound pressure of a sound source producing a sound pressure level of 28 dB SPL, what sound intensity level does the sound source now produce? Why? 

28 dBIL; because we’re looking at one sound, my sound pressure level and sound intensity level are the same logarithmic scale of intensity.

Look Out for TRICKS:

3 pigs oinking at 60 dBSPL

7 goats mehing at 60 dBSPL

All animals is 66 dBSPL and 66 dBIL .

We need to consider the _____ ____________ between the two sources when we combine two equal sound levels (two seperate sound sources). 

Phase relationship

What happens if the sound sources are 180-degrees out-of-phase?

(180 out-of-phase: mirror images, hitting opposite points of maximum displacements and equilibriums) Cancelling of SPL or IL = 0 dB {would still be a sound, just not high in pressure or intensity}

What happens if the sound sources are in-phase?

(in-phase: these two sounds are hitting all maximum displacements and equilibriums at the same time) Doubling of SPL or IL

What happens if the two sound sources are out-of-phase?

(out of phase: does not match in the slightest) An increase of 3 dB, regardless of SPL or IL

What happens if the phase relationship changes randomly over time?

They average (or increase) by 3 dB. 

Combining two independent sound sources will result in a perception of change in _________.

Intensity

Doubling the number of equal intensity sound sources results in a doubling of sound _________ NOT a doubling of sound ________. 

Intensity ; Pressure

Why does doubling the number of equal intensity sound sources result in a doubling of sound intensity and NOT a doubling of sound pressure? 

I need a greater change in physical stimulus. You will perceive pressure before intensity.

Example Problem: Two independent sound sources each produce a sound intensity of 60 dB.  What is the sound intensity level when we combine the two sources?

63 dB IL

Example Problem: Two independent sound sources each produce a sound intensity of 60 dB. Two new speakers, each producing 60 dB intensity levels, are added to the original two.  What is the new sound intensity level?

DOUBLE IT TWICE:

Sound #1: 63 dbIL

Sound #2: 63 dbIL

            = 66 dBIL

What are some practical and common applications of the decibel scale?

1. Sound measurement and hearing protection

2. Audio engineering

3. Physics and acoustics 

Define the praticality of this common application of the decibel scale: Sound Measurement and Hearing Protection

(such as noise levels, headphone and speaker output, workspace safety, hearing health) WE CARE ABOUT THESE BECAUSE THESE HELP PROTECT FROM DANGEROUS AND DAMAGING LEVELS OF SOUND

Define the praticality of this common application of the decibel scale: Audio Engineering

(such as sound engineering) WE CARE ABOUT THESE FOR THE QUALITY OF THE SOUND WE HEAR

Define the praticality of this common application of the decibel scale: Physics and Acoustics

(such as acoustic energy) WE CARE ABOUT THESE FOR HEARING AIDS TO PRODUCE SAFE SOUND WITHOUT DAMAGING YOUR HEARING FURTHER

What is the relationship between power and sound intensity and sound pressure level?

If we increase the power, we increase both the sound intensity and the sound pressure levels as well. This is because sound pressure levels are the logarithmic scales measuring the magnitude of change in the local atmospheric pressure by the vibration of a sound source; the sound intensity levels are the logarithmic scales measuring the amount of sound power that travels through a specific area.

What is the relationship between sound intensity and sound pressure?

Sound intensity and sound pressure are two levels of power for the same sound source.

T/F: If I double sound intensity, then I also double sound pressure.

False: doubling the sound intensity level increases the sound intensity by 3 dB, and doubling the sound pressure level increases the sound intensity by 6 dB, they are not the same physical quality so they are not calculated the same.