The Decibel Scale

Decibel Scale - brief

the system we use to describe different sound energy levels and to measure hearing threshold

Why do we Care?

-SLP: understanding dB allows us to accurately interpret hearing tests which in turn can give them **insight into their patient’s access to hearing

-AuD: dB is central to diagnosing hearing loss, programming hearing aids, and monitoring noise levels

Important Concepts - Force

force: the energy required to displace air molecules 

• measured in dynes for small changes in force

• human ear is very sensitive to sounds so only a small amount of force is required to stimulare hearing

Important Concepts - Amplitude

amplitude: the maximum distance that a molecule can travel from its resting position

• the height of a sine wave; what we perceive as loudness

• greater the force, greater the displacement and amplitude of wave

Important Concepts - Sound Pressure

sound pressure: variation in air pressure caused by the compressions and rarefactions of a sound wave  

• micropascals (μPa)

• force/area

• **as force increases, pressure increases (and vice versa)

-**larger amplitude → larger changes in air pressure → higher sound pressure

Important Concepts - Intensity

intensity: the amount of acoustic energy per unit area carried by the sound wave

• watt/cm²

• higher force → higher amplitude → higher pressure → higher intensity → more energy hitting your ear drum

Important Concepts - Loudness

loudness: the perception of intensity by the human auditory system

-force on the sound source → amplitude of vibrations → sound pressure variation in air → intensity of sound wave → perceived loudness

Problem with scaling sounds

-human ears hear an incredibly wide range when expressed linearly

-there is no absolute lower limit of hearing so no easily identified 0 point

-solution: a measurement scale that addresses a wide range conveniently and address the absence of an absolute 0

• logarithmic scale: equal steps represent equal ratios/multiplicative changes

  • ex. richter scale, pH scale, dB scale

Ex. Logarithms

10,000

• log(10,000) = 4

0.01

• log(0.01) = -2

1

• log(1) = 0 (***)

The Decibel Scale

-the bel is a unit based on the log scale

-a “Bel” is too large for the sensitivity of human hearing so we use decibels (1/10 of the bel)

-**decibels measure sound pressure and intensity

-**it requires a reference (it’s a comparative scale rather than absolute)

Intensity Level as a Reference

-I₀ : measured intensity

-I_R : reference intensity

-dB IL = 10 x log(I₀/I_R)

• I_R = 10^-16 watt/cm²

Sound Pressure as Referencee

-P₀ : measured pressured 

-P_R : reference pressure

-dB SPL = 20 x log(P₀/P_R)

• P_R = 20 μ Pa

Important!!

-0 dB IL and 0 dB SPL do NOT mean the absence of sound

• the reference value represents the softest sound that an average healthy human adult can hear

dB HL

-human hearing demonstrates differential sensitivity across 20-20,000 Hz

-dB HL adjusts dB SPL values to account for human hearing sensitivity across frequencies

-** 0 dB HL = average hearing threshold of young adults at that frequency (this makes it easy to understand what’s normal)

dB HL vs dB SPL vs dB IL

-dB HL is used to measure pure tones so we can get a flat and interpretable audiogram that we can compare to normal thresholds

-dB SPL and dB IL are used to measure complex sounds 

• why? because SPL and IL are physical, not normalized - they capture the actual acoustic energy of real world sounds

-dB SPL: real-world sound measurement (speech, music, environmental sounds)

-dB IL: engineering/acoustics

-dB HL: audiometry with pure tones