Decibel ( dB)

1. Decibel (dB) • dB is a logarithmic value that expresses difference and not absolute values • A corresponding reference scale is required • For sound, sound-pressure level (SPL) is used as the reference scale (dB SPL) • The threshold of hearing (0 dB SPL) is used as the reference or zero point

2. Decibel (dB) • Example dB levels (approximate) • 0 dB = threshold of hearing • 30 dB = soft whisper (5 ft.) • 70 dB = conversation (1 ft.) • 90 dB = subway train (20 ft.) • 130 dB = pain threshold

3. Changes in Decibels (dB)

4. Changes in Decibels (dB)

5. Decibel (dB) and Distance • Doubling (2x) the distance = decreases the apparent signal level by 6-10 dB • Halving (1/2) the distance = increases the apparent signal level by 6-10 dB • There is a 6-10 dB change for every doubling or halving of the distance • For example • 4x the distance = 12-20 dB decrease • 8x the distance = 18-30 dB decrease • 1/4 the distance = 12-20 dB increase • 1/8 the distance = 18-30 dB increase

6. Auditory Perception • Human ears are non-linear • The sound arriving at your ears is not the sound you actually “hear” (psycho acoustic effects) • Frequency response is not flat and humans hear mid-range frequencies better especially at lower sound levels

7. Response Variation by Frequency

8. Auditory Perception • Harmonic distortion is added at louder levels – perceived tones change with sound level • Frequency response changes with sound level • Lower sensitivity to low and high frequencies at low listening levels • Fletcher-Munson Equal Loudness Contour Curves illustrate the difference in sensitivity to different frequencies at various sound levels

9. Fletcher-Munson Equal Loudness Contour Curves

10. Equal Loudness Examples • At 50 dB SPL, compared to a 1 kHz pure tone • 40 Hz tone will be 30 dB lower and 10 kHz tone 13 dB lower • At 110 dB SPL, compared to a 1 kHz pure tone • 40 Hz tone will be 6 dB lower and 10 kHz tone 4 dB lower • *1 phon = 1 dB SPL at 1 kHz for a pure tone

11. Sound – Loudness and Pitch • Loudness of a tone affects perceived pitch • A 100 Hz tone at 100 dB SPL will sound 10% lower in pitch than at 40 dB SPL • A 500 Hz tone at 100 dB SPL will sound 2% lower in pitch than at 40 dB SPL • Loud headphones or room sound levels can make vocalists sing flat and instrument tuning more difficult

12. Sound Level Considerations and Warnings • Often monitor at 75-85 dB to minimize equal loudness effect, reduce pitch errors, reduce hearing loss with prolonged exposure and to mimic common listening levels • Prolonged exposure above 85 dB can cause gradual hearing loss • Regular exposure for more than 1 minute above 100 dB can result in permanent hearing loss

13. Sound Envelope (ADSR)

14. Sound Envelope • Attack – how quickly the sound reaches full volume after the sound source is activated • Decay – how quickly the sound drops to the sustain level after reaching the peak or full volume • Sustain – how long the sound remains at near constant volume after decaying (may not be present) • Release – how quickly the sound fades to zero

15. Speed of Sound • Speed of sound in air at 70°F is 1130 ft/sec (343 m/s) • Two sounds <15 msec apart will be heard as one distinct sound • ~15-35 msec apart will sound doubled • ~25-35 msec or more apart will be heard as two distinct sounds

16. Frequency and Wavelength • Wavelength = 1130 feet per sec* / Frequency (cycles/sec) • For a 40 Hz sound wave: 1130 / 40 = 28.3 feet per cycle • * at 70°F

17. Standing Waves and Room Nodes • If any room dimension is less than a wavelength, generation of standing waves at the corresponding frequency and below will occur • Standing waves result in room nodes which can boost or attenuate frequencies, thus changing the perceived sound • Rarely a problem for high frequencies because of their very small wavelengths • The room nodes change as you move in the room (i.e., location dependent)