Basic Acoustics & Terminology Flashcards
What is sound?
Vibratory energy transmitted by pressure waves in air or other media that is the objective cause of the sensation of hearing
What are 4 sources of sound?
Taut cords
• Taut membranes
• Air in tubes
• Knocking
What do all sources of sound involve?
All involve moving or vibrating structures – energy is transferred from mechanical energy into sound energy
In sound transmission what happens when a sound source vibrates?
air molecules around the source vibrate and pass on the vibrations to the adjacent molecules
What is the result of sound transmission?
waves of high pressure (compression) and low pressure (rarefaction) travel through the air
What happens as a sound wave moves out from its source?
energy is dissipated in the form of heat.
What happens when energy is dissipated in the form of heat?
the energy of the sound wave decreases as the distance from the source increases.
Does certain media pass sound more easily?
Yes
What is the speed of sound in air?
340m/s (760mph)
What is the speed of sound in water?
1500m/s (3350mph)
What can a vacuum not pass?
Sound energy
What is a sinusoid?
• Physically: a sine wave is made up of just one frequency of vibration of the air molecules
• Subjectively: the sound is clean and sharp – a “pure tone”
What produces a sinusoidal wave? (Sine wave)
A tuning fork produces the most basic sound.
What are the 3 physical characters of a sinusoid?
• Amplitude (Intensity)
• Frequency
• Phase
What is the amplitude of a sound?
a measure of the magnitude of pressure changes in the medium concerned (Sound Pressure Level SPL)
What is SPL?
Sound Pressure level
What is the simple answer for amplitude?
Loudness
What is amplitude measured in?
Newtons/m2= Pascal (Pa) or micropascals (uPa)
What does 0.000001 Pa =?
1uPa
What is the quietest sound heard for the human ear?
20uPa
What is the loudest sound tolerated for the human ear?
100,000,000 uPa (100Pa)
What is used to measure a sound level?
Decibels
What is dB derived from?
a ratio and must be referenced to an absolute amount
What does dB=?
20log10(P/Pref)
What does reference pressure is human 1kHz threshold=?
20uPa
What are the advantages of the decibel scale?
• It has a zero baseline
• The just noticeable difference ~1–3dB
• Similar to human subjective perception of loudness, i.e. twice the dB level = twice loudness
• Human hearing range 0–140dB
What is frequency?
Frequency is the number of wave cycles in one second.
What is frequency in subjective terms?
Pitch
What is frequency specified in?
Cycled per second- Hertz
When does the frequency of a sound increase?
When the number of cycles per second increases
What is the normal human hearing range in Hz?
20 t0 20,000Hz
What can harmonics be found in?
• Musical instrument
• Electronic amplification
• Human voice
What is the tone with lowest frequency called?
Fundamental
What are other tones called?
Overtones
What does Overtone with frequency that is a multiple of the fundamental frequency?
Harmonic
What is the normal human ear able to do between many sounds?
detect, identify, and differentiate
What is Pure tone Audiometry used for?
To assess patients’ hearing thresholds (quietest sound they can hear) over a range of frequencies
What is the range of frequencies in air conduction?
250-8000Hz
What is the range of frequencies in bone conduction?
500-4000Hz
What range does Pure Tone Audiometry cover?
Speech
What is a phase?
A description of where, at a point in time, the sound wave is during its repeating cycle
What does Phase measure?
degrees or radians
What can two waveforms have the same and different?
Same- frequency and amplitude
Different - phrase
What happens if two sinusoids of the same frequency and same phase from different sources coincide?
The regions of positive pressure combine, as do the regions of negative pressure – constructive interference.
What is destructive interference?
if the two signals are out 180o of phase the positive and negative portions cancel each other out
What may Patterns of destructive and constructive interference lead to?
“dead” spots and “live” spots in auditorium acoustics
What are room acoustics based on?
- Sound reflection
- Sound absorption
- Reverberation
• Attenuation
What happens when sound meets a change in surface density?
some sound energy is absorbed, some is reflected and some is lost as heat.
What is reverberation?
The sound remaining after the sound source has stopped
What is reverberation time?
• the time taken for the sound intensity to drop by 60dB from it’s original level
What does direct sound always do?
arrives first, and always from the correct direction.
What are there lots and lots of in precedence effect?
reflections from all surfaces and objects in the
room (i.e. reverberation)
What is attenuation?
The stopping of sound by enclosure or shielding
What does a dosimeter measure?
Level and frequency of sounds over time
When must ear protection must be used when using a dosimeter?
80dB
