Basic Acoustics Flashcards
What is Sound?
A longitudinal-wave disturbance in any compressible substance,
especially in air. Perceived as an object “heard” in the brain when these waves stimulate human ears.
Sound propagation
how it travels from one place to another: waves
Sound perception
how it affects the senses and emotions of a listener
Longitudinal waves (compression waves)
wave motion and particle motion are parallel to each other (lengthwise waves on a slinky spring, sound waves).
Transverse waves
wave motion and particle motion are perpendicular to each other (waves on a rope, electromagnetic waves, e.g. radio, light, x-rays).
Wavelength λ (lambda)
The distance from one crest to the next (water waves) or from one compression to the next (sound waves)
The speed of sound
344 meters per second: v = 344 m/s (dry air at room temperature (T = 20° C)
Approximate speed of sound in other media
- carbon dioxide 270 m/s
- helium 1000 m/s
- water 1500 m/s
- steel 6000 m/s
When the temperature is higher
the random molecular motions are faster, the molecules collide more often and pass the sound disturbance faster from one region to another.
Speed of sound and air temperature
by every Celsius sound changes by 0.6m/s (21c = 344.6 m/s, 19c = 343.4m/s)
Displacement amplitude
distance each bit of air moves to either side of its normal position during its vibration (usually on the order of millionths of a meter).
Pressure amplitude
maximum increase of air pressure (above normal atmospheric
pressure) in a sound wave compression
Musical time scales
long = entire song, section, or phrase
medium = individual notes or chords
short = individual sound vibrations (percussion)
Frequency
measured in Hz, Frequency is the repetition rate of a vibration. In musical terms, one octave is a doubling of frequency.
Frequency range
The range of audible frequencies covers about 10 octaves, and extends from approximately 20 Hz up to 20,000 Hz or 20 kHz (kilohertz).
waveform
The waveform of a tone played on a musical instrument is usually obtained by converting the pressure fluctuations of the air into an equivalent electric voltage with a microphone. This voltage can be recorded or directly displayed with an oscilloscope.
periodic sound
is perfectly steady and keeps repeating the same vibrational pattern indefinitely;
transient sound
is not sustained, but quickly dies away (percussive sound).
Phase
A complete cycle has 360 degrees of phase, Therefore the positive peak of the cycle is at 90 degrees, the zero point is 180 degrees, and the negative trough is at 270 degrees
Wavelength = Speed divided by frequency. W = S/F
Assume a wave has a frequency (speed) of 100Hz.
Then W = 344/100
therefore: W= 3.44 metres
This equation can be used to measure any one of these three parameters, assuming that the other two are known.
absorption
The drainage of sound energy into a surface
reflection
Harder materials such as concrete will absorb very little energy because the densely packed particles in these rigid surfaces are not so easily jarred into motion and will instead bounce off
normalised sine wave calculation
sin*degrees of phase/sin()
0 degrees = 0
90 degrees = +1
180 degrees = 0
270 degrees = -1
diffusion
is when waves encounter a rough surface with bumps
about as large as the wavelength of the waves. The wave energy is scattered in many different directions by different parts of the surface. Irregularities that are ¼ of a wavelength, or greater, will start to cause diffusion.
Multiple reflections
allow sound waves to travel from the source to a different room, for instance someone calling you from downstairs, however waves become weaker at each reflection, because part of their energy is lost to absorption.
concave wall reflection
The performer must be a little closer than half the radius of the walls curvature so sound can spread nearly uniformly.
Refraction
is a change of direction of wave travel due to difference in its speed from one place to another. (shouting against the wind will increase refraction)
Diffraction
The bending of waves around small obstacles and the spreading out of waves beyond small openings. An ambulance siren can be heard around a corner.
Diffraction through spaces
Waves coming through a narrow opening (compared to their wavelength) spread out almost equally in all directions. Waves coming through an opening that is large compared to their
wavelength most continue in a straight line
The inverse-square law.
energy twice as far from the source is spread over four times
the area (squared), hence one-fourth the intensity. if we measure the intensity (i) of a sound at distance (d) from its source, at a distance of 2d, the intensity would be i/4. 4d therefore would equal i/16.
excitation
The action of setting an object into vibration
Idiophones
sound is produced by the instrument ‘on its own’
Membranophones
sound is produced by a vibrating membrane, stretched over an opening (percussion)
Chordophones
string instrument
Aerophones
sound is produced by a vibrating column of air
Electrophones
(electronic instruments)
Sympathetic vibrations
When a sound wave of one frequency strikes a surface (a violin, for example) it will vibrate naturally at the same frequency. For example, an A string at 440 Hz will cause an E string at 330 Hz to resonate, because they share an overtone of 1320 Hz (the third harmonic of A and fourth harmonic of E).
Natural modes
a motion in which every part of the system oscillates sinusoidally at the same frequency.
Resonance
means the comparatively large-amplitude vibration that results whenever the frequency of some driving force closely matches a
natural oscillation frequency of the system on which it acts.
The Law of Superposition
As we add integer harmonics to a sine wave, the shape of the
wave changes.
Vibraphone bars
help produce musically helpful frequency ratios. Thinning a bar at the center reduces the stiffness and lowers the frequency of the first mode to change the frequency ratio
Acoustic reflection
The reason string instruments produce loud volume is because its sound reflects of the body of the instrument, causing the soundwave to invert and therefore doubling its amplitude.
The Law of Superposition
if two waves in a positive phase occur at the same time and point, we can simply add their values to calculate their combined effect.
If two waves occur at opposite phases to each other, we subtract their values therefore resulting in nil.
Open end tube
is when air (sound) can freely vibrate when traveling to the end of a tube. If both ends of the tube are open, the musical instrument is said to contain an open-end air column.
Factors that affect the sound of a mic
polar response: how mics responds to sound from different directions
The frequency response: the accuracy with which the mic outputs frequencies across the audible range
The sensitivity: the measure of the strength of output derived from any given strength of input
The distortion and noise output figures
Dynamic Microphone (velocity sensitive)
In a Dynamic Microphone, sound waves cause movement of a thin metallic diaphragm and an attached coil of wire. A magnet produces a magnetic field which surrounds the coil, and motion of the coil within this field causes current to flow, the amount of current is determined by the speed of that motion.
Condenser (Capacitor) Microphone
In a condenser microphone, the diaphragm is mounted close to a rigid back-plate. A battery is connected to both pieces of metal, which produces an electrical charge. The amount of charge is determined by the distance between the two. This distance changes as the diaphragm moves in response to sound. When the distance changes, current flows in the wire as the battery maintains the correct charge. However the current is so small that it must be electrically amplified before it leaves the microphone.
Electret Condenser Microphone
uses a material (Electret) with a permanently imprinted charge for the diaphragm. The major disadvantage of electrets is that they lose their charge after a few years and cease to work.
Mic faults
Condenser require batteries or power from the mixing console to operate.
Dynamic mics require shielding from stray magnetic fields, which makes them a bit heavy,
Overload characteristics
Any microphone will produce distortion when it is overdriven by loud sounds. With a dynamic, the coil may be pulled out of the magnetic field; in a condenser, the internal amplifier might clip.
Noise prevention tricks
The basic technique, called shielding, or screening, is to surround the wires that carry the current to and from the mic with a flexible metallic shield, which deflects most radio energy.
Balanced Lines
Use two cable cores to carry the signal, these are in inverse phase to each other. When one core signal is phase reversed, the signal now exists in phase with itself, and the noise is out of phase. Therefore the mic signal is preserved, but the noise is phase cancelled.
Polar Patterns
The capability for mics to pick up sounds elsewhere from its front. There are 5 patterns: Omni, Bi-directional, Cardioid, Hyper-cardioid and shotgun.
Decibel measurements
Are measured using a Logarithmic scale, whereby the scaling is in powers of ten.
Intensity vs Pressure
Pressure is the measurable change in air pressure caused by a wave.
Intensity is the amount of energy directed at a given area per second.
intensity = pressure squared.
Intensity and pressure equation
Intensity = log(?)x10
Pressure = log(?)x20
Therefore intensity is always half pressure and pressure is always double intensity
Threshold of Hearing
intensity =1x10-12 watts/metre squared (known as 0dB SIL)
pressure =2x10-5 newton/metre squared (known as 0dB SPL)
