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发表于 2008-2-12 18:22
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part 6
6] Miscellaneous Questions
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*** 6.1 What is active noise control ?
ANC is an electronic method of reducing or removing unwanted sound by the production of a pressure wave of equal amplitude but opposite sign to the unwanted sound. When the electronically produced inverse wave is added to original unwanted sound the result is sound cancellation.
This method of noise control is becoming increasingly popular for a variety of uses. It is sometimes considered a miracle "cure-all" for noise problems which, at the present time, is not the case. For example noise cancellation in 3D spaces, such as living areas, is very difficult to achieve. However it can be more successful locally, eg for a passenger sitting in an aircraft or car. There are many institutions and companies around the world working on the technology to increase the circumstances where ANC can be used effectively. The award winning Active Noise Control FAQ is maintained by Chris Ruckman and available at a number of sites worldwide including: http://www.xis.com/~ruckman/
*** 6.2 What causes a sonic boom ?
(from "Aircraft Noise" by Michael T Smith, Cambridge, 1989)
" .. When the speed of an aircraft is supersonic, the pressure waves cannot get away ahead of the aircraft as their natural speed is slower than that of the aircraft. Slower, in this context, means just over 1200 km/hr at sea level and about 10% less at normal cruising altitude.
Because they cannot get away, the pressure disturbances coalesce and lag behind the aeroplane, which is in effect travelling at the apex of a conical shock wave. The main shock wave is generated by the extreme nose of the aeroplane, but ancillary shocks are generated by all the major fuselage discontinuities. .. "
Ken Plotkin (kplotkin@access2.digex.net) on 24th July 1995 wrote:
[snip] .. A body moving through the air pushes the air aside. Small disturbances move away at the speed of sound. Disturbances from a slowly moving body go out in circles, like ripples from a pebble in a pond. If the body moves faster, the circles are closer in the direction of travel. If the body is supersonic, then the circles overlap. The envelope of circles forms a cone. The angle of the cone is determined by its vertex moving in the body's travel direction at the body's speed, while the circles grow at the sound speed. [snip] The existence of the "Mach cone", "Mach waves" and the corresponding angle, was discovered by Ernst Mach in the nineteenth century. [snip]
*** 6.3 Can you focus sound ?
Sound can be focused like light, but in the case of sound the "optics" must be much larger because you are dealing with longer wavelengths. The effect is heard in some domed buildings such as the Capitol in Washington, and St Paul's Cathedral in London providing noise background conditions permit.
Large parabolic reflectors can be used very effectively to send and receive sound over significant distances. Check out your local science museum or exploratorium - there may be a demonstration. It is also possible to refract sound and focus it using a lens. The lens is constructed from a large thin bubble, say 2 metres across, filled with carbon dioxide. The effect is not very pronounced.
Sound can be directed by making use of constructive and destructive
interference. This idea is used in column speakers, and commercial
systems for reducing noise levels outside the dance floor area of
discos.
*** 6.4 What is sonoluminescence ?
In the early 1930s Frenzel and Schultes discovered that photographic plates became "fogged" when submerged in water exposed to high frequency sound. More recent experiments have succeeded in suspending a single luminous pulsating bubble in a standing wave acoustic field, visible in an undarkened room. Generally sonoluminescence is light
emission from small cavitating bubbles of air or other gas in water or other fluids, produced when the fluid is acted upon by intense high frequency sound waves. The mechanism is not completely understood, but very high pressures and temperatures are thought to be produced at the centre of the collapsing bubbles.
See "Science" 14 October 1994 page 233, "Scientific American" (International Edition) February 1995 Page 32 or "Physics Today" September 1994 Page 22, all quite readable articles.
See also the following URLs:
http://ne43.ne.uiuc.edu/ans/sonolum.html
http://www.wdv.com/Sono
James Davison (TKGN58A@prodigy.com) on 28th June 1995 wrote:
[snip] .. I have been sufficiently interested to reconstruct the apparatus for producing this effect -- using a pair of piezoelectric transducers, an old oscilloscope and a signal wave generator --materials costing only a few hundred dollars.
I am proud to say that tonight I managed to reproduce this effect -- the tiny bubble has the appearance of a tiny blue star trapped in the middle of the flask. It is distinctly visible to the unadapted eye in a dark room, and it is a very startling thing to see. [snip]
*** 6.5 Why does blowing over a bottle make a note ?
Resonance in acoustics occurs when some mass-spring combination is supplied with energy. Many musical instruments rely on air resonance to improve their sonority. If you blow across the mouth of a bottle you can often get a note. The bottle behaves as a Helmholtz resonator. The main volume of air inside the bottle is analogous to a spring, whilst the "plug" of air in the neck acts as an attached mass. The resonant frequency is roughly given by:
f = { c sqrt (S/LV) } / 2pi
c is velocity of sound
S is the surface area of the neck opening
V is bottle volume
L is the effective length of the neck ie the actual length plus ends correction. Ends correction ~ 1.5 times radius of neck opening
Example: A 75 cl (7.5E-4 m^3) wine bottle with neck diameter 19 mm, bottle neck length 8 cm, air temp = 20 degC calculated resonance = 109Hz (actual resonance was 105Hz)
Helmholtz resonators are sometimes employed as a means of passive noise control in air conditioning ducts. They may also be hidden in the wall design of auditoria and offices in order to improve the acoustics.
*** 6.6 What is pitch ?
The term "pitch" has both a subjective and an objective sense. Concert pitch is an objective term corresponding to the frequency of a musical note A (at present 440Hz). Using such a standard will define the pitch of every other note on a particular musical scale. For example, with Equal Temperament each semitone is higher or lower in frequency than the previous semitone by a factor of 2^(1/12). An octave is a pitch interval of 2:1. Many sounds with no obvious tonal prominence are considered by musicians to be of indeterminate pitch; for example, the side drum, cymbals, triangle, castanets, tambourine, and likewise the spoken word.
Pitch is also a subjective frequency ordering of sounds. Perceived pitch is dependent on frequency, waveform and amplitude or changing amplitude. Numbers can be assigned to perceived pitch relative to a pure frontal tone of 1000Hz at 40dB (1000 mels) thereby establishing a pitch scale.
Further info and examples on pitch from URL:
http://www.music.mcgill.ca/auditory/Auditory.html
*** 6.7 What are musical intervals ?
An interval is the ratio in frequency between musical notes. These intervals are sometimes called a second, third, fourth, fifth etc. which refers to the position on the scale that the note is to be found. In the scale of C major: C D E F G A B C, the note 'E' is the third note of the scale and the interval from C to E is therefore called a third. For the scale D major: D E F# G A B C# D, the third will be F#. The term 'interval' can also be used to indicate that the notes are sounded together, in which case there are consonant intervals and dissonant intervals.
The ratio of frequency intervals for Just Intonation is demonstrated below in the scale of C major, though the same ratios apply to all the major keys:
C
(9:8)
D
(10:9)
E
(16:15)
F
(9:8)
G
(10:9)
A
(9:8)
B
(16:15)
C <- Octave
The interval between E & F and between B & C is a semitone, whilst the other intervals are tones. The interval between any two notes above can be found by multiplying the intervening ratios; thus if all the aboveratios are multiplied together the resultant is 2 because an octave is
twice the original frequency.
The notes of minor scales differ from their major counterparts; one important difference being the flattened third. E flat is a minor third above the note C.
The use of Just Temperament causes serious problems of intonation when music modulates between keys. Equal Temperament is nearly always used as a compromise to the problem of tuning (see question 6.6).
*** 6.8 What causes "helium voice" ?
Many people, on hearing the voice of someone who has breathed helium, believe that the person's speech pitch has increased.
WARNING - Breathing helium can be very dangerous.
^^^^^^^
A cavity will have certain resonant frequencies. These frequencies depend on the shape and size of the cavity and on the velocity of sound within the cavity. Human vocal cords vibrate non-sinusoidally in the vocal tract, giving rise to a range of frequencies above the fundamental. The vocal tract mainly enhances lower frequency components imparting the recognizable voice spectrum.
The velocity of sound in helium is much greater than in air, so breathing helium will raise the vocal tract's resonant frequencies. Although the vocal cords' vibrational frequencies are little affected by helium, the effect of higher cavity resonances is to alter substantially the relative amplitudes of the voice spectrum components thus leading to apparent pitch change.
*** 6.9 What is structural acoustics ?
Structural acoustics is concerned with the coupled dynamic response of elastic structures in contact with non-flowing fluids. (The fluid, although non-flowing, undergoes small-amplitude vibration relative to some equilibrium position.) For heavy fluids like water, the coupling is two-way, since the structural response is influenced by the fluid response, and vice versa. For lighter fluids like air, the coupling may be either one-way (where the structural vibration affects the fluid response, but not vice versa) or two-way (as occurs, for example, in the violin).
Structural acoustics problems of interest involving water include the vibration of submerged structures, acoustic radiation from mechanically-excited, submerged, elastic structures; acoustic scattering from submerged, elastic structures (e.g., sonar echoes); acoustic cavity analysis; and dynamics of fluid-filled elastic piping systems. These problems are of interest for both time-harmonic (sinusoidal) and general time-dependent (transient) excitations. Water hammer in pipes can be thought of as a transient structural acoustics problem.
Structural acoustics problems of interest involving air include determining and reducing noise levels in automobile and airplane cabins.
Reference (for simple geometry problems):
"Sound, Structures, and Their Interaction," Second Edition, by M.C.
Junger and D. Feit, MIT Press, Cambridge, Mass (1986).
*** 6.10 What is the doppler effect ?
When a sound source is moving, a stationary observer will detect a different frequency to that which is produced by the source. The speed of sound in air is approximately 340 m/s (see 2.11). The wavelength of the sound emitted will be foreshortened in the direction of motion by
an amount proportional to the velocity of the source. Conversely the wavelength of a receding sound source will increase. The doppler effect may be noticed as a marked drop in pitch when a vehicle passes at high speed.
Example 1: A sound source, S, emits 1000 waves per second (1 kHz) and is moving directly towards an observer, O, at a speed of 100 metres per second (equivalent to approx 225 miles per hour).
After 1 second the wave front, which is travelling at the speed of sound, will have travelled 340 metres from the original source position. Also after that second the sound source will have moved 100 metres towards the observer.
0 m 340 m
S | | | | | | | | | O
<-------------- 1000 waves ------------------>
100 m 340 m
S | | | | | | | | | O
<------- 1000 waves --------->
Therefore the same number of waves will occupy a space of 340-100 = 240 metres and the wavelength will be 240/1000 = 0.24 metres. To the observer the frequency heard will be the speed of sound divided by its wavelength = 340/0.24 = 1416.7 Hz.
Example 2: An observer moving at 100 metres per second directly approaches a stationary sound source, S, which is emitting 1000 waves per second (1 kHz). In this example there is no change in wavelength. In one second, the observer will hear the number of waves emitted per second plus the number of waves which s/he has passed in the time (1000+100/0.34) = 1294.1 Hz.
Note the interesting result - a stationary observer with moving source will not hear the same frequency as a would a moving observer with stationary source.
*** 6.11 What is white noise, pink noise ?
The power spectral density of white noise is independent of frequency. Since there is essentially the same energy between any two identical frequency intervals (for example 84-86Hz and 543-545Hz), white noise narrow band FFT analysis will show as flat. However octave band analysis will show the level to rise by 3dB per octave because each band has twice the frequency range of the preceding octave.
Pink noise is often produced by filtering white noise and has the same power within each octave. Narrow band analysis will show a fall in level with increasing frequency, but third-octave band or octave band analysis will be flat.
see Joseph S. Wisniewski's Colors of noise FAQ at:-
http://capella.dur.ac.uk/doug/noisecols13.txt |
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