What is the difference between sound and noise?

• Short but somewhat simplistic answer: sound is characterised by a wave that is periodic in time, noise by an irregular, disordered wave.
• The real answer is long, because the concepts of sound and noise depend not only on the objective features of the perceived sound wave, but also on the perceiving subject. That is, they depend on the interaction between the wave and the perceptual system that picks them up, transforms them and analyses them. Any answer that points to a single physical parameter characterising the wave independently of the perceptual system is hopelessly imprecise.

• For a fuller explanation, with sound examples, diagrams and a dialogue on the subject, see the pages sound or noise?, different types of noise and perception of pitch.
• See the section general features of sound for an introduction to the subject, and the section from sounds to music for the relation between noise and sound in music.

In ripple tank images with two sources, does constructive interference correspond to the bright zones or to the dark ones?

To neither in particular. In a ripple tank, light passes through the water and is projected onto a screen: the crests of the waves act as converging lenses and produce bright stripes, while the troughs make the light diverge and produce dark stripes. The contrast between bright and dark is therefore greater where the amplitude of the wave is larger.

• In the zones of constructive interference the water oscillates with maximum amplitude: there one sees a strongly contrasted alternation of bright and dark, which moves as the waves advance (in photographs it appears as a row of alternating bright and dark spots).
• In the zones of destructive interference the water stays almost still: there one sees a stripe of uniform, grey brightness, which does not move.

The "grey, still" lines radiating from the region between the two sources are therefore the nodal lines (destructive interference), while the high-contrast zones between them are those of constructive interference. The same phenomenon can be observed in the virtual laboratory (see interference between two point sources) and on the page on interference.

If sound is a wave, is it possible to build acoustic "lenses" or "mirrors"?

Certainly! Typical examples of both devices can be seen in concert halls designed specifically for good acoustics. In these halls an optimal reverberation must be achieved, allowing the audience, even in very large halls, to enjoy unamplified music produced by instruments or voices without compromising its intelligibility. Mirrors and deflectors (which are nothing but smooth, rigid wooden panels of various shapes) are placed where the sound needs to be deflected or diffused so as to reach listeners in all positions as uniformly as possible. Sound-absorbing panels (those perforated panels of porous material that absorb sound instead of reflecting it) have the opposite effect. They are used to damp unwanted reverberation and resonances, for example in cinemas, where the power of the loudspeakers is considerable compared with the volume of the hall.

The Kammermusiksaal of the Berlin Philharmonie. The building was designed with particular attention to acoustics, as the official home of the Berliner Philharmoniker. Right above the area reserved for the orchestra you can see large, slightly curved panels resembling sails, which reflect the sound as uniformly as possible into the surrounding space. Note that the audience sits all around the orchestra without any substantial difference in how its sound is perceived. On the right, a detail of the sails.

Apart from curved acoustic mirrors (typically the set of panels placed behind an ensemble performing in a theatre), which, precisely because they are curved, act like lenses, acoustic lenses proper consist of layers of shaped materials in which sound changes its propagation speed, as happens in optical lenses. They are used especially in the sensors and signal generators of SONAR, to concentrate and amplify the signal. In ultrasound transducers too, used for example in medicine for ultrasound scans, matching layers (and a gel) are used to match the acoustic impedance of the probe to that of the human body, which is mostly water, while an acoustic lens on the probe focuses the beam.

Can sound be focused with a "zone plate", as is done with light?

Yes, but with important limitations. A zone plate (also called a Fresnel zone plate) does not use refraction, like the acoustic lenses described in the previous answer, but diffraction and interference.

• The plate is a screen made of concentric rings that are alternately open and closed. Each point of the open zones behaves as a secondary source. The contributions of all the open zones reach the focal point with similar phases and add up constructively. The closed rings block precisely the contributions that would reach the focus in phase opposition: without the closed rings these would partly cancel the others.
• Sizing the rings only requires a condition on the path lengths. If D is the shortest path from the source to the focus, the boundaries between one ring and the next lie where the path is D + λ/2, D + λ, D + 3λ/2 and so on, where λ is the wavelength to be focused. The open zones are those where the path exceeds D by an amount between 0 and λ/2, between λ and 3λ/2, between 2λ and 5λ/2 etc. (or, alternatively, all the others). In the simplest case, a plane wave falling on a flat screen with the focus at distance f, the radius of the n-th boundary is

${\displaystyle r_n=\sqrt{n\lambda f+\frac{n^2{\lambda }^2}{4}}}$

The same condition also holds for a nearby source or a curved screen, but then the calculations are more complicated.

There are two limitations.

• The plate works for one wavelength only. Since the focal distance is approximately ${\displaystyle f\approx r_1^2/\lambda }$, different frequencies are focused at different distances: doubling the frequency also doubles the focal distance. With a complex sound, such as music, each component is concentrated at a different point. There are also weaker secondary foci at distances f/3, f/5 etc.
• For sound the sizes are huge. At 1000 Hz the wavelength in air is about 34 cm: with the focus at 10 m the first boundary has a radius of about 1.85 m, the second 2.63 m, the third 3.23 m. Low frequencies would require structures the size of buildings. Moreover, the "focus" is not a point but a region at least as wide as the wavelength.

For this reason zone plates are used mainly where wavelengths are small, as for X-rays (for which ordinary lenses do not work) or ultrasound.

Does sound always propagate in a straight line?

• The answer is no, for all types of waves. The propagation of a wave depends on the characteristics of the medium in which it propagates; in particular, it is straighter the more homogeneous the medium is in the region of space where the wave propagates.
• In a homogeneous medium (that is, without obstacles and with properties that do not change from one point of space to another), sound waves, like light waves, propagate in straight lines; but in the presence of obstacles, or of changes in density or other properties of the medium, the wavefronts can bend. In the first case the phenomenon is called "diffraction", in the second "refraction".
• Light waves have wavelengths of the order of half a thousandth of a millimetre, so very small obstacles are needed to see these deviations from straight-line propagation (see the section diffraction). Audible sound waves, on the other hand, have wavelengths of the order of a metre, so their interaction with practically all everyday objects distorts the wavefronts. Clear evidence of this is the ability of sound to get around obstacles such as people, cars and sometimes buildings.
• See also the sections diffraction and diffraction of sound, refraction and refraction of sound.

You can also check these phenomena for yourself interactively in the virtual laboratory. Follow the instructions on the pages Virtual laboratory: experiments on refraction and Virtual laboratory: diffraction experiments.

Is it possible to deflect a sound with another sound?

Under normal conditions, no. At the levels we meet in everyday life sound propagates in the linear regime. The superposition principle therefore holds: two waves that cross do not collide like two billiard balls, but pass through each other. In the region where they overlap an interference pattern forms (zones of reinforcement and zones of weakening), but beyond that region each wave continues as if the other had never been there. So it is not possible, for example, to "push away" traffic noise with a beam of sound waves, audible or not.

Sound can instead be deflected by acting on obstacles or on the medium in which it propagates:

• by reflection, as noise barriers or acoustic mirrors do (see the question on acoustic lenses and mirrors);
• by diffraction, which makes sound "bend" around the edges of obstacles;
• by refraction: the speed of sound depends on air temperature and on wind. Their variations with height above the ground bend the sound "rays" (see also the question on sound mirages).

One sound can affect another only at extreme intensities, when the medium no longer behaves linearly (see non-linear effects). "Parametric" loudspeakers are based on this principle: they emit an intense and highly directional beam of ultrasound, inside which the non-linearities of the air generate an audible sound that stays concentrated in a narrow zone. In this case, however, an existing sound is not deflected: a new one is created.

Finally, a noise can be reduced by superposing an identical wave in phase opposition (see the question on active noise control). The method works well only in small spaces, as in headphones. In any case it does not move the noise elsewhere.

Do sound "mirages", like optical mirages, exist?

• Yes. Both kinds of mirage occur when waves (sound or light) propagate along curved paths in a place where we would expect them to propagate in straight lines, such as a large open space.
• In a broader sense, a mirage is an optical illusion, and here too the parallel holds, since there are acoustic illusions

• For a general description of the bending of rays, see the section on refraction, and experiment in the interactive laboratory following the instructions on the page mirages and other strange phenomena.
• For the specific case of sound mirages, and the conditions in which they can occur, see the section refraction of sound.
• To listen to various acoustic illusions see the pages acoustic effects and illusions, critical bands,

masking and beats.

Why does lightning create the sound of thunder?

As we know, lightning is an intense electrical discharge passing through the atmosphere. A very large potential difference produces a discharge that ionises the air it passes through, instantly turning it into an incandescent plasma at a temperature of tens of thousands of degrees Celsius. This produces a very rapid expansion, and consequently a shock wave that propagates into the surrounding air. The pressure level of this shock wave easily reaches 120 dB, and can therefore cause permanent damage to the ear. However, its energy is quickly dissipated, and at some distance the shock wave turns into a true sound wave, which is perceived as the sound of thunder.

Why is it that when lightning strikes nearby we hear a sharp sound but when it strikes far away, the sound is not just weaker but also longer and blurred?

1. As explained in the answer on the origin of thunder, thunder is initially a shock wave, not a sound wave. The difference is that in a shock wave the properties of the medium (such as the pressure) change abruptly (discontinuously) across the wavefront. The noise of explosions or a supersonic "bang" have similar characteristics. Close by, therefore, the ear is subjected to a strong stress in a very short time, and perceives the noise only at the instant the wavefront passes. At some distance, however, the energy of the front tends to disperse (see dispersion) and spread out in space. The wavefront becomes smoother and more continuous, and the energy reaches the ear over a longer time. This is why the corresponding noise sounds less "sharp".
2. In places with obstacles (as when a storm breaks out in a valley), the reflections (see reflection) of the wave from the obstacles also help to blur the sound of thunder, creating secondary waves that reach the ear later than the direct wave.
3. A final remark: distant thunder also sounds much "duller" (a kind of rumble). This means that it consists only of low-frequency waves which, along the way, are absorbed by the air much less than those of higher frequencies (on this point see power of a sound source)

What would someone hear while moving away from an orchestra faster than sound?

It can be calculated with the Doppler effect. If a listener moves away at speed u from a stationary source emitting at frequency f in still air, they hear the frequency

${\displaystyle f^{\prime }=f\frac{c-u}{c}}$

where c is the speed of sound.

• If u is less than c, all sounds are lower and the music slower.
• If u equals c, the listener travels along with the waves and hears nothing new.
• If u is greater than c, the listener catches up with waves emitted earlier and meets them in reverse order: they hear the music backwards.

Lord Rayleigh noted that at speed u = 2c the factor (c − u)/c equals −1: the listener would hear the piece at the same speed and the same pitch as the original, but played backwards. Of course the experiment is impossible: at supersonic speeds aerodynamic noise and shock waves (see the supersonic bang) would drown out any music.

And relativity? Relativistic corrections to the Doppler effect become important only at speeds close to that of light, hundreds of thousands of times greater than the speed of sound in air: for sound waves the classical formula is always adequate. Relativity does, however, have a curious musical consequence. Time runs more slowly for someone travelling at speeds close to that of light, so if two twin musicians began playing the same piece, one staying on Earth and the other leaving on a very fast journey, on returning the traveller would find themselves many bars behind. Not because of a fault in their instruments, but because less time has passed for them.