Domande tecnologia en
From "Physics, Waves, Music": a website about the physics of waves and sound, acoustics of musical instruments, musical scales, harmony and music.
| AI-translated from Italian. Original page: Domande_tecnologia |
I know that two pure sounds with opposite phases cancel each other out; but is it really possible to cancel a sound by adding one with the opposite phase to it?
It is possible, and the mechanism is the basis of so-called active isolation. It is used both for sound insulation and for isolating machines or structures from vibrations.
In both cases a microphone picks up the signal and drives a suitable actuator, which generates the same signal but with inverted phase. By the superposition principle, the algebraic sum of the two signals must be zero. In practice it is fairly simple to apply the technique within a small space, such as a pair of headphones, greatly improving their insulating properties. As for vibrations in structures, the most frequent case is the need to isolate particular frequency bands that are especially annoying, or harmful to the structure itself.
These pages do not analyse active isolation, but you will find more information on passive isolation on the pages
See also, in the page on interference, the paragraph where does the energy go? about the superposition of two pure sounds in antiphase.
What are a "tweeter" and a "woofer" and what are they used for?
While an electrical circuit can carry signals at all acoustic frequencies (20-20000 Hz), and well beyond, the same cannot be said of the physical structures that must turn the electrical signal coming from the amplifier into vibrations of the air, that is, into sound waves. The response times of a mechanical oscillator are determined by its mass and elasticity, and are generally much longer than those of an electrical oscillator. So oscillators of different sizes are not equally efficient at all frequencies; in particular, large-diameter cones are suited to radiating low frequencies, while smaller cones are needed for high frequencies. These considerations become particularly critical in systems that must reproduce sound with high fidelity (hi-fi), which is why loudspeakers often consist not of a single driver but of several, each assigned a frequency band. The tweeter is the smallest driver, for frequencies from 2 kHz to 20 kHz (and sometimes beyond), while the woofer is the largest, covering the band between about 40 Hz and about 1 kHz. There is often also a further cone for the intermediate frequencies, and one for those below 40 Hz (subwoofer).
|
Here is an example of a sound that covers the whole audible range while keeping a constant amplitude. If you listen to it through headphones, then through your PC speakers, and finally through a hi-fi system, you will notice that the sounds are quite different. They are reproduced with different degrees of fidelity in the three situations, solely because of the different frequency response of the loudspeakers. For example, without a suitable woofer the lowest frequencies are never transmitted. Listening carefully, you will also notice that the sound seems to change in loudness with frequency. This is the combined effect of the different response of the loudspeakers at different frequencies and of the different response of the ear at different frequencies. |
See also the pages on
Why is it that when I hear my recorded voice it sounds so different from my real voice?
If the recording was made, and is played back, in high fidelity, your voice sounds different to you because it reaches your ears in a different way. While we speak, the vibrations produced by our voice reach the eardrum not only from outside the ear, but also directly through the bones and cavities of the body. Every medium the sound passes through acts to some extent as a filter on the original sound, since it transmits its various frequency components differently (we say that the impedance of a medium depends on the frequency of the sound passing through it). Consequently, the original voice has components of rather different amplitudes depending on the channel it travels through from the source to the receiver. When the two are the same person, this effect is particularly evident.
A further source of distortion may lie in the transmission channel itself. If the recording is not played back in high fidelity, the playback equipment can also introduce filtering and distortion. This is particularly evident in narrow-band channels such as AM radio and, even more, the telephone. It is indeed harder to recognise a person's voice on the telephone and, even when we can recognise it, it sounds distorted and nasal.
See also
Can a weak sound, such as a heartbeat, be amplified with a cone or a megaphone?
Not in the sense of increasing its energy. A horn or a megaphone is not an amplifier: it adds no energy, but improves the matching between the source and the air, so that a larger part of the energy of the vibration is radiated. It also concentrates the sound in one direction. If the source is weak, the sound remains weak.
In the case of the heartbeat there are two further difficulties.
- The source is weak and low-pitched. Heart sounds have frequencies mostly below 200 Hz, where the ear is not very sensitive (see perception of loudness).
- A horn radiates well only above a cut-off frequency, which depends on its size: the mouth must be comparable to the wavelength. At 50 Hz the wavelength in air is almost 7 m: a mouth about 2 m in diameter would be needed. Moreover, efficient horns are not simple cones: their profile widens more and more rapidly towards the mouth ("exponential" or "tractrix" profiles, or the bell of brass instruments). Simple cones were used in old megaphones.
The stethoscope works because it does not radiate the sound into the room: a closed tube carries the vibration directly into the listener's ear canal. To let a whole room hear the heartbeat a real amplifier is needed: a microphone (or an electronic stethoscope), an amplifier and a loudspeaker suited to low frequencies (see the question on tweeters and woofers).
