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Sound

Beginning questions:

What is sound?
How do we hear sounds?
Can we hear all kinds of sounds?

Have you ever thought, how important sound is to people? What would life be without any sounds? Sound profits us in numberless of cases. It’s also the fastest and easiest way to communicate.

The origin of sound

Sound gets its origin in a sound source that pulsates and creates tactful and known fast air pressure variations and sends them to the environment. We hear those pressure variations as sound. The air pressure variations are often called longitudinal wave motion. Good examples of sound sources are loudspeakers, vocal cords, musical instruments’ strings and air columns etc.

Sound travels

Sound needs an intermediate agent to advance. So in a vacuum that has no materials, for example space, there’re no sounds. In the earth there’s air that sound can use by advancing. Air consists of chemical elements like oxygen (21%), nitrogen (78%) and some air pollutions (1%). Every material has mass because of the force of gravity. Also air has mass (1,3 kg per 1000 litres) and that’s why it stays in the atmosphere. Air transports sounds from sound sources to our ears. If sound waves hit a firm obstruction, they don’t penetrate, instead they bounce back. That event is called echo.

Sound velocity

Because sound is longitudinal wave motion, it has all the qualities of wave motion like wavelength, frequency and velocity.

Sound hasn’t a known velocity. Sound velocity depends on the material that sound uses by advancing. Rule of thumb is the thinner the material the lower sound velocity. Sound velocity depends also a bit on the temperature of the material. In air (+20ºc) sound travels 340 m/s, in water 1 485 m/s and in steel 5 100 m/s.
Sound intensity

Sound intensity is usually informed as decibels. The bigger motions sound source makes the louder sound it creates. If sound intensity is too loud, hearing can be damaged. If you hear repeatedly sound that is over 80 dB or once over 120 dB, your hearing gets weaker. Also some divers that don’t use needed diving suits will destroy their hearing, because the deeper in water you go the bigger is the water pressure that tries to squeeze everything that has smaller pressure inside than water’s.

Low and high sounds

If a sound source pulsates for instance a hundred cycles per second, it creates low sound that contains bass, but if it pulsates thousands of cycles per second, it creates high and treble containing sound. If a sound source pulsates 500 whole cycles in one second, its frequency is 500 Hz (hertz). The lowest sound that we can hear is 16 Hz and the highest is 20 000 Hz. Sounds under 16 Hz are called infrasound and sounds between 20 000 Hz and 1 000 millions Hz (1 000 MHz) are called ultrasound. Some animals can create and observe sounds between 16 and 150 000 Hz.

A sound that includes all frequencies sounds like static and it’s called white sound. White sound is created, when for instance a radio tuner is left between two stations. It’s called white sound, because the light that contains every wavelength is called white light, for example sunlight (sun radiates all colours of light and we see that combination as white light).

Resonance

Resonance means that a sound source causes the sound receiver oscillate in the same frequency with it. Resonance is possible only, if the source and the receiver can pulsate in the same frequency. You can notice resonance in practise: If you turn on your stereo, make the volume hard and bring a piece of normal paper close to the woofer, you can see paper pulsating in step with the bass sounds.

Resonance is very important to us. It’s the way that we hear sounds. Our eardrums have adjusted to pulsate in a large frequency response (16-20 000 Hz). The strongest resonance of a human’s eardrum is between 500 and 5 000 Hz. You can notice that in practise: Mobile phones’ ringing melodies that have high notes sound like louder than low-noted melodies, because the highest notes have over 500 Hz.
Same hertz amount, different sound

Every musical instrument can create notes that have known hertz amounts each. For instance every instrument surely has normal C-note that has always 262 Hz. So why does that note sound different in separate instruments? That’s founded in the same way as we recognize each other’s voices. Every sound source sounds like itself because of the weak-powered upper tones of sound waves. Every sound that we hear is a combination of the key-tone and upper tones.

Sound recording and reproduction

When an electrical sound source was invented, it was possible to put radio technology into effect. Electrical sound sources and receivers are called oscillators. The way to record sound was discovered in 1888 by the American engineer Oberlin Smith and the first recording gadget was prepared in 1928 by German F.W.O Bauch. Since that time sound recorders and reproduction gadgets have begun to develop.

Sound can be recorded in analogical or in digital format. In practise those formats don’t have many differences but in analogical format the quality of sound degenerates with time and digital format stays always the same. Nowadays the digital format is the most used recording format. For instance CDs, MP3 and Minidisks exploit the digital sound technology and C-cassettes exploit analogical.

Loudspeakers are the only way to listen to recorded sound and they are planned to repeat sounds that we can hear. Every loudspeaker that repeats strong bass and still very clear sound has more than one speaker. There’re at least two speakers: a treble speaker and a woofer. Some loudspeakers have also more than one woofer and some have a middle sound speaker (middle sound area is either within the woofer or in a separate speaker). Different speakers are planned to repeat different frequencies. For instance treble speakers’ metallic elements repeat high frequencies, woofers’ rubber bass elements repeat low frequencies and middle sound speakers’ elements that are usually made of polymer cardboard repeat a bit of both frequency classes. Every polyphonic speaker has a frequency filter. The filter assorts high sounds for treble speaker and low sounds for the woofer. That filter is very important, because without it the sound wouldn’t be fine and the woofer would suffer because of trying to repeat too high frequencies.

Stereos are quite common nowadays. The secret of the sound that reminds real sound is in two speakers that have own sounds both. The sounds sound the same, but they still have some differences. Recording stereo sound resumes two microphones that should be placed in separate sides of the recording object and in a stereo set the speakers should be in the same order than the microphones in the recording area. Nowadays there’re also Dolby Digital™ and Dolby Digital EX™ sets that have a woofer five and or six Surround speakers (the amount depends on system) and every speaker has its own sound. Recording that kind of sound resumes five or six microphones that have to be in the very same order that the loudspeakers in the theatre. The woofer whose sound is a combination of others is near to the object, three Surround speakers are in of it, and the rest two or three are in the back corners of the object.

There you had some knowledge about sound.

Sources:
Aine ja energia – Fysiikan tietokirja
Sony Style magazine 03 kevät/kesä 2002, Sony Car Hifi 2002-2003
The main part of the text is writer’s own output.

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