Wednesday, 12 March 2014

Week Two - Engage!

The second week of our audio engineering studies started as good as the last week ended. We continued speaking about hearing damages on Monday. The Incredible Tinnitus made his return once again, accompanied by his evil cousin Sudden Deafness (Hörsturz, Ohrinfarkt or Managerohr).

But a big new topic arose: oscillations, which can be described as "maximum displacement from equilibrium repeating itself in certain time intervals", which sounds awfully complicated at first, but believe me when I say it is not. Inherent to this chapter in audio basics are the parameters to describe oscillations, of which there are 5:
a) main oscillation type (sine, saw, triangle or square - hybrids are possible but not relevant as of now)
b) frequency
c) amplitude
d) cycle duration
e) polarity (phasing/"Phasenlage"). 

Lo and behold! The same edit area but from a different angle.

The next big topic we addressed and discussed were the different kinds of crosstalk between two frequencies. We started with something easy, superposition of two identical waves (same frequencies, amplitudes and polarity/phasing). The result is quite unspectacular: the consequent wave is the same but louder by 6 dB. This is the maximum in amplitude a wave can gain by doubling it with an identical wave. So, take two waves with 100Hz & 10dB each for example and what you get is a wave with 100Hz & 16dB. Logarithmic scales are the secret when talking about absolute loudness. If we're talking about perceived loudness a gain of +10dB is needed to double the loudness of a wave. You'd have to nearly triple a wave to get the desired result. I want to point out once again, that this is for the PERCEIVED loudness, not the absolute or physical loudness.
The next kind of crosstalk was as easy as the first one: wave cancellation. This happens if you have two identical waves but the polarity of one wave is reversed by 180°. As the name suggests both waves cancel each other out. The wave has ceased to be. It has gone to meet its maker. It is an EX-wave. Nothing will remain of both waves if the polarity of one wave shifts by 180°. 
 The second last kind of crosstalk was "polarity shift" (all shifts, but 180° or 360°!) which results in some cases in massive changes in the sound. It would be too much to explain it all. As cool as a blog about sound/acoustic physics would be, it's not what I set out to write.
The last crosstalk is called frequency modulation or beat ("Schwebung"). This happens when you have two waves with different frequencies, i.e. 100Hz + 105Hz. What you get is a constant change of constructive and destructive polarity, meaning some frquencies are boosted some are cut and this changes permanently as time goes on. What you will hear is a kind of wibbly-wobbly that accelerates if the difference in frequencies increases. If anyone of you is playing any kind of instrument with a drone (i.e. bagpipes, hurdy-gury, ...) you know this sound. If you don't, switch on your drones and try tuning them for Cthulhu's sake!

After school I did the next aural training unit. And you can guess what it was about. Did I hear "frequencies" in the last row? Right. Frequencies again. Pinpointing one out of 10 possible frequencies and whether it is boosted or cut is something I need to practise some more. But as they say: no one masters anything without hard work. And that's what I'll do: work hard so my not-to-shabby hearing will become better.

This is where aural training takes place. A comfy couch.

Tuesday started out pretty relaxed. The first topic was "different auditory events". Since I don't know the correct English terms for the six events I'll just give you the German ones:

1. Ton
A pure sine sound with just one frequency. A "Ton" doesn't occur in nature at all.

2. Tongemisch
Sound comprised of two or more "Töne", but then again: pure sine. Nothing else. For example: 100Hz + 147Hz, 100Hz + 231Hz + 387Hz, ... . You get the idea.

3. Klang
"Klang" is a special type of Tongemisch and has a harmonic spectrum. Its ingredients are: 1st harmonic (or fundamental) and several other harmonics (or overtones), which are multiples of the 1st harmonic/fundamental, i.e. 100Hz (1st harmonic/fundamental) + 200Hz (2nd harmonic) + 300Hz (3rd harmonic) + 400Hz (4th harmonic) + 500Hz (5th harmonic). Instruments always produce "Klänge".

4. Klanggemisch
A Klanggemisch is, as you can possibly imagine, produces by mixing two or more Klänge. They don't have to result in a chord. Soeven the combination of C + C# + D is a Klanggemisch.

5. Geräusch
This is a special kind of Tongemisch with a continous spectrum, i.e. you have a sound with 500Hz as the lowest frequency and 12kHz as the highest frequency. A Geräusch contains ALL frequencies in between 500Hz and 12kHz, usually with varying amplitude.

6. Rauschen (or "noise")
Again a special type of Tongemisch with a continous spectrum, but with ALL audible frequencies (20Hz-20kHz). There are different kinds of noise, but the most important ones right now are "white noise" (all frequencies have the same amplitude) and "pink noise" (every octave/doubled frequency is cut by 3dB).
Following this block our teacher Michael introduced us to "waves", what waves are and how we could calculate the wavelength. Just to sum it all up: waves describe the propagation of oscillations in an elastic medium. The speed of sound is quite vital for this and we've learned that the density of a material, the temperature and even humidity and CO2 content have more or less influence on the speed of sound, which is important if you're working at a big open-air festival.

Lastly we started learning about sound propagation in air. To start with there are two types of sound fields:
 - free sound field and
 - diffuse sound field.

Let's take a quick look at the free sound field. In a free sound field there are no obstacles for acoustics noises whatsoever. Sound can propagate freely. As with the "Ton" it doesn't occur in nature, because there are always some obstacles. The closest you can get in nature is a snow-covered area. If you've ever been to the mountains and stood on such a snow-covered area, you have surely noticed how hollow and strange everything sounds.

Next one. The diffuse sound field is what you usually encounter in your everyday life. As I wrote above sound always encounters various obstacles in its path, i.e. people, lamps, desks, chairs, trees and so on. Because of those obstacles, the material they are made of, their size and various other factors certain interesting things happen. To be precise those things are: reflection, absorption, diffraction, rebounding and interference (Reflexion, Absorption, Brechung, Beugung und Interferenz). But I will talk about those in my next entry.

Well, that's all for today. Cheers!
Yer poking fun at us audio engineers, laddie? =)

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