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08 APRIL 2017

written by Mike

FREQUENCY OF SOUND

 

 

What is the first thing that comes to your mind when you think about the waves?

How it travels?

What are the lowest pressure levels humans can hear?

How can we surf them?

None of the above and I’ve only tried surfing once so I can’t advise on that. I’m going to give away the answer; it’s the Hertz.

The Hertz, named after scientist Heinrich Rudolf Hertz indicates how often the particles vibrate when sound energy passes through a medium. Medium is air, water, steel and so on. Remember that vibrating particles don’t move, they pass the energy forward, sort of like an audience wave during a football match.

A hertz is a unit of vibration.

1 Hz = 1 vibration / 1 second

What you need to remember is that the particles will always vibrate at the same frequency. So, for example, you struck an awesome guitar note at 1000Hz. As the sound waves travel through the air particles will interact with each other (creating compressions and rarefactions) but the frequency stays the same, 1000Hz.

And every particle on the way will vibrate at 1000Hz. Energy from the source will have the same frequency when it gets into your ear.

Easy to remember. It stays the same.

Now, one vibration is called period. A period is either peak to peak or trough-to-trough. The period indicates one complete cycle of vibration.

So when the Moon travels around Earth, it completes its cycle. But 27 days (around that number) is a period of the Moon’s orbit. Next important thing to remember is a relationship between frequency, pitch and directionality.

High frequency will have a high pitch and will be more directional.

Low frequency will have a lower pitch and will be less directional.

So something like a bass in an EDM can be less than 200Hz with low, pumping “thump” and omnidirectional flow. It travels in every direction possible. That is why you can hear it through your neighbour’s wall the most. It is also because low-frequency waves are much longer that high-frequency waves. So in an instance of distant explosion, you are more likely to hear the low rumble rather that the full frequency blast.

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 From the acoustics point of view, it is quite hard to control low frequencies. The most common solution is installing so-called “bass traps” at the end of your room, often in the corners. Their job is to absorb and dampen the low-frequency nightmare.

On the other hand, you have high frequencies. A scream of a small child will be much more directional, shorter and easier to control. From the acoustics point of view of course.

High frequencies travel in short waves and installing few diffusers and absorbers will do a decent job of stopping them. High frequencies can also add ‘air’ or ‘breath’ into the mix but more than often a nasty sibilance will drive you mad.

We also have mid-range, which are middle frequencies. These are frequencies that our ear is most sensitive to. That’s why all the instruments and vocals will fight for a place in your mid-range. I will talk more about mid frequencies when we get to EQ overview. For now, just remember that clashing mid-range will “muddle” your mix. I know, it’s a super scientific term to describe it.

Ok, so when we talk about frequency we always say about the range. We tend to divide frequencies in low, mid and high range. It helps to know these guidelines when we get to EQ.

The human ear can detect a lot of frequencies. Our listening device is so sensitive that we can detect frequency difference of 2Hz. I’m talking here about people trained in music but most of us can still detect small frequency changes.

To generalise, our frequency range is around 20Hz to 20kHz.

That is in our prime age too. When we get older, we tend to detect less and less high frequencies so the range can fall to 17kHz or less. That’s why it is important always to take care of your hearing, take breaks and wear earplugs when necessary.

That not only applies when you work with live sound. Working as a re-recording mixer for twenty or thirty years will take a toll on your hearing too. Do you know how many times I had a conversation with a mixer that went something like that:

“What do you mean it’s saturating? I can’t hear anything there!”

So yeah, it can be quite interesting.

Frequencies below our range of hearing (20Hz) we call infrasound. By using the special device a scientist can detect geophysical changes and monitor activities of volcanoes, earthquakes or avalanches.

Frequencies above our hearing range (20kHz) we call ultrasound. You may recognise the name from pregnancy or other medical tests.

High frequencies can create an image of inside organs in our body or an image of a baby by using a sonogram. Sonars in submarines also use ultrasound to detect underwater things. They send off the signal that bounces off anything that interrupts its travels, just like bats do.

It’s important to note that animals don’t perceive sounds in the same ways as we do. Elephants can go as low as 5Hz, dogs detect sounds from 50Hz to 45kHz, and cats can reach around 85kHz. There are other animals that can go extremely high such as bats (120kHz) or dolphins (200kHz). In contrast, blue whales are known to use infrasound to communicate overlong distances underwater.

It must be quite handy for them as sound travels much faster in water too.

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Ok, let’s go back to differences between frequencies. As you know, it’s quite rare to hear a single frequency. Most sounds are made of low, medium and high frequencies and they are all different.

Some frequencies, when played together sound nice, other can be a cacophony. These relationships are a basis for the music system and music intervals.

Nice sounding frequency interference is called consonant, horrible ones we call dissonant.

Let’s have a look at the ratios and frequency relationships in music intervals.

Octave – 2:1 – 512Hz/256Hz

Third – 5:4 – 320Hz/256Hz

Fourth – 4:3 – 342Hz/256Hz

Fifth – 3:2 – 384Hz/256Hz

So as you can see two sound waves played at the same time can create a pleasant sound.

Not just intervals, but chords, solos and music scales are all built on frequency relationships. You don’t need to know precise frequencies to play the piano, but the knowledge becomes handy when you want to record and mix it.

Awesome tool that lets you to input your pitch data and show you the exact frequency of that note.

Pitch to frequency calculator

Before we finish, let’s quickly look at the other characteristic of a sound wave – its power. Decibel or dB is a unit used to measure the intensity of a sound (sound pressure level – SPL).

0dB is a near silence, the least audible sound.

A 3dB increase will double the level of the signal; a 3dB decrease will reduce the level by one-half.

We can describe the levels of signal with a power of ten.

10dB is 10 times as powerful as 0dB.

20dB is 100 times more powerful than 0dB

30dB is 1000 times more powerful than 0dB.

A whisper will be around 15dB, but normal chitchat around 60dB. The jet engine we can describe with around 120dB, a gunshot less 140dB.

All of we measure with SPL, sound pressure level. SPL is an acoustic pressure built up within a defined atmospheric area. So moving away from the sound and doubling the distance will reduce the power of a signal by 6dB. Moving closer to the source and halving the distance will increase the power by 6dB.

I will go more into the depth of sound pressure and waveform characteristics in the future articles.  Just like with the rest of the topics, basics are important; you can easily find a deeper analysis of them on the Internet.

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