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How Sound Works – Amplitude, Perceived Amplitude & Decibels

Volume, or amplitude, is the loudness of a sound. 

Amplitude is measured in decibels (dB), which we’ll learn about further below. The larger the amplitude is, the louder the sound will be. The smaller the amplitude, the quieter the sound. A jet taking off will have a decibel level of 140-160dB, while a normal conversation is 60dB in amplitude.

This is how this looks in a waveform:

amplitude

The human ear can cope with loud sounds for short periods of time only. It is recommended that we only listen to sounds of 80dB for the maximum of an hour before we’ll start to damage our ears. This is about the volume of loud traffic. Any seasoned sound engineer will tell you that your ears are your greatest tool so protect them! It’s ok to listen to loud music for a short period of time, but make sure you give your ears a break.

give your ears a break

When you’re having a party, your neighbours are going to hear mostly the thump thump and low hum of your super awesome subs, but not any of the high frequencies of the song. This is because these frequencies are much more powerful and able to travel through your walls. The lower the frequency the more powerful it is. Sound is reflected off surface, which creates echoes and reverb, but high frequencies reflect better, and low frequencies are more likely to absorb and travel through the wall.

reflection of frequencies

Now to make things even trickier, we don’t actually perceive the volume of different frequencies equally. Our ears are made to hear frequencies that give clarity to a person’s voice the best, which is between 2kHz and 6kHz no matter what volume it’s at. So low or very high frequencies must be much louder for us to perceive them as the same volume as a mid frequency. 

In the below chart, you will see the volume of the sound in dB SPL on the left and the frequency in Hz running along the bottom. The red lines indicate how loud the frequency must actually be for us to perceive it equal to the reference frequency.

perceived amplitude of frequencies

Quite often in a balanced sounding mix, the low frequencies will in fact be a lot louder in dB than everything else.

So if you’re trying to mix a band pretty loud but there are noise restrictions and the venue is trying to catch you out by testing the volume of your mix, turn the low frequencies down a little. It might just get you out of trouble!

Now lets talk about decibels.

Decibels are a level of reference, meaning that they aren’t a fixed measurement – a dB is the comparison between one signal level and another. We first must work out what 0dB is, then the dB level is just relative to that.

The type of dB we’ve been talking about above is dB SPL. This describes sound pressure levels, like the air pressure of the sound on our eardrums. This is a comparison where 0dB is where we start to hear. And using this reference level, we can work out what is a safe listening level. 

dB SPL levels

However, there are also 4 other main types of dB levels that refer to the dB ratios when it comes to electrical signals: dBm, dBu, dBV and dBW. These are all ratios of dB levels that compare the signal to electrical current, resistance, voltage or impedance and different equipment will use a different ratio. Just know that this is a comparison between the level and a reference point. You can’t know for sure that 10dB on one mixer is using the same reference point as 10dB on another mixer.

As aptly said by the Yamaha Sound Reinforcement Handbook, 

“The key concept is that “dB”, in itself, has no absolute value… “The console’s maximum output level is +20dB.” That statement is meaningless because the zero reference for “dB” is not specified. It’s like telling a stranger “I can only do 20,” without providing a clue as to what the 20 describes.”

Instead we need to work out what the power capability of the console is and use that as a reference. If you need to convert between these different dB levels there are some basic formulas you can follow along with the dB guidelines in your equipment manual.

Another thing to consider is that faders on a mixing desk are often logarithmic as opposed to linear. This means that as you turn up a fader, or rotate a knob with a logarithmic taper, it does not turn up at an even rate. 

In the below graph, the red line represents the change in volume of a linear volume knob, while the blue line represents that of a logarithmic volume knob. This means that you are less likely to notice the difference in volume between a fader that is 1/3 of the way up and 1/2 of the way up, while the difference between the fader being 1/2 the way up and 3/4 of the way up will be much louder.

Get to know the gear you use all the time, and don’t presume that the levels on another desk will sound the same, or the faders will react at the same ratio.

logarithmic fader

2 comments for “How Sound Works – Amplitude, Perceived Amplitude & Decibels

  1. April 10, 2019 at 11:03 am

    This is truly useful, thanks.

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