Acoustics Write up

The physics of sound

Sound is an energy that travels regularly through a channel. When sound is created the ripples it makes in the air are likewise to the waves created when a stone hits water, so we use the term sound wave to describe the movement of sound. Sound travels away from the source that created it with equal energy in every direction. When sound energy moves in every direction it’s called Omnidirectional. A sound wave is a visual image of what is really an outwardly pulsing sound bubble.

Sound waves spread through the air. Air is a mix of different gases: oxygen and nitrogen, which is made up of atoms and carbon dioxide, which is made of molecules. When a sound source creates a disorder of these particles it then creates a chain reaction, in which the atoms and molecules hit and bounce off each other by vibrations and oscillations. When you hit a garden fork on concrete it vibrates and this then creates frequent moments of high and low pressure. These moments are called compressions and rarefactions of the air molecules, and will produce a frequency.

One compression with one rarefaction is known as a cycle. The amount of completed compressions and rarefactions that happen in a second is called frequency; these are measured in Hertz (Hz). The perfect human hearing range from lowest to highest pitch i.e. our frequency spectrum is 20Hz to 20,000Hz. This however does fade over time. Musical notes are set to certain frequencies. An A note is 440Hz, when an A note is played an octave higher the frequency is doubled so it is 880Hz.

You may have noticed that when a police car goes past you when you are on the street the siren is a first high pitched when it is close to you, but then it goes lower in pitch as it gets further away. This effect, where there is a change in frequency and wavelength, is known as the Doppler Effect. When a source goes near the observer, the observed wavelength decreases and the frequency increases. When a source moves away from an observer the observed wavelength increases and the frequency decreases.

Waveforms are a visual representation of sound waves and are curves, showing the shape of the wave at any given time.

Sound travels at different speeds through different materials:

Air- 344m/s

Petroleum- 1290m/s

Speed through human body- 1530m/s

Iron- 5850m/s

Aluminium- 6260m/s

The speed of sound is measured in meters per second

To work out the speed of sound you have to use the formula- Velocity (344m/s) divided by frequency and wavelength.

Envelope- ADSR attack, decay, sustain and release. The envelope of an instrument is its trait relating to time and force; it’s often called its shape. We can see the envelope of instruments by looking at their waveforms. For example if the amplitude of the waveform is bigger at the start of the wavelength then the source of the sound has a strong attack and if the crests of the waves are close together then they have a high frequency and if they are far apart then they have a low frequency.

Amplitude- The amplitude of a wave is the distance from the centre line to the top of the crest.

Decibels- The decibel is used to measure sound level. The unit used when referring to decibels is dB. The human ear is very sensitive; they can hear everything from your fingertip brushing lightly against your skin to a jet engineIn terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest perceptible sound. On the decibel scale the smallest perceptible sound is 0dB, a sound ten times more powerful is 10dB.

The Cocktail Party effect- We’ve had that moment when you’re at a party and you’re talking to someone and you completely ignore your surroundings. Then you hear your name in another area of the party and you immediately divert your attention to that area. This is what scientists call the “Cocktail party effect”. The ability to tune in and out of the sounds is a result of the way our hearing works. We are able to focus on a conversation while still unknowingly looking around our surroundings. Then when we hear something important i.e. our name behind us then we immediately hear it.

Incorporating sound to logic

Phase- Whenever one or two waveforms are in a single location, out of phase, their relative signal levels will be added as one to create a joint amplitude level at that one point in time. Whenever two waveforms that have the same shape, peak and amplitude are completely in phase, which means they don’t have any relative time difference, the newly made waveform will have the same shape, frequency and phase, but will have two times as much amplitude. If the same two waves are combined out of phase, then they will end up cancelling each other out when they are added. This evidently creates a single straight line of zero amplitude, if the second wave is only a little out of phase then the levels will be added at the point where the joint amplitudes are positive, and reduce in level where the joint amplitudes are negative.

Harmonics- The ear understands frequencies that have “ratios that are whole multiples of the fundamentals as being specially related”. Since these frequencies are even multiples of the fundamental, they are known as even harmonics and frequencies that are odd multiples of the fundamental are call odd harmonics.

Even harmonics are seen to give a sound that it pleasing to the ear where as odd harmonics gives a more discordant and harsh sound

Beats- Two tones that differentiate only a little in frequency and have around about the same amplitude will produce and effect known as beats. This effect sounds like rhythmic volume surges that are the same as the frequency to differ between these two tones. The incident is often used as an aid for tuning instruments because as the beats slow down, the two notes draw near the same pitch and finally stop when the pitches match. On the whole beats happen because the ear is unable to distinguish closely pitched notes. This then creates a third frequency created from the phase sum and difference values between the two notes.

EQ (Equalisation)-An equaliser is a device or software that lets us control the relative amplitude or the different frequencies in the perceptible bandwidth. EQ allows you to correct specific problems in a recorded sound, like to make a sound go back to its original tone. It can also allow you overcome deficiencies in the frequency response of a mic or in the sound of an instrument. As well as this it can allow you to blend together contrasting sounds for many instruments within a track or mix and fix a sound purely for musical or creative reasons.

There are two main types of EQ that are used, these are Shelving EQ and Bell (or Peak) EQ.

Shelving EQ refers to a increase or drop in a frequency response at a certain frequency which tapers off to a current level and continues at that level until the

end of the audio spectrum.

Bell EQ is one of the more common EQs it is created by a peaking filter and as its name implies a peak shaped bell curve can either be boosted or attenuated

around a selected centre frequency.

A semi parametric EQ is a bell EQ without the quality side of things. Therefore the bandwidth of the bell curve will remain stable. A parametric EQ has the ability to widen or narrow the selected bandwidth.