Unit 1 Acoustics Article 2 PDF

Daisy Grace Wakefield 14 April 2015

The Ears

There are three main parts to the human ear;

Outer ear

Middle ear

Inner ear

The outer ear consists of the Pinna, Ear Canal and the Eardrum.

The Middle ear is made up of the Eardrum and Ossicles[1], the

three smallest bones in the human body called the Malleus, Incus

and Stapes[2]. Lastly, the Inner ear is made up of The auditory

nerve to the brain[1]. Sound waves enter the ear canal causing BTEC Music Technology 1

Human Hearing How do the human ears work and how to characterisJcs for a

recording studio or venue.

Figure 2.1


the eardrum to pulsate. Because the eardrum is moving, this

causes a chain reacJon to move the three bones known as the

ossicles. The malleus is moved by the eardrum, the incus is

moved by the malleus and the stapes moved by the incus. The

stapes hits the membrane of the cochlea causing

the uid inside to move [1]. The cochlea consists

of three chambers. The stapes causes pressure

waves to travel through the cochlea. Inside the

cochlea there are an esJmated 20 to 30

thousand bres. As vibraJons travel down this

chamber, they meet with bres that have the

correct resonant frequency and energy is

released[2]. When these bres resonate they

cause the hair cells located next to them to

move which sends electrical impulses to the

cochlea nerve and up into the brain. The pitch of a sound wave

can cause the bres to resonate in specic locaJons and the

louder the sound then more hairs will move. Our brain will sort

through all this informaJon allowing us to hear[2]. Sound is 22

Jmes greater when the sound reaches the brain than when it rst

entered the ear[1].

Psychoacoustics

PsychoacousJcs is the scienJc study of sound percepJon[3].

Subjects that are associated with psychoacousJcs include;

Sound LocalisaJon

Cocktail Party Eect

Pitch PercepJon

Auditory Maskins

The Doppler eect

BTEC Music Technology 2

Figure 2.3


Sound engineers use psychoacousJcs to help them create

more realisJc sound space experiences for music, movies, and

concerts[9].

Sound Localisation

Sound localisaJon is to do with guring out where sounds

originate form. This could be in front of you, behind you, to the

side of you and in any other direcJon possible, even right

above you. Humans are able to locate noise almost precisely to

where it is coming from[7]. The brain is able to do this as it can

interpret informaJon coming from both ears. Imagine that there

is a ring around your head just like the planet Saturn and there is

a speaker closer to your le\ ear than your right that is playing

music. Your brain is able to pick up that the sound is ge^ng to

your le\ ear quicker than it is ge^ng to your right ear[7]. The

brain is then able to compare the dierences in Jme to work out

where exactly the sound is coming from[7].

There are two cues that the brain looks for to help determine the

sounds locaJon. The rst is amplitude. The dierence in

amplitude between the two ears can help determine which side

the sound is closer to and Jme dierence.

The Jme taken for the sound to reach one ear

compared with the other is another way the

brain can detect where the sound is coming

from.

The Cocktail Party Effect

The cocktail party eect is a phenomenon or

occurrence where in a noisy room, at a party or

at a sporJng event with lots of people, you are able to focus on

one parJcular voice and ignore all the others and these blend into


Figure 2.2

Figure 2.4


background noise [10]. The human brain can only completely

focus on one thing at a Jme so it gives its aènJon to one person

and dismisses all other voices[10]. In other words, we can tune

in and tune out of dierent conversaJons. This phenomena

works beèr as a binaural eect, meaning it works beèr with

both ears funcJoning and is related to sound localisaJon. People

with only one funcJoning ear have found it dicult to stay tuned

in to one voice [4].

Auditory Masking

Auditory masking is when one signal is

masked or covered by another which

has a greater amplitude, is louder, or has

a greater frequency range. There are two

types of auditory masking;

Temporal Masking

Simultaneous masking

Temporal masking is when there are two sounds but they are not

occurring at the exact same Jme. It is best to demonstrate this by

using an example. A bass drum is hit, and then there is a clap

straight a\er. The clap may be masked because the bass drum

has a larger amplitude or is louder than a clap. This example is

called forward masking as the sound that is being masked by the

louder amplitude happens around 200 milliseconds a\er the

sound with the louder amplitude is being played. Backwards

masking is when the sound with the smaller amplitude happens

20 milliseconds before the sound with the higher amplitude is

played, in other words, the clap happens 20 milliseconds before

the drum is hit [5][6].

Simultaneous masking is when two sounds occur at exactly the

same Jme but the masking signal covers the original signal by

increasing in amplitude. An example would be talking to someone

in a room where there is music playing in the background and


Figure 2.5


then someone decides to turn the volume up. Your voice, the

original signal, is being masked by the masking signal, the music.

Sound engineers can be aected by this as certain sound can be

masked or hidden by others which can aect the overall mix[5].

The Doppler effect

This occurrence does not actually happen inside of the brain but

is linked to sound localisaJon. This is to do with moving sounds.

As sounds get closer to us, their amplitude increases, so it gets

louder, and if the sound moves

further away, the amplitude

decreases, gets quieter[7]. As

the sound moves closer the

wavelength decreases and the

frequency decreases, and as

the sound moves away the

frequency decreases and the

wavelength increases[8]. An

example would be a police car

driving past you with its sirens

on. When it is far away from

you, the siren is quite quiet but

as it gets closer and drives right past you, its amplitude increases.

Due to sound localisaJon, we are able to tell where this siren is

coming from and move out of the way allowing the car to drive

past, but the amplitude of the siren increasing and decreasing

happens outside of the brain unlike the cocktail party eect

where the brain focuses on one thing at a Jme and blocks

everything else out[7].


Figure 2.6


DESIGNING A

RECORDING STUDIO Standing Waves

A standing wave is when there is a wave that is travelling in one

direcJon and bounces o of something such as a wall and then

travels back to its source[13]. Standing waves are associated with

boundary condiJons [15]. Standing waves within a room are also

called room modes [19]. There are parts of this wave that are not

moving and these are called nodes. There are the parts of the

wave where the wave going forward and the wave coming

backwards meet. There is also something called anJ-node, which

is where the peak or trough moves up and down[13].

Standing waves can be separated into two categories;

Ridged boundaries

Open boundary condiJons or free boundary condiJons

Ridged boundary condiJons are associated with, for example,

stringed instruments where the medium, the string, is

constrained so that it cannot move and nothing can aect it other

than in-between the boundaries. Open boundary condiJons

refers to standing waves that are not pinned down and so are free

to move about, for example in a recording studio or music venue

such as the O2 arena [14].

Standing waves will occur for all frequencies whose wave lengths

are mulJples of the room dimensions [16]. These can occur

between the two parallel walls and with the oor and ceiling. We

can calculate which frequencies would create standing waves

within a room with a simple formula [16];


Figure 2.7


1130 divided by 2L

What this means is that 1130 feet relates to the speed of sound in

that it travels 1130 feet per second, and the 2L is the room

dimensions Jmes by 2. For example, if the distance between two

parallel walls is 13 feet, the frequency wavelength is double this

number, 26\.

1130 divided by 13 X 2 = 173.846154.

This is approximately 174 Hz. If we then double this number,

348Hz, the second harmonic, this frequency will also create a

standing wave, including the third harmonic, 522Hz and the

fourth, 696Hz and so on [17]. Standing waves are much worse

when the dimensions of the room are mulJples of each other like

8 X 12 X 16. Also the worse shape to have a recording studio or a

venue would be a cube as this is perfect for standing waves to

occur [17].

When two or more waves meet and are in phase with each other

at a specic frequency, you will have a peak in response. When

they meet and are out of phase with each other, they cancel and

you end up with a dip or null in response [19]. Standing waves

can make the sound being generated weaker or stronger as these

waves are able to cancel each other out and double up [19].

Recording Studio

There are golden raJos that can help you to create a room that

will minimise the eects of standing waves. When building a

recording studio from scratch, standing waves have to be taken

into account before building starts. Any way you can edit the

room to avoid the creaJon of standing waves is helpful. As

standing waves occur between parallel walls, when the

dimensions are mulJples of each other and when the room is

cube shaped, it might be an idea to have one of the walls at a

slight angle so that there is less chance for the standing waves to

occur. Also, once the studio is built, you could add in some curved


Figure 2.8


wooden panels on the walls as this will change the dimensions of

certain parts of the room and help to get rid of standing waves

[20].

When building the room, you need an outer wall and a inner wall.

The outer wall would be, if we looked at a house, the side that is

outside or exposed to the elements where as the inner wall is the

one facing inside the house. But in-between the outer and inner

wall there will be an array of dierent materials used to

soundproof the room [20].

Materials can include;

Dry wall

Wooden frame/studs

Pockets of air

Sealant

Fibre glass

Other foam insulaJon

Once the outer walls are built, with the following dimensions for

example; height at 11 feet, width at

18 feet and length at 16 feet, you can

start to add in the insulaJon. Firstly

dry wall[18] is added the outside wall

then another layer is added but with a

small air gap between the two for

more sound proong. A\er, a wooden

frame or stud is placed upon the walls

and bre glass or other foam

insulaJon is placed in-between the

wooden pockets. Next you a`ach

sound clips, metal clips, to the

wooden frame and add hat channels,

long rectangular sheets of metal to


Length = 16 FeetWidth = 18 FeetHeight = 11 Feet

Having one wall at an angle can help to get rid of standing waves and this will aect the dimensions and therefore which frequencies will occur in the space.

Figure 2.9

Figure 2.10

Figure 2.11


the clips then another layer of dry wall is added. Adding

another wooden frame with more breglass and another layer

of drywall will help lock in the dierent frequencies.

This process can also be repeated on the ceilings and the oor but

with ooring, adding an extra layer of dry wall will help insulaJon.

Adding a carpet and then a hard wooden oor is be`er for sound

insulaJon. With ceilings,

adding another layer of dry

wall is preferable but having

a gap of air will help with

soundproong [21][22][23].

Inside the completed studio,

materials such as fabric

curtains and chairs can be

added to help absorb sound

[12].


Figure 2.12

Figure 2.13

Figure 2.14

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