Properties of sound

THE PROPERTIES OF SOUND

Remember calculus at school? How you said “I’m never gonna need this”

THIS SOUNDS LIKE SCIENCE! WHY DO WE NEED TO KNOW THIS?

BECAUSE TO BE A ROUNDED COMPOSER/PERFORMER/ MUSICIAN IN THE CURRENT ENVIRONMENT, YOU NEED TO UNDERSTAND THE TOOLS YOU WILL SPENDING MOST OF YOUR TIME USING:

RECORDING STUDIOS MIXING CONSOLES MICROPHONES INSTRUMENTS (AND YES THIS INCLUDES VOICE) PA SYSTEMS EQUALISERS AND SPATIAL EFFECTS PROCESSORS

Why do we record sound?

So that an event can be re-created again.

Sound is...

“Vibrations transmitted through an elastic solid or a liquid or gas, with frequencies in the approximate range of 20 to 20,000 hertz, capable of being detected by human organs of hearing.”

“Sound” is a word that describes the brain’s perception and interpretation of a physical stimulus that arrives at the ears...”

Huber, D.M. (2001). Modern Recording Techniques. 5th Ed. Focal Press. p23

Sound Waves

Sound travels through the air like ripples on the surface of water when a pebble is dropped into it.

As a sound is made (e.g. a drum is struck, someone speaks, etc), the displacement of air molecules create what is called a sound wave.

Sound Waves

Sound arrives at the ear in the form of periodic variations in atmospheric pressure called :Sound Pressure Waves. (Modern Recording Techniques, p33).

Sound Waves

are divided into:

Compression - air molecules are forced together to form a wave.

Rarefaction - the wave then dips down, creating negative air pressure, reducing the density of the wave.

The wave continues it’s cycle of compression and rarefaction as it travels.

http://en.wikipedia.org/wiki/FrequencySinusoidal waves of various frequencies.

Periodic Waves in shallow water

http://en.wikipedia.org/wiki/File:Periodic_waves_in_shallow_water.png

Sound MovementSoundwaves moving objects.www.acoustics.salford.ac.uk

Instruments Vibratingwww.acoustics.salford.ac.uk

Sound Wave Characteristics

www.youtube.com/watch?v=REqxyVIT45M&feature=related%00

Hearing

These sound waves eventually enter the ear canal and apply pressure to the Tympanic Membrane - better known as the “skin” on the ear drum.

This membrane registers the specific pressure applied to itself and in turn converts the information into electrical impulses.

These impulses travel to the brain and are interpreted as perceived sound.

Hearing

Vibration! Audio soundwave!Audio soundwave travels

into the ear!

Audio soundwave converted !to electrical signal!

Brain recognises the electrical signal as “sound”!

Look after your hearing

Waveform Characteristics

A graphic representation of sound’s volume and pitch.

These characteristics allow one waveform to be distinguished from another.Huber, D.M. (2001). Modern Recording Techniques. 5th Ed. Focal Press. p25

Waveform CharacteristicsAmplitudeFrequencyVelocity

WavelengthPhase

Harmonic contentEnvelope

The most fundamental of these are amplitude (volume) and frequency (pitch). Huber, D.M. (2001). Modern Recording Techniques. 5th Ed. Focal Press. p25

Waveform Characteristics

Amplitude = Volume/Loudness

Amplitude is the amount of energy carried in a soundwave.

The greater the energy carried, the greater the amplitude, therefore the greater the volume.

The amplitude is the distance above or below the centre of a waveform.

The Decibel = Sound Pressure Level (SPL)

The decibel (abbreviated dB) is the unit used to measure the intensity (SPL) of a sound.

Loudness doubles with each increase of 10dB in intensity.

The higher the SPL level, the louder the sound.

Near total silence = 0 dB

A whisper = 15 dB

Normal conversation = 60 dB

A lawnmower = 90 dB

A car horn = 110 dB

A rock concert or a jet engine = 120 dB

A gunshot or firecracker = 140 dB

Any sound above 85 dB can cause hearing loss, and the loss is related both to the power of the sound as well as the length of exposure.

You know that you are listening to an 85 dB sound if you have to raise your voice to be heard by somebody else.

Eight hours of 90 dB sound can cause damage to your ears; any exposure to 140 dB sound causes immediate damage (and causes actual pain).

Look after your hearing

Practice mixing and recording at levels no louder than between 85 - 95 dB. (VU meter demonstration)

Take ‘quiet’ breaks every hour or so to rest and recalibrate your ears.

Have your hearing regularly checked by an audiologist (free at Wintec Health Centre).

Frequency = PitchWe measure the quality of sound waves in frequency.

Frequency is the number of cycles that occur within a second.

Frequency is measured in Hertz.

1 Hertz = one complete cycle.

440Hz is the A below middle C on a piano.

A sound wave’s frequency determines its pitch; low frequency = low pitchhigher frequency = higher pitch

Double the frequency is 1 octave higher.

Hearing & Hertz

The human ear can only register sounds in the range of 20 Hertz to 20,000 Hertz.

As we get older we tend to lose the ability to hear the higher frequencies.

The frequencies that determine the audibility in the human voice are in the 1000 - 4000 Hertz range. The human ear is the most sensitive in those frequencies.

Hearing & Frequency Response

www.youtube.com/watch?v=a2cGQDCSanw

Velocity - The Speed of Sound

Sound travels through the air at approximately 344 meters per second.

This equates to 1,236 kilometers per hour (768 mph), or about one kilometer in three seconds.

Wavelength

The wavelength of a waveform is the physical distance in medium between the beginning and the end of a cycle.

The physical length of a wave can be calculated using:

Wavelength = Velocity / Frequency.

Wavelength cont.

The time it takes to complete 1 cycle is called the period of a wave.

Eg. a 30 Hertz sound wave completes 30 cycles per second.

Why design a sound room with walls and the ceiling at different angles?

Reflection of Sound

The way that sound reflects off surfaces is important to consider when recording.

Never have a square room for a recording studio, otherwise sound will reflect off walls and cause phase problems.

A “Dead” room with no reflection is best for recording. (But not too dead)

When recording, don’t sit close to any glass surfaces. Sound will reflect off the surface and confuse your ears. (There are exceptions to this rule. E.g. When recording drums)

Sound reflects off a wall at an angle that is equal to its initial angle of incidence.

This is one of the cornerstones of the complex study of acoustics.

Huber, D.M. (2001). Modern Recording Techniques. 6th Ed. Focal Press. p39

Diffraction of Sound

Uniquely, sound can reconstruct itself after it bends around an object.

This allows us to hear around corners, through walls etc.

Huber, D.M. (2001). Modern Recording Techniques. 6th Ed. Focal Press. p40

Phase

Phase describes where in its cycle a waveform is at any given time.

Phase cancellation occurs when two signals of the same frequency are out of phase with each other resulting in a reduction in the overall level of the combined signal.

If two identical signals are 100% or 180 degrees out of phase, they will completely cancel one another if combined.

Sound waves which are exactly in phase add together to produce a stronger wave (and greater volume).

Sound waves which are exactly inverted, or 180 degrees out of phase, cancel each other out and produce silence. This is how many noise-cancellation devices work.

Sound waves which have varying phase relationships produce differing sound effects.

Phase

Why phase matters...

When we use more than one microphone to record a sound, the sound will arrive at each source at different times.

Thus phase problems may occur, certain frequencies may be cancelled out or amplified creating an undesirable sound.

Phase problems may arise due to reflected sound as well.

Microphone Technique 3 to 1 Rule

If the first mic is 20cms from a source, the second mic should be placed 60cms from the second mic.

Using the 3:1 rule will minimise phase problems created by the time delay between sound reaching the mics.

Timbre A trumpet sounds

different to a violin!

!

A clarinet sounds different to an flute. A clarinet sounds different to a flute

Harmonic Content

Sine waves are composed of a single frequency that produces a pure sound at a specific pitch. (The Fundamental).

Fortunately, musical instruments rarely produce pure sine waves. If they did, all instruments would sound the same.

The factor that helps us differentiate between the tone or sound of different instruments is the presence of HARMONICS.

Harmonics are additional frequencies (called ‘partials’) that exist with the fundamental pitch being played.

Each instrument has a different ‘harmonic series’ which gives the instrument it’s particular ‘sound’.

http://en.wikipedia.org/wiki/File:Moodswingerscale.svg

Fundamental pitch1st harmonic2nd harmonic

3rd harmonic

4th harmonic

5th harmonic

6th harmonic

Harmonics of 440 Hertz

440 Hertz = Fundamental pitch

880 Hertz = 1st harmonic

1320 Hertz = 2nd harmonic

1760 Hertz = 3rd harmonic

2200 Hertz = 4th harmonic

Sound Envelope

Harmonics are not the only characteristic which help us ‘hear’ the difference between instruments.

Each instrument has a different ‘sound envelope’ which also differentiates it’s sound from other instruments.

Sound Envelope

Sound Envelope - ADSR

Attack refers to the time taken for sound to build up

Sustain refers to the increases and decreases in volume

Decay refers to the reduction in level

Release quickly the sound fades away

Waveform Envelopes

(a) trombone

(b) snare drum

(c) Cymbal crash

How awesome was that?