AUDIO BASICSFREQUENCY BY: ANDRÉS F. REGALADO BUCHELI

“Hi I’m Andrés F. Regalado

Bücheli from a small town inColombia called Pasto. Thislesson is for week 1 ofIntroduction To MusicProduction at Coursera.org.I will teaching about a veryimportant property of soundwhich is FREQUENCY andi hope you enjoy this lessoneven english is not my nativelanguage.

FREQUENCY

(SOME PHYSICAL

DEFINITIONS)

In this lesson we will learn a little bit more about frequencyfrom the point of view of audio engineering and this is thesubject that concern to us, but by first we have to review somedefinitions which are related with physics of sound.

Wavelenght (𝝀): as you can see in the picture, is the lenghtbetween two points of a wave with the same characteristics (inthis case we have two peaks.

FREQUENCY

(SOME PHYSICAL

DEFINITIONS)

Amplitude (𝑨): as you can see in the picture, is the

lenght between the maximum or the minimun of a

wave and the central point (average of the wave)

Period (𝑻): as you can see in the picture, is the elapsed time

between two equivalent points of a wave.

Frequency (𝛎): is the number of oscillations per unit time (is

the number of times a wave repeats.

FREQUENCY

(SOME USEFUL EQUATIONS IN

AUDIO ENGINEERING

I)

IMPORTANCE OF PHYSICS IN AUDIO ENGINEERING:Knowledge of the physics of sound in the audio devices design(amplifiers, sound effects for electric guitars, etc) is veryimportant and in this section we will see some useful equationsrelated with frequency.

FREQUENCY

(SOME USEFUL EQUATIONS IN

AUDIO ENGINEERING

II)

Wavelenght (𝝀)

Period (𝑻)

Frequency (𝛎)

Angular frequency (𝝎)

𝝂 =𝟏

𝝀

𝒇 =𝟏

𝑻

𝝎 =𝟐𝝅

𝑻= 𝟐𝝅𝒇

FREQUENCY

(SOME USEFUL EQUATIONS IN

AUDIO ENGINEERING

III)

When we are talking about sound waves, wavelength is related to the frequency by the speed of sound in air.

For example at room temperature ~ 𝟐𝟎𝟎𝑪

Speed of sound in the air c = 343 m/s

Density of air 𝝆𝟎 = 𝟏, 𝟐𝟏 𝑲𝒈/𝒎𝟑

Relationship between wavelenght and frequency of sound in air:

𝝀 =𝒄

𝒇

FREQUENCY

(HOUSE TO EQUATIONS IN

AUDIO ENGINEERING )

We can use the above equations in some useful calculations in audio engineering:

For example if we need to calculate the wavelenght at a frequency of 20 KHz we can use:

𝝀 =𝒄

𝒇

𝝀 =𝟑𝟒𝟑𝒎/𝒔

𝟐𝟎𝟎𝟎𝟎 𝑯𝒛= 𝟎. 𝟎𝟏𝟕𝟏𝟓𝒎 = 𝟏. 𝟕 𝒄𝒎

FREQUENCY

(SOME USEFUL TIPS ABOUT

FREQUENCY IN AUDIO

ENGINEERING I)

Audible range of the human ear:𝟐𝟎 𝑯𝒛 ≤ 𝒇 ≤ 𝟐𝟎 𝑲𝑯𝒛

As we calculate in the previous slide, at a frequency of 20 KHz, which is the same of 20000Hz, a sound wave have a lenght of 1,7 cm, and that means that at high frequencies, thelenght of a wave is very short.

• If we use the same equation, we can calculate the lenght of a wave at 20 Hz and it is 17meters, and that means that at low frequencies, waves are very long.

The previous tips are related with the human hability to localice sound.

The average distance between human ears is approximately 22-23 cm.

The wavelenght that fits between the ears correspond to a frequency of 1500 Hz.

FREQUENCY

(SOME USEFUL TIPS ABOUT

FREQUENCY IN AUDIO

ENGINEERING II)

Humans determine directionality of sound by two basicmethods:

ITD (Interaural time difference): For f < 1500 Hz. (Time thattakes to time to get one ear and then the other ear).

IID (Interaural intensive difference): For f > 1500 Hz. (The brainlocalice a sound source because the head and the skin absorbsound pressure and a sound is perceived higher intensity in oneof the ears)

• Using this two properties of human hearing, engineers cancreate audio illusions as audio 3-D

FREQUENCY

(CABINETS AND LOCATION OF

LOUDSPEAKERS)

Why are the transducers different sizes inspeaker design?

The small transducer is for high frequencysound radiation and the larger driver is forlow frequency sound radiation.

FREQUENCY

(GUITAR EFFECTS)

Some guitar effects as Wah Wah work as an electronictreatment of frequency, using a high pass filter with a variablecutoff frequency to produce its typical sound.

FREQUENCY

(ACKNOWLEDGEMENTS AND REFERENCES)

This work is based on some definitions consulted in Wikipediaand in the online coursera course “Fundamentals of audio andmusic engineering: Part 1 Musical Sound & Electronics” offeredby Rochester University.