Audio Noise-Hiss,Hum,Rumble & Crackle

This is a 4 series article where we will look at the various aspects of audio noise, its causes and ways to remove them. In this article we will focus on what is noise and the different kinds of noise.

In acoustics, noise is usually referred to as an objectionable sound that is unwanted. In more technical terms, a sound that is added unintentionally to a recording is audio noise.
Sound waves are formed by a chain of analog sine waves. The characteristics of sound depend upon the combination and blending of these sine waves. This determines whether the sound will be pleasant or unpleasant in listening.

While performing an audio recording, audio noise can often be found on analog tapes as well as digital recordings. In most cases, a layer of hiss can be found on every recording of an audio cassette. In modern recordings, the main source of noise is due to faulty recording equipment, which introduces low hum and broadband hiss. The environment of the recording also plays major part in how clean the sound is. A totally soundproofed room/studio will have almost no noise, while those recorded near fans, traffic etc. will have maximum noise.

There are 4 major kinds of noises, hiss, rumble, crackle and hum.

Hum: Hum is a continuous low frequency sound, similar to the whirring of a low pitched motor. It has a typical frequency of 40-80 hz. Hum is generally caused by electrical interferences or when the recording equipment is not properly grounded. Cheap audio interfaces and microphones also introduce hum in the recordings

Hiss: In electronic devices, hiss is often caused when random electrons deviate from their intended path under the influence of heat. These deviated electrons manipulate the output signal voltage and thus an audio noise is created. Low quality recording equipment generally results in hiss. Hiss can also be due to environmental factors, like a/c, fans etc.
In magnetic tapes, noise often results because of the granular composition of the medium. The larger is the size of the magnetic particles, the more is the possibility of audio noise to appear in the medium.

Rumble: Another common audio noise. It is a low-frequency audio noise which results due to a random sound wave that exists between specific limitation points. A rumble is most noticeable in turntables where ball bearings create the noise. In order to avoid or minimize the rumble, only high quality turntables with slide bearings should be used. In some instruments, rumble filters are used to remove the noise. This low-frequency audio noise can create problems while playing records on audio systems and hence needs to be corrected.

Crackle: crackles are discontinuous and non-musical sounds. These irregular explosive sounds are very much similar to the sound that we often here during wood burns. Such type of sound distortion is usually caused by explosive aperture of small airways.. Crackles can be further divided into two categories considering their acoustic properties and they are fine and coarse. Fine crackles are generally highly pitched and less intense audio noises while coarse crackles are of low pitch and last for longer durations.

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In next article we will discuss how to record without noise. Till then happy recording!

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