Nature of Sound

• Sound is a longitudinal wave motion consisting of a train of Compression and rarefaction travelling in a medium. When these waves strike the eardrum these are converted into signals which are carried to the brain by the auditory nerves and are finally interpreted into what we call Sound.

SOUND WAVESOUND WAVE

MOLECULES MOLECULES

1.2. Soundwaves

Source of

Sound

Region of High Pressure (Compression) and Region of low pressure (Rarefaction)

Pressure and intensity of Sound waves

• Sound waves produce variation of pressure in the medium in the form of compressions and rarefactions in quick successions. Sound pressure variation is therefore represented by newton per square metre (N/m2), or pascal (Pa). In terms of micro-bar (dyne per Sq cm) one Pa is equal to ten micro-bars.

Sound & Sensitivity of human-ear:

• Human ear is very sensitive to sound intensity & can detect sound as low as 0.1 pW/m2 (or 10 dB below the threshold of hearing).

• The ear cannot distinguish difference of intensity of less than 1 dB between two sounds.

• Minimum level which can be comfortably detected over threshold of hearing is 3 db for speech or music.

• The ear possesses characteristics of masking that is the louder sound reaching

the ear can suppress the weaker sound.

• The ear judges’ direction of sound from the first received even if it is weaker.

Electrical Representation

COMPONENTS OF SOUND IN AN ENCLOSED SPACE

- Direct Sound- Reflected Sound

Loudness & Phon

• Loudness is defined as the intensity of sound as judged by the ear. It needs higher intensity at low frequencies than at high frequencies to impart same sensation of loudness.

• The intensity of 60 dB at 40 Hz and of 0 dB at 1000 Hz imparts the same loudness.

Phon :The intensity in dB with reference to Threshold of hearing as perceived by the ear at 1000 Hz is called phon (P). If it is 0 dB then loudness is 0 phon, if it is 40 dB then loudness is 40 phon.

Sone: It is found that a 10 dB increase in sound level corresponds approximately to a perceived doubling of loudness. One sone is defined as the loudness expressed by a person listening to a 1000 Hz tone of 40 phon loudness level. Similarly 50 phons would have a loudness of 2 sones, 60 phons would be 4 sones, etc. The relation between sone (L) and Phon (P) is given by-

10logL= (P - 40) log2

FrequencyFletcher-Munson Curves for loudness vs. Frequency

Frequency range for Speech

Audible frequencies range from 16Hz to 20000 Hz. For satisfactory transmission of speech two factors are very important.

• Intelligibility- It is defined as the clearness of one’s speech determined through the test of articulation.

• Energy-The energy in speech is contained mostly in the low frequencies.

Overtones and Timbre• Sound waves produced by Speech and

musical instruments are not pure sine waves, but are complex waves consisting not only the fundamental frequencies (tones) but also of their harmonics, and other frequencies, called ‘overtones’. The proportion of tones & overtones present in the sound that helps us to identify any particular voice is called Timbre.

• Intervals: It is defined as the ratio of two frequencies.

Example-Interval of 400Hz &100Hz is 4.

• Octaves: An interval of 1:2is called an OCTAVE.

Example: One octave of 200 Hz is400 Hz

• Harmonics: It is an integer ratio between two frequencies.

Example: 2nd harmonicof 100Hz is 200Hz

Pitch It generally represents the perceived

fundamental frequency of a sound. Sound waves with a longer wavelength don't arrive at the ear, as often (frequently) as the shorter waves. The shorter the wavelength, the higher the frequency, and the higher the pitch, of the sound. In other words, short waves sound high; long waves sound low.

Acoustic Reverberation• The term acoustics has been derived from

the Greek word akoustos, meaning “hearing.” It is the area of science devoted to the study of the production, transmission, reception, and effect of sound.

• In the field of broadcasting the broadcast studios are the originating place of the program to be broadcast live, or recorded for future use. Hence proper care should be taken in their designing and construction.

Reverberation• The persistence of sound, caused due to

repeated reflection is called reverberation.

• In an studio the sound is received directly from the source as well as sound reflected from walls, floor, ceiling, etc. The sound persists for a noticeable time even after the original sound stops. It fades away gradually.

MicrophoneMicrophone is a transducer.It converts sound wave

(acoustical energy) into electrical energy

Five important characteristics of microphone

• Operating principle

• Frequency response

• Directionality of microphone

• Electrical output and

• Physical design of the microphone

Microphones

• Acoustical classification

• Electrical classification

• Polar pattern-wise classification

Pressure-operated Microphones

• Sound pressure is applied on one side of the diaphragm.

• Electrical output is proportional to the sound pressure.

• Theoretically omnidirectional.• However,the physical size of

microphone works as an obstacle at high frequency.

• Sensitivity at HF decreases. Operating Principle

Pressure-operated Microphones-cont’d

• Examples-Moving Coil, Carbon, Crystal and Condenser Microphones

Typical Polar Pattern

Pressure Gradient(velocity)Operated Microphones

• Both sides of the diaphragm are exposed to the sound pressure.

• Hence, the electrical output is proportional to the difference in pressure on the two sides.

Operating Principle

Combined-operation Microphones

• The principle of pressure-operated microphones and pressure gradient operated microphones are combined to get maximum sensitivity in one direction and minimum sensitivity in the opposite direction.

• Unidirectional characteristics.

An Acoustic Labyrinth

Electrodynamic Microphone with Moving Coil

• A dense coil of wire moves within a strong electromagnetic field.

• Induced current is proportional to displacement velocity.

• Great mechanical robustness.

.

Operating Principle

Electrodynamic Microphone with Moving Coil - cont’d

• The dense coil of wire makes it less sensitive especially to very soft sounds or high pitch sounds (sibilants or harmonics).

• Used in live performance where rough handling is common.

• Examples- AKG D-202, D-222, D-900, D-770, D-190E, SM58, SM57, SM48 etc.

AKG D-770Instrument/Vocal

Microphone

Electrodynamic Ribbon Microphone

• The coil of moving coil microphone is replaced by a corrugated thin strip of aluminium.

• Very sensitive to shock and large sound volumes.

• Very delicate.• Unusable in windy

condition. Operating Principle

Electrodynamic Ribbon Microphone- cont’d

• Very low impedance and hence uses in-built transformer.

• Inherently bidirectional.

Electrostatic(Condenser) Microphones(single diaphragm)

• A thick metallic plate as a diaphragm is fixed to a perforated metal back plate.

• Electrical charge is applied to either or both plates.

• Sounds pressure changes the distance between two plates and causes variation in the capacitance.

• Requires in-built pre-amp Operating Principle

Electrostatic(Condenser)Microphones(single diaphragm)-cont’d

• High sensitivity and very good frequency response.

• Smaller and lighter diaphragm gives more accurate and pin-sharp sound.

• Requires external power supply popularly known as “Phantom Power Supply” - 9v to 52v.

• Very sensitive to humidity and temperature.

• Less sensitive to shock.

• High cost.

Electrostatic(Condenser)Microphones(single diaphragm)-cont’d• Tends to record sound as it

really is.• Large diaphragm gives flattering

response.• Resonant frequency at the upper

end of audio spectrum.• Examples- C2000B, C3000B,

SM86, SM94, SM81 etc.

SHURE

SM94

Instrument Mic.

Electret Microphone

• It is a modified form of condenser microphone.

• It does not use external power supply.

• Either diaphragm or back-plate is permanently charged.

• However, pre-amp requires power supply.

Electret

Electret Microphone- cont’d

• The output is independent of the diaphragm surface area.

• Light and small in size.

• Excellent quality/price ratio.

• However, not suitable for bass.

Omnidirectional Microphones

• Gain remains constant in all directions-It is unity.

• However, true Omni is possible at low frequency only.

• Moving coil and Condenser microphones are designed to work as omnidirectional microphones.

Omnidirectional

Omnidirectional Microphones-cont’d

• Pickup angle-360 degree.• No proximity effect.• Used for recording ambient

and background sound and also for vocals.

• Examples- C4000B, C414B, C577WR, D230

• C4000B has dual large diaphragm.

Bi-directional Microphones

• Figure of 8 pattern.• Gain is proportional to cosine

of angle of incident of sound.• Pickup angle-90 degree(120

degree at -6dB)• Limited use.• Example- Ribbon Microphone,

Condenser Microphone

Cardioid Microphones

• Gain= 1+cosine of angle of incident of sound.

• Heart -shaped.• Pickup angle(Coverage angle) -

130 degree.(180 degree at -6dB)

• Null angle - 180 degree.• Degenerates towards omni

(Hypocardioid) at low frequency.

Cardioid

Cardioid Microphones- cont’d

• More directional than required at high frequency.

• Favoured for stage use as it prevents feedback.

• Examples- AKG D770, C3000B, C2000B etc.

Hypercardioid Microphones• Gain= 0.5+cosine of angle of incident

of sound.• Cardioid +Bidirectional• Pickup angle - 105 degree.(140

degree at -6dB)• Null angle - 110 degree.• Narrower pick-up than cardioid,

hence feedback rejection improves even further.

• The least overall pickup of ambient sound.

• Examples- C1000S, D660S

hypercardioid

Supercardioid Microphones

• Pickup angle - 115 degree.• Null angle - 125 degree.• Maximum ratio of on-axis

pickup to ambient pickup.• Useful for more distant

pickup or in higher ambient noise level.

• Examples- D880/D880S, Beta58A, Beta87A etc.

Unidirectional Microphones• The term is used for ‘Gun’

microphone.• Long and rod shaped.• Good for recording single

voice in noisy locations.• Also used for picking up

voice from long distance.• Shotgun(18 in. long), Long

gun(36 in. long).• Long gun pick-up-20 to 25

feet. • Example- D900.

s

GUN MICROPHONE

Electrodynamic Microphone for recording of voice in hand held condition

Uses • Recording of interview of players and sports authority. • Less sensitive so suitable for noisy situation • Mechanically strong and robust Examples- AKG D-202, D-222, D-900, D-770, D-190E, SM58, SM57, SM48

SHURESM58VocalMicrophone

AKG D190ESpeech/InstrumentMicrophone

Condenser Microphone

Uses• Tends to record sound as

it really is.

• Broad Frequency spectrum.

• Uses to record when sound source is not near

• Examples- C2000B, C3000B, SM86, SM94, SM81 etc

SHURESM81

Lip Microphone

• A close talking microphone.• Designed to ensure constant

spacing between the microphone body and the lips of the user.

• Also known as noise canceling microphone

• Similar to ribbon microphone. • Used for radio reporting and

sports commentary. Lip Microphone

Gun Microphones• Highly unidirectional • Long and rod shaped.• Good for recording single

voice in noisy locations.• Good for recording sound

effect from a far distance. • Also used for picking up

voice from long distance.• Shotgun(18 in. long), Long

gun(36 in. long).• Long gun pick-up-20 to 25

feet. • Example- D900.

GUN MICROPHONE

Lapel Microphones • It is usually electret

microphone.• Based on Lavalier Technology.• Omni directional polar pattern.• Very small and light- weight.• Offers reasonable intelligibility

in fairly noisy surrounding.• High frequency loss when

hidden behind clothing.

Lapel Microphone

Lapel Microphones-cont’d

• Also called as neck microphone.

• May be corded or cordless.

• Worn at the chest or clipped to clothing.

• It can be powered by batteries or phantom power.

• Suitable for running commentary or a lecture.

• Can also be used for wind instrument.

Parabolic Microphone• A cardioid microphone is

placed at the focal point of a parabolic reflector.

• The parabolic reflector is made of sheet metal or stratified polyester or glass fibre.

• Low frequency pickup is proportional to the diameter of the reflector.

• Used for recording faint sounds such as birdsong.

Operating principle

Boundary Microphone• A small capsule microphone

usually an electret, is housed in a flat receptacle.

• The flat receptacle works as a plane reflective surface.

• The directivity is hemispherical at all frequencies.

• More dynamic range and clarity.Omnidirectional.

• Also known as PZM.

Pressure Zone Microphone

Contact Microphone

• Size is small. • Attached with the

sound source itself. • Pickup vibration

pulsing through solid.

• Attach to a point so that it should not come in the view of camera.

• HF response is good but LF response is bad.

Wireless Microphones• These are ordinary

microphones with an FM transmitter.

• Provides complete freedom of movement.

• Lapel type or handheld.• Omnidirectional.• Interference from outside

source.• Suitable for stage

performance.• Suitable for places where

laying of microphone cable is not possible

SHURE MICROPHONE

Lapel Mic

Transmitter

Receiver

Handheld Mic.

Placement of Microphones• Placement of microphones depends on the acoustic

nature of the sound source and the acoustic characteristics of the microphones.

• Always use minimum number of microphones.• Microphone should be placed with its 0 axis facing

the source of sound to avoid off axis colouration.• When two or more microphones are used, it should

be ensured that their outputs are in phase.• The Microphone should not be too close or too far

to the sound source.

Placement of Microphones-cont’d• When the Microphone is close to the sound source,

direct sound is predominant and hence the output appears to be ‘dry’. When it is far away from the sound source, the reverberant (indirect) sound is predominant and hence the output lacks in clarity.

• Microphone should not be placed very close to a reflecting surface like bare walls, hard tables etc.

• Directional microphones should not be placed too close to the source of sound to avoid boosting of low frequencies due to a phenomenon called proximity effect.

Placement of Microphones-cont’d This effect should be normally avoided by placing

the microphones fairly away (30-45 cm) from the source of sound.

• For spoken word recordings, microphones should not be placed directly in line with the mouth since it will result in ‘p’ blasting.

• A talker should not hold the script between his face and the microphone otherwise shadowing effect will occur at high frequencies.

Placement of Microphones-cont’d• For stringed instruments (violin,sitar,sarangi,etc.) 0

axis of the microphone should be preferably placed normal to the front face of the instrument.

• For instrument with large sound output (like drums and other percussion and bass instrument) the microphone should be placed well away from the source of sound.

• For wood wind instrument where the instrument is not particularly direction (such as flute) the microphone may be placed about 60 cm.away .

Placement of Microphones-cont’d• In outdoor locations because of the higher ambient

noise level, the working distance must be kept less than the corresponding distance when working indoors.

• When using multiple microphones, to reduce acoustic interference between the microphones THREE-to-ONE rule should be followed, which states that the microphone-to- microphone distance should be at least three times the source-to- microphone distance.

Placement of Microphones• Microphone

should be placed with its 0 axis facing the source of sound to avoid off axis effect.

• The Microphone should not be too close or too far to the sound source.