Octave band.pptx

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1. Octave band The audio frequency range is divided into 10 standard octave bands with center frequencies 31.5, 63, 125,250,500, 1000,2000,4000,8000 and 16000 Hz Each center frequency is double the preceding center frequency and each bandwidth doubles the preceding one

2. The upper and lower limits of frequency of each band Because Bandwidth for an octave is

3. One-third octave band One-third octave bands are formed by subdivided each octave band into three parts The successive centre frequencies increase in intervals by cube root of 2 Upper and lower frequencies are related to center frequencies as Also Bandwidth is

4. Decibel additions , subtraction and averaging Let L p1 , L p2 , L p3 , … L pm are the sound pressure levels We have Total sound pressure

5. subtraction If total sound pressure level L pt and the sound pressure level of back ground noise or any other device is L pB , the sound pressure level of source or another device is determined using

6. Averaging In certain cases SPL at any location is measured several times and average SPL has to be determined. Average SPL

7. Find the total sound pressure level due to 96 dB, 87 dB and 90 dB Measurements indicated L pt =93dB at a specific location with a lathe in operation. When the lathe is shut down the background noise measures at 85 dB. What is the sound level due to the lathe? Determine the average sound pressure level L p at a location for a series of measurements taken at different times: 96,88,94,102 and 90 dB.

8. How sound is heard Sound wave enters the outer ear and cause the ear drum to vibrate These vibrations pass along the middle ear via three bones known as ossicles Ossicles amplify the vibrations and transmit them to the cochlea of inner ear Hair cells in the cochlea moves in response to the vibrations and send signals to the brain through auditory nerves, which helps to understand the sound

9. The human ear does not possess uniform sensitivity across the audio frequency range The frequency response of the ear is not constant, but changes slowly with ageing Rapid reductions in sensitivity may occur if we are exposed to damaging noise levels

10. Masking Speech which is perfectly audible in quiet surroundings may become unintelligible as the ambient noise level increases. Masking depends not only on the relative pressure levels of two sounds, but also on their frequency content. A pure tone is more effectively masked by a second pure tone of similar frequency than by a pure tone of quite different frequency. Low-frequency noises mask high-frequency noises more than high-frequency noises mask low frequencies.

11. Threshold of hearing and hearing area T he threshold of audibility of a normally hearing person for pure tones Sensitivity of hearing is maximum between 3000 and 4000 Hz The sound pressure levels which will give painful sensation The region between the threshold of hearing and the threshold of pain is the ‘auditory area Auditory area contains the frequencies and pressure levels of all pure tones which our hearing can receive without causing any harm to our ear

12. Audiometry Audiometry is the measurement of hearing sensitivity. The instrument used is called an audiometer. The subject wears headphones and listens to a series of pure tones at pressure levels which can be adjusted over a wide range. He is asked to indicate the threshold of perception for each tone used, and the test is repeated separately for each ear. Audiometry is done in a sound proof room in order to avoid masking

13. Audiometer result

14. Loudness Loudness is subjective The loudness of an auditory stimulus is a psychological, not physical attribute of the stimulus L oudness is the listener's subjective description of the strength or volume of the stimulus It is the perceived volume of sound and perception varies from person to person It is the subjective perception of changes in amplitude of sound Unit of loudness is sone

15. It is the perceived magnitude of sound as estimated by listeners having normal hearing based on the acoustic properties of the sound and its manner of presentation to the listener. Loudness depends primarily on the sound pressure level , it also depends on the frequency, waveform, bandwidth and duration of sound One sone is the loudness of sound whose loudness level is 40 phon A sound that is twice as loud as another sound has double the number of sone

16. Loudness is a perceptional concept and not a physical concept People are not equally sensitive to sounds of all frequencies so perceived loudness of a tone in fact depends on frequency as well as intensity Two sounds can have the same physical sound pressure levels but if they are of different frequencies, they are often perceived as having different loudness.

17. Loudness level Sensitivity of human hearing is frequency dependent Response of the ear depends on frequency as well as pressure amplitude Equal loudness contours(Fetcher-Munson curves) Subject is presented with a 1kHz tone, which is the reference tone, and a second sound The subject is asked to adjust the level of sound until it sounds same as that of the reference sound Contours are drawn in this manner.

18. Equal loudness contours(

19. Hearing loss Excessively loud noise can damage the structure of the ear, which results in temporary or permanent loss of hearing. Mechanical damage of hair cells, seriously impairs the normal function of the ear Sudden exposure to very intense sound damages hair cells Regular exposure to excessive noise results in the formation of harmful molecules in the inner ear as a result of stress caused by noise-induced reductions in blood flow in the cochlea. The harmful molecules build up toxic waste products known as free oxygen radicals which injure essential structures in the cochlea, causing cell damage and cell death, resulting in noise induced hearing loss This type of hearing damage is often accompanied by permanent tinnitus or “ringing in the ears”.

20. Trauma High-intensity sounds like explosions or jet exhausts can rupture the eardrum. Damage of Ossicles, sensory hairs on the basilar membrane or cochlea also lead to this permanent damage Occurs suddenly and is irreversible Sudden exposure to noise of SPL around 150 dB may lead to this

21. Chronic hearing loss The gradual loss of hearing caused by persistent exposure to high noise levels, for those people working in factories and workshops. Limited time exposure of loud noise results in temporary threshold shift (TTS) For a person with normal hearing, the threshold of hearing is 20micropascal or 0dB at 1 KHz The hearing threshold can rise by up to 20 dB at 4 kHz after exposure to loud noise. If the exposure is short, loss is temporary else permanent loss develops over a period of time It is irreversible also Greatest reduction in hearing occurs at 4kHz

22. Frequency weighting-necessity Human perception of loudness depends on the frequency of sound Perception of human ear is high in the mid frequency rather than low or high frequencies eventhough all are of equal energy A noise whose energy is concentrated mostly in the mid frequency of the audio spectrum is perceived as louder than noise of equal energy concentrated in low or high frequency region

23. This effect is more pronounced for soft sounds than loud sounds Thus loudness control is provided on some audio amplifiers, which provides greater amplification to the high and low frequency contents

24. In order to account for the difference in the actual sound pressure level and the perceived sound pressure level, frequency weighting is done. ‘A’ weighting is exclusively used in measurements that involve human response to noise It is reported in dB(A) or dBA

25. For 1kHz frequency band actual and perceived sound pressure levels has no difference ‘A’ weighting is introduced for sound levels below 55dB (low sound), ‘B’ weighting for levels between 55dB and 85 dB (moderate sound) and ‘C’ weighting for levels exceeding 85 dB(high sound)

26. Frequency weighting curves

27. Conversion table of sound levels in 1/3 rd octave band

28. Performance indices for environment noise For quantifying noise exposure performance indices are using These indices are single number criteria used for assessing environment noise Three indices based on A-weighted measurements are L N (gives the levels exceeded N% of the measurement time) L eq –the equivalent continuous sound pressure level in dB(A) Ldn- the day-night average sound level in dB(A)

29. L N criteria are specified in terms of sound levels that exceeded 10%, 50%, and 90% of the time Level corresponding to percentage of total observation is determined using the probability of exceeding each range of decibel levels Sound levels that are exceeded a particular percentage of time is estimated.

30. A histogram showing probability of exceedance for plant noise

31. Equivalent sound level L eq It is the sound energy averaged over a given period of time T It is the rms or mean level of the time varying noise

32. Day-night equivalent sound pressure level L dn It is a modification of L eq Mainly used for evaluating community noise problems A night time a penalty of 10dB is imposed on measurements between 10 pm and 7 am

33. Find L eq in the case where L n = 90.5, 95, 103, 88 and 98 dB(A) are obtained as the respective average levels for five short, equal time intervals. Find L dn for the situation where the day time equivalent sound level is 82 dB(A) and the night time equivalent sound level is 76 dB(A)

34. Noise and number index(NNI) NNI was used to assess aircraft noise disturbance near major airports in UK since 1963 It was formerly used in the United Kingdom as a measure for evaluating the long term average sound pressure levels from aircraft near airports It was used to measure the subjective noiseness of aircraft It was devised by the Wilson Committee of noise in Britain in 1963

35. It was the degree of annoyance It was prepared by relating people’s reactions to aircraft noise, at various positions near Heathrow airport using measured noise levels and periodicity of flights in those positions

36. It uses the PNdB (peak nose dB) as a basis and takes into account the number of aircraft per day (or night) as the key annoyance factor NNI=L+15(log 10 N)-80 where L is the logarithmic average of aircraft noise level heard and N is the number of aircraft heard during the day between 0700 and 1900 hours local time on an average summer day