Free Essay

In: Science

Submitted By zhaoshuai6cs32

Words 4667

Pages 19

Words 4667

Pages 19

Unit 4

Frequency analysis,

Frequency bands,

Decibel scales,

Descriptors for time varying noise levels Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-1

Contents

Frequency Analysis of Sound Pressure Signals

Constant Proportion Bandwidth Frequency Bands

Constant Bandwidth Frequency Bands

Decibel Scales

Descriptors for Time Varying Noise Levels

Equivalent Continuous Sound Level

Sound Exposure Level

Percentile Exceeded Sound Level

4-2

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

Frequency Analysis of Sound Pressure Signals

A microphone is constructed to produce a voltage proportional to the sound pressure of interest.

These voltage signals, which are functions of time, are referred to as sound pressure signals. It is necessary to have knowledge of the frequency composition of these signals. Fourier Series and the Fourier Transform can be used to mathematically relate functions in the time and frequency domains. The use of these relationships in the practical frequency analysis of signals will not be discussed in detail in this course. A less mathematical and more physical description of elementary frequency analysis is given in this section. The purpose of an elementary frequency analysis is to determine how the “strengths” of the components of the sound pressure are distributed as a function of frequency.

Consider first a pure tone. At a particular point in space, the time history of the sound pressure of the pure tone is given by: p(t ) = P cos 2πft

(4.1)

The elementary frequency analysis of this pure tone shows that at a frequency of f, it has a strength or amplitude (peak value) of P. It is often more convenient to measure the strength of a component by its root mean square value, prms, which here is given by P / 2 . The time domain representation of the root mean square value of a pure tone is shown in Figure 4.1, and the frequency domain representation of this pure tone is shown in Figure 4.2.

Figure 4.1

Time domain representation of a pure tone.

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-3

Figure 4.2

Frequency domain representation of a pure tone.

When the sound is a simple pure tone, its magnitude may be represented by its amplitude, which in this case is also the same as its peak value. The average value of the sound pressure over any period of more than a few cycles will be zero because the positive half-cycles will cancel the negative pressure half-cycles. For more complex waveforms, such as harmonic, transient or random noise, the expression of magnitude is not as simple, but the time-averaged value is still zero.

A commonly used expression of magnitude is the RMS value of the sound pressure. This gives a non-zero average, corresponding to the square root of the mean (average) of the square of the pressure. Figure 4.3 Processing stages of a complex sound pressure signal to determine its RMS value.

Consider now a complex sound pressure obtained by the superposition of two harmonically related pure tones of frequencies mf and nf. p(t ) = Pm cos 2πmft + Pn cos 2πnft

(4.2)

The frequency domain representation of this acoustic pressure is shown in Figure 4.4.

It is evident that the strengths of the individual components of frequencies mf and nf, in terms of their RMS values are pmrms = Pm / 2 and pnrms = Pn / 2 .

4-4

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

Figure 4.4

Frequency domain representation of a complex acoustic pressure.

A measure of the total strength of the sound pressure is useful, and the RMS value is again appropriate. The total RMS value of the sound pressure whose time history is given by

Equation (4.2) can be found by squaring Equation (4.2) and averaging it over a time equal to l/f, resulting in:

(

2

2

2 prms = pm rms + pn rms

)

(4.3)

It is of interest to consider the significance of this result in terms of sound energy. An expression for the sound energy density (energy per unit volume) of one dimensional plane acoustic waves associated with a simple harmonic variation of the sound pressure can be written in terms of the RMS sound pressure as:

E =

2 p rms ρc 2

(4.4)

2

The sound energy density associated with the component whose frequency is mf is pm rms ρc 2

2

and for the component whose frequency is nf the sound energy density is pn rms ρc 2 .

Sound energy and sound energy density are scalar quantities and so the sound energy densities associated with the two components can be added to give a total sound energy density of

2

2 pm rms + pn rms ρc 2 . The total sound energy density associated with the total sound pressure is

(

)

2 given by prms ρc 2 . Equating these two expressions for the sound energy density leads to

Equation (4.3).

It can also be concluded, from energy considerations, that, even if the frequencies are not harmonically related, provided that they are all different, the total RMS value of a sound pressure which is composed of N simple harmonic components of different frequencies whose

RMS values are p1 rms , p2 rms , ...., p N rms is given from:

(

2

2

2 prms = p12rms + p2 rms +....+ p N rms

)

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

(4.5)

4-5

Constant Proportion Bandwidth Frequency Bands

Many commonly occurring sound pressures are composed of a large number of components and it is not feasible to try to individually describe all these components. Instead, the RMS values of the sound pressures in frequency bands are used to present frequency domain information. The most widely used frequency bands for analysing sound pressure signals are the octave bands.

The word octave means a doubling of frequency. The ideal octave band filter allows frequency components greater than a lower frequency f l and less than an upper frequency f u = 2 f l to pass through the filter unimpeded while other components are completely blocked. The centre frequency f c of the pass band is geometrically defined by: fc fu

=

fl fc (4.6)

fc and f u = 2 f c . The

2

centre frequencies of adjacent bands are then an octave apart. The preferred octave bands have centre frequencies of 31.5, 63, 125, 250, 500, 1000, 2000, 4000, 8000 and 16 000 Hz. The audible frequency range from about 20 Hz to 20 000 Hz is therefore covered in 10 octave bands. Use of the result f u = 2 f l in this equation leads to the results f l =

Usually, octave band sound pressures are obtained from measured sound pressures by amplifying the voltage signal obtained from the microphone, passing it into octave band electrical filters and then measuring the RMS value of the filtered voltage.

Octave bands are quite wide in a frequency sense. The 1000 Hz octave band extends from

1000 / 2 = 707 Hz to 1000 × 2 = 1414 Hz , that is, it is 707 Hz wide. The widths of octave bands are in fact equal to the low frequency limit of the band.

Frequently, it is desirable to make analyses in narrower bands, and hence one third octave bands are used. One third octave bands, as the name implies, are one third of an octave wide in a frequency sense. Three one third octave bands are contained in every octave band. This is done

1

by setting the upper frequency limit of the band f u to 2 3 f l where f l is the lower frequency limit of the band. The centre frequency of the band f c is again defined geometrically by f f f Equation (4.6), that is, c = u , and so f l = 1 c6 and f u = 21/ 6 f c . The preferred one third

2/

fl fc octave bands have centre frequencies of 16, 20, 25, 31.5, 40, 50, 63, 80, 100, 125, 160, 200, ...

Hz.

Actual octave and one third octave band filters do not provide the characteristics of the ideal filters discussed so far. The pass bands of the ideal and typical octave filters are shown in

Figure 4.5.

4-6

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

Figure 4.5

Ideal and typical octave pass bands.

The octave and one third octave bands which have been discussed so far are examples of constant proportion bands. The widths of the bands are proportional to the centre frequencies of the bands. Frequently, narrower constant proportion bands are used and a 6% bandwidth is commonly used. The bandwidth percentage is the ratio of the bandwidth to the band centre frequency, expressed as a percentage.

% bandwidth =

bandwidth (Hz )

× 100% centre frequency (Hz )

(4.7)

Example 4.1

A complex sound pressure is known to be composed of five individual pure tones. The frequencies and amplitudes of the individual components are given in the following table.

Frequency

(Hz)

100

150

160

210

250

Amplitude

(Pa)

2

4

6

2

1

Determine the total RMS value of the complex sound pressure, and the RMS value of the components in the ideal 125 Hz octave band.

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-7

Solution:

The RMS values of the individual components in ascending order of frequency are 1.414,

2.828, 4.242, 1.414 and 0.707 Pa. Application of Equation (4.5) gives:

(

prms = 1.414 2 + 2.8282 + 4.242 2 + 1414 2 + 0.707 2

.

)

1

2

Pa = 5523 Pa

.

The ideal 125 Hz octave band extends from 125 / 2 = 88.4 Hz to 125 × 2 = 176.78 Hz . Thus the first three of the tabulated frequency components are in this band. Application of Equation

(4.5) gives:

(

p rms = 1.414 2 + 2.828 2 + 4.242 2

)

1

2

Pa = 5.291 Pa

Exercise 1

1.

Two complex sound pressure signals have RMS values p1 rms and p2 rms . The smaller signal has an RMS value of 0.1 times that of the larger signal. What error will be introduced in using the RMS value of the larger signal to estimate the total RMS value of the combined signals?

Exercise 2

1.

4-8

Determine the percentage bandwidths of octave and one third octave band filters.

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

Constant Bandwidth Frequency Bands

It is sometimes an advantage to analyse sound pressure signals into narrow frequency bands of constant bandwidth and not constant proportion bandwidths. Reasonable estimates of the power spectral densities of sound pressure signals can be obtained with constant bandwidth analyses.

The power spectral density of a sound pressure signal at a frequency of f can be interpreted as the mean square value of the sound pressure signal after it has been filtered by an ideal pass band filter which is 1 Hz wide and is centred at a frequency of f .

The power spectral density is a function of f and is generally denoted S ( f ) . The contribution to the mean square value of a sound pressure signal from components between two frequencies f 1 and f 2 is given by the integral of S ( f ) between f 1 and f 2 .

If the power spectral density is relatively uniform between two frequencies the sound pressure is said to be “white” between these frequencies.

The mean square value associated with this white portion of the sound pressure is simply given by the product of the power spectral density and the frequency range.

Mean square value of white noise = S ( f ) × ( f 2 − f1 )

(4.8)

Example 4.2

A narrow band frequency analyser with a constant bandwidth of 10 Hz is used to analyse a sound pressure signal. It is found that when the centre frequency is 1000 Hz the measured RMS value is 0.1 Pa. Determine the power spectral density at this frequency.

Solution:

The mean square value in this frequency band is (0.1)2 Pa2. The band is 10 Hz wide and so the power spectral density at this frequency is given by:

(01) 2 Pa 2 10 Hz = 0.001 Pa 2 Hz

.

Exercise 3

1.

The power spectral density of a sound pressure varies linearly between 1000 Hz and

5000 Hz. At 1000 Hz the power spectral density is 0.001 Pa2/Hz and at 5000 Hz the power spectral density is 0.0001 Pa2/Hz. Determine the RMS value of the sound pressure between these frequency limits.

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-9

Decibel Scales

Quantities such as sound pressure, sound intensity and sound power vary over wide ranges and it is convenient to express these quantities on a logarithmic scale. The decibel scale, which is based on logarithms to base 10 is widely used. It is convenient in discussing the decibel scale to begin by considering its application to describing sound powers.

A sound source which is radiating W watts of sound power has a sound power level of LW relative to a reference sound power of Wref watts defined by:

LW = 10 log10

W

Wref

(4.9)

LW has the “units” of dB re Wref . It is international practice to set Wref to 10-12 watts. Suppose, for example, that W is 1 watt. LW is then 120 dB re 10-12 watts.

Sound intensity, being the sound power flow through a unit area is closely related to sound power. The intensity level LI for a sound wave whose intensity at a specified point is I , is defined by:

LI = 10 log10

I

I ref

(4.10)

LI has the “units” of dB re I ref . It is international practice to set I ref to l0-12 watts/m2.

Sound intensity is proportional to the square of the sound pressure ( I ∝ p 2 ). Hence, the sound pressure level Lp corresponding to a sound pressure p can be defined by:

L p = 10 log10

2 p rms

2

p ref

(4.11)

Equation (4.11) can be written as:

Lp = 20 log10

prms pref (4.12)

Lp has the “units” of dB re pref . It is international practice to set pref to 2×10-5 Pa. Suppose,

for example, that p is 2 Pa. Lp is then 100 dB re 2×10-5 Pa. Often, the “re 2×10-5 Pa” term is not included as it is considered to be implied. Further, this reference pressure is taken to be the

RMS value. It is of interest that the threshold of hearing at 1000 Hz for a young listener with good hearing occurs at a sound pressure of approximately 2×10-5 Pa, that is, pref corresponds to the threshold of hearing.

4-10

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

The reference quantities for sound power, sound intensity and sound pressure have been selected so that the corresponding levels are inter-related under certain conditions. An expression for the intensity of a travelling plane wave can be expressed in terms of the RMS value of the sound pressure as:

I =

P2 p2 = rms ρc 2 ρc

(4.13)

When ρc is 415 rayls and I is set to the reference sound intensity of 10-12 watts/m2, the corresponding sound pressure is 2.057×l0-5 Pa. This value is close to the reference sound pressure of 2×l0-5 Pa. Thus LI is approximately equal to Lp for a travelling wave.

Other quantities can be expressed on the decibel scale. For example, the calibration constant of a measurement microphone can be expressed on a decibel scale, as can a voltage. Suppose that a voltage signal has a value of V volts. This voltage can be expressed in terms of a level relative to some reference voltage V0 by use of an equation similar to Equation (4.12):

LV = 20log10

V

V0

(4.14)

LV has units of dB re V0 .

Care must be used in adding and subtracting quantities expressed on decibel scales. The safest procedure to follow when quantities expressed on decibel scales must be added or subtracted is to express the quantities in physical units and then these quantities can be added or subtracted in the appropriate manner. Sound powers and intensities, when expressed in the physical units of watts or watts/m2 can be added or subtracted directly. However, sound pressures, when expressed in the physical units of Pa must be added or subtracted by adding or subtracting the mean square pressures and not the root mean square pressures.

Example 4.3

The calibration constant of a microphone is known to be 0.001 V/Pa. Express this in dB re l V/Pa.

Solution:

The calibration constant in dB re l V/Pa = 20log10

0.001

= − 60 dB re l V/Pa.

1.0

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-11

Example 4.4

An octave band analysis of a sound pressure gives the following results.

Octave Band Centre Frequency

(Hz)

31.5

63

125

250

500

1000

2000

4000

8000

16000

Sound Pressure Level ( Lp )

(dB re 2×10-5 Pa-2)

80

90

100

90

100

90

90

110

80

60

Determine the RMS value of the total sound pressure and the corresponding sound pressure level. Solution:

Application of Equation (4.11) allows the mean square sound pressures in each of the octave bands to be found. The mean square sound pressures in each octave band are given in the following table.

Octave Band Centre Frequency

(Hz)

31.5

63

125

250

500

1000

2000

4000

8000

16000

Mean Square Sound Pressure

2

( prms ) (Pa2)

0.04

0.4

4

0.4

4

0.4

0.4

40

0.04

0.0004

∑ = 49.6804 (Pa)2

These mean square sound pressures can then be summed by use of Equation (4.5).

The total RMS sound pressure is prms = 49.6804 = 7.05 Pa.

The corresponding sound pressure level can be found by application of Equation (4.12) to be:

L p = 20 log10

4-12

prms pref = 20 log10

7.05

2 × 10

−5

= 110.9 dB re 2×10-5 Pa.

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

Exercise 4

1.

At a point midway between two typewriters the sound pressure level when both typewriters are working is 86 dB. What will be the sound pressure and sound pressure level at this point when only one typewriter is working?

2.

A compound 10m x 10m contains 10 barking dogs. At a distance of 50 m from the centre of the compound the sound pressure level with the dogs barking is found by measurement to be 60 dB. An additional 40 dogs are introduced into the compound.

What is the increase in the sound pressure level due to the additional dogs?

3.

Two sound pressure levels differ by x dB. Derive, as a function of x, an expression for the quantity which must be added to the larger sound pressure level to give the sound pressure level of the combined sounds.

Descriptors for Time Varying Noise Levels

So far it has been assumed that the sound is steady, and it can be represented by the overall sound pressure level Lp , or the A-weighted sound pressure level L A .

Frequently the sound is not steady and this causes difficulties. The first attempt to overcome this problem is to use the Fast (F) or Slow (S) time constants on the Sound Level Meter. The time constant “F ” is designed to approximate the response of the ear.

There are a number of more sophisticated measures which attempt to quantify sound which is not steady by a single number. These are the equivalent continuous sound levels ( Leq , LAeq ), and the sound exposure levels ( SEL , LAE ).

Equivalent Continuous Sound Level

Sound power is proportional to p 2 . Consider a sound pressure p(t ) whose level fluctuates in some way over a time T. It is useful to find an equivalent sound pressure whose level is constant over the time T such that the total sound energy (power × time) associated with the equivalent sound pressure is the same as that associated with the fluctuating sound pressure. It is thus necessary to have

T

p × T = ∫ p 2 (t )dt

2

eq

0

⇒

2 peq 2 pref 1

=

T

p 2 (t )

∫ pref dt

2

0

T

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-13

⇒

⇒

Leq ,T

2

peq

1

= 10log10 2 = 10log10

p

T

ref

1

Leq ,T = 10 log10

T

n

∑ T × 10

Leq ,Ti / 10

i

i =1

T

∫

0

p 2 (t ) dt

2

pref

(4.15)

(4.16)

Usually, the A-weighted equivalent level LAeq is of interest. It is found by A-weighting p(t ) and so it is given by

1

LAeq ,T = 10log10

T

⇒

1

LAeq ,T = 10 log10

T

2 p A (t ) dt

2

pref

T

∫

0

n

∑ T × 10 i =1

L Aeq ,Ti / 10

i

(4.17)

(4.18)

LAeq is used as a rational means of obtaining a single number to describe a sound of fluctuating

level as shown, for example, in Figure 4.6.

Figure 4.6 Leq as a measure of fluctuating level.

Leq measurements are used to describe fluctuating machinery noise, eg. the above Lp vs time

plot could be noise produced by a machine which works on a cycle.

Leq (or more particularly the LAeq ) measurements are useful in assessing long-term noise

exposure associated with occupational and environmental problems.

Often for these purposes, LAeq is measured over a time T and so LAeq is written as LAeq ,T .

For occupational purposes T is commonly 8 hours, but for environmental purposes different values of T are used. Thus LAeq ,9 might be measured from 22:00 to 07:00 to describe the noise levels during sleep times.

4-14

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

Sound Exposure Level

The Sound Exposure Level ( SEL , also denoted LE ) is similar in form to Leq , but is used for discrete noise events. It is defined as the constant sound pressure level which, if maintained for a period of one second, would deliver the same noise energy to the receiver as the original event itself. Thus t2 p 2 × T0 = ∫ p 2 (t )dt t1 T0 = 1 second

where

t 2 − t1 = a stated time interval long enough to encompass all significant sound of the event

⇒

⇒

SEL = 10log10

t2

1

T0

p 2 (t ) dt 2 pref ∫

t1

1

SEL = 10log10

T

0

n

(4.19)

∑ T × 10

Leq ,Ti / 10

i

i =1

(4.20)

Usually the pressures are A-weighted in which case LAE is used for sound exposure level,

LAE

⇒

1

= 10log10

T0

t2

∫

t1

1

LAE = 10log10

T

0

2 p A (t ) dt 2 pref n

∑ T × 10 i =1

i

(4.21)

L Aeq ,Ti / 10

(4.22)

It can be seen that either SEL or LAE gives a measure of the total energy of a single noise event, whereas either Leq or LAeq gives a measure of the average sound power of a sound.

LAE is useful for describing transient noise events such as vehicle drive-by noise and aircraft fly-overs. Figure 4.7 shows an example of a measurement beginning and ending in background noise.

LAE does not depend on the time of measurement whereas LAeq does.

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-15

Figure 4.7 Comparison between LAE and LAeq for a transient noise event.

It is also useful to be able to calculate an LAeq value applying for a given period from data on individual noise events which may occur in the period. Examples could be when different types of vehicles drive by or if a number of different types of machines are used in, say, an eight hour period. The LAE value and number of occurrences of each event must be known. For a single event occurring in a time period T (where T is in seconds), it can be shown that LAeq is related to LAE by:

LAE = LAeq ,T + 10 log

⇒

LAeq = L AE − 10 log

T

T0

where T0 = 1 second

T

T0

where T0 = 1 second

(4.23)

(4.24)

For n events with individual sound exposure levels given by L AEi

n

LAeq = 10 log ∑ 10 i =1

4-16

L AEi

10

− 10 log

T

T0

where T0 = 1 second

(4.25)

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

Percentile Exceeded Sound Level

When noise is fluctuating, the energy averaged Leq alone does not indicate the level of annoyance as no information is given concerning the degree of fluctuation or the maximum and minimum values and their duration. Statistical analysis of the noise is a convenient way of quantifying the temporal variation of the noise level over the measurement period.

The common statistical descriptor of fluctuating noise levels is the Percentile Exceeded Sound

Level L N which is the noise level exceeded for N% of the time over which the measurement was made. Figure 4.8 shows how L N is related to the noise signal. To be rigorous, the notation

LN ,T should be used where T specifies the time of measurement. Usually the sound is Aweighted and the notation used is LAN ,T .

Often LA 90,T is used to estimate the residual background noise level, that is, L90 represents the noise level exceeded for 90% of the time.

LA10,T is used to estimate the maximum levels, that is, L10 represents the noise level exceeded for 10% of the time.

Figure 4.8 Percentile exceeded level related to noise signal.

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors

4-17

Exercise 5

1.

A sound level meter is set on the ‘linear’ scale and has been used to obtain the following data for a sound source totally in the 1000 Hz octave band: time period

0 seconds to 5 seconds

meter reading

Lp = 70 dB re 2×10-5 Pa

5 seconds to 15 seconds

Lp = 110 dB re 2×10-5 Pa

15 seconds to 30 seconds

Lp = 90 dB re 2×10-5 Pa

30 seconds to 100 seconds

Lp = 70 dB re 2×10-5 Pa

a)

Determine Leq and SEL .

b)

Determine Leq and SEL if the final time period were from 30 seconds to 200 seconds (still with Lp = 70 dB re 2×10-5 Pa). Hence comment on which measure of sound is the more appropriate.

Exercise 6

1.

Consider a noise event lasting for ten seconds. As shown on the graph below, the noise level increases from 60 dB(A) re 2×10-5 Pa to 70 dB(A) re 2×10 5 Pa during the fourth second and again increases from 60 dB(A) re 2×10-5 Pa to 90 dB(A) re 2×10 5 Pa during the eighth second.

LA [dB(A) re 2*10-5 Pa]

100

90

80

70

60

50

40

0

1

2

3

4

5

6

7

time [seconds]

8

9

10

11

Determine:

(a) L Aeq for the ten second period

(b)

(c)

(d)

(e)

(f)

4-18

LAE for the first five second period

LAE for the second five second period

LAE for the ten second period

LA 90 %, 10 seconds

LA 10 %, 10 seconds

Fundamentals of Acoustics and Noise: Unit 4 – Frequency Analysis, Decibel Scales, Special Descriptors…...

Premium Essay

...Noise 1. Calculate for the noise power if the temperature of the conductor is 290 K, the bandwidth is 200kHz. PN=kTB PN=1.38x10-23290(200kHz) PN=8.004x10-16 W 2. With a resistance of 200k ohms, a bandwidth of 500khz and a temperature of 500 K calculate for the noise voltage. VN=4kTBR VN=41.38x10-23500(500k) VN=1.17x10-7 V 3. Given a noise power of 3000kW and a bandwidth of 2.5Mhz. Compute for the noise density No=PNB No=3000kW2.5Mhz No=1.2 WHz 4. Given a noise temperature of 350K calculate for the noise factor. Teq=ToF-1 F=TeqTo+1 F=350290+1 F=2.2 5. With a bandwidth of 450 hz and a current of 50mA. Compute for the shot noise. IN=2BIq IN=(2(450)(50mA)(1.6x10-19) IN=2.68 nA 6. Calculate for the Bandwidth if the noise power is 500kW and the temperature is 279K PN=kTB B=PNkT B=500kW1.38x10-23279 B=1.3x1026Hz 7. If the noise voltage is 20V and the resistance and temperature is 200 ohms and 273 K respectively, calculate for the bandwidth. VN=4kTBR B=Vn24TR B=2024200(273) B=1.83x10-3Hz 8. Given a signal voltage of 55V and a noise voltage of 35 V calculate for the signal-to-noise ratio in decibel form. SNdB=20logVsVN SNdB=20log5535 SNdB=3.93 dB 9. Given a signal power 60W and a noise power of 43W. Calculate for the signal-to-noise ratio in decibel form. SNdB=10logPsPN SNdB=10log6043 SNdB=1.45 dB 10. Given a signal power 80W and a noise power of 33W. Calculate for...

Words: 293 - Pages: 2

Free Essay

...J O U R N A L O F M A T E R I A L S S C I E N C E 3 4 (1 9 9 9 ) 4995 – 5004 Tribological behaviour and acoustic emissions of alumina, silicon nitride and SAE52100 under dry sliding H. S. BENABDALLAH, R. J. BONESS Department of Mechanical Engineering, Royal Military College of Canada, PO Box 17000 Stn Forces, Kingston, Ontario, Canada, K7K 7B4 E-mail: benabdallah-h@rmc.ca The friction, wear and acoustic emission behaviour of various combinations of alumina, silicon nitride, and SAE52100 steel, operating under dry sliding conditions, was investigated. A designed ball-on-ﬂat-disc type of tribometer was used to conduct these experiments. This apparatus, equipped with a force sensor, using silicon strain gauges, measured simultaneously the normal load and friction force. Both forces were used to determine the real-time value of the dynamic coefﬁcient of friction. The AE signal arising from the interaction of the surfaces in dynamic contact was also detected and a data acquisition system was used to gather this signal as well as the outputs from the force sensor, at high frequency. The effects of test duration, sliding speed and normal load on the above mentioned tribological parameters were evaluated. The interest of this study further extended to assess the correlations that may exist between the integrated rms acoustic signal (AE) and the friction mechanisms, wear volume, friction work as well as the material removal power. Under the speciﬁc conditions of the present......

Words: 6799 - Pages: 28

Free Essay

...different types of noise. Noise can be identified as anything in the process of communication that manages to interfere with the intended receiver getting and understanding the message properly. Environmental/physical noise This refers to anything external to the sender and receiver of a message that disrupts the sending and/or receiving of the message. Examples of this could be: - a stereo playing loudly - a phone ringing - being seated a long way away from the speaker - people chatting loudly around you - cars driving or honking their horns Physiological noise This refers to a physical interference that can prevent a message getting through properly. This might include: - hearing loss - poor eyesight - blindness - memory loss Psychological noise This is a mental interference between sender and receiver. This might occur if feelings such as love, hatred or irritation distract one of the communicators, causing the message not to be conveyed or received accurately. There is psychological noise present if one of the communicators is daydreaming rather than paying proper attention. Emotional distress or relationship problems might affect someone and cause messages not to get through properly. Semantic noise This type of disruption in the communication process can happen when the sender and receiver have a different understanding of the same word or phrase, which causes them to apply meaning to words differently. This sort of semantic noise......

Words: 298 - Pages: 2

Free Essay

...Loud Noises Just make it stop, please, let it stop. I'm spending every minute awake wishing I was asleep, and every minute asleep in complete terror. Every breath seems to drain more life from me than the last. I would take or do anything to make it stop. I would sell my soul to the devil himself if I could. I can't take this anymore, I'm giving up. I was never a normal child growing up. Well I was always told by my superiors that I wasn't a normal child. My parents always telling me how unruly of a child I was. I was described as a ball of hell fire, who spat at the rules like they didn't apply to me. I was diagnosed With every hyperactive disorder the doctors had, Bipolar, ADD, ADHD. I felt like I wasn't wanted by my family. It was emptying and caused me to lash out more. Instead of just giving me attention they just shoved medication onto me. When I wasn't drugged out I was making sure my hatred for my parents was well heard. Some of the medication I was on made me feel like I was outside of my body watching someone else live my life. I never interacted with other children because my intelligence dwindled. I was always in a fog, It took more energy to communicate with others than any kid should have to. It was like trying to swim through high tide just trying to talk to others. My mind strained, my muscles ached, all tastes and smells were dulled down. I stopped enjoying food and stopped eating. I lost interest in sports, especially since I could barely pull myself out...

Words: 1917 - Pages: 8

Premium Essay

...CONTENTS S.No Titles Page No Introduction 1. Classification of Noise 1 1.1 Community Noise 2 1.2 Occupational Noise 2 2 Effect of Noise on Public Health 3 3 Noise Standards in Pakistan 4 3.1 Motor Vehicle Rules, 1969 5 4 Studies and Data for Removal of Pressure Horns in Different Cities of Pakistan 6 4.1 Environmental Protection Agency, NWFP 6 4.2 Environment Protection Department, Punjab 6 5 Major Steps Taken by Ministry of Environment 7 8 References 8 INTRODUCTION Noise is an unwanted, unpleasant and annoying sound caused by vibration of the matter. Vibrations impinge on the ear drum of a human or animal and setup a nervous disturbance, which we call sound. When the effects of sound are undesirable that it may be termed as “Noise”. Noise from industry, traffic, homes and recreation can cause annoyance, disturb sleep and affect health. Thus, sound is a potentially serious pollutant and threat to environmental health. The response of the human ear to sound depends both on the sound frequency (measure in Hertz, Hz) and the sound pressure, measured in decibels (dB). A normal ear in healthy young person can detect sounds with frequencies from 20Hz to 20,000 Hz. Noise measurements are expressed by the term Sound Pressure Level (SPL) which is logarithmic ratio of the sound pressure to a reference pressure and is expressed as a dimensionless unit of power, the decibel (dB). The reference......

Words: 2146 - Pages: 9

Premium Essay

...noise pollution definition of noise noise is a sound ,especially one that is loud or unpleasant or that causes disturbance. Sources of noise pollution 1-Household sources Like food mixer, grinder ,vacuum cleaner ,washing machine and air conditioners can be very noisy and injurious to health. others include loud speakers of sound systems and TVS, ipods and ear phones. Another example may be your neighbor’s dog barking all night everyday at every shadow it see. 2-Social events Places of worship, parties and other social events also create a lot of noise for the people living in that area. In many market areas , people sell with loud speakers , others shout out offers and try to get customers to buy their goods. 3- transportation Think of aero planes flying over houses close to busy airports ,over ground and underground trains, cars , vehicles on road – these are making a lot of noise Effects of noise pollution Generally, problems caused by noise pollution include stress related illness, speech interference .hearing loss, sleep disruption , and lost productivity. Most importantly , there are two major effects 1- Hearing The immediate and acute effect of noise pollution to a person, over a period of time, is impairment of hearing. Prolonged exposure to impulsive noise to a person will damage their eardrum, which may result in permanent hearing 2- Effects on general health Include anxiety and stress reaction . the physiological manifestations are......

Words: 360 - Pages: 2

Free Essay

...Signal Processing and Noise Reduction, Second Edition. Saeed V. Vaseghi Copyright © 2000 John Wiley & Sons Ltd ISBNs: 0-471-62692-9 (Hardback): 0-470-84162-1 (Electronic) 2 NOISE AND DISTORTION 2.1 Introduction 2.2 White Noise 2.3 Coloured Noise 2.4 Impulsive Noise 2.5 Transient Noise Pulses 2.6 2.7 2.8 2.9 2.10 Thermal Noise Shot Noise Electromagnetic Noise Channel Distortions Modelling Noise N oise can be defined as an unwanted signal that interferes with the communication or measurement of another signal. A noise itself is a signal that conveys information regarding the source of the noise. For example, the noise from a car engine conveys information regarding the state of the engine. The sources of noise are many, and vary from audio frequency acoustic noise emanating from moving, vibrating or colliding sources such as revolving machines, moving vehicles, computer fans, keyboard clicks, wind, rain, etc. to radio-frequency electromagnetic noise that can interfere with the transmission and reception of voice, image and data over the radio-frequency spectrum. Signal distortion is the term often used to describe a systematic undesirable change in a signal and refers to changes in a signal due to the non–ideal characteristics of the transmission channel, reverberations, echo and missing samples. Noise and distortion are the main limiting factors in communication and measurement systems. Therefore the modelling and removal of the effects of noise and distortion have......

Words: 4459 - Pages: 18

Free Essay

...Name Instructor Course Date Architecture Part 1: Qualitative and Design process considerations 1.1 Diagrams Fig 1. Design process consideration for a high performance acoustic studio building Fig2: Qualitative and Design process considerations 1.2 Daylight design process The day lighting design process involves the identification of a lighting need and the selection of the most appropriate method for delivering the appropriate day lighting in this case. The process begins with determining the building type. The type of building gives the set of visual tasks that have an associated lighting criteria which is used in the design process. In addition, it indicates the likely size and mass of the building. The feature is important because the lighting concepts that are generated for single-storey buildings will be remarkably different from those that are used in high-rise offices (Yancey 9). Moreover, there are numerous feedback loops that can be used during the design process. For instance, as I generate day lighting system concepts I can re-evaluate the resource measurements to identify their adequacy in giving me the information that I need to produce an effective process. A good example is that a day lighting system that relies upon sunlight cannot be evaluated using the average sunlight intensity collected over a wide range of cloudiness. As a result, the appropriate time scale that should be used in each scenario is determined by the......

Words: 618 - Pages: 3

Free Essay

...Telecommunication Engineering Principles Introduction Why need to understand noise? Limit the performance of a device or system Limit the sensitivity of measurements, and the transmission rate to lower the bit error rate Efficient product development requires quantification of noise from components calculation of noise effects on system performance One of the main impact on system cost Incompetent noise specifications can irritate customers Relationship of Noise and Decibels What is decibels and noise? Decibel (dB) Uses ratios and logarithms Compress wide span of magnitude into smaller and easier-tomanipulate range of numbers Uses to measure one signal against another signal of defined value Simplifies the measurement of relative signal gain and loss Allows total gain through multiple stage to be calculated by simple addition Relationship of Noise and Decibels Noise Interfered or unwanted signal Measures in decibels in relation to the desired signal Produces its own noise in any circuit The overall system performance is calculated by adding the amount of noise from the circuit itself with the noise of the received signal Decibels for Power and Voltage Power and dB To compare two power signal/level dB 10 log10 Uses P Po two power value instead of single absolute value such as watt Use base 10 logarithm, to compress wide......

Words: 817 - Pages: 4

Free Essay

...engines on takeoff. Noise pollution became an unfortunate side effect of the jet age. The U.S. began enacting legislative controls with the Aircraft Noise Abatement Act in 1968. This authorized the FAA to prescribe standards for the measurement of aircraft noise. This act was later modified by the Noise Control Act of 1972 which now required consultation with the Environmental Protection Agency (EPA). "The Congress declares that it is the policy of the United States to promote an environment for all Americans free from noise that jeopardizes their health or welfare" (Noise Control Act of 1972). Other federal programs provided money for noise reduction projects such as sound proofing nearby buildings to include homes and schools, and land acquisition to acquire homes surrounding airport property and relocating them to quieter locations (Government Accountability Office, [GAO], 2001). Land use guidelines and zoning laws addressed land use in the vicinity of the airport. They idea behind land use planning is that residential development should not occur in areas near airports exceeding a described decibel level, as well as limiting airport expansion in existing residential areas. Additional acts required aircraft designers and manufactures to follow FAA Aircraft Certification standards to achieve noise reduction. These standards were classified as “stages” with early aircraft classified as stage 1. These stages were based on noise limits according to......

Words: 1386 - Pages: 6

Premium Essay

...The Effects of Population Density and Noise Paper PSY460 Introduction Living different types of lifestyles can be from being raised in an urban or rural environment. Daily stressors are formed from the simplest things in life to the most over populated part of a city. During this paper we are able to learn about the concept of each affects that territory, privacy and personal spaces. How nature can affect the way of living to urban areas or how it can help other areas in the urban city. Noise to most people can be understanding and yet very draining depending on the situation. Learning about different environments makes a better understanding of how each person reflects on life and how they live it. Territory A territorial area is what is most important to humans, their property that they want to keep secluded from others. This is easily the privacy and personal space of humans that the owner wants others to see being territorial ties into all aspect s of privacy and personal. Not only are humans territorial but so are animals. There are many different signs for animals to show another animals territory. With birds singing, bears making claw marks in the trees to cats peeing to mark their territory making others know where they stand. As the cultures of others become to clash and become different is when people start to mark they land, they property making it know that what once was welcomed is not trespassing. Most people who have territory make it know with......

Words: 1485 - Pages: 6

Premium Essay

...Basic Acoustics Acoustics is the science of sound and can help determine the quality of sound transmitted in a room or space. Understanding the sonic characteristics of a room or space will result in better decisions when recording or mixing. Surfaces and Materials Different surfaces and materials can affect the acoustics in a room. Too much material applied to a wall or surface is generally destructive to a sound. Professional recording environments are typically constructed with many types of surfaces and materials. 1. Carpets 2. Wood Floors 3. Bricks or stone wall 4. Diffusers 5. Concave or convex walls *Note: Professional recording spaces are rarely perfect squares with low ceilings, because parallel walls and surfaces create standing waves, or room modes. Basic Acoustic Terms * Reflections: when sound strikes a wall, ceiling, or any object, the sound is reflected at an angle equal to, and opposite of, its initial angle of incidence. * Absorption: is the process in which acoustic energy is reduced when a sound wave passes through a medium or strikes a surface. The amount of energy absorbed compared to the amount reflected is express as a ratio known as the absorption coefficient. * Diffraction: the ability of sound to bend around an object and reform itself. * Diffusion: is a way to control reflections and standing waves. A diffuser can be used to break up standing waves by reflecting sound at a wider......

Words: 527 - Pages: 3

Premium Essay

...The effects of populations and noise go hand-in-hand, or have a cause and effect relationship. If there are larger amounts of people in a smaller space, there will be more noise. The amount of noise a human can tolerate changes from each person’s own perspective. Most people have a desired level of noise, and a desired amount of personal boundaries. Cultural factors affect the way human beings determine what is appropriate personal space, or what is uncomfortable for each individual. Environment has a huge impact on what one considers privacy. The comfort level in which individuals interpret their own privacy varies, also has to do with how one is brought up and what their personal culture has taught him or her or privacy. For example, in certain cultures the people feel it is appropriate to keep the women covered up as much as possible. Traditional Muslims wear head pieces that cover everything except their eyes. African tribal groups will show their women wearing next to nothing, they may have their breasts exposed or it may be customary for the whole tribal community to wear nothing at all. If there is no stigma taught by the social norms of that society, then the people do not feel as though they are exposing themselves. Individuals are a product of their environment. If what they are doing is taught to be right, then they will not have the feeling that they are doing something wrong. Human territoriality is the predisposed feeling to control one’s own surroundings and...

Words: 1674 - Pages: 7

Premium Essay

...The Effects of Population Density and Noise Paper Your Name PSY460 Date Instructor Name The Effects of Population Density and Noise Paper * Concepts of Territoriality, Privacy, and Personal Space Territoriality refers to a limited district for a person or animal and the tendency to defend an area of territory (Oxford, 1042); whereas personal, or one’ private space indicates the region humans or animals designate as their own. The relationship between the thoughts of territoriality, privacy, and personal personal space to population mass is associated with behavior. For example, Rubenstein (1980) examined changes in population mass and supply patterns, and noted how these changes affects the hostile behavior of the small sunfish. According to Rubenstein, the means by which small sunfish contend for nourishment is swayed by the mass of the inhabitants, the dispersal of victims, and the gender domination position of a person. Studies on territorial rights showed that Increases in prey dispersion reduced the proportion of battles over possessions in both low and high mass populations (Morrell & Koko, 200). Morrell & Koko sought to explain how animals establish territorial ownership, which is viewed as a major determinant of fitness in territorial animals. Some other behavioral characteristics associated with the establishment of territories include: (a) subordinates initiate fights over resources; (b) initiators are unlikely to contest prey in low mass......

Words: 1512 - Pages: 7

Free Essay

...such as decreased privacy and excessive population noise. In most cases, as population density increases, that larger population will produce more noise and more areas will be crowded. Individuals that live in areas where high population density exist also have a harder time maintaining privacy, so person's territorial tendencies must change and adapt. In order to fully grasp the concept of how population density affects the average person the concepts of personal space and privacy, territoriality, and noise must be understood. Noise The idea of noise is very subjective issue. What is considered noise to one person can be music to another person ears. Technically speaking the definition of noise is sound or a sound that is loud, unpleasant, unexpected, or undesired. When discussing noise it is so important to identify and classify noise by different characteristics such as amplification, Timbre, and pitch which are all physical attributes of noise. Decibels are used to measure how loud a noise is or its amplification. Hertz is used to measure the frequency of a noise. It can be classified as anything such as the sound that comes from airplanes, trucks, trains, cars, to a rock concert and party, construction work, and many other sound producing activities. Individuals that live by airports for example are 50% more likely to be bothered by the noise that that airport produces and generally experience four times the amount of noise that occupants other different areas......

Words: 1445 - Pages: 6