6.1 Noise - basic concepts

Question 1

wavelength

Answer 1

The wavelength (λ) determines the pitch of the sound. Wavelength is the length of
one complete cycle, and is measured in metres (m). Its relationship to the frequency
and speed of sound can be expressed as:

Wavelength (λ) in metres =
Speed of sound (c) / Frequency (f)

Long, slow waves are a low pitch (like a fog horn).
Short, fast waves are a high pitch (like a whistle).

Question 2

Frequency

Answer 2

Frequency (F) is the number of times a complete wave passes a point. It is
measured in hertz (Hz), or cycles per second. The slowest, lowest sound a human
can hear is approximately 20 Hz. The highest sound a human can hear is
approximately 20,000 Hz (or 20 kilohertz - kHz).

Question 3

period

Answer 3

The period (T) is the time it takes to complete one full cycle, it is proportional to the 
frequency - T = 1/f (seconds).

Question 4

A-weighting:

Answer 4

 Reduces the importance of lower frequencies at 500 Hz or less. The lower the
frequency, the greater the A-weighted correction factor becomes (see Figure
6.4).
 Slightly increases the overall magnitude of the mid to high frequencies (2,000-
4,000 Hz).
 Reduces the very high frequencies as they extend beyond normal hearing.

Question 5

C-weighting:

Answer 5

Used principally for the evaluation of impulse noise and for hearing protection. It was
originally intended to be used when measuring high sound pressure levels such as
aircraft noise.

C-weighted correction values show significantly less low frequency roll-off relative to
the A-weighted correction values.

Question 6

The strength or loudness of a sound is determined by

Answer 6

the amplitude or height of

the sound waves. Tall waves are loud; short waves are quiet.

Question 7

Amplitude is a convenient measure of the magnitude of the sound and can be
related to its intensity and loudness and ultimately the effect it has on the human ear.

There are various options for determining the amplitude: 3

Answer 7

 The peak value does not relate closely to the subjective impression of the sound.
 An average value may be more appropriate but due to the symmetrical shape of
the pressure wave positive sides of the wave ‘cancel out’ the negative and the
resultant ‘average’ is zero.
 The measure which best takes into account the magnitude of the sound pressure
fluctuations, but not the direction, is the root-mean square (or RMS) sound
pressure.

Question 8

Sound power (PWL)

Answer 8

Sound power is the total sound energy generated by the source per unit of time
expressed in units of watts (W). The sound power of a source output is constant,
regardless of its location although, as will be shown, the sound intensity and sound
pressure will change as a function of the environment in which it is located.

Question 9

Sound intensity

Answer 9

Sound intensity is sound power per unit area (W/m2). It is a vector quantity, i.e. is
specified by direction.

Note: Sound intensity is proportional to sound pressure squared.

Question 10

Sound pressure (SPL)

Answer 10

The sound pressure level (SPL) is the variation of pressure superimposed on the
atmospheric pressure.

Sound pressure is expressed as force per unit area, and the preferred unit is the
Pascal (Pa) (or Newton per square meter N/m2).

When measured in sound pressure the hearing scale runs from

20μPa (20 × 10−6Pa) at the threshold of hearing

to 200 Pa (200 000 000 μPa) at the threshold of pain.

Note: compared to static air pressure (101.325 kPa – often approximated to 105 Pa) these variations are very small.

Sound pressure is the ‘effect’ of a disturbance (what is heard). The actual ‘cause’ of
the disturbance, and the resulting reaction effect, is due to the sound power.

The sound pressure equates to the sound power plus a constant (k) which is
dependent upon the acoustics of the environment, the directivity of the sound and
the distance from the source.

Question 11

Rules of the decibel scale 2

Answer 11

10dB expresses a 10 times increase in sound intensity but, to the subjective listener,
seems about twice as loud, i.e. it would take ten violins to sound twice as loud as
one violin.

3dB expresses a doubling in sound intensity (i.e. If one machine gave a sound
pressure level of 75 dB two identical machines would give a reading of 78 dB)
although this would give rise to a just noticeable change.

Question 12

LEPd

Answer 12

daily personal noise exposure

Question 13

Leq

Answer 13

To determine the average dose received over a given time, an integrated sound level
meter balances out the peaks and troughs to calculate a single figure that would give
the equivalent dose over that time. This equivalent level is known as an Leq

Question 14

Noise exposure levels equivalent to 80dB(A)LEPd (the lower exposure action value
– see Table 6.5), can be calculated using the ‘rule of three’.

Answer 14

80dB(A) over 8 hours is the same noise dose as 83dB(A) over 4 hours. Double the 
noise level (increase by 3dB) over half the time gives the same dose.