Technical data

56 | Project planning manual for heat pumps for heating and domestic hot water preparation | 01.2012 www.dimplex.de
5 Noise emissions from heat pumps
5 Noise emissions from heat pumps
Every noise source - a heat pump, a car or an airplane - emits a
certain amount of sound. Thus, the air surrounding the source of
noise is turned into vibrations and the pressure spreads out in
waves. On reaching human ears, this pressure wave creates vi-
brations in the eardrum, which triggers the hearing process.
Sound field parameters are used to describe this so-called air-
borne sound. The sound pressure and sound power are two of
these parameters.
The sound power is a theoretical dimension typical for a sound
source. It can be mathematically calculated from measurements.
The sound power is the total acoustic energy emission in all di-
rections.
The sound pressure is the change in air pressure as a result of
vibrations in the air caused by the noise source. The greater the
change in the air pressure, the louder the noise will be perceived.
Physically speaking, sound is caused by the propagation of pres-
sure and density fluctuations in a gas, liquid or solid. Generally,
sound is absorbed (i.e. heard) by human beings in the form of air-
borne sound as a noise, a tone or a bang. Pressure changes in
the range between 2*10
-5
Pa to 20 Pa can be detected by human
hearing. These pressure changes correspond to vibrations with
frequencies between 20 Hz and 20 kHz and represent the
human audible sound or the range of audibility. The frequencies
result in individual tones. Frequencies that are above of the
range of audibility are referred to as ultrasonic sound, those
below as infrasonic sound.
The sound transmissions from noise or other sources of sound
are given or measured in decibels (dB). This is referred to here
as a reference value, in which the value 0 dB generally repre-
sents the auditory threshold. A doubling of the sound level, i.e. by
a second source of sound with equal sound transmission, corre-
sponds to an increase of +3 dB. For the average human sense
of hearing, an increase of +10 dB is necessary so that a noise is
perceived as twice as loud.
There are two types of sound propagation.
Solid-borne noise
Mechanical vibrations are started in bodies such as machines
and building sections, transmitted within these bodies and par-
tially emitted as airborne sound at a different point.
Airborne Sound
Sound sources (bodies triggered into creating vibration) create
mechanical vibrations in the air, which spread like waves and are
detected by the human ear.
5.1 Sound pressure level and sound power level
The terms "sound pressure level" and "sound power level" are
frequently confused and compared with each other. In acoustics,
sound pressure refers to the measurable level that is triggered by
a sound source at a certain distance. The closer the sound
source, the higher the measured sound pressure level and vice
versa. Thus, the sound pressure level is a measurable dimen-
sion, dependent upon distance and direction; it is, for example a
decisive factor for adherence to the immission standards accord-
ing to the German government's TA-Lärm-Technical Instructions
for Noise.
The total air pressure given off in all directions by a sound
source, is denoted as sound power or as sound power level. In-
creasing distance from the sound source causes the sound
power to spread out upon an ever-increasing surface. The value
will remain unchanged as long as the total emitted sound power
is taken into consideration with reference to the enveloping sur-
face at a specified distance. Because the sound power emitted in
all directions cannot be precisely calculated metrologically, the
sound power must be determined from a measured sound pres-
sure at a specified distance. Thus, the sound power level is spe-
cific to the source of sound, independent of distance and direc-
tion, and can only be only be determined via mathematical
calculation. Based on the emitted sound power level, sound
sources can be compared with each other.