Measurement report:
Compressor bearing for reducing
primary airborne noise
Quieter equipment thanks to efficientvibration isolation of the compressor

Room dimensions: 4.08 m × 9.14 m

MP 6

MP 3Compressor

MP 4Measurement configuration of an air conditioning unit showing measuring
point 6 (a microphone) and measuring points 3 and 4 (geophones)

Set-upBenefits

Measurements were carried out on a compact air conditioning unit in order to compare the effect on primary airborne noise of the differing elastic bearings ofa compressor. A GEA Bock compressor (see Fact box)
is the main source of vibrations. Noise and vibration
measurements were carried out at a number of measuring points. To prevent distortion from any standing
waves that might occur, the device was positioned parallel to the wall. The microphone at measuring point 6
was aligned perpendicular to the top of the housing at
the same level as the compressor.

— Quieter equipment with a verifiable reduction in airborne noise of more than 7 dB(A)— Better working environment in adjacentparts of the building due to lower structure-borne noise propagation— A lower level of housing vibration increases
the service life of electronic components inthe control unit

The compressor is equipped with a frequency converter to enable the rotational speed of the compressor tobe modified. The working range lies between 30 and70 Hz. The rotational speed of the fan was set to atleast 30 % of its maximum and held constant during
all the measurements.

Type:

Fact box

Pmax:
V/h:
Weight:

GEA Bock
Reciprocating piston compressor
Semi-hermetic
HGX22P/160-4S
18/28 bar
13.7 m3/h
80 kg

Investigated bearing types
To provide a meaningful comparison, the rubber-metal bearing fitted in the device was compared with Isotop DMSN vibration dampers, a steel spring/Sylodamp®
combination and Isotop MSN-DAMP. When selecting theappropriate Isotop products, the stability of the overallsystem is an important factor. For the Isotop DMSN,
for example, a higher deflection than with the Isotop
MSN-DAMP has to be considered.

Isotop MSN-DAMP

Rubber-metal bearing

Isotop DMSN

Measurement results
Structure-borne noise
Secondary airborne noise is generated by the vibrations
introduced into the floor. Propagation into the building
was determined as a function of frequency using twogeophones at measuring points 3 and 4. Over this measurement, the rotational frequency of the compressor wasincreased steadily from 30 to 70 Hz.

With reference to the technical rule ONR 1990551, it canbe assumed that the reduction in the level of introduced
vibration will also produce a marked reduction in theemission of secondary airborne noise.

At low compressor speeds, a marked reduction in structure-borne noise emissions was observed, showing an improvement across the whole operating frequency range.

MP4: Floor away from bearing

85Vibration velocity level dBv

Vibration velocity level dBv

MP3: Floor near the bearing

807570657065605560505545405030354045

Existing rubber bearing
Isotop MSN-DAMP
Isotop DMSN

1755055706065
Inverter frequency in Hz

30354045

Existing rubber bearing
Isotop MSN-DAMP
Isotop DMSN

ONR 199005 - Calculation of the secondary airborne noise level from vibration measurements, Austrian Standards Institute, 2008

5055606570
Inverter frequency in Hz

Measurement results
Airborne noise
The airborne noise level was also measured as a functionof frequency. The signal was recorded at measuring point
6, and the A-weighted airborne noise level was calculated.
To start with, the airborne noise level was measured
when using the rubber-metal elements fitted as standard(average of four measurements). The maximum noise
emissions at operating frequencies of 30, 56 and 69 Hz
are clearly evident. The rubber-metal elements were thenreplaced by Isotop DMSN and Isotop MSN-DAMP fromGetzner, and the test was repeated. This revealed asignificant improvement in the emitted airborne noise
figures across the entire frequency spectrum. The measured reduction at the three maxima (30, 56 and 69 Hz)
was 7.7 dB(A) and 8.9 dB(A) lower with Isotop MSN-DAMPand Isotop DMSN respectively.

60

Reduction of airborne noise level in dB(A)Airborne noise level dB(A)A-weighted airborne noise level at MP6
Average values from four measurements

595857
5655
5453

98765
4352

251

150

Reduction inairborne noiseof more than7 dB(A)100354045

Existing rubber bearing
Isotop MSN-DAMP
Isotop DMSN

5055606570Inverter frequencyLooking at the effectiveness of the Isotop bearing at
an operating frequency of 30 Hz in the frequency
domain (third-octave spectrum), a significant improvement can be observed compared to a conventional rubber-metal bearing. Particularly noteworthy is
the fact that an almost uniform improvement occurs
across the entire audible frequency range.

30 Hz56 Hz

69 Hz
Inverter frequency in Hz

Isotop MSN-DAMP
Isotop DMSN

Airborne noise in the third-octave spectrum
at a low inverter frequency of 30 Hz
Airborne noise level dB(A)3060555045403530252015102031.563100

Existing rubber bearing
Isotop MSN-DAMP
Isotop DMSN

2004008001250 2500 4000 6300
Third-octave band in Hz

Summary
1. If structure-borne noise, i.e. the propagation of vibrations into the housing of an HVAC device, is reduced, theequipment will be quieter.
2. Providing efficient vibration isolation of the compressor produces a verifiable reduction in airborne noise of morethan 7 dB(A).
3. Accurately designed PU bearings give better results than conventional rubber-metal bearings – over the entire lifeof the device.

Put us to the test!
Every unit is different. The configuration and foundation also differ.
We would be delighted to support you in selecting the right bearing
for your unit or in conducting measurements.

Isotop products have well-defined properties, so to come up with a design using standard products is a very
simple matter. Register, and take advantage of EquipCalc, our free online selection tool.

MB compressors airborne noise en © Copyright by Getzner Werkstoffe GmbH | 02-2023

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