Proceedings of the Institute of Acoustics

 

 

Elastic hangers for suspended ceilings – are they really needed?

 

Bengt Johansson¹, Efterklang (Part of AFRY), Solna, Sweden

 

ABSTRACT

 

Suspended ceilings made of several layers of gypsum boards are a common way to increase the sound insulation of slabs in buildings. The ceiling is typically hanging 100-500 mm below a slab. There are many manufacturers of the hangers and most of them supply elastic hangers, made of rubber or metal springs. The elastic hangers are very costly and there are very little scientific data of the effect of these elastic hangers, especially at low frequencies and especially for concrete slabs, which are the most common slab type. The sound insulation of normal buildings is increasing with frequency and the low frequency region is the most interesting region. The author has for more than 30 years been part of many cinema projects where elastic hangers have not been used. Still, the sound insulation at low frequencies is excellent. What are the most important parameters? Could it be that elastic hangers are even reducing sound insulation due to resonances at low frequencies? In this paper we will discuss measurement data both from laboratory and real buildings with focus on air-borne sound insulation at low frequencies. We will also discuss calculations methods.

 

1. INTRODUCTION

 

Suspended ceilings made of several layers of gypsum boards are a common way to increase the sound insulation of slabs in buildings. The ceiling is typically hanging 100-500 mm below a slab. There are many manufacturers of the hangers and most of them supply elastic hangers, made of rubber or metal springs. Unfortunately, there are very little measurement data of the sound insulation of suspended gypsum board ceilings from concrete slabs, especially if the ceiling is suspended without any elastic springs.

 

The aim of this paper is to investigate the available prediction methods and compare with measurement data. Since the most common slab type is made of concrete, the focus will be on gypsum board ceilings suspended from concrete slabs.

 

The low frequency region is often the most problematic region, since the sound insulation is low by nature is this region. Due to this the focus will also be on the low frequency region.

 

2. PREDICTION METHODS

 

2.1. Theoretical methods

 

In the book “Sound Insulation in Buildings” [1] by Rindel one method is described for calculating the air borne sound insulation of double constructions with bridges. The bridges can be line- or point-bridges with or without elastic elements.

 

The total sound reduction can be devided into the sound transmission through the cavity and the sound transmission via the sound bridge.

 

 

Figure 1: The total sound reduction can be devided into the sound transmission through the cavity and the sound transmission via the sound bridge.

 

 

𝑅_𝑖 is the sound reduction of panel i.

 

The mass-air-mass resonance 𝑓₀ can be calculated from:

 

 

𝑚_𝑖 is the surface weight of panel i [kg/m²], d is the panel distance [m], 𝜌_𝑑 is the density of the gas in the cavity [kg/m³], 𝑐_𝑑 is the speed of sound of the gas in the cavity [m/s]. If we have air in the cavity 𝑐_𝑑 = 𝑐 (342 m/s) and 𝜌_𝑑 is 1,2 kg/m³.

 

The knee frequency for standing waves in the cavity:

 

 

We rewrite the sound reduction through the bridge as:

 

 

𝑅_{(1+2 )}is the sound reduction of panel 1 and 2 close together.

 

 

𝑓_𝑐𝑖 is the coincidence frequency of panel i.

 

κ_p is the resonant radiation factor  

 

N_p is the number of connections

𝛾_𝑝 is the coupling factor A3

 

𝜎₂ is the radiation efficiency of panel 2.

 

𝜂₂ is the loss factor of panel 2.

 

𝑘_𝑑𝑝 is the stiffness of the connection [N/m]

 

We want the sound reduction through the bridge to be high.

 

ΔR_m is large if N_p is small, i.e. number of connections is small

 

ΔR_m is large if 𝑓_𝑐𝑝 is large:

 

This means 𝑓_𝑐1 and/or 𝑓_𝑐2 should be high and mi should be low

 

ΔR_m is large if the coupling factor is small where is small, i.e. stiffness 𝑘_𝑑𝑝 is low. At frequencies below 𝑓_𝑘𝑝 the sound bridges behave as stiff connections.

 

ΔR_m is large if the resonant radiation factor is small.

 

The radiation factor 𝜎₂ should be low (f << f_c) and internal loss factor 𝜂₂ high.

 

Please notice that 𝛾_𝑝 is 1 for a stiff point connector which means Δ𝑅_𝑚 is (almost) independent on frequency.

 

Calculation example:

• 200 mm concrete slab. 468 kg/m²

• 200 mm cavity with 200 mm mineral wool.

• 2x13 mm gypsum board suspended in point connections. 18 kg/m²

 

 

Figure 2: Example slab with suspended gypsum board ceiling.

 

In diagram 1 we can see the calculation results according to ref [1] for a 2x13 mm gypsum board ceiling suspended 200 mm below a 200 mm concrete slab. The distance between point connections are 600 mm. The stiffness of the elastic coupling is chosen to give 14 Hz mass-spring resonance. If we look at the individual paths, airborne through the cavity and structure borne through the connections we can see that the transmission through the stiff connections is higher than the air borne path through the cavity for the whole frequency range. On the other hand, the transmission through elastic point connections is lower that air borne transmission if the stiffness is sufficiently low.

 

 

Diagram 1: Calculation results according to ref [1] for a 2x13 mm gypsum board ceiling suspended 200 mm below a 200 mm concrete slab. 600 mm distance between point connections. Sound reduction through elastic point connections is higher than the air borne sound reduction.

 

In diagram 2 the calculation results are presented for hangers with different stiffness. The stiffness is chosen to represent the basic resonance for the mass-spring system of the actual ceiling.

 

 

Diagram 2: Calculation results according to ref [1] for a 2x13 mm gypsum board ceiling suspended 200 mm below a 200 mm concrete slab. 600 mm distance between point connections with different stiffness. Total sound reduction including both air borne and structure borne paths. The sound reduction with elastic point coupling can be almost as good as without connection.

 

2.1. Software

 

There are few commercial software that can be used to predict the sound insulation of a slab with suspended gypsum board ceiling.

 

2.1.1 Insul

 

Insul [2] is the most common prediction tool, sold in over 4 000 licenses worldwide. It is developed by Marshall Day Acoustics in New Zealand. Insul can predict sound insulation of single, double or triple constructions with many types of studs and bridges. In diagram 3 prediction results with different types of point connections are shown. For low frequencies there are almost no effect of elastic hangers compared to stiff.

 

 

Diagram 3: Prediction results from Insul for a 2x13 mm gypsum board ceiling suspended 200 mm below a 200 mm concrete slab. 600 mm distance between point connections. Almost no effect of elastic hangers at frequencies below 100 Hz compared to stiff hangers.

 

2.1.1 ENC

 

ENC [3] is software containing calculation methods from the book “Engineering Noise Control” by Bies & Hansen. Module 5 deals with sound insulation calculations with theories from Davy and Sharp. In diagram 4 prediction results with Sharp theory with different stiff point connections are shown.

 

 

Diagram 4: Prediction results from ENC for a 2x13 mm gypsum board ceiling suspended 200 mm below a 200 mm concrete slab. 600 mm distance between point connections. Elastic coupling is not possible for Sharp theory.

 

In diagram 5 prediction results with Davy theory with different stiff point connections are shown. The effect of elastic point connections are only visible for high frequencies.

 

 

Diagram 5: Prediction results from ENC for a 2x13 mm gypsum board ceiling suspended 200 mm below a 200 mm concrete slab. 600 mm distance between point connections. Different types of elastic couplings are possible for the Davy theory, but a strange gap in the results is present between 160 Hz and 630 Hz. The elastic coupling only affects the high frequency region and not below 125 Hz.

 

2.1.1 NorFLAG

 

NorFLAG [4] is software that is based on the Transfer Matrix Method, TMM. It is developed by Tor Vigran in Norway. The TMM technique is often used for prediction of sound absorption but can also be used for sound insulation prediction. Norflag has the possibility to add rigid or flexible connections, of both point type and line type. In diagram 6 prediction results are shown for point connections with different elasticity.

 

 

Diagram 6: Prediction results from NorFlag for a 2x13 mm gypsum board ceiling suspended 200 mm below a 200 mm concrete slab. 600 mm distance between point connections. User defined spring constant of the elastic couplings are possible.

 

3 MEASUREMENTS

 

There are surprisingly few available measurement results of gypsum board ceilings suspended from concrete slabs. This includes both field measurements and laboratory measurements. The reason for this is not clear.

 

3.1 Measurement methods

 

When measuring vertical sound insulation in a laboratory, the test slab is often positioned inside an opening in the laboratory slab. In order to limit the structural coupling, there is an elastic connection to the rest of the slab.

 

The measurement methods are standardized in ISO 140-3 or ASTM E90. One of the suppliers below have used an alternative method with a tapping machine. It is not clear how well this method works.

 

3.2 Problems with measurements

 

All laboratories have a certain limit for the maximum sound insulation possible to measure in the lab, without having flanking sound affecting the measurement result.

 

Since the sound insulation of a concrete slab with suspended gypsum board ceiling is very high, it is important that the limit of the lab is not affecting the measurement result. This limit is often indicated in the measurement result.

 

This problem is an even bigger with field measurements, where the flanking transmission can be severe, especially with poured concrete walls. The flanking issue can often be less problematic with gypsum board walls (Dry walls).

 

3.3 Measurement results

 

3.3.1 Measurement of AMC hangers [5]

• Laboratory: Labein Tecnalia in Spain

• Original slab: Ceramic hollow block with concrete according to Figure 3.

• Surface mass 356 kg/m²

• Measurement method: EN ISO 140-3

 

 

Figure 3: The tested slab with suspended gypsum board ceiling.

 

Tested ceilings:

3x15 mm gypsum board suspended 255 mm from slab. Rockwool, 40 mm 160 kg/m³ + 50 mm 20 kg/m³.

• M6 rod (stiff)

• Elastic hangers “Akustik 4’ (45 shore A)

• Elastic hangers “Akustik 3 + Sylomer 30 Type B’

 

The predictions are made for a massive concrete slab with the same surface weight of 356 kg/m².

 

 

Diagram 7: Prediction results from NorFlag and Insul of original concrete slab compared to measurement result

 

 

Diagram 8: Prediction results of concrete slab with 3x15 mm gypsum boards ceiling suspended 255 mm below a concrete slab in stiff point couplings, compared to measurement result.

 

 

Diagram 9: Prediction results of concrete slab with 3x15 mm gypsum boards ceiling suspended 255 mm below a concrete slab in elastic point couplings, compared to measurement result.

 

 

Diagram 10: Prediction results of insertion loss, ΔR, of the ceiling with 3x15 mm gypsum boards ceiling suspended 255 mm below a concrete slab in elastic point couplings, compared to measurement result. None of the prediction methods work well.

 

 

Diagram 11: Prediction results of insertion loss, ΔR, of the ceiling with 3x15 mm gypsum boards ceiling suspended 255 mm below a concrete slab in stiff point couplings, compared to measurement result. Measured 30 dB insertion loss for stiff point connections at 1 kHz is impressive.

 

Comments:

 

There were no measurement results below 100 Hz. According to diagram 8 to 11 the measurement result is much better than all prediction methods for frequencies above 200 Hz. Insertion loss of 30 dB at 1 kHz is impressive for stiff point connections, see Diagram 11. The insertion loss is less for frequencies above 1 kHz and 0 at 5 kHz, which may be due to measurement problems but is not commented in the measurement report.

 

3.3.2 Measurement of Vibratec VT-SFC hangers [6]

• Laboratory: Akustikverkstan, Sweden

• Original slab: 160 mm concrete

• Surface mass 374 kg/m²

• Measurement method: Tapping machine

 

 

Diagram 12: Prediction results of insertion loss, ΔR, of concrete slab with 2x13 mm gypsum boards ceiling suspended 250 mm below a concrete slab in elastic point couplings, compared to measurement result. Measurement result show good effect above 200 Hz, in alignment with Rindel predictions. Below 200 Hz little effect and much less effect than Rindel prediction.

 

Comments

 

The measurement result show very good effect at frequencies above 200 Hz, but very little effect below 200 Hz. The reason for this result is unclear but may be due to the chosen measurement method with tapping machine. The measured insertion loss (ΔR) corresponds well with predictions from Insul and Rindel above 200 Hz. No measurement data is reported for frequencies above 1 kHz.

 

3.3.3 Measurement of Kinetic Noise ICC Isolation hangers [7]

• Laboratory: NRC, Canada

• Original slab: 150 mm concrete

• Surface mass 356 kg/m²

• Measurement method: ASTM E90. Excitation with 4 speakers. 9 microphone positions

 

 

Diagram 13: Prediction results of 150 mm concrete slab, compared to measurement result. Measurement shows very good agreement with Insul prediction.

 

 

Diagram 14: Prediction results of concrete slab with 2x15 mm gypsum boards ceiling suspended 330 mm below a concrete slab in elastic point couplings, compared to measurement result. Measurement shows very good agreement with Rindel prediction.

 

 

Diagram 15: Prediction results of the insertion loss (ΔR) of the ceiling with 2x15 mm gypsum boards suspended 330 mm below a concrete slab in elastic point couplings, compared to measurement result. Measurement shows very good agreement with Rindel prediction.

 

Comments:

 

The measurement result show good agreement with the Rindel method. There are few point connections separated 1200 mm, which give low structure borne transmission through the connections. This allows the prediction for the structure borne path to be less accurate without affecting the total result.

 

3.3.4 Measurements of stiff vs elastic hangers

 

There are very few measurements of gypsum board ceilings suspended in elastic hangers from concrete slabs. There are even fewer measurements for stiff hangers. This means there are very few measurement results available to prove if the elastic hangers actually are efficient.

 

To prove this we need the same configuration with and without the elastic element. We need the identical slab, ceiling and cavity.

 

Only one measurement has been found and the slab is unfortunately not a massive concrete slab.

• Laboratory: Labein Tecnalia in Spain

• Original slab: Ceramic hollow block with concrete. See Figure 4.

• Surface mass 356 kg/m²

• Measurement method: EN ISO 140-3

 

 

Figure 4: The tested slab with suspended gypsum board ceiling.

 

Tested ceilings:

3x15 mm gypsum board suspended 255 mm from slab. Rockwool, 40 mm 160 kg/m³ + 50 mm 20 kg/m³.

• M6 rod (stiff)

• Elastic hangers “Akustik 4’ (45 shore A)

• Elastic hangers “Akustik 3 + Sylomer 30 Type B’

 

 

Diagram 16: Measurement result of the insertion loss (ΔR) of the ceiling with 2x15 mm gypsum boards suspended 255 mm below a concrete slab in point couplings. Elastic hangers seem to be about 2-6 dB better than stiff hangers above 160 Hz. Very little effect of elastic hangers at is seen frequencies below 160 Hz.

 

 

Diagram 17: Measurement result of the insertion loss (ΔR) of the AMC elastic hangers compared to stiff point connections. The rubber hanger shows negative effect at 125 Hz, meaning amplification of low frequency noise.

 

Comments:

 

There seem to be between 2 and 6 dB to gain from the AMC elastic hangers compared to stiff hangers. From diagram 17 it is seen that the rubber hanger gives -2 dB insertion loss at 125 Hz, meaning amplification compared to stiff m6 rod. The Sylomer hangers gives 2 dB ΔR at 100 Hz and 125 Hz. Unfortunately, no measurement data is available at frequencies below 100 Hz.

 

3 CONCLUSIONS

 

Calculations according to ref [1] show elastic hangers should increase sound insulation in the whole frequency range 50 Hz - 5000 Hz, if the stiffness is selected well.

 

Predictions with Insul show that elastic hangers do not increase sound insulation below 100 Hz. At higher frequencies elastic hangers increase the sound insulation depending on the elasticity. Predictions with ENC show elastic hangers increase sound insulation at high frequencies only. Predictions with NorFLAG show very lite benefit from elastic hangers at frequencies below 100 Hz and no benefit at 50 Hz.

 

There are very limited measurement data that can show increased sound insulation for elastic hangers compared to stiff hangers. Only two measurements of AMC hangers were found. One of the elastic hangers shows 2-6 dB increased sound insulation at frequency 100 Hz – 1 kHz. The other one show decreased sound insulation at 125 Hz compared to a stiff hanger. Unfortunately, no measurement data for these hangers was available at frequencies below 100 Hz.

 

Measurement data of suspended gypsum board ceilings show that even with stiff hangers the ceiling can improve sound insulation with up to 30 dB, which is much higher than calculations and predictions show. Adding elastic hangers only improves sound insulation with 2-6 dB compared to stiff hangers. Considering the cost of elastic hangers compared to stiff hangers the price per dB is very high.

 

The author is aware of the limited number of measurements the conclusions are based on. The suppliers of elastic hangers are suggested to perform measurements also with stiff hangers and also for the original slab without the ceiling.

 

4 REFERENCES

 

1. Rindel, J H. Sound Insulation in Buildings, Chapter 9 (2018).

2. INSUL version 9.023, Software for sound insulation predictions, Marshall Day Acoustics, www.insul.co.nz

3. ENC version 5.5, Software based on the book Engineering Noise Control, by Bies, Hansen. www.causalsystems.com

4. NorFlag, software for calculation of sound absorption and sound transmission with Transfer Matrix Method. https://web2.norsonic.com/product_single/norflag/

5. Measurement result AMC, LABEIN report 91.2184.0-IN-CT-10/28, B0082-IN-CM-26-M53-I, - M56-I, -M59-I

6. Measurement result Vibratec VT-SFC, Akustikverkstan report 18-718-R3

7. Measurement result Kinetic Noise ICC, NRC Canada, report B-3448.12, B-3463.1

 

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¹ bengt.johansson@efterklang.org