PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Extraction of high contributing sound of Air Handling Unit andnoise reduction using transfer path analysis
Takuya HIRAO1; Yousuke MIYAMOTO1; Junji YOSHIDA1
1

Osaka Institute of Technology, JapanABSTRACT
Air handling unit (AHU) is generally built in a machine room of a building and controls the temperature inmultiple office spaces. The motors of AHU generate sound and vibration and have a possibility to cause noiseproblem in office. Accordingly, it is necessary to extract high contributing part of the AHU and apply
intensive countermeasures to the part quickly. In this study, we attempted to reduce the noise by applying theoperational TPA to the simple AHU model. In the test, the radiated sound in front of the model was recorded
and the frequency band which increased the overall noise level was extracted. In addition, operational TPA
was applied to find out the main contributor to the radiated noise. As the result, the noise at 200 Hz band waslarge and the motor radiated noise was found to be the main contributor. Furthermore, the noise was increasedin the unit case by the cavity resonance. Then, the inside volume of the AHU was separated using a plastic
plate for the reduction of the motor noise in the unit. The result showed that the unit radiated noise at thefrequency band could be decreased well.
Keywords: Air handling unit, Radiated noise, Transfer path analysis

1. INTRODUCTION
Air handling unit (AHU) is a system for controlling temperature, humidity and air flow of multiple
office spaces (1). This system is generally built in a machine room of the building placed on near
office. Hence, the system may cause the noise and vibration problems to the office spaces bytransmitting the noise and vibration of the unit. In the noise reduction activity of AHU, finding out
the main contributors to the radiated noise and applying intensive countermeasure to the part isessential to realize the effective countermeasure. However, when the noise problem occurs and thepeople in office recognize it, the unit is already installed and the office is already running. Accordingly,
we cannot use adequate duration for the analysis and the countermeasure. In such a case, an instantanalytical technique for obtaining the high contributing part is required.
In this study, we attempted to utilize operational transfer path analytical (TPA) method to theoriginal AHU model to realize the effective noise and vibration analysis for AHU. In addition, we
actually tried the radiated noise reduction using the analytical results and a fundamental CAE
technique to verify the effectivity of the method.

2. NOISE MEASUREMENT OF AIR HANDRING UNIT
In this study, simple AHU model was made to analyze the radiated noise of AHU as shown inFig. 1 (a). This model was a small size based on a compact AHU system having frame, two fun motors
and the air ducts. The length, width and height were 400 x 1600 x 800 mm. Each part and the air flow
image are described in Fig. 1 (b).

1junji.yoshida@oit.ac.jp1263

(a) –Outside view(b) –Internal parts and air flow
Figure 1–Air handling unit model

The radiated noise was recorded at first to grasp the sound characteristics. In the recording, soundpressure signal was measured using a microphone positioned at 1 m from the front of the AHU model
at a height of 1 m as shown in Fig. 1 (a). As the AHU condition during the recording, the supply airmotor (SA motor) and the return air motor (RA motor) speed were increased from 0 to 3600 rpm for
20 s. After then, the speed was constant at 3600 rpm for 10 s. Figure 2 shows the averaged A -weighted
SPL of the radiated noise.

Figure 2 – Averaged spectrum of the radiated noise of AHU
Horizontal and vertical axes show frequency and A-weighted SPL, respectively. From this figure,A-weighted SPL peaks were found at 100 Hz, 200-250 Hz, and 300 Hz bands. These peaks seem tobe the main factors deteriorating the quietness of the AHU. Hereafter, we analyze the main
contributing parts to the radiated noise at the frequency bands using TPA for the effective
countermeasure to improve the quietness of the AHU model.

3. FACTOR ANALYSIS OF NOISE USING OPERATIONAL TPA
3.1 Transfer Path Analysis
In this section, we applied TPA to find out the main contributing parts to the radiated noise of AHU.
In many kinds of TPA methods (2-5), we applied operational TPA (OTPA) (4, 5). This TPA method
was known to be able to calculate the contributions with smaller man -hour comparing with the othermethods. In the OTPA, input force are not required to be measured but reference point vibrations orsound pressures at close to input points and the response point (radiated noise) are necessary to bemeasured simultaneously. The contributions of the response point from the reference p oints arecalculated as follows.
3.2 Procedure of OTPA (4)In the OTPA, the individual contributions are calculated by two-step processes. In the first step,the transfer function from the reference point to the response point is calculated using various

1264

operational signals measured at the reference and response points. In the second step, eachcontribution is obtained by multiplying the reference point signal by the calculated transfer function.
The transfer function in this method is calculated using a principal component regression method. The
calculation procedure is shown in Fig. 3.

Figure 3 – OTPA model using principal compornent regression method
At first, a response signal and multiple reference signals are transposed to frequency -domain
signals from time-domain signals by fast Fourier transform (FFT). The response signal matrix [Aout]
can be expressed by the reference matrix [Ain ] and the transfer matrix [H] for each frequency, as
follows:[Aout ] = [Ain ][H](1)
The (i, j)-th element in the reference matrix [A in] is the data at the j-th reference point in the i-th
fast Fourier transform (FFT), and the (i, k)-th element in the response matrix [Aout ] is the data at thek-th response point in the i-th FFT. The (j, k)-th element in the transfer matrix [H] is the transfer
function from the j-th reference point to the k-th response point. Following singular value
decomposition (SVD), the reference matrix [Ain ] can be expressed as follows:[Ain ] = [U][S][V]T(2)
The principal component (PC) matrix [T] is then obtained using the results of SVD, this procedureis the same as principal component analysis (PCA):[T] = [Ain ][V] = [U][S](3)
Where [V] is the coefficient matrix for transposing the reference matrix [Ain ] to the PC matrix [T],and the (i, m)-th element of [T] is the m-th PC in the i-th FFT. Next, multiple regression analysis(MRA) from the PC matrix [T] to the response matrix [A out ] was performed, and the regression
coefficient matrix [B] was obtained as follows:[Aout ] = [T][B](4)[B] = ([T]T [T])-1[T]T [Aout ](5)
Where [B] is the coefficient matrix used to transpose the PC matrix [T] to the response matrix[Aout], and the (m, k)-th element of [B] is the regression coefficient from the m-th PC to the k-thresponse point. The transfer function [H] is obtained by multiplying the coefficient matrix [V] withthe coefficient matrix [B], as follows:[H] = [V]([T]T [T])-1[T]T [Aout ](6)[H] = [V][S]- 1[U]T [Aout ](7)
The contribution to the response signal from each reference signal [A out cont ] can be obtained bymultiplying the measured reference signal [A cont] at the contribution separation target condition by thecalculated transfer function [H]:[Aout cont ] = [Ain cont ][H](8)
The (i, k)-th element in the contribution matrix [Aout cont] is the contribution at the k-th response point in
the i-th FFT. This procedure was used to obtain the contributions in this study.

1265

3.3 Analysis of High Contributing Part to the Radiated Sound
Here, we applied OTPA to find the main contributing parts to the radiated noise. We then used theradiated sound pressure recorded in front of the AHU model as the response signal. As the reference
signals, front-back vibration, up-down vibration of SA motor and those of RA motor were employed
as shown in Fig. 4. In addition, radiated noise near SA motor and RA motor were also used as thereference signals. These response and reference signals were measured simultaneously to obtain eachcontribution by OTPA.
As same as the previous test condition, the radiated noise was recorded when the SA motor andRA motor speed was increased from 0 to 3600 rpm for 20 s and the speed was constant at 3600 rpm
for 10 s as the operational test condition.

Figure 4 – Measurement points for OTPA
To assess the contribution from each reference signal easily, we calculated group contribution
according to the parts position as shown in Fig. 5. In this case, SA motor group contribution and RA
motor group contribution were calculated respectively to determine the high contributing part between
SA and RA motor. The calculated group contributions are shown in Fig. 6.

Figure 5 – Contribution of each reference point and group contribution to the response point

Figure 6 –SA and RA motor contributions

1266

Horizontal and vertical axes of right diagram of Fig. 6 show the frequency and the sound pressurelevel, respectively. Gray solid and black dotted lines show the SA and RA group contribution. As theresult, the RA motor contributions were observed to high at 100 Hz, 200 -250 Hz and 300 Hz bands.
In addition, the SA motor contributions were also high at 100 and 200-250 Hz bands. Then, each
motor sound and acceleration contributions were calculated to obtain the detailed contribution of themotor respectively. The calculated contributions are shown in Fig. 7 and 8.

Figure 7 – SA detailed contribution

Figure 8 – RA detailed contribution
Figure 7 and 8 show the contribution of motor vibration and sound of SA and RA motors,respectively. From the SA detailed contributions in Fig.7, the SA motor sound contribu tion wascalculated as very high at 100 Hz and 200-250 Hz bands. About the RA detailed contribution inFig. 8, the motor sound contribution was also high at 100 Hz, 200 -250 Hz and 300 Hz bands.
From these results of OTPA, both motor sounds were found to be dominant source to the radiatednoise in front of the AHU at 200-250 Hz. Hence, we focused on the frequency band in the following
analysis. Figure 9 (a), (b) and (c) shows the SA and RA sound contributions and the sound pressurelevel and transfer function of SA and RA motor sound which compose the contribution.

1267

(a) –SA contribution(d) –RA contribution(b) – SA sound pressure signal(e) –RA sound pressure signal(c) –SA transfer function(f) –RA transfer function
Figure 9 – Contribution, sound pressure signal and transfer function of SA and RA motor
As shown in Fig. 9, the contribution peak at around 200-250 Hz band of SA and RA (Fig. 9 (a) and(d)) were found to be made by the sound pressure signal (Fig. 9 (b), (e)) and the transfer function(Fig. 9 (c), (f)) because both elements had peaks at around the focused frequency. From the results,
we supposed that the cavity resonance in the unit case of SA and RA motors increased the soundpressure level near the motors and the transfer functions to the response point.

4. COUNTERMEASURE OF AHU NOISE USING OTPA RESULT
4.1 Eigenvalue Analysis by CAE
In the previous OTPA, we estimated the main factor increasing the front radiated noise at 200 250 Hz band as the resonance in cavity by utilizing the calculated contribution elements. Here,acoustical eigenvalue analysis was carried out to evaluate the estimated factor by using CAE.
Figure 10 shows the acoustical mode of SA cavity at 213 Hz. Noting that the cavity volume and shapeof RA was identical of that of SA, hence, the resonance frequency and the acoustical mode becamethe same.

Figure 10 – Acoustical mode of SA and RA cavity at 213 Hz obtained by CAE

1268

As shown in the figure, the acoustic mode along the top-down direction was obtained at 213 Hz in
the SA and RA cavity. This frequency met where the SPL and transfer function of SA and RA motors
had peak. Hence, the cavity resonance was clarified to be the main factors of the AHU noise peak at
200-250 Hz band.
4.2 Countermeasure
For the reduction of the AHU noise at the target frequency, we then divided the cavity into 2 spaces
by using a plastic plate to move the resonance frequency to high as shown in Fig. 11.

Figure 11 – Modified model by separating the cavity using plastic plate
We again carried out the operational test again before and after the modification to verify theeffectiveness of the countermeasure. Figure 12 (a) and (b) shows the comparison of the averaged Aweighted SPL near the SA and RA motor before and after the countermeasure.

(a) –SA motor SPL(b) –RA motor SPL

Figure 12 – Comparison of SA and RA motor SPL before and after countermeasure
Black solid and dotted lines show the motor sound before and after countermeasure. As shown inthese figures, the SPL near SA and RA motors were observed to decrease about 10 dB at the target
frequency band. This indicates that the cavity resonance was actually the main factor increasing theSA and RA motor noise at 200-250 Hz.
Then, we recorded the front side SPL of the AHU before and after the modification. Figure 13
shows the comparison of SPL.

1269

Figure 13 – SPL comparison of the front side radiated noise of AHU
Black solid and gray dotted lines are the averaged SPL of the front side radiated noise of AHU
before and after the countermeasure, respectively. As shown in the figure, the SPL only at 200-250 Hz band could be reduced about 5 dB by taking countermeasure to the cavity resonance estimatedby using OTPA and CAE. From these results, the high contributing part to the radiated noise of AHU
could be found by applying OTPA and CAE.

5. CONCLUSIONSIn this study, we attempted to utilize an OTPA method to the original AHU model to realize theeffective noise reduction of the AHU. In addition, we applied a fundamental CAE technique toevaluate the estimated high contributing part in detail.
Through the radiated noise measurement at the front side of the AHU, the SPL was large at
100 Hz, 200-250 Hz and 300 Hz bands in the operational condition. Then, we applied OTPA to theradiated noise of the AHU model to find out the main contributing parts. As the result, both SA andRA motor sounds and the transfer function to the front side radiated noise of AHU were calculated tohave high contribution at 200-250 Hz band. From the analytical results, we estimated the cavity
resonance in the unit case of SA and RA motor were the main factors of the AHU SPL peak.
Subsequently, we applied an acoustical eigenvalue analysis to the cavity to consider the main factorof the AHU noise peak at 200-250 Hz in detail. As the result, the cavity resonance frequency was
213 Hz and the acoustical mode in cabin was obtained. Accordingly, we separated the cavity volume
by inserting a plastic plate in the SA and RA motor cavity for the verification of the analysis.
Consequently, the radiated noise could be decreased about 5 dB only at the target frequency by thecountermeasure. This revealed that the method could indicate the correct high contributing part to
AHU radiated noise.
From these results, the OTPA and CAE method were clarified to have ability to analysis theimportant contributing part to the AHU radiated noise for carrying out quick measurement, analysis
and countermeasure.

REFERENCES
1. Arizono S., “Function and basic structure of air conditioner AHU (air handling unit) – FCU (fun coilunit)”, BE Construction equipment, 2008, p.171.
2. Van der Auweraer H., Wyckaert W., Van Der Linden P., “Noise and Vibration Transfer Path Analysis,
Lecture series - van Kareman Institute for fluid dynamics”, van Kareman Institute, Rhode-Saint-Genèse,Belgium, 1979.
3. Van der Auweraer H., Mas P., Dom S., Vecchio A., Janssens K., Van de Ponseele P., ͆Transfer PathAnalysis in the Critical Path of Vehicle Refinement: The Role of Fast, Hybrid and Operational PathAnalysis͇, SAE 2007-01-2352㸪Proceedings SEA Noise and Vibration Conference, St.Charles (IL), USA㸪2007, pp.15-17.
4. Noumura K., Yoshida J., “Method of Transfer Path Analysis for Vehicle Interior Sound with No Excitation
Experiment”, FISITA2006 Proceedings F2006D183, 2006, pp.1-10.
5. Yoshida J., Yoshida S., Okajima M., Hosokawa A., N., Hujii T., “Contribution analysis of drum typewashing machine noise during drying using multi-layered operational TPA”, Inter Noise17, CHINA, 2017,
pp.1195-1204.

1270