Cabin noise analysis of an H120 B helicopter for active noise control applications Florian Ernst 1 Delf Sachau 2 Helmut-Schmidt-University / University of the Federal Armed Forces Hamburg Holstenhofweg 85 22043 Hamburg, Germany

ABSTRACT

Helicopter pilots are exposed to high noise levels during flight induced by the rotor, engine and airflow. To reduce this noise exposure, headsets with active noise control functions are increasingly being used. In addition to the passive noise reduction these systems reduce low frequency noise by local destructive interference of the sound waves. The development of active noise control (ANC) systems requires detailed analysis of the acoustic environment, which is rarely available for helicopters. For this reason, sound measurements were taken for different flight parameters on board an H120 B helicopter. Multiple microphones were placed at the crew and passenger positions and in the center of the cabin. In addition, a head and torso simulator (HATS) was placed in the copilot seat to measure the passive attenuation and the ANC performance of a commonly used commercial ANC headset in flight. The results of the analysis provide useful information about the noise characteristics for different flight conditions of the H120 B and identify the most critical noise components.

1. INTRODUCTION

Helicopters are widely used for their special ability of vertical takeoff and landing as well as for hovering flight. Due to this capability, they are often used for special operations, such as search and rescue, surveillance and related tasks. With vertical takeoff and landing abilities, helicopters can operate even in urban regions where normal airports can not operate. This however, comes at the price of high noise emission, mainly induced by the helicopter’s rotor and engine. To reduce noise, special designs for helicopters aiming to reduce noise emission have been developed [1-3]. Noise emission is however not limited to the surrounding area but to the helicopter cabin as well. While classic passive dampening can be applied in the helicopter, lightweight construction is very important in aviation applications. For this reason, weight expensive noise reduction measures are not feasible. To counter high noise levels in the cabin, crew and passenger use passive damping headsets. Noise can be further reduced by using active noise control (ANC) headsets, that further attenuate low

1 ernstf@hsu-hh.de

2 delf.sachau@hsu-hh.de

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frequency noise. Even with this measure, noise levels for the passenger are still very high, leading to high risk for hearing loss or damage, especially for daily operating crews [4].

In previous studies a digital active noise control headset has been developed and tested in an anechoic chamber [5]. During the development, estimates of a sound field in a helicopter were used, as detailed analysis of the acoustic environment is rarely available for helicopters. Publications on helicopter sound measurements can be found for example in [4, 6, 7, 8], although these refer to different classes of helicopters and sound files are not available. For this reason, sound on board of an Airbus Helicopters H120 B helicopter was measured during this study using multiple microphones in different positions within the cabin. Different flight parameters were used to show sound spectra for various flight conditions.

The Airbus Helicopters H120 B (formerly known as Eurocopter EC120 B) is a small type helicopter with a capacity of up to 6 persons on board, depending on the configuration used. It is powered by a single twin-spool free-turbine turboshaft engine, powering a three-blade main rotor with a diameter of 10.0 m [9, 10]. To counter the main rotors torque, the H120 B is equipped with an enclosed 8-blade anti-torque tail rotor. This arrangement is called Fenestron TM and leads to a reduction of the tail rotor noise due to the special design. The enclosure in the fin allows shorter blades which are operated at higher speeds. During forward flight, the fin helps to stabilize the aircraft thus reducing the necessary tail rotor power and noise [1]. The division into several blades leads to a higher blade pass frequency, while an asymmetrical arrangement of the blades further reduces noise emissions. In contrast to conventional tail rotors with two-blade propellers, this makes the helicopter much quieter for an observer on the ground, since high frequencies are attenuated more over distance. However, the higher blade pass frequency lies in the frequency range that is well perceived by humans. Such a tail rotor is therefore perceived quieter from distance, but louder in the cockpit.

In addition to the sound measurements, a widely used commercial ANC headset is used to measure the noise reduction performance and serve as a benchmark, as well as to identify optimization potential for further development. 2. MEASUREMENTS

In order to determine the acoustic conditions on board, an H120 B helicopter is equipped with calibrated measurement microphones at 8 positions as shown in Figure 1. These positions include all crew and passenger positions as well as a centered microphone (4) between the front pilots. A Brüel & Kjaer (B&K) Type 5128 head and torso simulator (HATS) is placed in the copilot’s position (1, 2). During flight a widely used commercial ANC headset is placed over the ears of the HATS, to measure the passive reduction and ANC performance of a standard headset. For the acoustics measurements, B&K 4986 omnidirectional Microphones are used. These microphones have a nearly flat frequency response up to 20 kHz. The signals were recorded by a battery powered frame module containing one B&K 3050 and one B&K 3053 LAN-XI-Frontend using a sample rate of 56 kHz. All microphones are calibrated using a Gras 42AP Calibrator.

According to ISO 5129:2003-10 [11], all ventilation ports are closed and no external sound sources are active during the record periods. There is no nonstandard interior fitted inside the cabin during flight. For the inflight measurements, the pilot is in the front right position, the HATS is in the front left position and two engineers are placed in the rear left and rear right position. The microphones 3, 5, 6 and 8 are fitted on top of the headsets within 0.1 m apart from the typical ear position. The rear middle seat is left vacant and is fitted with a microphone (7) in the typical head position.

Figure 1: Measurement setup in the helicopter Airbus Helicopters H120 B with indication of the microphone positions

Different flight parameters are used during the measurements representing different flight phases that commonly appear in helicopter missions. All measurement durations are at least 30 s. The flight parameters are listed in Table 1. For cruise flight conditions, three cruise speeds v cr are used during level flight at an altitude of 1000 ft above mean sea level (MSL). The segments climb and descend are performed with an indicated airspeed (IAS) of 80 kt and a rate of climb or descend of 500 ft per minute respectively. For the turn segment, a 180° rate 1 turn, equals 3° per second, is carried out at an altitude of 1000 ft MSL. Apart from cruise flight, measurements are taken during hover in ground effect (IGE) above grass surface, while taxiing to the runway over asphalt surface and during hover out of ground effect (OOG) at 1000 ft MSL. Lastly the takeoff phase is measured, while using the speed for best vertical climb rate v y of 65 kt IAS.

Table 1: Flight parameters used during measurement flight

Flight Condition Description Duration

Hover IGE Grass Surface 30 s

Taxi Asphalt Surface 30 s

Take Off v y = 65 kt 60 s

Cruise Flight v cr = 100 kt, A = 1000 ft MSL 60 s

Cruise Flight v cr = 80 kt, A = 1000 ft MSL 60 s

Cruise Flight v cr = 60 kt, A = 1000 ft MSL 60 s

Climb R/C = 500 ft/min, v cr = 80 kt 60 s

Descend R/D = -500 ft/min, v cr = 80 kt 60 s

Turn Rate 1 Turn, 3° / sec, right 60 s

Hover OOG A = 1000 ft MSL 60 s

3. ANALYSIS OF MEASUREMENT DATA

The evaluation is divided into three sections. In the first section, the sound pressure level spectrum is shown for the different microphone positions during cruise flight, from which the individual frequency components can be identified. These frequency components are assigned to the corresponding noise sources and explained in more detail. Subsequently, the sound spectra for the different flight parameters are shown and described. In addition, the overall sound pressure levels for the individual flight conditions are each given unweighted and with applied A-weighting.

In the last section, the performance of the ANC headphones is analyzed and the sound spectra in flight without headset, with headset and with activated ANC function are presented and evaluated.

3.1 Sound pressure level spectra for different flight parameters

The measured sound pressure level (SPL) spectrum of the helicopter during cruise flight is shown at different microphone positions in Figure 2, giving an example of the different sound spectrum components. The contributing components are additionally listed in Table 2. The most visible and loudest spectral component is the blade pass frequency of the main rotor at 20.3 Hz. This matches the expected main rotor frequency of 6.77 Hz multiplied by the number of rotor blades, which is 3. The main rotor frequency does not change during flight as its revolutions are controlled by a so-called governor system. By using this system, the pilot does not need to continuously control the power setting of the engine, reducing the overall workload and leading to less fatigue.

Figure 2: SPL spectrum at different microphone positions during cruise flight at v cr = 100 kt

The next two spectral peaks at 40.6 Hz and 61 Hz are both harmonics of the blade pass frequency of the main rotor. To counter the rotational force induced by the main rotor, helicopters use an anti- torque tail rotor to prevent the helicopter from spinning around its vertical axis. As stated earlier, the H120 B uses a fan in fin construction named Fenestron TM , leading to less thrust needed to counter the main rotor torque, as the fin stabilizes the rotorcraft during forward flight [3]. The tail rotor is driven by the tail rotor shaft, which again is coupled via the main gear box to the engine. With this system, the main rotor and the tail rotor are physically connected to each other and always increase and decrease their rotational speed by the same factor. The frequency of the tail rotor is visible in the spectrum and is approximately 75 Hz. The tail rotor blade pass frequency is determined by multiplying this frequency by the number of tail rotor blades (eight), resulting in a frequency of 602 Hz. This frequency is as well highlighted in the spectrum. Like the main rotor, the harmonics of the tail rotor blade pass frequency at 1204 Hz and 1806 Hz are clearly visible in the spectrum and contribute to the overall sound pressure level.

‘SPL [dB] 110 100 90 80 70 60+ 40 Front Middle Front Left Front Right Rear Left Rear Middle Rear Right 30 10 20 50 200 500 Frequency [Hz] 1000 2000

The helicopter is powered by the twin-spool free-turbine turboshaft engine with a nominal revolution for the gas generator of N G = 54.117 rpm which is approximately 902 Hz [9, 10]. This frequency peaks out in the spectrum and is marked in Figure 2. The power turbine nominal speed is N F = 44.009 rpm which is approximately 733 Hz. Although it does not exactly match this value, the peak of the spectrum at 738 Hz can be aligned with the rotation of the power turbine. The power turbine shaft is coupled to the main gear box with a speed of 6.000 rpm or 100 Hz.

Table 2: Main identified frequencies and the inducing components

Freq. [Hz] 6.77 20.3 40.6 61 75.25 100

Description Main Rotor BPF Main

1. Harm. BPF

2. Harm. BPF

Main Rotor Tail Rotor Engine MGB

Rotor

Main Rotor

Coupling

Freq. [Hz] 602 738 902 1204 1482 1806

Description BPF Tail

Rotor Power Turbine Gas Generator

1. Harm. BPF

1. Harm. Power Turbine

2. Harm. BPF

Turbine

Tail Rotor

Tail Rotor

The SPL spectrum contains multiple frequency peaks above 1 kHz consisting mostly of harmonics from the components and systems described above. In addition, the hydraulic system and aerodynamical noises overlay the sound spectrum as broadband noise.

The identification of the sound sources helps to estimate, how the different systems contribute to the overall sound spectrum at different power settings and flight phases. This is especially important in regards to the development of ANC systems as stationary sound is easier to attenuate than transient sound sources.

The sound pressure level spectra are evaluated next at the microphones for the different flight phases and presented in Figure 3. Throughout the flight only minor differences in the basic structure of the spectrum occur. This is mainly due to the fact, that the main rotor is regulated to maintain a steady rotation speed at any time during flight. Therefore, the main peaks in the spectrum do not change in frequency, even though changes in amplitude can be observed.

SPL [dB] 110 100 90 80 70 60 50 40 30 Front Middle Rear Left Rear Right Front Left Front Right Rear Middle 100 1000 10000 Frequency [Hz]

b) Cruise Flight v cr = 80 kt

a) Cruise Flight v cr = 100 kt

SPL [dB] 110 100 90 | 80 70 60 50 40 30 Front Middle Front Left 10 Frequency [Hz] Rear Left Front Right L Rear Right Rear Middle 100 1000 10000

SPL [dB] 110 100 90 80 70 60 50 40 30 Front Middle Front Left 10 Frequency [Hz] Rear Left Front Right L Rear Right Rear Middle 100 1000 10000

c) Cruise Flight v cr = 60 kt

d) Right Turn

SPL [dB] 110 100 90 80 70 60 50 40 30 Front Middle Front Left 10 Frequency [Hz] Rear Left Front Right L Rear Right Rear Middle 100 1000 10000

SPL [dB] 110 100 90 80 70 60 50 40 30 Front Middle Front Left 10 Frequency [Hz] Rear Left Front Right L Rear Right Rear Middle 100 1000 10000

e) Climb

f) Descent

SPL [dB] 110 100 90 80 70 60 50 40 30 Front Middle Front Left 10 Frequency [Hz] Rear Left Front Right L Rear Right Rear Middle 100 1000 10000

SPL [dB] 110 100 90 80 70 60 50 40 30 Front Middle Front Left 10 Frequency [Hz] Rear Left Front Right L Rear Right Rear Middle 100 1000 10000

g) Hover OOG

h) Hover IGE

SPL [dB] 110 100 90 80 70 60 50 40 30 Front Middle Front Left 10 Frequency [Hz] Rear Left Front Right L Rear Right Rear Middle 100 1000 10000

i) Taxi

j) Takeoff

SPL [dB] 110 100 90 80 70 60 50 40 30 Front Middle Front Left 10 Frequency [Hz] Rear Left Front Right L Rear Right Rear Middle 100 1000 10000

Figure 3: SPL spectrum at different microphone positions for various flight parameters

SPL [dB] 110 100 90 80 70 60 50 40 30 Front Middle Front Left 10 Frequency [Hz] Rear Left Front Right L Rear Right Rear Middle 100 1000 10000

In order to structure the evaluation, the differences between individual microphone positions will be discussed first. This is followed by a discussion of the individual flight parameters.

From the different sound pressure level spectra in Figure 3 it can be seen that the spectral characteristics of the individual microphone positions are generally similar. In the tonal peaks, however, larger deviations between the microphones can be seen in some cases, for example in (a) during cruise flight. Considering the highest amplitude in the spectrum, the blade pass frequency of the main rotor at 20 Hz, it can be seen, that the front right microphone is the loudest, followed by the rear right, front middle, rear middle, front left and rear left microphone in this order. The difference between the highest and lowest SPL at this frequency is 7.4 dB during cruise flight at 100 kt. While the amplitudes change at various flight phases, the order of the loudest microphone does not change significantly.

Outside of these tonal peaks, the microphones do not differ significantly from each other during the flight. The largest deviations are seen in the frequency range from 50 to 200 Hz with up to 10 dB difference between the rear middle microphone and the front right microphone. In this frequency range, the microphones in the rear of the cabin measure higher values, which correlates with the smaller distance to the engine compartment and rotor. This observation is constant over all flight parameters (a-f).

It appears that the microphones positioned in the right-hand side of the helicopter measure higher sound pressure levels. While the reason for this cannot be explained directly from the data, there are some features that could lead to this observation. One reason might be the asymmetrical layout of the cabin with respect to the cabin doors. The left side has a large door for the front seats and a large sliding door for the rear seats, whereas the right side also has a large door for the front seats but only a small door for the rear row of seats. This can result in the right side of the cabin being noisier. Another reason might be the air intake on the top of the cabin, which is slightly offset to the right.

In the frequency span, where less tonal components contribute to the spectrum (e.g. 150-500 Hz), the spectrum consists mainly of broadband aerodynamic noise. Within this region, the microphones in the front of the cabin are loudest in this frequency range during cruise flight. During hover flight (g), where no airstream is present, the SPL of this frequency span is also lower compared to cruise flight conditions (a-c).

Since the rotor speed remains constant during flight, more lift is generated during takeoff (j), climb (e), and fast forward flight (a) by increasing the main rotor blade angle, resulting in an increase in engine power. This leads to an increase in the amplitudes of the BPF and its harmonics, as well as the amplitudes of engine frequencies. With higher airspeed, the overall noise floor increases as well. These progressions are well seen in the comparison of the spectra from the hover OOG (g) and the increased velocities 60 kt (c), 80 kt (b), and 100 kt (a). While the spectra of the descent (f) and climb (e) do not seem to differ much at first sight, the higher engine power during climb leads to higher amplitudes in the frequencies of the gas turbine (740 Hz) and power turbine (902 Hz). In general, more power is needed in a turn to maintain speed and altitude. However, the spectrum of turning flight (d) at 80 kt is not noticeably different from cruise flight at 80 kt (b). While this is surprising, the evaluation showed that the target speed of 80 kt was not maintained during the turn and therefore less power was needed.

Comparing the hover flight states out of ground effect (g) and in ground effect (h) it can be seen that higher amplitudes are measured in ground effect, even though through this effect less power is generally needed. This can be explained by sound reflections from the ground that don’t occur in flight out of ground effect. Comparing (h) to (i), this effect is even more visible, due to different ground surfaces. The hover flight measurement was performed above grass surface, while there was asphalt surface during taxi, leading to higher reflections [12] and thus higher SPL.

The prescribed effects contribute to the overall sound pressure level, which is presented in the following section.

3.2. Global sound pressure levels

In addition to the previously shown spectra, the overall sound pressure levels evaluated at the rear middle microphone are shown in Table 3. The selection of this microphone was based on the fact that the differences between the microphones are small, as shown previously, and according to the requirements of ISO 5129:2001 [11] , no person is present at this position. For each flight phase the unweighted SPLs are presented. In addition, acoustic A-weightings are applied to the SPL, to better respect human hearing.

Table 3: Sound Pressure Levels for different Flight Phases

Flight Phase L [dB] L [dB(A)]

Cruise 100 kts 111.5 95.9

Cruise 80 kts 111.6 94.8

Cruise 60 kts 110.1 93.4

180 Right turn 111.1 94.4

Climb 80 kts, 500 fpm 112.4 94.9

Descend 80 kts, 500 fpm 111.7 94.6

Hover OOG 104.6 94.4

Hover IGE 108.6 94.5

Taxi 111.9 95.3

T/O 111.7 96.6

The overall SPL at the rear middle microphone varies between 104.6 dB during hover OOG to 112.4 dB during climb. As neither aerodynamic noise nor reflections from ground surfaces occur, hover flight is the quietest state during the helicopter flight. With an increase of 4 dB the hover IGE above grass surface follows next. During taxi over asphalt surface, higher reflections and aerodynamic noise increase the overall sound pressure level to 111.9 dB, which is even higher than in cruise flight at 60 kt with 110.1 dB. The sound pressure levels of the other flight states do not vary much, leaving climb flight state the loudest at 112.4 dB.

While these sound pressure levels are very high, it is interesting to take the human perception into account, which can be achieved by applying A-weighting to the SPL spectrum. As the human ear perceives low frequencies much less, this leads to drastically changes in the A-weighted spectrum, as the main helicopter components lay in this frequency range. The unweighted and A-weighted spectra measured at the rear middle microphone are shown in Figure 4 for cruise flight at 100 kt.

Figure 4: Unweighted and A-weighted SPL spectrum at rear middle microphone during cruise flight at v cr = 100 kt

While the A-weighted spectrum above 600 Hz is nearly identical to the unweighted one, the difference increases with lower frequency up to 50.4 dB at 20 Hz within the main rotors blade pass frequency. From the A-weighted spectrum it can be seen that the tail rotor blade pass frequency and its harmonics are perceived loudest by a human listener. As the highest peak in the spectrum mainly determines the overall sound pressure level, the A-weighted overall SPL is closely related to the tail rotor SPL. The tail rotor is connected to the main rotor thus also kept at a steady rotation speed during flight. Therefore, larger variations in the A-weighted overall SPL do not occur, as seen in Table 3. The overall A-weighted SPL ranges from 93.4 dBA during cruise flight at 60 kt to 96.6 dBA during takeoff. Compared to the SPL spectra from Figure 3 (c, j) these values correlate to the first harmonic of the tail rotor blade pass frequency. Even in the quietest flight phase, the sound pressure level is at a dangerously high level, with high risk of ear damage if the crew is exposed to this noise for longer time. It is therefore necessary to use adequate hearing protection. The noise reduction performance of such a headset is evaluated in the following section.

SPL [dB] L Unweighted —— AWeighted 20 50 100 200 500 Frequency [Hz] l 1000 2000 5000 10000

3.3. ANC headset noise reduction

For reference, a commercially available ANC flight headset was used during flight to measure the noise reduction performance. The headset was therefore placed on a head and torso simulator in the front left seat in the helicopter cabin. Measurements were taken during all flight phases each once with activated ANC, deactivated ANC for passive attenuation and once with the headset removed for reference measure. In this study the noise reduction is analyzed for 100 kt cruise flight condition. The unweighted sound pressure level spectrum of the left HATS ear is shown in Figure 5.

Figure 5: Unweighted SPL spectrum at left HATS ear during cruise flight at v cr = 100 kt

SPL [dB] L 20 50 100 200 500 Frequency [Hz] i 1000 2000 5000 10000

With headset in place, low frequency components are slightly amplified below 100 Hz. Passive attenuation starts at 180 Hz and increases with higher frequencies up to 4 kHz at which point it remains constant with an attenuation of about 40 dB. While all measurements were made with the same flight parameters, they were taken at different times. Therefore, slight differences occur, as it can be seen at 600 Hz, where the peak without headset is smaller than in the measurement with headset on.

With active noise control enabled low frequency components are further reduced with attenuations of up to 34 dB compared to passive attenuation alone. The headphones are not capable of reproducing low frequencies well, leading to a smaller reduction at low frequencies. The largest reduction, averaging over 25 dB, occurs in the range from 50 to 300 Hz. With increasing frequency, the reduction decreases again, up to approx. 1000 Hz. From this point on, passive reduction becomes predominant. While low-frequency noise is effectively reduced with ANC, some amplification occurs at higher frequencies. This is particularly noticeable in the tonal peaks, e.g. at 1220 Hz. In this case, the ANC system amplifies the tonal peak by up to 7 dB.

The overall sound pressure level within the shown frequency range at the HATS ear without headset is 110 dB. With the headset in place, the amplification of low frequency components leads to a slightly higher overall SPL of 112 dB. With ANC activated, the SPL reduces to 96 dB.

Figure 6: A-weighted SPL spectrum at left HATS ear during cruise flight at v cr = 100 kt

Considering human hearing perception, the spectra from Figure 5 are shown A-weighted in Figure 6. As previously described in 2.3, the A-weighting strongly reduces low-frequency components, so that the focus tends to shift to the higher-frequency components. As seen in Figure 6 and mentioned before, the frequency components of the tail rotor as well as the higher harmonics contribute significantly to the overall sound level. Since the A-weighting shifts the amplitudes of individual frequencies, the relative reductions are also the same in the A-weighting. However, the A-weighting has a great influence on the calculation of the total sound pressure level. The overall sound pressure level within the shown frequency range without headset is 99 dBA. With the headset in place, the overall SPL reduces to 85 dBA. While low frequency components are still slightly amplified, these components no longer contribute as much to the overall SPL due to the weighting as they are perceived quieter. With ANC activated, the SPL further reduces to 80 dBA. At this level, sufficient noise reduction is achieved to provide adequate hearing protection for the crew. However, the evaluation also shows that the loudest perceived noise components lie outside the range controlled by the ANC system. In fact, the ANC even amplifies these frequency components to some extent. Nevertheless, the ANC system achieves a lower noise level compared to passive attenuation because the low-frequency noise range is not sufficiently passively attenuated. The higher the frequency, the more difficult it is to control it with an ANC system. Considering the high noise levels above 1 kHz, it would be beneficial to optimize the ANC system for these frequencies.

80 SPL [dBA] S Q S 3 nN J Without Headset Passive Attenuation _ Active Noise Control 20 50 100 200 500 1000 Frequency [Hz] Ll 2000 10000

4. CONCLUSIONS

In this study sound measurements taken in different locations within the cabin of an H120 B helicopter where analyzed. It can be seen, that the overall sound pressure level does not change significantly during different flight phases. The sound pressure level varies between the different locations in the cabin, which might be explained by the asymmetric cabin design. Using the sound pressure spectrum, individual helicopter components of the engine and rotor could be identified and analyzed during different flight parameters. The observed changes are mostly restricted to changes in amplitude, as the main and tail rotor speeds are controlled to be constant in all flight phases.

Using a head and torso simulator, the noise reduction performance of a commercially available ANC headset was analyzed. It could be seen that major components of the sound pressure spectrum inside the cabin are above 1 kHz, which are not attenuated by the active noise control system. Low frequency components are significantly reduced by the ANC headset. However, this only leads to a small reduction of the A-weighted overall SPLs due to the unattenuated high frequency components of the tail rotor.

In future studies, the recordings and results of this analysis can be used to reproduce the sound field of a helicopter more realistically in the laboratory and thus to test ANC systems. Such ANC systems should consider the main helicopter frequency components identified in this study to further reduce noise in the cockpit.

5. ACKNOWLEDGEMENTS

This research paper is funded by dtec.bw – Digitalization and Technology Research Center of the Bundeswehr in the project MissionLab which we gratefully acknowledge.

6. REFERENCES

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