Design and fabrication of active noise control system for a grass cutter

Abstract

Active noise control technique was used to reduce noise generated by a grass cutting machine. Grass cutting machine running on a diesel engine generates loud noise of about 105 dBA. Based on the spectral analysis of engine noise, it was observed that frequencies in the range of 440–5000 Hz were having more noise power. An active noise control circuit was designed and fabricated using operational amplifiers. The active noise control circuit was tested with the help of a duct made of thermocol. Results show that reduction in noise up to 10 dBA was obtained when the active noise control circuit was used with a duct made of thermocol, while a reduction up to 5 dBA was obtained when used on a grass cutting machine. The active noise control system developed may be used to reduce noise generated by a grass cutting machine.

Keywords

Active noise control active noise control circuit noise level grass cutter duct digital signal processing

Introduction

Grass cutting machines are widely used in various applications like agriculture, gardening, landscaping, etc. The level of noise generated by these machines is of very high intensity and may be a health hazard for workers using them.^1-4 Mallick et al.⁵ investigated the effect of two types of grass-trimming machine engines noise on the operators. It was found that sound pressure level (SPL) of the grass-trimming machine engines was higher than the limit of noise recommended by ISO, NIOSH, and OSHA (1971) for an 8-h workday. Tengku et al.¹ while determining noise exposure among grass cutters showed some evidence of mild to moderate hearing impairment in some workers. Although the OSHA (1971) guidelines existed for workers using such machines, there is no protection for the workers in grass mowing workplace. In urban areas, lawn maintenance is a very common activity. Noise levels generated by grass cutting machine were measured using sound level meter at the operator’s ear at a distance of 10 ft and was found to be in the range of 95–100 dBA which were more than permissible limits set by OSHA and NIOSH; therefore, there was a need to bring the noise within the acceptable limits. Both passive and active noise control (ANC) techniques are used for bringing the level of noise down. ANC has better advantage over passive control. Many researchers have used ANC techniques in the past successfully in various applications like reducing car engine noise.^6,7 Shi et al.⁸ observed that ANC was ideal for reducing low-frequency noise and was an incomparable addition to passive noise control. Narula et al.⁹ demonstrated ANC using Simulink and data acquisition toolbox; adaptive filters were used in real-time environment. Cuesta et al.¹⁰ designed two ANC systems to attenuate low-frequency noise, that is, hybrid passive–active control system and a single-input single-output ANC system. Reduction of more than 20 dBA was obtained using the two systems. Kuo et al.¹¹ developed an ANC system which used filtered-x LMS adaptive algorithm. The system leads to the reduction of 7 dBA in the SPL. Tiseo et al.¹² observed that passive noise control was most effective at mid and high frequencies, while ANC techniques are better effective at low frequencies. Iwai et al.¹³ proposed a multichannel feedforward ANC system for reduction of acoustic broadband noise by using multiple reference microphones located close to noise sources. Following methodology was adopted in the present work.

Methodology

The noise level was measured using a sound level meter-Pulsar model 33. General features of the model are as follows: “The model 33 is a powerful, user-friendly instrument for acoustic measurements. It can work as a type-1 integrating-averaging sound level meter that complies with IEC 60651, IEC 60804, and IEC 61672 international standards and their EU counterparts EN 60651 and EN 60804. The model 33 also complies with the American standards ANSI S1.4 and ANSI S1.43. The model 33 is also a spectrum analyser measuring in real time and in one-third octave bands and octave bands with type-1 filters, which comply with IEC 61260 and EN61260. They also comply with ANSI S1.11. The noise level meter was having a microphone (Class 1 MK: 224 Class+ Electret Prepolarised Condenser, Class 2 MK: 216 Class+) with a measuring range from 22 dB (A) to 137 dB (A) with noise floor (typical) as 17 dB (A). The frequency weighting and time weighting were dB (A) to IEC 6065 1 type 1 and F, S, and I to class 1 tolerances, respectively.

A survey was carried out to determine the level of noise generated by a grass cutter. The level of noise was measured using a sound level meter (Pulsar model 33). Spectrum analysis of noise generated by the grass cutter was done. The grass cutting machine and noise level meter used in the present study have been shown in Figure 1. It was seen that frequencies in range of 1000–5000 Hz were contributing more toward noise. Simulation for ANC was done using MATLAB and Simulink library. Different models were created and tested by generating various levels of noise of different frequencies. The model was compiled to check for any error. The position of the speaker and noise level meter has been shown in Figure 2. Level of noise was checked by sound level meter. The resulting noise was checked by sound level meter and reduction in decibel was evaluated.

Figure 1.

Grass cutter used for analysis and sound level meter for noise measurement.

Figure 2.

Position of speaker and sound level meter.

Simulation model

Simulink software was used to design and simulate the system prior to fabricating the hardware. This facilitated the creation of a virtual prototype for building the model. The software offers a simulation environment with hardware validation. In Simulink, Digital Signal Processing System Toolbox was used to implement various models. Different types of filters like low-pass, high-pass, and band-pass were used for filtering specific frequency range. In low-pass filters, a cutoff frequency is specified and the filter blocks all frequencies above the cutoff frequency, while high-pass filter does the reverse, that is, it blocks all the frequencies below this frequency. Band-pass filters pass the specific range of frequencies given by the lower and upper cutoff frequency and block the rest (Figure 3). Two loudspeakers (make creative) were used in the simulation process. Speaker 1 was used as the noise generating source and the microphone recorded the noise and sent it to the Simulink circuit where the anti-noise generated was played by speaker 2.

Figure 3.

Active noise reduction simulation using filter.

Hardware design

To cancel the noise, an op-amp-based circuit is used which can be divided into two stages. The first stage is a non-inverting preamplifier followed by an op-amp-based inverting amplifier. The circuit used is simple, as it requires op-amp, resistors, and capacitors apart from the power supply. The signal picked up by the microphone is weak and before any subsequent processing is applied on it, requires amplification. The preamplifier (Figure 4) stage boosts the signal level, and the gain of this stage depends on the ratio of R3 and R2. In order to cancel the noise, an opposite phase signal is to be generated having the same frequency components. In the second stage, inverting amplifier, as shown in Figure 5, achieves this by giving a closed-loop voltage gain as

\frac{V_{output}}{V_{input}} = - \frac{R_{f}}{R_{i}}

Figure 4.

Preamplifier op-amp circuit.

Figure 5.

Inverting op-amp circuit.

Since no amplification is required at this stage, the values of $R_{f}$ and $R_{i}$ were chosen equal. A switch is used to select either the raw noise signal or its phase inverted version. This helps to measure the noise level in the presence and absence of the proposed circuit. A variable resistor is used to fine-tune the overall gain of the entire circuit for and to choose the setting for maximum noise cancellation.

Experimentation

For conducting experiment on grass cutting machines, the noise generated by the machine was assumed to be a point source and sound travels in all directions in 3D environment. The grass cutter engine is the primary source of noise to workers performing the task. The noise generated by the various components of the lawn mower also contributes to the increase in noise level. The noise generated by these sources is multidirectional. The ANC can only work effectively if the noise emitted is unidirectional. In an ANC system, the purpose of the duct/box is to make the noise flow in one direction instead of multiple directions. So, a box of plywood was made to render the noise generated as a point source. The plywood box may not reduce the noise significantly because it only made the noise flow one way.

A hole was drilled in the enclosure through which a microphone was passed; this microphone recorded the sound generated by the machine and sent it to the ANC circuit. The experimental setup has been shown in Figures 6 and 7.

Figure 6.

Enclosure to make sound unidirectional.

Figure 7.

Complete setup for experimentation and position on microphone.

Observations

Noise emitted by a grass cutter was recorded for no load, moderate load, and maximum load conditions as shown in Figure 8. It can be observed that at no load conditions, the higher frequencies were dominant, while at maximum load, lower frequencies were prevalent. Inverted noise was played through speaker placed in the path of original noise and reduction in noise level was measured. Reduction in noise level is shown graphically in Figures 9–11.

Figure 8.

Noise generated by grass cutter at different loading conditions.

Figure 9.

Frequency spectrum of noise of 1500 Hz without and with active control.

Figure 10.

Frequency spectrum of noise of 2000 Hz without and with active control.

Figure 11.

Frequency spectrum of the noise (a) without and (b) with active control for 4000 Hz.

Results and discussion

After conducting experiments, observations were recorded and have been presented in Table 1. It may be seen that considerable reduction was obtained in level of noise of various frequencies. When the circuit was assembled on breadboard, the reduction was obtained in noise level. However, a humming sound was produced in the circuit. When the same circuit was assembled on a printed circuit board, the humming sound was eliminated. A reduction of up to 10 dBA was obtained using a duct fabricated in the present study. The circuit designed showed good results when it was used on the grass cutter and a reduction of up to 5.5 dBA was obtained. The actual reduction in the noise level at the operator’s ear was obtained as 4 dBA. The difference in reduction in the level of noise was due to the fact that complete destructive interference did not occur because machine sound waveform was random in nature. Also, ambient noise caused corruption of the inverted waveform resulting in the difference in noise reduction. The ANC circuit implemented on PCB for ANC of the grass cutting machine is based on simple op-amp, which is readily available and cheap. As seen from Table 1, substantial reduction in the noise level is achieved over a wide range of frequencies. It is important to note that the maximum reduction of 12 dBA in the noise level is achieved at 3000 Hz, a frequency at which the human ear has maximum sensitivity. Thus, the simple and cost-effective ANC circuit proposed can be used to reduce noise in the perceptually important frequency range in which the human ear has high sensitivity, thereby reducing the risk of damage to the operator’s ears.

Table 1.

Reduction in the level of noise achieved using active noise control.

Frequency, Hz	Sound level, dBA	Sound level with active control, dBA	Reduction achieved, dBA
440	71	60	11
2000	70	62	8
3000	86.5	74	12
4000	89	80	9
5000	83.2	75	8.2

Conclusion

Present study resulted in the significant reduction in noise levels generated through a grass cutting machine. Results may be improved using various filters like high-pass filters, band-pass filters, etc. The ANC system presented in the present research may be used for reducing noise in various grass mower machines.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Mohammad Muzammil

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