Low-frequency noise or low-frequency sound waves typically in the 50 to 100Hz range, are consistently the most difficult to absorb or manipulate. They have strong penetrating ability, long propagation distance and not easy to decay, so it is very difficult to control them effectively.
The emergence of artificial sound-absorbing materials and metamaterials has made effective sound attenuation possible, but such passive acoustic structures are still limited by structural size in the low frequency range.
Active noise reduction/noise reduction technology is to interfere with and eliminate noise waves in a certain volume by transmitting sound waves with exactly the same spectrum as noise but with opposite phase (difference of 180°). This technology is widely used in headphones and cars.
Note: The picture is from the Internet
Piezo ceramic actuator is a kind of actuator with large output and relatively small displacement. Its output can reach tens of thousands of Newtons, and the displacement can vary from a few μm to more than 260μm. It has a variety of different sizes, the larger the outside diameter, the greater its output; The higher the height, the longer its displacement will be.
Model |
Displacement |
Push/pull force |
No-load resonant frequency |
PSt150/5/60 VS10 |
57μm |
550/100N |
15kHz |
PSt150/7/80 VS12 |
76μm |
1200/200N |
12kHz |
PSt150/10/100 VS15 |
95μm |
2300/250N |
10kHz |
PSt150/14/120 VS20 |
114μm |
4700/700N |
8kHz |
PSt150/20/140 VS25 |
133μm |
7300/1000N |
6kHz |
PSt150/20/200 VS25 |
190μm |
7300/1000N |
3kHz |
PSt1000/10/150 VS18 |
150μm |
4000/700N |
5kHz |
PSt1000/16/60 VS25 |
60μm |
12000/1500N |
27kHz |
PSt1000/25/40 VS35 |
40μm |
25000/4000N |
30kHz |
PSt1000/35/20 VS45 |
20μm |
50000/6000N |
35kHz |
The principle of low-frequency noise control through the piezo ceramic actuator is to adjust the vibration displacement of the wall in the noise space through the piezo actuator, such as displacement frequency and phase, so as to change the impedance of the wall and other surfaces. When the impedance of the wall matches the air, the noise will be absorbed completely, and when the impedance is zero, the phase will be reversed, causing total reflection. In addition, the amplitude of the reflected wave can also be modulated.
To achieve the effect of the piezo ceramic actuator tuning, the acoustic sensor and the piezo controller can form a closed-loop control, the acoustic sensor (such as microphone, etc.) to receive and process the noise, and generate the corresponding electrical signal, the electric signal can be fed back to the piezo controller, the piezo controller for receiving and processing. Then the voltage signal corresponding to the tuning displacement is sent to the piezo ceramic actuator, and the piezo ceramic actuator generates the corresponding vibration displacement, so that the wall driven by it generates vibration displacement and is tuned to achieve full absorption or full reflection of noise.
Low frequency noise generally has a long wavelength and requires a large displacement of piezo ceramic actuator. Therefore, it is necessary to amplify the displacement of piezo ceramic actuator. But also because of its low frequency, time delay is longer than high frequency, so that the noise sensor can have more processing time, and piezo ceramic actuator also has more displacement adjustment time, displacement vibration power is relatively lower.
The operating power of piezo ceramic actuators is usually less than 5W and can be estimated by U^2fC.
According to the requirement of large displacement, the mechanical amplification structure can be used to amplify the displacement of piezo ceramic actuator to meet the requirement of low frequency wavelength.
In addition, the piezo amplified actuator can also be directly used to drive the wall. Its output is relatively small, but its output displacement is larger, up to 2mm.
Model |
Displacement |
Blocking force |
Non-load resonant frequency |
95A10 |
95μm |
540N |
350Hz |
100A7 |
100μm |
155N |
1950Hz |
500A10 |
500μm |
22.5N |
390Hz |
2000A10 |
1600μm |
11.2N |
190Hz |
Driven by a piezo actuator, the walls reflected by sound waves are actively tunable. In order to absorb or reflect all the noise, it needs to have the same frequency and amplitude as the noise, but the phase is opposite. Firstly, the phase of the wall displacement coincides with the phase of the noise wave and is locked. Then, the piezo ceramic actuator is used to continuously adjust the displacement of the wall to find the best position for noise reduction.
In this process, the vibration adjustment of the wall, which means, the surface impedance adjustment, needs to be completed before the noise arrives. The transmission rate of electrical signal is much faster than that of acoustic signal, and the signal of sensor will arrive earlier than the noise, which provides an advantage for its subsequent noise reduction. In addition, the closer distance between the acoustic sensor and the noise will also improve the feedback and processing speed of the sensor signal. The signal of the acoustic sensor should arrive in advance, generate the required driving signal, and reach the piezo ceramic actuator, to adjust the amplitude, phase, and so on, so as to adjust the wall impedance. In the case of multiple sound sources, it is necessary to tune for the composite track.
The noise cancellation is targeted at low frequency sounds. For the control of high frequency sound waves, a similar method can be adopted, but passive acoustic metamaterials may have better results. For wide acoustic wave control, active tuning and passive control can be combined.
Conclusion
By adjusting the rapid displacement amplitude of the piezo ceramic actuator, the wall impedance can be adjusted in the low frequency range of 50 to 100HZ, and the effect of full absorption or full reflection can be achieved. The displacement of piezo ceramic actuator is linearly amplified to meet the requirement of low frequency wavelength and realize the purpose of wall surface impedance modulation. Even the small size of the wall will produce better noise reduction effect, and effectively reduce noise for low-frequency sounds, to provide better indoor hearing experience, which is very suitable for all kinds of low-frequency noise reduction scenes.