Chinese New Year's Eve, my sister-in-law showed up at my house on time with her son and daughter. The boy is seven or eight years old, addicted to iPad, watching animation; The girl is about three years old, energetic and likes to jump up and down. At the moment, they are all doing what they are good at - screaming.
Luckily, I still had a bit of sobriety, turned the noise-canceling headphones out of my bag, put on a metal music and turned the volume up to 8 bars.
The phone prompted me to "the headphone volume is high", it doesn't matter, the world is finally "quiet". No, why can I still hear my sister's screams? Do noise-canceling headphones still treat them differently when they "isolate" external sounds?
Don't blame my sister, it's the noise-canceling headphones that are wrong
I borrowed a few small noise-cancelling headphones and went out to give them a try. Chinese New Year's Eve, there are not many cars on the roads around the community, and only two shops are still "hawking" on one road.
I used the Apple Watch "noise" feature to test traffic noise between 65-67, and a short honk pulled decibels above 70. When the Bose QuietControl 30 (in-ear) noise cancellation level is maximized, it visibly filters out engine sound, leaving more shrill and astringent friction between the wheels and the surface.
First of all, we must understand the principle of noise-canceling headphones. The core is "Active Noise Cancellation" (ANC) technology. Through the microphone equipped on the headphones, the external sound is collected, and then the reverse sound wave with the same waveform and phase difference of 180 degrees (that is, the opposite) is generated through the noise reduction chip, and the two waves cancel each other to achieve the purpose of noise reduction.
The principle of active noise reduction丨Image source: Physics Book
Here's the problem. The low-frequency sound wave period is long, and the waveform is relatively regular; High-frequency sound waves have fast frequencies, short sound waves, and unpredictable changes. Active noise cancellation technology is difficult to quickly identify high-frequency noise and generate an active signal with an accurate difference of 180 degrees to counteract it. The shielding effect of low-frequency noise is better, and the higher the frequency of the sound, the worse the shielding effect on it.
This technology has been developed for half a century. In the 50s of the 20th century, a doctor filed a patent on active noise reduction. In the 70s and 80s, both Bose and Sennheiser were working on making special noise-canceling headphones for pilots.
The human ear can hear frequencies between 20Hz-20000Hz. Among them, 30-150Hz is the low frequency band, 150-500Hz is the medium and low frequency band, 500-5KHz is the medium and high frequency band, 5K-16KHz high frequency band, 5K-20KHz very high frequency band.
The frequency range of human voices is 100Hz to 8000Hz, and most of them are in mid and high frequencies.
Helicopter noise is loud at frequencies below 250Hz, and when frequencies exceed 250Hz, the volume tends to decrease significantly. Most of the low-frequency noise is distributed between 10-40Hz, and the sound law in the aircraft environment, fan, combustion chamber, turbine, jet noise, these categories, are low-frequency, low-frequency noise. So this is why, active noise canceling headphones are first applied from this field.
According to the noise reduction ability test of AirPods Pro 2 by the evaluation community Woodenear, when ANC is turned on, it can be reduced by up to nearly 45dB in the low and mid frequency bands. As the frequency of the sound increases, the noise reduction ability is significantly weakened. Even today's mainstream noise-canceling headphones, the performance gap between mid-low frequency and high frequency is still obvious.
The green line is when noise cancellation is on, and the blue line is when noise reduction is off丨Image source Woodenear
Just as I was immersed, two little boys "threw a shot" at my feet, caught off guard in a quiet sound environment. I threatened to snatch two from them and ran to a place where no one was doing an experiment.
I recorded the sound of the falling cannon and analyzed it with sound software. It was found that the sound of falling guns was concentrated between 1KHz-10KHz (mid-high frequency), and the fluctuations were more violent. No wonder, according to my actual listening experience, the sound of falling cannons is "highlighted" in the noise-canceling headphones.
Band analysis of the sound of the crashing gun丨Image source: author
Well, I know, when I meet the "bear child" again, I might as well cover my ears.
Active noise reduction with "rebound", passive noise reduction by "cover"
"Covering the ears" is actually a passive noise canceling (PNC) method, which is to physically isolate the human ear from external noise. To put it bluntly, when the noise reduction headphones are not turned on for active noise cancellation, they play a passive noise reduction role.
I was standing next to the "Michelle Ice City" wearing a Sony WH-1000XM5 (Baotou type). Feel the effect of passive noise reduction, more like "covered with a layer" of overall noise reduction, car driving, street music have decreased to a certain extent, but still heard clearly. The sound of music from afar, because the ears were "covered", were almost isolated.
The passive noise reduction effect depends largely on the product form.
Covered ear headphones are used to "wrap" the ears to isolate noise, while the in-ear type fills the ear canal with ear tips made of silicone, sponge and other materials. Flat head plugs are not as strong as in-ear noise reduction. Professional PNC soundproof headphones can block medium and high frequency noise by about 10-40 dB according to different frequency bands.
Of course, it is also related to the noise band.
One difference between low-frequency and high-frequency noises is whether they attenuate as they propagate. High-frequency sound penetration is poor, and once the distance is far, it will be significantly attenuated when encountering obstacles. So physical "isolation" is effective. Low-frequency sound has a long transmission distance and strong penetration. Active noise cancellation is better to "cancel".
I used to wear traditional flat-head plug-up headphones (without active noise cancellation), riding the subway, and I couldn't wait to turn up the volume. In this way, hearing damage can be caused in the long term.
Long-term high decibel environment to harm the human body丨Image source: Chinese Preventive Medicine Association
According to scientific research, one threshold for damage to hearing cells is: 85 decibels for 8 consecutive hours. Therefore, in recent years, active noise cancellation technology has been replaced.
Noise canceling headphones, microphone + chip + algorithm
If you are also a "heavy user" of noise-canceling headphones, you may remember the uncomfortable experience of slow response, noise floor, poor anti-wind noise, and strong negative pressure at the moment of wearing the noise-canceling headphones in the early years.
The "orientation" of the microphone has a big impact. The feed-forward internal structure is relatively simple, placing the radio microphone on the outside of the headphones to detect the surrounding sound environment. The signal is then transmitted to the processor for processing, and the speaker emits reverse sound waves to "neutralize" the noise.
The feedback type is to place the microphone on the inside of the headphones, near the speaker, closer to the sound that the human ear can hear. The disadvantage is that music sounds similar to noise are treated as noise.
When both single-feed noise cancellation methods have drawbacks, the hybrid type places a microphone both outside the headphones and near the speakers. This is also the way that the flagship models of large manufacturers are adopted today. For example, the Sony WH-1000XM5 is equipped with 4 microphones (on one side). The external 3 microphones will deal with ambient noise first, canceling the noise. 1 internal feedback microphone detects residual noise near the ear canal and performs another noise reduction.
Limited by the form factor limitations of wearable devices. Analogous to smart phones to "computational photography", noise-canceling headphones are also improving chip computing power, and software algorithms, under the premise of limited internal space, to achieve noise reduction capabilities and sound quality iteration. Some manufacturers in China will preset different noise reduction modes and automatically switch according to the environment. The algorithm can adjust the ANC controller parameters and optimize wind noise leakage according to the wind noise detection results.
The AirPods Pro2 is equipped with a radio microphone inside and outside. Because of the chip capability of H2, the noise reduction depth of AirPods Pro2 is twice as high as that of the first generation, but the "ear press" is not uncomfortable. The H2 chip and the optimized acoustic algorithm give a suitable noise reduction environment to achieve a balance between noise reduction depth and comfort.
But at the moment of "violent level" decibels, this is useless.
Fireworks丨Image source: author
According to the American Chinese Speech and Hearing Association (ASHA) on the volume level of common sound sources in daily life, the sound intensity of fireworks within one meter can reach 150 decibels.
In a small alley, I came across a family setting off red firecrackers. After almost 5 meters, after the noise reduction of AirPods Pro 2, Apple Watch detection is still as high as 81 decibels, which is a noise level of "need to flee the scene immediately".
Band analysis of red-hung firecrackers丨Photo @ Jimmy Ng HK Chinese New Year 2012 - Day 2 - firecrackers
Because I escaped too quickly, I didn't record it. Later, I found a video recorded by netizens from the Internet and analyzed it. It can be seen that the sound of the "red hanging firecracker" is concentrated in dense fluctuations between 500 Hz - 7500 Hz (belonging to the medium and high frequencies).
There are "hard wounds"
Why is the processing power of the chip improving, or can not solve the "high-frequency bug"?
Woodenear tested the AirPods Pro2's transparency mode (also known as transparent transmission mode), the principle of transparency mode is that after the microphone collects external sound, it is played back in the headphones, and the acquisition ability of the noise-canceling headphone microphone can be seen from the side.
We see that in the low and mid-frequency bands, the sound pressure (the amount of pressure change caused by vibration as the sound wave passes through the medium) after transmission mode is almost unchanged. And at 2KHz there is shaking, and by 5KHz it is shaking violently. It proved that it was not possible to capture the mid-high frequency bands well.
AirPods Pro2 transparent transmission mode analysis丨Image source: Woodenear
That is to say, even the H2 chip equipped with AirPods Pro2 can perform 48,000 noise processing per second. Even if the chip capacity is enough, the "hard injury" appears on the microphone, and it is distorted in the acquisition of high frequency bands.
Including AirPods Pro, many mid-to-high-end noise-canceling headphones use MEMS microphones. Due to the application demand of wearable devices, MEMS microphone package size is getting smaller and smaller, but too small package size will bring performance and reliability degradation. Data from a MEMS microphone supplier (Infineon XENSIV MEMS) shows that these microphones have a very flat response at low frequencies but rise significantly to high frequencies.
The low-frequency response is very flat, but the float to the high frequency rises significantly丨Image source Infineon
Even with an ear implant equipped with a 1/2-inch (diaphragm area) microphone, accuracy and sensitivity are still limited. LEC (International Electrotechnical Association) also only standardizes the acoustic impedance characteristics of the implant ear to 100Hz-10000Hz. In comparison, AirPods have a length, width and height of 3*2*0.98 mm, and the volume is less than the size of a grain of rice.
Towards night, there were more firecrackers outside. The wrestling cannon of a naughty child, mixed with the second kick of a brave youth, and from time to time there are monkeys in the sky, and fireworks into the sky.
The monkey is dense in the mid-high frequency and high-frequency sound segment丨Image source: Author
If you are at the firing site, noise-canceling headphones can only help you filter out a small amount of "boom" sounds, which will not help the "heu~u~" and "simply crackle" spiked noises. Of course, for the "level" of artillery battles such as two kicks, even if the sound is concentrated in the low midrange, I wear noise-canceling headphones, and I am still "liver trembling".
Second, the kick is in the low and mid frequencies, and the volume of the middle frequency is relatively large丨Image source author
If you're fleeing a firecracker scene like me, I recommend that you completely block your ears to protect your hearing.
Apple Watch noise detection丨Image source: author
Back home, the fireworks in the next community, through the physical layer of weakening, transmitted to the ears of the low frequency sound waves, at this time with noise-canceling headphones, the sound of firecrackers heard is similar to that of the air conditioner.
But it couldn't stop the sentence, "Sister, you're finally back~" This sound is extremely loud.
bibliography
[1] https://techteller.com/sci/anc/
[2] https://www.headphonesty.com/2022/11/active-noise-cancellation/
[3] https://www.airitilibrary.com/Publication/alDetailedMesh?docid=a0000157-199412-29-6-452-456-a
[4] http://www.gov.cn/govweb/fwxx/kp/2013-08/20/content_2470212.htm
[5] https://picture.iczhiku.com/weixin/message1593447396323.html
[6] https://www.woodenears.com/article/b8246101-be09-44f3-ab33-f46300b10f94
Author: Cake grade frozen fog
Editor: Shen Zhihan