Introduces technical terms related to sound and audio technology, essential physics in sound and audio research and development, terms used in mathematics, and terms on technology that are attracting attention as a trend of sound and audio technology.

Keywords: Active noise canceling, digital signal processing, digital filter

Recently, noise canceling technology has become widely recognized, even with low -priced wireless earphones.
The theory is very simple, but it is a technology that can be expected to develop in the future.

In fact, this noise canceling technology was the reason I was interested in sound, signal processing, and spatial sound.
In the introduction of the laboratory in college, I showed me a demonstration of noise canceling technology, but when I put my head into the specified space, the sound I heard so far disappeared, and said, "Like this magic. I still remember being shocked!
However, when I actually designed noise canceling with the ZE8000, I realized that it would not work so well, and that it was necessary to try and error in various restrictions.
I felt again that it was necessary to work hard, and that it was the only technology that the know -how and knowledge obtained in the development were accumulated.
We believe that it is important to understand the contents correctly in order to proceed with technical development in the previous era of such noise canceling.

・ Difference between passive and active
There are two major types of noise canceling technology that suppress noise.
One is passive and the other is active.


Simply put, it is a method of blocking the intrusion of noise, and is also called an earplug effect.
The physical seal of headphones and canal -type earphones enhances the sound insulation effect.
Just block the noise? It seems that the effect is very high, and if it is designed firmly, it will be enough to suppress the noise.
Mainly, the noise of the midrange to high range can be suppressed.


This is a method of erasing the invading noise by digital signal processing.
In general, it is often called ANC.
We use a mechanism to eliminate noise by raising the noise with a microphone attached to earphones and headphones, generating and adding the sound signal of the sound signal and the reverse phase.
The mechanism is very simple, but the effect changes depending on the performance of the microphone that earns noise and the digital filter applied to the sound signal, so you need a high level of technology.
If you wear earphones and headphones and listen to music in an environment with a large noise (for example, on a train, near a car, etc.), it will be difficult to hear music due to the external noise. Active noise canceling is used to suppress this noise. Although the text is written as noise, it means noise specifically.

・ The principle of canceling with the wave of the opposite phase
It is very simple as a mechanism.
First, the sound is a wave that is proposed by pressure changes in the air.
By overlapping the sound wave of the opposite phase of 180 °, the sound can be erased as shown in the figure.
This is the principle of noise canceling.

(Fig. 1) Silence by overlapping sound waves 

・ ANC method
Active noise canceling (ANC) has three main types.
Feed forward, feedback method, hybrid method.
Each will explain the characteristics.


Microphone is installed outside the headphone, and the external sound is yielded with the microphones, and noise is removed.
The feature of the feed forward system is that it can remove noise shallowly in a wide band.
From the perspective of the impact on the music signal, the feed -forward system only has an external noise and adds the signal of the inverted phase to remove the noise in principle. Does not spread.
The signal that removes noise is generated by applying a digital filter to the external noise.
This digital filter is designed by calculating the difference between the transmission function from the noise source to the eardrum position with the earphone attached, and the transmission function from the noise source to the eardrum position with the earphone is turned on to the earphone. Masu.
The transmission function varies greatly depending on the jig to measure the transmission function and its accuracy.
Be careful as digital filters may not be designed in the first place.
In addition, the design itself can be done, but it may not be felt in the hearing effect.
In addition, the type and amount of digital filters may be restricted.
If an approximation according to the resource is required for the filter calculated from the transmission function, even if the transmission function from the noise is accurately measured, the filter characteristics change due to this approximation of the filter. You need to be careful because the performance changes greatly.
It is also necessary to recover from the production of jigs to the transmission function, and the digital filter design many times to increase the accuracy.

(Fig. 2-1) Mechanism of feed forward system

(Fig. 2-2) Features of noise suppression by feed forward method [Horizontal axis: frequency, vertical axis: gain]

Micropones are installed inside the headphones, observing the state in which the sound from the outside leaks and reaches the eardrum, and removes noise.
The feature of the feedback method is that the micro -hon is placed near the eardrum, and the noise reaching the eardrum can be accurately accurate to measure the accurate transmission function, so that the noise is removed deeper to the low range. You can do it.
From a viewpoint of the impact on the music signal, the feedback method also decerses the bass of music because the sound of regeneration sounds and removes noise.
Therefore, it is necessary to lift the low range of the music signal only in advance. This may deteriorate the SN ratio and adversely affect sound quality.
In addition, since it is extremely difficult to place a microphone at the eardrum position, measurement errors at the observation point may affect the noise removal performance.
Therefore, in development, it is necessary to measure the characteristics with the target housing and adjust the coefficient and gain value of the digital filter.
This parameter adjustment greatly changes the noise removal performance and the quality of music, so it is necessary to start over and increase the accuracy.

(Fig. 3-1) Mechanism of feedback method


(Fig. 3-2) Features of noise suppression by feedback method [Horizontal axis: frequency, vertical axis: gain]


The hybrid system can take advantage of both characteristics by combining the feed forward method and the feedback method.
However, since it is necessary to install a microphone on both the outside and inside, the number of microphone increases. In addition, it is necessary to consider the location of the microphone, which affects the housing design.
The impact on the music signal and the precautions when you actually design are the same as what you mentioned in the above two methods.
The difficulty of design is high because it is necessary to establish a feed -forward system and a feedback method at the same time.

(Fig. 4-1) Hybrid mechanism

(Fig. 4-2) Features of noise repression by hybrid system [Horizontal axis: Vertical axis: gain]


・ Actual ANC design
Wireless earphones and wireless headphones are the basic configuration in which the driver unit outputs a signal from the driver unit through a digital filter for ANC.
There are various factors, such as microphones, performance, driver units and housing design, digital filter design, measuring equipment for filter design, gain tuning, etc. there is.
Another IC configuration is also important.
Finally, the signal output from the driver unit is "a signal that cancels the regenerative signal + noise." At the same time as receiving a recycled signal from the smartphone via Bluetooth, it can be completed with one chip if you can generate a signal that cancels noise.
As a result, the circuit area can be reduced and smaller.
Alternatively, it is possible to add an ANC dedicated IC separately from the Bluetooth IC.
The circuit area increases and the power consumption increases, but it is possible to increase the performance.
In this way, it is not necessary to consider only the performance of the ANC, but it is necessary to determine the circuit area, the number of microphones, the number, and the driver unit, and determine the comprehensive configuration by the product concept.
In the ZE8000, the sound is collected with a microphone located outside the earphone, an ANC digital filter is applied inside the Bluetooth IC, a signal that cancels the noise is generated, and a constitution that outputs a signal output from the driver unit along with the regenerative signal. Masu. We think of music playback as a top priority and select a feedforward method that does not affect the music signal. In addition, ANC processing is performed inside Bluetooth IC to reduce the circuit area as small as possible. We make ourselves for the measuring jigs for filter design, and achieve a natural ANC effect without pressure.

(Fig. 5) internal structure of ZE8000



・ Numerical performance and measurement system as marketing
Have you ever seen an ANC performance as "Maximum ** DB" in the earphones equipped with anc?
Or buy an ANC earphone with good numerical performance, that? Isn't it as expected? Have you ever felt?
The value of "maximum ** DB" is used as one element of marketing.
In other words, it is an index to convey the value of the product in an easy -to -understand manner.
The higher the number, the higher the noise repression performance, but it is necessary to correctly understand the meaning of this number.
First, this figure is the value of "peak value at a certain frequency".
In other words, it is the maximum performance that comes out when the pure sound of a specific frequency is played.
But think about it. In the first place, there is not much noise environment of pure sound.
In other words, just because this number is good, it is not always possible to say that the sound is high in the actual noise environment.
As an index to convey the value of Anc, the reality is that this is written because it is simple and easy to understand.

(Fig. 6) ANC numerical performance as marketing


So, what should I do to correctly evaluate the noise repression performance?
The method of measuring and evaluating the noise suppression performance by physical volume is stipulated in Jeita as RC-8142A [12].
As for the measurement environment, the sound energy field, which has a uniform acoustic energy at each measurement point, and the flow of acoustic energy, is required in all directions.
However, it is not easy to measure in such a special environment.
And even if it can be measured under the above conditions, it is not clear that the evaluation index matches the hearing impression of a person.
We are thinking about "What are the evaluation indicators that match the hearing impression?"
This is because there is a desire to use the evaluation indicators that match the hearing impression so that customers can understand the correct understanding.
And we believe that this correct understanding is important for the development of earphones and headphones to the previous era.

(Fig. 7) Characteristic measurement system example [Source: JEITA RC-8142A]


・ About future ANC
Recently, an adaptive ANC technology that changes the applicable amount of noise cancellation has appeared according to the surrounding noise, and the optimal noise canceling for the environment has been realized. In addition, there is an earphone that can select noise canceling with different parameters from the preset according to the scene.
In the future, the technology to analyze the surrounding environment will develop, and by applying ANC accordingly, I think that you will be able to enjoy music comfortably in any environment. In addition, I think that ANC accuracy can be further enhanced by digital signal processing and improving Bluetooth IC performance.
I believe that with a sense of purpose and further technology development, we can further improve user audio experiences.
We will continue to develop products while watching the technical trends, so please look forward to it.


[1] "New Edition Active noise Control, Japan Society of Society, Masaharu Nishimura/ Takeshi Usagawa/ by Shiro Ise/ Yoshinobu Kajikawa
[2] JEITA RC-8142A Noise Canceled headphones and earphones
[3] https://acoustics.jp/qanda/answer/196.html

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