VoiceGurus

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Noise 101

噪声常识

This article will briefly outline the use of a mobile phone call when the phone surrounding noise environments. During the call the most important thing is to protect the speaker’s voice, which is obvious, but we want to avoid all unnecessary noise. However, to achieve this is not simple, because all kinds of noise around mobile phones have different characteristics, which increases the difficulty to eliminate noise. This article describes the various noise during a call may be involved, and a brief overview of the characteristics of various noises.

1. Noise knowledge

For mobile phones, the speech and noise received by one or more microphones. However, people feel a little surprised that not all of the signals received by the microphone has a sound source. For a more complete description of all sources of noise and speech, we will integrate a variety of sources to the following figure.

sources of noise and speech

2. unnecessary noise

Noise classification is necessary, when we classify the main basis for two very important features:

Static / non-static. Static noise changes will not (at least not soon) occur, such as the hum of air conditioning or the operation of the machine issued. Non-stationary noise will always change, such as the sound of music, conversation was issued or vehicles on the road.

Diffusion / non-proliferation. Diffusion noise source means simultaneously from a plurality of directions or a plurality of sound sources, and the sound source is far away from the device. For example, many people at the same conversation in a restaurant, or the sound of vehicles on the road in the factory sound. Refers to the non-proliferation of noise sources are local small range of sounds, from a single source close to the device. For example, of voices, sound or radio sound projector someone. The most obvious characteristic diffusion noise is unable to locate where the sound comes from, because it comes from all directions.

Of course, not all of the noise can be simply divided into two categories, they can also be a blend of these two characteristics of the hybrid.

Unwanted Noises

2.1 local noise sources

Local noise sources clearly discernible from a single location, such as a radio, projectors or someone is banging the keyboard. Because of local noise sources from a single location, so depending on the orientation of the device, a microphone will receive different signals. If the device away from the noise source is close enough, then some of the microphone received signal strength may be higher than other microphones.

2.2 intrusive speaker

Interfering speech belonged local noise sources. That is close to the device’s single speaker. This is a special type of noise, because between voice and we want to convey it has many of the same characteristics. But in handset mode, this type of noise is unwanted sound interference. In hands-free mode, the kind of noise that we want to communicate with the voice has the same characteristics, it is difficult to distinguish. For example, several people in the meeting room of a device using a conference call, so that everyone could be the speaker!

2.3 instrument noise

Acoustic instrument noise is not noise, but this noise will affect the microphone signal. For a well-designed mobile phones, this noise is very small, but still exists. The reason for this noise is not the same, so it may be static or non-static, it may be the proliferation of non-proliferation, are more likely to be of mixed type.

2.4 wind noise

Wind noise more features, because the wind itself does not belong to the noise. Wind turbulence created around the microphone, thereby generating noise. Because of this noise is very unstable, changes very frequently, often turn beep on each microphone, noise and sometimes manufactured simultaneously on multiple microphones. Especially at low frequencies, the sound will be great.

2.5 Noise body

Holding a cell phone with this action will produce noise. For example, hand scraping device, or change in position when holding the phone.

2.6 Echo

Echoes the call from the other, the other’s voice through the phone’s internal speaker repeated play. Such voices belong to the local sound source, because it comes from a single location clearly discernible. It is located in the phone shell, it is very close to the microphone. This is a special kind of local noise sources, which we have more understanding, and we will refer to this as a remote signal. In handset mode, because the speaker is very close to the ear, so the volume is very low replay. In hands-free mode, since the speaker is very far away from the listener’s ears, so the replay need to try to increase the volume. So you can imagine, the echo problem is more obvious in the hands-free mode.

3. want to convey voice

For the successful conclusion of the discussion, we must also pay attention to the small girl’s voice in Fig. The little girl’s voice how the phone will receive depends largely on the phone is used. We usually consider two main use cases that the handset mode and handsfree mode.
Handset model has several advantages as follows:

  • Gripping the phone mode shown in Fig.
  • Receive a high degree of voice you want to convey, it is generally very good signal to noise ratio, that is the voice of the volume to be greater than the noise.
  • Different microphones are very different on the receiving degrees, since the relative distance from the mouth of the girl, the far distance between the microphone. When the girl’s voice and to distinguish more distant noise sources on the degree of difference in the reception helpful.

Challenges faced by many hands-free mode:

  • Cell phone in hand, arm length away from the face about, or placed on the table.
  • Receiving the degree you want to convey voice is lower than the handset mode. SNR lot worse, or if you need a much quieter environment to get the same signal to noise ratio.
  • Differences in the different microphones voice reception is very small, or no difference. The relative distance from the girl’s mouth, close proximity between the different microphones.
  • Changes in voice characteristics vary with distance. When the speaker away from the device, the sound will be more reverberant sounds, the larger the room, the more obvious repercussions.

Echo is not the noise source, but the rebound cumulative noise in the room, it would have been reflected and slowly weakened. We want to eliminate unwanted noise and echo, because it also affects the quality of the call. Farther away from the speaker phone, echo (the speaker) is generated even more apparent. When the speaker is very close to the phone, the speaker’s voice will become a source of local non-proliferation. When the speaker away from the phone, the speaker’s voice becomes more diffuse, the concentration decreases, so like a lot of unnecessary noise source.

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