Microphone array

With the constant enlargement of the internet bandwidth, the internet video conference has become possible. However, the wide bandwidth does not mean high quality of sound. The consumers usually need to wear the earphones to get the clear sound effects, and it is really inconvenient for them. Therefore, VISTA has already started to support the function of microphone array, which help to record sound more clearly, and can eliminate the noise and wind shear sound in the background. Naturally, besides the support of the operating system, the computer hardware also needs to be matched. Consequently, with the collocation of the microphone array and the DSP, the consumers can easily enjoy using the web phone such as SKYPE without wearing the headsets.

The theory of the microphone array is to set more than two pieces of microphones to record the sound separately, and than the DSP will pick the tonic we want. The simplest design is to pair a unidirectional microphone and an Omni directional one, which can limit the sound receiving range at the width of 120-140 degrees in front of the microphone, and then the SPL will be reduced over 20dB beyond this conical area. This is the reason why the ambient noises can be reduced widely and hardly to be heard.

How to choose a Buzzer (Transducer)

How to choose a buzzer

There are many different kinds of buzzer to choose, first we need to know a few parameters, such as voltage, current, drive method, dimension, mounting type, and the most important thing is how much SPL and frequency we want.

Operating voltage: Normally, the operating voltage for a magnetic buzzer is from 1.5V to 24V, for a piezo buzzer is from 3V to 220V. However, in order to get enough SPL, we suggest giving at least 9V to drive a piezo buzzer.

Consumption current: According to the different voltage, the consumption current of a magnetic buzzer is from dozens to hundreds of mill amperes; oppositely, the piezo type saves much more electricity, only needs a few mill amperes, and consumes three times current when the buzzer start to work.

Driving method: Both magnetic and piezo buzzer have self drive type to choose. Because of the internal set drive circuit, the self drive buzzer can emit sound as long as connecting with the direct current. Due to the different work principle, the magnetic buzzer need to be driven by 1/2 square waves, and the piezo buzzer need square waves to get better sound output.

Dimension: The dimension of the buzzer affects its SPL and the frequency, the dimension of the magnetic buzzer is from 7 mm to 25 mm; the piezo buzzer is from 12 mm to 50 mm, or even bigger.Connecting way: Dip type, Wire type, SMD type, and screwed type for big piezo buzzer are usually seen.SPL: Buzzer is usually tested the SPL at the distance of 10 cm, if distance double, the SPL will decay about 6 dB; oppositely, the SPL will increase 6 dB when the distance is shortened by one time. The SPL of the magnetic buzzer can reach to around 85 dB/ 10 cm; the piezo buzzer can be designed to emit very loud sound, for example, the common siren, are mostly made of piezo buzzer.

About microphone sensitivity

The sensitivity of the microphone is one of the most important factors, which affects the receiving result. The higher the sensitivity is, the higher the SPL can be received.Different from the production process of other products, the microphone is selected and sorted by its sensitivity after producing. We cannot get 1000pcs of what we need in sensitivity from just inputting 1000pcs of microphones’ material on the production line.
Therefore, in order to avoid discontinuing the production, we had better choose the spec. usually with the sensitivity of -40 to -44, which is popular and easily produced.About the volume, we can still improve it by modifying the circuit, such as the impedance, the voltage, and the amplification factor.We suggest that do not choose a special spec. with very high or low sensitivity at first, or the problem of lead time and lacking of material might happen.

A&B Components Co. Ltd.

Why the spekaer emit noise?

To analyze the usage, there are a few reasons explain why the speakers emit buzz sound or noise:1. Speaker is a passive component, and the drive circuit decides the sound. Therefore, giving the correct power and waveform to the speaker is the basic request.The too large power will lead to the too big amplitude of the diaphragm. The slight problem is distortion, but the serious problem is that the diaphragm might hit the mechanism or the magnet and cause the buzz sound. The basic waveform to drive the speaker is sine wave, if we give square wave or other waves, the efficiency of the speaker will reduce and easily get heated, the buzz sound may happen as a matter of course.2. The enclosure design of the product will also affect the quality of the sound. The well designed enclosure can improve the frequency response curve of the speaker.The area of sound apertures of the enclosure at least request over 1/8 area of the surface. We need to keep enough vibration space between the case and the diaphragm to avoid the diaphragm hitting the case when it works and emits the noise.

Soldering and assembling condenser microphone

Soldering manufactureThe diaphragm of the microphone is a very thin mylar membrane, it is easily being contracted and distorted with the high temperature. Therefore, we can only use hand-welded method to process, and the temperature of soldering iron cannot over 330°C, the welding work should be done within 2 seconds, the faster the better.

The best way is arranging the skilled welders to solder one wire/ pin first, and then finish the left one after the microphone cool down, this can avoid the high temperature causing distortion of the diaphragm, the worse sound receiving results or even breaking down.Otherwise, you can entrust the manufacture to us, our factory has special manufacturing tools to help the microphone cool down. Besides, we will test the sensitivity again after soldering in order to keep the qualification and the reliable quality.

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Small speaker with high SPL?

In order to increase sound pressure level, we need to vibrate more air molecule by more power under unit time.For the two sizes speakers, the area of diaphragms of the 15mm speakers are around 176mm square (7.5*7.5*3.14),and the 20mm size speakers are around 314mm square(10*10*3.14). The 20mm speakers are only bigger by5 mm/ 25%, however, the diaphragm are bigger by 78%. Therefore when we input the same power, the 20mmspeakers can vibrate more air at the same time, and it result in higher sound pressure level.Generally, the speaker in bigger size can provide lower frequency response which is better for human’saudition. On the other side, we can also provide the smaller speakers more power to make higher soundpressure level, but it is limited by,1. Acceptability of the diaphragms2. Thermolysis of the speakers3. After increasing the power, frequency response will change too(because thicker diaphragm, heavier coil..etc)Therefore, what we can do is reform the material and the structure of the speakers to increase sound pressure level,but if it is possible to choose the bigger size, the problem of the lower sound pressure level will be improved immediately.

The buzzer has noise

When the sound which the buzzer generates is not a clear beep sound, we call it noise or broken sound. There are two possible ways to cause the noise.Improperly used If the voltage we input is too high, the diaphragm will easily touch other components, which causes the noise. When we set the wrong frequency instead of the rated frequency is easy to have noise. For example, if the rated frequency is 2730Hz, but the user input 2400Hz, the noise appears easily, especially within the range 10% ahead of the rated frequency.Disqualified A diaphragm of the buzzer is just set on the braces not firmly attached. Therefore the external force will move the diaphragm and result in the noise. When we assemble the buzzer, or in the process of user’s production line, if there are some iron chippings invade into the buzzer, the noise will also be caused.

A & B Components

Waterproof Speaker

Generally speaking, a paper diaphragm speaker is easily to have the problem of molding or distortion in a humid environment, and is certainly damaged in the water. On the other hand, the mylar diaphragm speaker is normally waterproof that it can work with drip, fog, or even slightly in the water if we use the rubber ring to fix the diaphragm instead of paper ring.Another important thing is that all the diaphragms of the speakers are very thin, and the pressure of water might distort even break the diaphragm if it is too deep.In conclusion, the mechanism design is also very important, especially the joint should be completely sealed in order to reach waterproof function.

Self Drive (DC) and External Drive (AC) Buzzer

The difference between the self drive and external drive buzzersDepend on the driving signal, the buzzer divide into self drive and external drive. However inner circuit/ no circuit, DC/ AC, and active/ passive are all mean the same.The design of the self drive buzzer is to set the circuit inside the buzzer and just offer the direct current to work.
The inner circuit has the designed frequency and voltage for the buzzer to generate sound, therefore the frequency of the self drive buzzer is fixed. However the frequency is still different from each buzzer, normally is 2300Hz, the acceptable range is +/-300Hz.However, the external drive buzzer needs to be drove by the signal of 1/2 square wave V 0-p (magnetic type), or square wave V p-p (piezo buzzer)It is convenient to use a self drive buzzer, but the disadvantages are the higher cost, the higher defective rate and the unchangeable frequency.

Buzzer drive circuits

For a self- drive buzzer ( DC/ circuit- built), either pizeo or magnetic just apply the rated current and voltage.
For the external-drive buzzer, it depends on,
1. We should give magnetic buzzer 1/2 square wave, and provide it at least 3 times the amount of the rated consumptive current.
2. Otherwise, we give square wave to the peizo buzzer instead of 1/2 square wave, because the half wave might cause the buzzer does not work.Therefore, voltage control is an important factor for a peizo buzzer which is driven by the voltage.

About microphone sensitivity

There are three reason might cause the problems,
1. Lower sensitivity of microphoneThe sensitivity of the condenser microphone is from -36 dB to -48 dB (0dB= 1V/Pa). If we use -44 dB microphones, the sound will be lower.
2. Defective design of amplifierThe impedance for the amplifier will affect the sensitivity and the frequency response.Normally, the higher the impedance is, the better the sensitivity will be. However if the impedance is too high to get enough voltage/current, the high frequency response may get worse.The lower the impedance is, the lower the SPL will be. The too much current will cause the insufficient voltage.Input higher voltage will increase the sensitivity. Depend on the design of the amplifier and the frequency, the sensitivity might increases around 0.3 dB to 0.6 dB by 1 V.3. Defective design of mechanismThe over-length or curved sound receiving holes leads to lower or unnatural sound.

Watt and sound output of speaker

Base on the premise that all the conditions are the same, if the drive circuit provides enough voltage, the speaker can generate higher sound pressure level. However, with the same voltage, but we use another speaker which applies higher rated power, it will be difficult to drive and output lower sound pressure level.For example, a circuit can only provide the power of 8 ohm 1 W, in order to generate higher sound pressure level we collocate a 2W speaker, the SPL will be lower instead of higher. It is because with the increasing of the power, the diaphragm should be thickened. The coil also needs to be replaced by thicker wires. However, the thicker wires results in lower impedance so we have to wind a bit longer more, and that makes the coil heavier than before. Oppositely, if we use another speaker which loads lower power or with lower impedance, it is likely to make a little bit higher SPL, however the noise or burning down will possibly happen in this condition.

Magnetic Buzzer and Piezo Buzzer

There are two types of buzzers, piezo and magnetic type.A piezo buzzer generates sound because of the piezoelectric effect from the piezoelectric ceramic which drives the metallic diaphragm to vibrate.

A magnetic buzzer is based on the electromagnetic principle, the metal diaphragm is pulled when input the power, and spring back without power.The piezo buzzer is driven by square waves, and the magnetic buzzer needs 1/2 square waves to drive.

We suggest inputting higher voltage for a piezo buzzer, normally over 9V to have enough SPL. On the other hand, a magnetic buzzer can be driven to generate 85 dB by only 1.5V, but the consumption of the current will be much higher than Piezo one.To compare the two types of buzzers, the magnetic type can have lower frequency response with the same dimension.Though being limited by the dimension, the SPL of a magnetic buzzer can only reach to 90 dB. The piezo buzzer can achieve the SPL of 120 dB in some spec., some larger size can also easily reach to 100 dB.

The Noise Reason of Electret Condenser Microphone

The noise of an electret condenser microphone mostly comes from:
1. Impure current source
If the direct current mix with the alternating current signal, we will hear a stable level, low frequency and continual noise. We can lead the power to the microphone directly, or set a non-join circuit to filter the AC signal to improve the situation.
2. Mechanism design
The faulty mechanism design might cause the wind sheer sound or resonant sound. Normally a deep curved sound hole is good to prevent the ESD, but it has some bad influence on the sound; for example, it is good for some frequency but some other frequencies might be declined. Therefore we may get unnatural sound with buzz or noise.
3. The noise from signal
Microphone is a component to receive sound, which can receive wider frequency range than human ears. Both high and low frequency signal will transform into noise as a form of harmonic waves after being received. An obvious example is that the noise happens when the cell phone and the blue tooth are set too closely. Therefore when we design the microphone circuit, we should keep it away from the antenna or other components which might radiate signal.
4. Outer noise
Generally, the main noise is the sound of the wind. The users complain about the unclear sound because the higher join capacity receives the sound of the wind or breath. Modifying the join capacity (normally around 0.022~0.1 u) can solve this problem.On the other hand, if the outer sound (exclude the main sound) is too louder, it means the sensitivity of the microphone or the loading impedance is too high, what we can do is change to a low sensitive microphone or to decrease the loading impedance.

Condenser microphone and Dynamic microphone

1. Transform sound into electric signal- The best design in theory:The design of electret condenser microphones is based on the theorem that we use charging and discharging motionbetween electric capacity conductors. When the ultra-thin or gold-plating Mylar diaphragm response to the soundpressure level, it changes the static electricity between the conductors into electric signal directly. Then we can gaina proper output of impedance and sensitivity by the electric circuit coupling.
2. The characteristic of reappearing the original sound:To reappear the original sound is the highest performance for the acoustic specialist to achieve. If we check the basicdesign of the microphone, we can find a electret condenser microphone can transform the sound into electric signaldirectly, because it is not only manufactured with accurate mechanism technique, but also combine complicated electriccircuit. Therefore, with the excellent characteristic in advance, the electret condenser microphone must be your best choice.
3. Very wild frequency response:Diaphragm is a main component of a microphone to sense the sound and to transform it into the electric signal.The material and the mechanical design of the diaphragm decide the acoustics of a microphone. Due to the materialof the diaphragm is very thin and light, and can transform the sound into electric signal directly; the range of thefrequency response can reach to the ultra-low frequency (under 10 Hz), and the high frequency can also gets tosupersonic easily, which shows the characteristic of wild frequency response.
The diaphragm of the dynamic microphone loads a sound coil which is hundreds times heavier than itself. (Left picture)The diaphragm of the electret condenser microphone works without any load. (Right picture)

Acoustic keyword

W ( Watt ): A watt = a voltage x an ampere. For example, 330 Watt= 110V x 3 A.
V ( Voltage ): V is the unit of voltage, when it needs one joule of work to sent one coulomb of electric from one point to another point, then the potential difference between these two points is one voltage. 
Ohm: Ohm is the unit of impedance in electronics, the higher the impedance is, the less electric current can pass through. 
Hz( Hertz): The frequency of the sound waves vibrate every second. For instance, 100Hz means 100 times vibration in one second. 
dB ( Decibel): dB is the unit of the sound pressure level, which is calculated with logarithm, because the susceptibility to the sound of human ears is not linear. 
Fo ( Resonance Frequency): Fo stands for the resonance point of the speaker. The lower the Fo is, the efficiency of the low frequency is better. 
S.P.L. ( Sound Pressure Level): The volume of the sound. 
Vo-p: The measurement from Vo to the peak, to drive the magnetic buzzer, the waveform is 1/2 square wave. 
Vp-p: The measurement from peak to peak, to drive the piezo buzzer. The waveform is square wave.

Microphone Sensitivity

The sensitivity of the microphone is one of the most important factors, which affects the receiving result. The higher the sensitivity is, the higher the SPL can be received.Different from the production process of other products, the microphone is selected and sorted by its sensitivity after producing. We cannot get 1000pcs of what we need in sensitivity from just inputting 1000pcs of microphones’ material on the production line. Therefore, in order to avoid discontinuing the production, we had better choose the spec. usually with the sensitivity of -40 to -44, which is popular and easily produced.About the volume, we can still improve it by modifying the circuit, such as the impedance, the voltage, and the amplification factor.We suggest that do not choose a special spec. with very high or low sensitivity at first, or the problem of lead time and lacking of material might happen.