Speaker orientation)

Most audio experts know that some rooms must have a directed speaker system, and some suggest an omnidirectional speaker system. The final choice of the system will depend on two factors: the location of the Speaker and the internal structure of the room. However, in general, the feedback is directly proportional to the Q aspect of the speaker, the maximum acoustic increase value can be obtained based on the equation below. In the following equation, Q refers to the ability of a speaker to transmit sound in a specific direction and prevent sound scattering. Under certain circumstances, Q is equal to the ratio of the square of sound pressure from the microphone to a fixed distance and the specific azimuth to the square average of sound pressure in all directions at the same distance. Q is related to the horizontal direction of the Speaker and the vertical radiation angle in ideal conditions. The formula is Q = 180/here θ = horizontal radiation angle, Phi = vertical radiation angle.

Although Q is useful in calculating sound gain, knowing the valid water level and vertical radiation angle at different frequencies is more helpful for system design. The "different frequencies" mentioned above is a key factor, because the directionality of any speaker or speaker system always changes with different frequencies. Next we will help you understand the changes in the direction at different frequencies, give an ideal parameter, and provide exaggerated comments on the aspect of the direction performance, so as not to trust the advertisement of the product manufacturer.

Hui Yun's Law
Any sound radiator with a constant phase on its surface, such as a low-frequency conical horn or an electrostatic plate horn, which is working at a low frequency, the sound is scattered across all sides. The higher the frequency, the stronger the direction. The reason for the directionality can be explained by the huygen law, that is, any reflection surface can be subdivided into several independent small planes, each of which is reflected to four sides. Assume that one of the conical surfaces is a and the other is B (2), then we can obtain the sound propagation effect between A and B and any specified receiving point P.

Figure 2: how to calculate the directionality

If the distance between the two points on the diaphragm and the receiving point is the same, the acoustic waves transmitted from these two points are superimposed at the receiving point. If at a certain frequency, the acoustic wave transmission time from point A is more than half a cycle than the acoustic wave transmission time from point B, the two acoustic waves at the receiving point will offset each other. By calculus, you can obtain the overall frequency response of a conical horn or any horn at any point of response. At a certain frequency, the overall frequency response of the conical horn can be divided into different targeted radiation modes (figure 4 ).

Figure 4 radiation direction

Figure 4 ka is classified based on different Ka values. Ka is the product of the radius and wave number of the conical horn. (K = 2 π F/C, where F is the frequency, C is the sound speed) through calculation, we found that Ka is only the ratio of perimeter to wavelength. We know that the wavelength of 1 kHz sound wave is about 1 feet and the wavelength is inversely proportional to the frequency. Therefore, for a 15 inch speaker, the circumference is about 4 feet, therefore, you can use the image with Ka equal to 1 on a 15 inch Hz 12 inch horn. If it is a 333 horn, the corresponding frequency should be Hz.

It should be noted that when the frequency is higher than the frequency value of Ka = 1, the sound will have a direction. Generally, at a frequency of Ka <= 1, the cone-shaped horn emits sound waves, just like the piston movement, and the whole vibration surface emits sound waves in the same direction. However, when the frequency is higher than the value mentioned above, in reality, the whole vibration surface of the conical horn will not emit sound waves, therefore, when ka> = 1, the graph in Figure 2 is basically not applicable in actual situations.

In this way, at a higher frequency, in reality, the direction of the conical horn is different from the theoretical mode of the piston horn of the same size. In general, at a higher frequency, the angle range of the conical horn (different parts of the Conical Horn are different from each other) is wider than the radiation angle of the real piston horn. At the same time, the direction of any conical speaker will become narrower when the frequency increases. This is the famous clustering effect.

Two methods
In order to control the sound propagation, we hope that the sound wave radiation of the Horn will be directed. Now there are two ways to achieve this effect: the horn-shaped horn and the array-shaped horn. A well-designed horn-shaped Horn has a very small array of film, but its performance is very powerful. It generates a spherical array. the radius of the sphere is mainly determined by the shape of the horn and the size of the sound exit. Array speakers use a group of smaller radiation bodies to generate the same type of arrays. In two examples, the direction is the only control label of the frequency, and its wavelength is equal to or less than the perimeter of the horn.

 

 

In a linear array horn, the line length must be about 0.6 times the wavelength. If the direction control is required, it will be longer. The two methods do not provide a better way to achieve the effect of low-frequency ing: in fact, a single speaker of the same size has the best effect at the lowest frequency. However, at a higher frequency, the direction of the horn-or array-Type Horn is better controlled than that of a single drive horn, and the design operation is more convenient.

Non-baffle speaker
Another type of radiation body can be used to meet the directed needs, and can also be used at low frequencies, even without a large amount of radiation. The simplest of the series is the non-baffle speaker, also known as the dipolar radiation body. Because the sound emitted from both sides of the speaker has the same amplitude, but the phase is opposite. They are orthogonal on the axial plane of the cone, and all the radiation sound waves are offset. In this way, its direction is shown in 8. If you place the appropriate sound resistance behind the conical horn, you will be able to get other directed patterns, such as super-heart-shaped or heart-shaped (like the microphone mode ). For a given low-frequency output, the technology is subject to a much higher conical offset. This technology has high requirements on the Mechanical and temperature design of speakers.

The horn is the channel through which the Sound passes, that is, a gradually expanding cross-sectional area from the driving end (throat) to the radiation end (mouth. At a very low frequency, the speaker is basically transparent to the sound, and the action of the driver is as if the speaker does not exist. However, when the frequency is greater than a specific frequency threshold, the speaker starts to play a role in sound conversion, lowering the sound resistance (low pressure, large volume rate) the mouth end is converted to the throat end with high sound resistance (high pressure, low volume rate. This type of conversion greatly increases the efficiency of the emission system, usually 12 dB or higher, and reduces the offset and related offset distortion (this is the most common distortion type ).

Between the mouth and throat, the sound is amplified when it passes through the horn. In this case, the speaker produces low-end threshold frequencies and directed radiation.

The directed radiation of the horn can be divided into three frequency areas for discussion (figure 6 ). In the lowest frequency area, the radiation is scattered from four to the exact direction. In the middle area, the horn guides the sound by checking its wall as you expected: the directionality of the straight wall is constant, the directionality of the curved four scattered walls increases with the increase of frequency. In the highest frequency area, the drive radius controls the direction.

Figure 6: directed radiation of horn

However, there are two other features that are not very obvious. The first one is: in the control area of the horn mouth, the direction is reduced by about half an eight degree, which is called the "belt ". Second, if the wall shape of the horn is not straight, the horn must work at a higher frequency. Basically, sound waves are consistent with the surface of the horn wall for the first half-wave transmission. Then, even if the horn wall is split, because it is in an exponential curve-shaped horn, the sound waves are no longer consistent with the horn wall. In this way, the speaker with slow expansion at the throat end is more directed at high frequencies than at low frequencies.

Radioactive Horn
The so-called radioactive horn, that is, by maintaining a constant conical acoustic radiation in the horizontal plane, is greatly separated from the conical surface in the vertical plane. In this way, the horizontal direction at different frequencies is almost constant, but on the vertical plane, the direction of the horn is very different. In some applications, the vertical angle is only 10 or 20 degrees, but in the horizontal direction, a 90 or wider angle is required. In this case, the radioactive horn is the most suitable.

The maximum direction of the horn is determined by the characteristics of the throat, which is usually defined by the opening of the compression drive. Such a speaker with a 25mm drive can have a wider high-frequency radiation mode than a 50mm drive speaker. This conversion occurs when the frequency is higher than a specific frequency (that is, the frequency at which the drive perimeter is equal to the wavelength): The 25mm drive is 5 kHz or the 50mm drive is 2.5 kHz. (Of course, at this frequency, the conversion is determined by the wall angle of the horn and the drive opening .)

One way to increase the high-frequency radiation angle is to use a "Bullet" at the front end of the drive's phase outlet ". When simulating and measuring the horn, we can see that these devices increase the irregular frequency response.

The bottom line is that a 90-degree * 40-degree Speaker only has a 90-degree * 40-degree radiation range in the intermediate frequency area mentioned above. Within the frequency range in which the speaker can provide direction control, the lowest frequency value in this range is the frequency when the speaker size is equal to 0.6 times the wavelength. In this way, if a common 1 inch * 10 inch horn in a low-price system is horizontally arranged in a long row, the approximate calculation method of its lowest frequency value with the direction control is: if the vertical length is 0.6 * the wavelength is 4 inch/0.6 = 6.67 inch, then the response frequency is f = C/λ = 13620 inch/6.67 inch = 2257Hz.

It is the best case when the horizontal length is 2.5 times of the vertical height, so the frequency is lower than 2.5 times (902Hz. In fact, most speakers have an exponential Radiation Surface (that is, a curved horn wall), so their radiation at high frequencies, such as 10 kHz, may fall to 40*20 degrees. The side wall of a CD speaker is relatively flat, so there is no great difference in the aspect direction within the frequency range of 10 hz-10khz. However, the throat design of a CD speaker often includes a narrow-Channel Terminal in the diffraction trough.

Generally, the channel size determines the maximum audio emission. For a better horizontal radiation range of 90-120 degrees at the highest audio location, it is difficult to break a slot to compress high-frequency speaker. Generally, this slot is about 1/2 inch wide, which prevents sound transmission at a frequency lower than 16 kHz.

When multiple speakers are arranged together to cover the same frequency range, the interference result is an irregular response in the overlap area (according to the resulting frequency response curvature shape, referred to as Comb Filtering ). The closer the speaker driver is, the higher the frequency of irregular occurrence, and the more unpleasant the sound. In the audio coverage area of the two speakers, many designers are very cautious about stacking the Horn's graphics because of a slight decrease (which means a lower echo ratio for the audience) it is more comfortable than the irregular frequency response caused by interference.

Figure 8: directed radiation of a linear signal source

Near and far Fields
The sound field status of any sound source radiation varies according to the answer distance. In general, the answer distance can be divided into near and far fields. The near field is the distance from the radiation source to the following: [L], where L is the largest radiation source size and distance, and l and λ are measured in the same unit. In the near field, the distance from the near field radiation is extended to an infinite distance. If the sound source is outside, the sound level from the source decreases at a rate of 6 dB at every double distance. In the room, the attenuation will be slower due to the impact of the room, basically at a rate of 3-6 dB at every double distance. These effects are important for measurement. In this case, the microphone should be within the near-field range at all relevant frequencies in order to work accurately. Similarly, this is also important for the evaluation and application of array speakers.

Linear Array
In the audio industry, people repeat a theory that was studied and patented before 1940s every few years, and then announce a major breakthrough, it may even be re-registered. Linear Arrays are such a re-Discovery Theory. A real linear array is a continuous stripe radiation body, which can be horizontal or vertical, generating a direction control through the interference effect between its unit components. As far as I know, there are only two real Linear Arrays (one formula ):

Electrostatic strip radiating body and electromagnetic stripe (band) radiating body. Many companies have produced many products with more or less Linear Arrays, but these are not real linear arrays.

A real linear array always emits the same phase in its length range. If the signal of each individual radiation body is in the same phase and the center interval of the adjacent device is within the 1/4 wavelength range of the highest frequency, an approximate linear array can be obtained. Arrays that do not meet this requirement will show irregular frequency responses.

Figure 8 describes the pointing feature of a real linear array. It should be noted that, apart from the main acoustic flap (longer than a wavelength in length) in the figure, the smallest acoustic flap can also appear. The audience in these angles will hear irregular frequency responses. However, it should be noted that, compared with the large flap, the level of the small flap degrades greatly. Approximate equation 6 indicates the angle range of the vertical linear array in the longitudinal radiation area: q = (the angle value is radians)

Amina DML panel with pictures printed on the surface

In addition, the direction varies with the frequency, but when the frequency increases, the direction will narrow. In some applications, this impact is not a big problem. For example, when the center of a linear array is placed on a plane with the audience's ears, a very narrow vertical direction is acceptable, all listeners can hear the voice of all frequency ranges clearly.

An Improvement on Linear Arrays is to split the arrays, so the center part of the radiation body will process all frequency ranges, the segment away from the center will input a low-pass signal (which has a lower threshold frequency ). In this way, when measured by wavelength, the effective length of the array should be consistent and generate a more consistent direction. This method is called the frequency shadow method. It has been demonstrated that the Bessel filter is the best technique used in the frequency shadow array. Philips owns the patents for Bessel arrays.

Another type of array design is the curve array, which provides lower-frequency emission.

To enable electronic control of the sound produced by a linear array, the array unit or driver can input signals through an independent amplifier, while different array units adopt different signal latencies. Most manufacturers' surround sound systems have this "controllable array" feature.

Transition from near-field to far-field
Another seldom-mentioned Linear Array application is influenced by the transition from near-field to far-field. If a linear array emits sound in a cylinder, when the listener is far away from the array, the sound level degrades at a rate of (assumed to be in a silent or outdoor environment ). If it is indoors, the attenuation rate will be slower. If the array is extended from the interior floor to the ceiling, it will transmit sound waves in all frequency ranges according to the real cylindrical radiation. In

The vertical direction range of the array at Hz is 40 degrees, and the sound pressure in the far field is decreased at a rate of 3 Gbit/s from each two-way, and the distance between the far field is greater than 4 meters at Hz. At 1 kHz, the point range is about 10 degrees, and the sound pressure outside 3 meters is decreased at a rate of 3 dB for each two-way distance. Therefore, when the distance of the 10 kHz listener is about 30 meters, the speed of the listener decreases by 3 Mbps. Pay attention to this trend. At a high frequency, when the drop-down rule of produces an effect, the listener must stay away from the array. At a close distance, the decreasing rate changes from 0 dB (distance between each two steps) at a very small distance to 3 dB at a far-field Distance Based on the frequency.

In applications, the array should complement the predefined frequency response, and it is necessary to confirm the reference answering distance. We hope to hear a flat response at this distance, because the distance between different listeners, the response plane will change within several dB ranges. When Linear Arrays are used indoors, this effect will be significantly reduced because indoor reflection tends to balance irregular reflection to a certain extent.

DML
A few years ago, some new Speaker products were available in the market, that is, distributed speakers, or DML. These speakers contain one or more converters carefully installed on the design panel. In use, the converter emits a curvature wave to the Panel, then the Panel emits sound waves to its perimeter distance, and then reflects back, so that it repeats several times until it degrades to 0. By selecting the appropriate panel material, the sound reflection duration in the Panel can be precisely controlled, so good frequency response is retained. However, sound waves are emitted in an incoherent phase, so the interface effects can be eliminated, and the Panel can generate a comprehensive flat response. Normally used panels have no backpane. In such applications, they can emit 360 degrees of radiation in free space.

When installed near a wall, they generate a pattern of nearly 180 degrees (hemisphere. Furthermore, when not only one panel is used in a single coverage area, their non-coherent radiation can free it from harmful interference effects (Comb Filtering ). In addition, they shake the room less than the related phase speaker.

Finally, due to the large area (which is useful for DML panels), the radiation is distributed, so the audience in the so-called near field will not be exposed to high voices. For example, a DML board can be used as a whiteboard and speaker in the conference room. When the presenter uses a wireless microphone to stand near the DML board, there is little feedback.

Of course, DML is not a cure for white diseases. This is because few applications require such a high degree of directionality to improve the comprehension of listeners. It cannot generate a concert sound grade. Its normal response sometimes does not have the highest fidelity. However, its unique features make it available in some special applications. These applications do not need ordinary speakers. For them, normal means boring.

 

 

From: http://www.ktv8848.com/news/20072/10720.shtml