【 Weak current College 】 determine speakers need of electric power

Speaker selection and layout is karaoke Hall sound design is an important part of several important acoustic indicators directly related with this.
1. theoretical calculation on Acoustics
OK Karaoke system power main consider sound pressure level and clarity on two factors. We know that in addition to direct sound outside the room, often also overlay because indoor many reflection of mixed sound field. Which mixed the sound field can reduce the room sound field uniformity, of course, at the same time, it will reduce the clarity of the language.
When used without directional sound source (speakers) stimulate the sound field, indoor range sound source distance r Department of sound pressure level:
Lp=Lw＋10lg（1/4πr2＋4/R）（1）
Type: R = Sa/(1-a) called room constants; Lw = 10lgWa + 120 as the source of the sound power level.
S is the overall surface area in the room, and a total for the room of the sound absorption coefficient, Wa for the desired sound source total power.
When directional sound source (speakers) stimulate the sound field, indoor range sound source distance r Department of sound pressure level:
Lp=Lw＋10lg（Q/4πr2＋4/R）（2）
Type: Q for speaker directivity factor, not only with the sound source directivity, also with the sound source position changes;
From (1), (2)-we can see that the room sound field by direct-to-noise (Q/4 r 2 π) and mixed noise (4/R) composed of two parts, with direct access to the square of the sound with the distance attenuation, and inversely proportional to the mixed sound is constant with distance-independent; sound source directivity effect only on the direct sound, mixing sound of little effect.
In the Interior, in order to guarantee a certain degree of clarity, the speaker distance effective service usually limited by the directivity, reverberation time and room size limit. Often take 3 to 4 times of critical distance, namely:
Rmax=（3～4）Dc=（0.3～0.4）（QV/πT）1/2（3）
Obviously, if the requirements in the karaoke Hall from the sound source r Department of sound pressure level LP, you can use (2)-seeking out sources of sound power required: Wa
Wa=1/（Q/4πr2＋4/R）?10（0.1Lp－12）（4）
If the ignore mixed noise could contribute, you can further simplify on-as:
Wa=4πr2/Q?10（0.1Lp－12）（5）
And then depending on the speaker's audio conversion efficiency, you can figure out the speaker's power required:
We=Wa/η（6）
In fact, in a free sound field, if you do not consider the speaker directivity, or (5) and (6)-together, directly by the speakers under type the desired power.
10lgWe=Lp＋20lgr－Lp9（7）
Type: We need for the speaker, the power (in watts)
Lp to specified location (distance speakers r m) on the required sound pressure level, (in DB)
Lp9 is speaker axial sensitivity, that is, it's 1 Watt 1 m condition generated by the sound pressure level, (in dB)
Note: 1) if the speakers are valid service distance less than audience distance sound source maximum distance, you must choose the nature of the more powerful sound source or decrease room reverberation time in order to increase the distance between the speaker and effective service. Of course you can also increase during and after field auxiliary speaker to increase during and after field of sound pressure level and clarity, that is, using a distributed PA system. 2) if the audience from the sound source maximum distance is less than the distance of your speakers effective service, it should be replaced by the actual distance Rmax for calculation.
Let's take an example to illustrate, for example a length, width and height respectively 30 m, 20 m and 5 m Hall, the requirements of the average sound pressure level of 80 DB, if you choose to use directional factor 5, electro-acoustic efficiency of 20 percent of the speaker, try to calculate the required power.
First, let's take a look at the rear to ensure (Rmax = 30 m) with sufficient clarity (allows direct access to sound mixing soundlow 12 DB) allowed in the reverb time. By (2)-you can work out room reverberation time is allowed:
T = 0.09 (Q/4 π r 2) = 0.48 seconds
Obviously, through sound absorption treatment methods to minimize the reverberation time this level is difficult. Then we can consider the nature of the more powerful speakers, or a pair of auxiliary speaker, and let the Lord speaker's service range appropriate to decrease. By calculation, if the nature of the factor of up to 12 other conditions remain unchanged, the reverberation time is allowed for 1.1 seconds. If we adopt the latter programme, called the primary speaker service distance up to 20 m, 10 m then after another on secondary speakers to meet. Clearly, if only the service scope to 20 m and other conditions, allows the reverberation time is 1.1 seconds.
Here we have to point to the sexual factor of 12, services in the range of 30 meters, the average sound pressure level at 80 decibels to calculate the required electric power:
Wa=Q/4πr2?10（0.1Lp－12）=0.0942（W）
We=Wa/η=4.71（W）
At this point the required electric power barely 4.71W! but it's far from simple, because these factors cannot be ignored.
● 1 for disco, powerful music, 80 DB average sound pressure level is far from meeting the requirements of listening. Should generally be to 90 ~ 100 DB sound pressure level.
○ 2 for normal PA systems, up to 10 DB of sound power, shall be considered as minimum requirements, and high-fidelity system you should reach 20 decibels.
The above two factors, the design-time reference sound pressure level must be: 100 to 110 decibels (although actual use is not possible to use, just in the form of an allowance to reserves), then increase to 20 to 30 decibels. From (3)-can be seen, each raising 1 sound pressure0 DB, the required power (or say power) must increase 10 times, the halls of power are required to reach 470 ~ 4700 Watts. Of course for stereo system, it refers to two-channel sum of the desired power.
Although the theory of computation are too complex and cumbersome, in engineering design of practical value, but to draw some conclusions on the us it is a revelation. That is:
• 1 large halls, purely from improving the way although the speaker power can improve the sound pressure level, but not necessarily improve the clarity of the back yard.
○ 2 determine speaker power not only with the size of the room, also with the arrival of the sound pressure level, speakers of points to factors, room reverb intensity, speakers of electro-acoustic efficiency factors.
○ 3 actually speaker power rating in quite a range to meet the needs of use, because the power each doubling of the sound pressure to raise 3 DB, power increase ten-fold, sound pressure is increased by 10 DB.
○ 4 in order to cope with high peak, come into consideration when the speaker power remain necessary power margin is necessary, however, the music of the dynamic range is wide, the average sound pressure level higher than 10 ~ 20 DB peak, there will be 10 to 100 times more power. Use appropriate based on actual need to decrease the volume, loss in sound quality is not great, can significantly reduce costs.
2, project estimation of simple
Actually, we can also take advantage of an easy way to calculate the power required for the speakers, it was largely based on the following two rules:
● 1 without considering the reverberation room and acoustic source directivity and other factors, the sound pressure decreases with distance from the square of the sound source by the inverse of decline, the distance and then doubled the sound pressure level dropped 6 DB.
○ 2 speakers the size of the radiation sound pressure and the input power is proportional to the logarithm, i.e. power each doubled, increasing their sound pressure level 3 DB.
In the speaker's specification, usually gives the sensitivity of this parameter. It is defined as a free sound field, that is, there is no room for various reflections on some distance from the speakers axis 1 metres of the effective sound pressure level and enter the power ratio, i.e.:
Ma=P/W
In the project, we usually use the sensitivity of this parameter using the "backwards" approach to estimate the required power speakers.
For example: If you consider the disco, a strong power output and the necessary power margin, we demand from the speakers 16 metres reaching 107 DB average sound pressure level, then go back on backstepping 1 metres should 107 + 4 × 6 = 131 DB sound pressure level; if the chosen sensitivity 101 DB (1 w, 1 m) speaker, 101 DB increased to a total of 131 DB also require additional 30 DB, its input power must reach 1024 Watts. In fact this power can be two or four speakers to share.
This shows that the speaker's sensitivity is also very important indicators, to achieve the same sound pressure level, if the speakers are the difference between the 3 DB of the sensitivity, the requirements of the difference between the input power is doubled.
There is also a more simple estimation method is also very practical, specific algorithms like this: first, find the interior decoration of the completion of effective volume, only for language amplified speakers room, the required power rating, press 0 per cubic metre. 3 watt average power to calculate; for karaoke Hall, night club, music system, then press 0 per cubic metre. The average power of 5 watts.
Identify the speaker system of power, according to the selected power amplifier output power.
3. determine the power amplifier output power
In principle, speaker of the rated power and power amplifier power rating. For power amplifier, it is only the actual power output and speaker impedance, and speaker of the rated power. Regardless of the speaker of the rated power and power amplifier power rating is the same, the speaker of the rated power of the power amplifier are little affected, just on the speaker's safety or quality. If the speakerinput impedance is too small, you may cause the actual power amplifier output power greater than its rated output power or maximum output, resulting in increased distortion or damaged power amplifier, speaker is not necessarily very safe.
Now we only discuss the impedance matching, how to match the power amplifier and speakers: If speaker impedance to meet the requirements, and rated power and power amplifier rated power small push power is sufficient, sounds, obviously feeling sound when power amplifier power margin, low distortion, especially to low-frequency component of music performance. Then, the power amplifier's rated output power rating than the speakers to the end to how much is good? this depends entirely on our design speaker that remain in power margin, if the selected speaker when its power is adequate allowance, the power amplifier output power greater than 30% of the speaker system, that is, in the course of the speakers and the amplifier is lower than the rated operating conditions. If you selected speaker when its power is less NPSH.of, power amplifier rated power can power rating than the speaker is two to three times, then work the speakers may reach rated State, and the power amplifier is always lower than the rated operating conditions. This is a good match for the performance of the music of peak signal very favorable, but particularly careful in the use of must, otherwise you may damage the speakers.
If the speaker of the rated power is power amplifier rated power, although both can work safely, but often work at power amplifier in the limit State, distortion is large, even sometimes feel that not enough hard drive showed strong bass.