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برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات. - حاصل آرایش چند اسپیکر در سالن ها و آمفی تئاترها+شکل

برق. قدرت. کنترل. الکترونیک. مخابرات. تاسیسات.

دایره المعارف تاسیسات برق (اطلاعات عمومی برق)

Specific Examples

Example #1: A Small Church

Description

The ceiling is low suspended acoustic tile over an open space covered with thin carpet. The RT60 (the time it takes the reverberant sound to decrease by 60 dB) is short, so controlling reverberation is not a problem for audio clarity. In fact, the room is a touch "dry" for music, and content of the worship service includes live musical performances. The sources of audio are the minister with a wireless microphone and the band. Additional sources are DVD/CD players and other devices as needed. Control is via a 24-channel mixer with all inputs used. Output is to a pair of powered speakers mounted high in the corners of the room in a stereo configuration. This installation was done by members of the congregation without consultation with an audio professional.

Figure 12. Stereo Speaker Pair Coverage

Problems

  • The quality of the audio is poor with numerous problems including uneven frequency response.
  • An experienced sound person is required to run the mixer for all audio system use.
  • There is poor coverage of the congregation from the stereo speaker pair. People sitting in the hot spots just in front of the speakers are blasted with excessive level, and the rest of the congregation is exposed to a strong interference pattern between the two speakers. The system is uncompensated for room modes, room response and speaker response irregularities. There is a small "sweet spot" in the center of the room where the two speakers combine coherently but there is an isle down the center of the seats. Since there are no chairs, no one is seated in the "sweet spot".

So does this audio system work the way it is? Yes, but even the pastor knows the congregation may not be receiving the best possible audio experience. This example is rich in possibilities.

Recommendations

Improvements to this system are accomplished in a number of ways. A DSP can be used for equalization, other processing and to add automation to the minister's microphone. The entire worship band could be run through a mixer with each individual input processed by an AGC. There are admittedly downsides to automating the audio mixing of a large group, as the automation is not as intelligent as an experienced sound person, but is possible in some cases.

The speaker system is examined to look at options that provide more even coverage of the congregation. Improvements to this audio system can be introduced in phases.

Phase 1

Add a DSP box between the output of the mixer and the feeds to the main speakers and on-stage monitors. Features added could be:

  • Parametric Wide-Band Equalization. This alone would greatly improve this system.
  • Parametric Narrow-Band Equalization. A short RT60 makes this unnecessary at this time. However, remodeling could increase RT60 to where narrow-band equalization would be needed. (This room could use bass absorbers).
  • High-Pass Filtering. If not in the 24-channel mixer already.
  • Compression. Always a good idea with microphones because of the inverse square law relationship between the preacher's mouth and the location of the microphone. See the Rane Pro Audio Reference entry for "Inverse Square Law."
  • Feedback Suppression. If needed.

Phase 2

Automation is incorporated with automixers and remote controls. There are many exciting ways to add these features depending on the needs of individual congregations. The most obvious upgrade would be to add the ability for a minister to turn on and control the main microphones from a simple control panel located in easy reach at the front of the room.

Phase 3

The very uneven coverage of the congregation by the stereo speaker pair needs to be addressed, as shown in Figure 12. The seats directly in front of the speakers have enough level to kill small animals.

If the audio system were perfect then each seat in the congregation would have the same audio level. In the author's experience, similar rooms have been controlled within a couple of dB. In this example, the seat closest to each loudspeaker is about 15 dB louder then the worst seat on the floor, and interference between the two speakers adds to a very lumpy and unpleasant frequency response. Another problem is that the FOH (Front Of House) Mixer is placed in a location for good sound, causing the levels at the ends of the front rows to be way too loud.

Line Array Speakers

One improvement is to remove the stereo pair of point-source loudspeakers, and install a floor-to-ceiling line array located in the center of the back wall as shown in Figure 13. Coverage of the congregation is more even, and the level at the FOH Mixer location is very similar to the coverage level over the whole floor of the congregation. The level of the stage monitors is greatly reduced and some of the stage monitors may no longer be needed depending on the individual needs of the musicians. Within the near field of the line array there is a range were the audio level will decrease by only 3 dB for each doubling of distance which greatly helps even the coverage across the entire floor. One other characteristic of this application is that the audio is distributed across the whole line so that even if a microphone is right next to the line there is little tendency to feedback.

Figure 13. Line Array Speaker Coverage

In this example, there is a low suspended-acoustic-tile ceiling that shortens the length of a line array speaker. This limits some of the good qualities of a line array so this might not be the best solution. If the room were remodeled so there was a high ceiling, then a line array would make more sense because a longer line array would fit. This is especially true if the newly remodeled ceiling was acoustically reflective causing the RT60 of the room to be much greater. The high directivity of a long line array greatly helps to project the audio out to the floor rather then have the audio directed toward the ceiling where it contributes to the reverberant energy and slap echoes in the room.

Supplemental Distributed Array Speakers

Because of the dropped ceiling, another option would be a distributed array of supplemental ceiling speakers in the back of the room as shown in Figure 14. The loudness level of the main stereo pair could be reduced by at least 12 dB. This would greatly diminish the effects of the hot spots in the front of the room but would leave the level at the back of the room way too low. Ceiling speakers can be added in the locations shown to fill in the audio in the back of the room.

It would be very important to include a speaker over the mixer location so the audio at that location matches the level in the congregation to aid in achieving an accurate mix.

Figure 14. Distributed Array Speaker Coverage

Why The Delay?

The ceiling loudspeaker signals should be delayed in time so their output combines coherently with the output from the point-source pair in the front of the room. If the rear loudspeakers are not correctly delayed then the loudspeakers in the room will not combine correctly.

This room is too small for audio from the front of the room to be perceived as a distinct echo. Applying a proper delay to the ceiling speakers can minimize the problem of localization confusion that occurs if the first arrival sound is coming from the overhead loudspeakers and not the front of the room.

Example #2: A Mid-Sized Contemporary House of Worship

Description

This second example is a medium sized house of worship. The vaulted ceiling is high and the floor in the congregational seating area is covered with hard-industrial vinyl. The RT60 is longer then the first example at approximately 1.5 seconds so reverberation is a problem in an empty room. The sources of audio are again ministers on a microphone and a worship band. Control is via a 32-channel mixer. The speaker system is an array of three large boxes mounted as a central cluster high in the peak of the ceiling. A professional audio company did the installation and calibration of the audio system.

The quality of the audio in this church is much better than in the first example. An interesting question is: how good is "good enough"? When interviewed, members of this congregation can usually hear. Rarely is the audio painful to listen to so some say that the audio quality is fully acceptable. This is a good time to reflect back on the example in the introduction where domed ceilings were held up as an icon of natural acoustic wonderfulness. Let's examine each individual audio characteristic previously discussed and see how this audio system installation stacks up.

Figure 15. Distributed Array Speaker Coverage

Problems

  • Reverberance is not well controlled and is dependent on the configuration and occupancy of the room. Low-mid frequencies are a particular problem as the energy builds up and is never trapped or controlled.
  • Clarity is fairly good and meets a minimum standard.
  • Articulation is acceptable but not outstanding. The ALCONs (Articulation Loss of Consonants) rating of this room is fairly low but in the acceptable range. However, there is room for improvement.
  • Listener envelopment is nonexistent and completely pales in comparison to the example of a domed ceiling.
  • Again, as in the first example, an experienced sound person is required to run the mixer for any use of the audio system, as there is no automation in the audio system.
  • There is good coverage of the congregation from the central cluster, but people sitting in the area where the coverage patterns between two of the speakers overlap experience uneven frequency response due to the comb filtering caused by the interference between these two speakers.
  • Bass response is particularly poor. The poor bass response leads to the impression that the system lacks sufficient power.

Recommendations

A DSP is already in the system and can be used for additional equalization and other tasks. The same recommendation applies to add enough automation so that a simple service can be done without bringing in a sound person.

The speaker system may already be fully adequate. The first temptation may be to add a subwoofer to add bass power, but after a quick survey it is probable that the buildup of mid-bass energy in this room makes the quality of the bass so poor that adding more bass will only make matters worse. To fix the room, the ceiling and walls could be completely covered in bass absorptive panels, but this is not really practical so a compromise is to add bass traps to the corners of the room and the ridge of the ceiling.

If it is not possible to tame the room with traps, then narrow-band filtering techniques could be employed. This is where the room is evaluated for the natural modes that build up energy in the room and these frequencies are notched out with a very narrow filter. A combination of some absorptive panels and narrow-band filters might be the best compromise.

There are regions (as shown in Figure 15) where the coverage from the individual speakers in the cluster interfere with each other rather than combine cooperatively. This interference is frequency-dependent. The solution is to reduce the contribution of some of the speakers of those problem frequencies so that interference is minimized.

The system would then require re-calibration to complement the above changes. That should do it.

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