Video Transcript
Design & Best Practices: Part 4
6m 46s
00:07
Welcome back! Let's pick up again where we left off.
00:12
Once a system is installed, it’s time for calibration and tuning.
00:16
Using a decibel meter set to the C-weighted setting Pink Noise is run through each loudspeaker individually,
00:23
or in groups of surrounds, which we’ll cover here in just a minute.
00:27
The most critical loudspeakers in the sound system are the Left, Center, and Right Screen Channels.
00:33
This is where most dialogue, music, and effects are localized. All three loudspeakers should be well-matched.
00:41
Any inconsistencies in these loudspeakers– especially in the human voice frequency range – can get really distracting for the audience.
00:51
Let’s look at the basic SPL targets for each loudspeaker type starting with these.
00:57
For each screen channel, the level should be set to achieve 85 dB C weighted at the 2/3 distance from the screen.
01:06
For surrounds in a 5.1 system, each Left and Right array should be set to 82 dB.
01:14
For a 7.1 system, Left side channel plus the Left rear channel, and Right side plus the Right rear channel should be set to 82 dB also.
01:24
For subwoofers, don’t use the SPL meter.
01:28
If you were to use a typical SPL meter, the narrow bandwidth of the subwoofer would read too low compared to a full range signal.
01:36
SMPTE Recommended Practice RP200 specifies a method using a Real Time Analyzer to set the subwoofer level.
01:44
Look at the level of the center channel on the RTA at mid frequencies and set the subwoofer level to be 10 dB higher.
01:52
The result is that the subwoofer ends up with the same level in its passband as the screen channel loudspeaker.
01:59
The most common calibration tool used in Cinema is the Real Time Analyzer, the RTA.
02:06
Most RTA’s use 4 or more microphones positioned in the seating area, a pink noise generator, and a multiplexer to create a “spatially averaged” measurement.
02:17
For screen channel loudspeakers, response should approximate the industry-standard “X curve”, which is defined in SMPTE standard 202.
02:26
You can see that the X curve has plus and minus 3 dB tolerance.
02:31
The good news is that for a properly designed, well-behaved loudspeaker, the high frequency loss due to sound through a typical perforated screen
02:40
along with the loss to natural “air” attenuation already approximates the X curve – without the need for a lot of equalization.
02:49
If a loudspeaker seems to require a lot of equalization – say, more than a few dB – there is probably something else wrong with the room acoustics,
02:59
the wiring of the loudspeakers, or maybe even the loudspeaker itself. In any of these cases, equalization will not fix the problem!
03:08
A high quality screen channel loudspeaker should look roughly like this on an RTA screen.
03:14
The Yellow line is the X curve, and the Red lines are the 3 dB tolerances.
03:20
This particular loudspeaker is a popular QSC model with our own Intrinsic Correction processing – and it required no additional equalization to approximate the X curve.
03:32
Now let’s take a look at how it all goes together in a system design.
03:37
This is just one example of a Basic 7.1 Sound System for a medium sized room, let’s say about 300 seats.
03:45
Three bi-amplified screen channel loudspeakers at Left, Center and Right. Two dual-driver subwoofers. And 18 surround loudspeakers.
03:55
Let’s assume this will be a fully-networked Q-SYS application, which provides all loudspeaker signal processing and routing,
04:02
and network control and monitoring of all of our system parameters.
04:07
Since we’re using our Q-SYS network amps, this entire system can be powered by just two 8-channel amplifiers.
04:15
Now let’s add some interconnections.
04:17
QSC recommends wiring the Left and Right loudspeakers together on one amplifier, and the Center loudspeaker on the second amplifier.
04:25
In the very rare case that an amplifier might fail, you can still have sound coming from the screen so it doesn’t ruin a show before the amp can be replaced.
04:35
The Center channel is powered by two channels of the second amplifer.
04:40
If there are multiple subwoofers, as we would recommend for this example room, you would also separate the subs onto the two amplifiers for the same reason.
04:49
Note that in this example, the subwoofers are each powered by two amplifier channels in parallel mode.
04:56
And we’ll use the same idea for the surround arrays.
05:00
The six rear wall surrounds can go on two channels of the first amp.
05:04
Surrounds for a 7.1 format system should be grouped and wired in parallel – and you can easily put 3 or 4 surrounds per channel on QSC amplifiers.
05:16
The remaining channels on the second amp feed the Left and Right wall sound arrays.
05:20
And you’re done! A simple and elegant, high performance 7.1 cinema sound system using just two amplifiers, with complete signal processing, control and monitoring.
05:33
Only possible, by the way, with the Q-SYS Ecosystem.
05:37
Once the products are selected, and the system is designed, installed, and calibrated - then it’s time for commissioning and final checking.
05:46
Always play some familiar content and walk around the entire room. Listen to every loudspeaker.
05:53
Listen to the dialogue. For even coverage around the room, for reflections and echoes, for noises that shouldn’t be there like buzzes, and rattles.
06:02
Look for any bright metallic surfaces from construction materials, or light sources through the perforations of the screen.
06:10
We realize that any room that is not designed as a purpose-built cinema will present a number of inherent challenges
06:17
that might make it impossible to adhere to all of these guidelines and best practices.
06:23
But the closer you can get to these guidelines, the better overall audio and visual experience you’ll be able to deliver.
06:31
And finally, don’t forget to check out the resources we’ve put in the Links & Downloads section.
06:38
Alright, that's a lot - I know. Let's take a break for now and come back again when you're ready.
Lesson Description
Design & Best Practices: Part 4
6m 46s
Specifics regarding system calibration and commissioning are covered along with interconections of a basic 7.1 system using Q-SYS.
Downloads and Links
Design & Best Practices: Part 4
6m 46s