Since there are many goals and and several different purposes for this project –
Goals: To make certain the Director’s Intent gets to the screen and speakers, and to make sure the audience is pleased with the results;
Purposes: To Learn, To Do Well, To Make the Bosses Look Good…etc., to be able to talk logically with the techs with information that you or the audience noticed about the picture and sound…things like that…
– there are many things that need to be done. Training is high on the list. Practice should be high on the list…but who has the time!?!? And, how horrible, you don’t have a tech or projectionist to teach you this week.
You can still download some audio and light measuring tools for your phone. Then you can experiment and practice with them until you can use them easily in a dark room.
There are many…too many, in fact. Some are extremely significant professional tools at great cost – not knocking them, since they are reasonable if you are a pro who needs the complexity. Some are too simple or have ads pop-up that will annoy you in the dark room. Some need several taps on the screen to set it up the way you need it for the auditorium measurement.
We recommend one that is not only free, but it is quick and it is simple to get good data from.
What we need is something that quickly tells us 2 numbers.
The first number is called Peak, which measures the sharp blast of energy that a clap of lightning or a single hand clap might have. The other number – the one we actually need most often – takes an average of all the sounds for a particular segment of time…a moving window of time, in fact. The name goes by the abbreviation of “RMS”, which are initials for a technical term that sounds like a rapper’s name on a science cartoon: Root Mean Square.
If you are thinking, “This sounds like math and Must. Be. Ignored.”, you are absolutely correct! In this case you can completely ignore the abbreviation and just know that you get a number that is a good average of the sound you are hearing.
Many of the tools will have a Peak and RMS number displayed quickly and at the same time. Sometimes this is what you want. Sometimes you want to ignore one or the other number. But it is good to get familiar with both of them. Usually, we will just report the RMS level.
But there is another layer of complexity. There are choices in how we measure RMS levels. The first choice is Fast or Slow. The second choice is A or B or C or Z weighting. Let’s cut to the answer and ignore the reasons:
Slow and C weighting.
We can study why in another lesson. Engineers talk for hours about these things. We will also skip to the recommended iPhone tool. The app that we keep coming back to is from dspmobile.com, named
Why? Because it has a very simple and clean interface that gives all the details you need very quickly. Most importantly; you can change the weighting and response speed from the front panel.
People may tell you that the iPhone isn’t exact, or argue that you need something more precise. But both of those statements are not true. We need something consistent. We measure today, we measure tomorrow. If the sound level is the same – when taken from the same position in the room – the measurement should be the same. Unless your iPhone gets thrown into a pizza – which will probably make the microphone read differently than the day before – you will get a consistent reading.
Which means that we can measure the speakers consistently, and tell if there is something different from day to day. When the sounds come from speakers, we can see their levels, mark the level down and compare the results from last week and last month…all the way back to the time when the room was set up by a technician. Oh…and sometimes we will measure silence.
Remember. Our only goal is: Finding differences. It does no good to tell a tech, “Hey, it’s only 80 dB in the room.” The tech will have no reference~! 80 for one room might be perfect, but it might be too soft for another room. It does good to say, “The left surround was 85 dB last week and is only 80 dB this week.”
And a quick hint. The person who is honored for his research in sound, and sound transmission across a distance (he was the first to get a telephone to work), is Alexander Graham Bell. That is why the ‘B’ is always capitalized. The ‘d’ is short for ‘deci’ meaning one tenth. Rarely do we use the term ‘Bel’ by itself. The typical measuring unit is the ‘decibel’ – a ‘dB’.
And finally. When you have finished measuring your auditoriums, turn off the dB Volume tool. It may stay running in the background. If so, it will cut your battery time.
Go and do well. Practice a lot! …then go and mark up those Manager’s WalkThrough Report forms.
Bonus points. The Society of Motion Pictures (Yeah!) and Television (Hiss!) Engineers is the group who studied all this and came up with the Standards and Recommended Practices for Cinema Auditoriums. If someone asks you, just say, “Yeah, dB Sound Pressure Level Slow Response C-Weighting is in Paragraph 5.5 of SMTPE Standard ST 202 SMPTE Standard for Motion-Pictures — Dubbing Stages (Mixing Rooms), Screening Rooms and
Indoor Theaters — B-Chain Electroacoustic Response.”