6) So Now You Want To Measure Sound Level

Hopefully you have read through the post named Basics: Audio (Sound), Part 1 and Part 2. It is good to have a basic understanding of frequencies and speakers and surround and amplifiers and level and Loudness. Hopefully you’ve noticed some of the many ways that the human hearing system has made things “interesting” in normal life, and while you have walked around your cinema theaters since learning more about these things.

For our testing, we are going to concentrate on one simple and free App for the Apple iPhone and iPad, dB Volume. It is from a company named dsp mobile. Click these words for the iTunes download link. There are many sound pressure level meters available, but this one has all the features we need, and is very simple to use…meaning, not only is everything that we need to see on the front screen, the ability to make changes is there also.

People may report back to you that there are internet sites that say that iPhone or other phone apps are not very accurate. That may or may not be true, but we don’t care. We just want a tool that is repeatable. Why? Because we are only interested in any changes that might happen from day to day, or week to week. Set-up engineers can worry about calibrated tools and measuring from 5 spots in the room, and other complicated things.

We only have to worry about doing the same thing each time we visit the auditorium and being able to communicate what we find effectively. What follows next is How To Do Those “Same Things”.

db_meter_screenshot_with_notes

The beauty of the dB Volume is that the controls we need are on the same page as the meter itself. We don’t have to go to an Preference page, then back to check that our setup is correct, or to change it. When a light and color test tool does the same thing, there will be angels singing.

Step 1 _ Controls.

Response ‘slow’ is what we want. Why? That is what the clever engineering types at the Society of Motion Pictures and Television Engineers have figured out, is the best answer. Generally, the reason is that we need to get a nice response from the meter that captures a stable range of the sounds, at all frequencies. If we used ‘fast’ the readings wouldn’t be as consistent from visit to visit. Click through to ‘slow’.

The same is true for ‘weighting’. We want to use ‘C’ weighting. Why? If you use ‘Off’ (which is sometimes called ‘Z’), or ‘A’ or ‘B’, the low frequency measurements will be lower and more inconsistent. So, click to ‘C’ on the 2nd button. Very important.

‘hold’ controls the Peak Number on the bottom right. Peak readings are valuable for some things, but not so good for measuring a cinema theater. Why? Because the design of the cinema auditorium is made to be a balance between hard and soft surfaces. Hard surfaces make too many reflections and distort the sound. To many soft surfaces, like the seats and the people and some of the wall treatment, and the sound will be dull. Peak readings measure the first clap of the thunder very well, but they don’t measure the average sound pressure level of the extended time of the thunder in the room. We want that averaged number.

If you poke around the internet, some people will tell you that 85 dB will harm your hearing. Yes, after 8 hours at that level, your hearing may suffer. But most people can tolerate much higher levels without harm for the length of a 2 hour movie. But, as you might say to your customers, if it is too cold in our rooms, please bring and wear a sweater. If performing these tests are too loud for your comfort, please bring and wear some ear plugs.

Finally, on the front panel is ‘Leq’, which is a measurement that averages the sound for a longer period of time than we need. It is good for something, but not these tests. Leave it set at “off”.

Step 2 – Position
Get settled in your listening position.
That position is 2/3rds back from the screen, and centered to the screen.

Most of the time “centered to the screen” is the same as being centered between the 2 side walls. But if the screen is not centered on the front wall (for example, if there is an exit door on the left that pushes the screen from the side wall), make certain that you are sitting in the seat that is in line with the center of the screen.

How do you figure 2/3rds? And how do you figure the center of the screen?
The most simple way is to look up and count ceiling tiles. For example, if there are 12 ceiling tiles from front to back, 2/3rds is under the back line of the 8th tile. And if the screen is about 12 tiles wide, then along the line of the 6th tile will be the center. Now look behind at where the lens of the projector is – just about all the time it will be “beaming” from the center of the screen.

Don’t be psychotic about the exact place, just be consistent.

Step 3 – Comfort and Repeatability.
The important thing is to get settled with your clipboard and pen and iPhone. Then hold the iPhone at a position that is about 8 inches (20 centimeters) above the seat in front of you. Why there? Because we want the microphone to be in a neutral spot. If it is too close to the seat, or to your head for example, the reflection or absorption will change the readings.

Turn your iPhone so that the microphone – which is at the end that you normally talk into – is facing up. The onscreen picture will rotate. Get out your pen and make sure you can see the place on the page that you will be writing.

Finally, have a place to put down the iPhone. You won’t be using the sound pressure level measuring tool, dB Volume, until the end of this process. Until then, you will use your ears and eyes to judge things.

So. Are you comfortable? There will be some light from the screen soon, but can you see in the dark? Do you need to turn on the light of your phone? (I found a pen that has a light at the point…works great.) Can you reach everything? Write the Auditorium Number and the date in the proper space on the Managers Report Form. Make a check that says which DCP is going to play. OK. That works. Let’s get busy.

Step 4 – Get out your radio or walkie and make certain your teammate has loaded the Playlist. We want to test everything in the chain of equipment, from the that runs one of the test DCPs for play through the media player and the projector.  Have them hit Play.

Step 5 – Watch the scale on the right side of the screen when the bell tones and the pizzicato string tones go by. The bar will go up, move around the scale a little bit, then go down.

It is going to take a bit of practice and then a bit of talent to know what the sound pressure level of those sounds are.

But one thing is more simple and quite a bit more important – Silence. We want to measure silence. Then write it down. Then compare it to previous Sounds of Silence in that room.

 

 

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