SPL measurement methodology

[normc]

I noticed in one the articles on measuring SPL, that it says to place the SPL mike in the same position as the ears, for obvious reasons. However, it also says to point the mike at the ceiling. This is not so obvious and I'd like to understand why, as opposed to pointing the mike at the front center for a 5.1 set up. What difference would it make and why is that considered the best orientation for the mike?

 

[Adam]

Welcome to the forum!

The instruction to point the mic at the ceiling is contrary to instructions that I've seen and followed. The instructions that I've used say to point the mic at an angle (45 degrees, if I remember correctly) towards the speakers. That's when using an SPL meter like those sold at Radio Shack that have what appears to be a directional mic. The mic that came with my receiver for its auto set-up system was more omnidirectional and was pointed upwards.

I'm not sure which orientation is better for a regular SPL meter. Some of the acoustic experts here might have some input for us.

 

[lsiberian]

I noticed in one the articles on measuring SPL, that it says to place the SPL mike in the same position as the ears, for obvious reasons. However, it also says to point the mike at the ceiling. This is not so obvious and I'd like to understand why, as opposed to pointing the mike at the front center for a 5.1 set up. What difference would it make and why is that considered the best orientation for the mike?

I suggest you experiment yourself with the MIC position and observe the differences. Share them with us and add to the wealth of knowledge.

You can do it both ways and see what results sounds better.

 

[MDS]

I've always pointed the mic straight up at the ceiling and everything I've ever read says to do so including the instructions for the RS meters. It's also best to place the meter on a tripod and stand behind it so your body does not cause reflections that could change the reading.

Pointing it at the ceiling makes sense because you want to measure both direct sound and indirect sound bouncing off the walls and furniture. The entire room and its furnishings affect the sound so you don't want to measure only the direct line from the tweeter to your ears especially if you may have other people in other seating positions in the room at times.

 

[highfigh]

I noticed in one the articles on measuring SPL, that it says to place the SPL mike in the same position as the ears, for obvious reasons. However, it also says to point the mike at the ceiling. This is not so obvious and I'd like to understand why, as opposed to pointing the mike at the front center for a 5.1 set up. What difference would it make and why is that considered the best orientation for the mike?

Mics used for measurement are usually omni-directional, but because incident sound is important to the results, they recommend this orientation. It's how the meter was designed to be used and if it's not oriented this way, it won't necessarily be as accurate.

 

[highfigh]

I've always pointed the mic straight up at the ceiling and everything I've ever read says to do so including the instructions for the RS meters. It's also best to place the meter on a tripod and stand behind it so your body does not cause reflections that could change the reading.

Pointing it at the ceiling makes sense because you want to measure both direct sound and indirect sound bouncing off the walls and furniture. The entire room and its furnishings affect the sound so you don't want to measure only the direct line from the tweeter to your ears especially if you may have other people in other seating positions in the room at times.

It's really best to not even be near the mic. Your body absorbs energy that the meter should be receiving.

 

[lsiberian]

It's really best to not even be near the mic. Your body absorbs energy that the meter should be receiving.

So your saying that we can only come out of the closet when our system is fully calibrated.

 

[highfigh]

So your saying that we can only come out of the closet when our system is fully calibrated.

You can come out of the closet anytime you want but just lurking will skew the results. Maybe stalking is a better word, eh?

 

[normc]

There is the school of thought that says you can't measure anything without changing it and highfigh is correct in that it would be best to not even be in the room. However, its neither practical or possible without a setup designed to achieve that.

In the absence of a qualified response I believe I'll follow Isiberian's suggestion and try the different orientations to see the difference. I was hoping to get a qualified response before doing that so I'd have some idea what to expect. I forgot to mention I'm using the Radio Shack analog meter so its pretty basic and I suspect the mike's HF roll off starts well before the system's does.

Will post again when done so we can all learn something from it. Although sometimes all we learn is that we still have a lot to learn.

 

[popotoys]

There is the school of thought that says you can't measure anything without changing it and highfigh is correct in that it would be best to not even be in the room. However, its neither practical or possible without a setup designed to achieve that.

In the absence of a qualified response I believe I'll follow Isiberian's suggestion and try the different orientations to see the difference. I was hoping to get a qualified response before doing that so I'd have some idea what to expect. I forgot to mention I'm using the Radio Shack analog meter so its pretty basic and I suspect the mike's HF roll off starts well before the system's does.

Will post again when done so we can all learn something from it. Although sometimes all we learn is that we still have a lot to learn.

Wouldn't you actually want people in your room when you do the measurement in order to reflect more accuracy. Don't good professional sound men\women change the eq as a bar fills up?

 

[Savant]

This topic has come up before:

http://forums.audioholics.com/forums/showthread.php?t=50359
http://forums.audioholics.com/forums/showthread.php?t=39582

IMO, pointing the mic at the ceiling (or in any other specific direction) is hogwash. My advice would be to position the sound level so that the display is easy to read.

 

[Birdman84]

Wouldn't you actually want people in your room when you do the measurement in order to reflect more accuracy. Don't good professional sound men\women change the eq as a bar fills up?[/QUOTE]most engineers I know will set up the eq first with the room empty then once the room has filled will tweak the sound to get the over all best sound. I have been doing it for over 30 years and find many things effect the final sound including the temperature of the room. but it comes down to how picky you are. I have meet very few people over the years the could really hear anyway. I once produced a show for Bruce Hornsby and his engineer spent most of his time making sure that the sound going to tape (live 48 track recording) was right that he paid little attention to the house sound which was terrible. But most of the people in the show did not know the difference.

 

[normc]

As promised, I did some further experimenting on the weekend and found as Savant has said, its a load of hogwash. I used 3 test tones (1kHz, 10kHz and 100Hz) as I varied the mikes direction from pointing horizontally at the speakers to vertically at the ceiling. The maximum difference was << 1db. Most of the time it was even less. Conclusion..... not worth worrying about unless someone's life is dependent on the results.