Using SPL meter -- why point it straight up?

[supervij]

Okay, so I was thinking about this the other day: why are we supposed to point the SPL meter straight up when taking measurements from our primary seated location?

The way our ears are situated on our heads -- doesn't that accentuate sounds coming from the front 180 degrees? And shouldn't we then hold the SPL meter on a slight angle forward when level matching our speakers, thereby resulting in an increase in our side and rear surround levels?

Am I out to lunch with this thinking? Is there something to it?

cheers,
supervij

 

[MDS]

If you point the meter directly at any of your speakers it will capture more of the direct sound coming from those speakers. At your listening position, your ears get a combination of the direct sound and the reflected sound that bounces off of the things in the room. Pointing the meter straight up more accurately captures what you will actually hear.

 

[supervij]

MDS, I'm not suggesting pointing the meter directly at any speaker. I'm talking about crouching down at the primary listening spot, holding the meter up, and angling it slightly downwards, i.e., if pointing straight up is 0 degrees, then letting it fall forward to maybe 10 degrees.

Considering the forward-pointing orientation of our ears, why would pointing the meter straight up be more accurate?

cheers,
supervij

 

[jonnythan]

MDS, I'm not suggesting pointing the meter directly at any speaker. I'm talking about crouching down at the primary listening spot, holding the meter up, and angling it slightly downwards, i.e., if pointing straight up is 0 degrees, then letting it fall forward to maybe 10 degrees.

Considering the forward-pointing orientation of our ears, why would pointing the meter straight up be more accurate?

cheers,
supervij

Because it gives the most accurate representation of the exact sound pressure at that point in space. It does not take into account human ear orientation.

Angling it forward slightly will make the measurement less representative of the actual sound pressure in empty space at that point. Your contention is that this alteration of the measurement will make it more representative of how humans actually hear, but I'm not at all convinced that's the case.

 

[j_garcia]

The unit is trying to read equally from all speakers for the exact position, so vertical should actually be a more "correct" reading. I have found though that the exact measurement it gives isn't exactly what sounds best to me, so I normally tweak after calibration to get my desired levels, which is generally about -1 to -2dB on the surrounds. The meter doesn't lie, but that doesn't mean you can't adjust to your personal taste.

 

[Savant]

If the measurement microphone is omnidirectional, you can point it any which way and get the same result.

I saw an alleged acoustics "expert" once demonstrate the "proper" way to align a sound level meter (with an omni mic) to get the "correct" results. Hogwash.

 

[supervij]

jonnythan: "Angling it forward slightly will make the measurement less representative of the actual sound pressure in empty space at that point. Your contention is that this alteration of the measurement will make it more representative of how humans actually hear, but I'm not at all convinced that's the case."

jonnythan, could you tell me why you don't think so?

j_garcia, I know I can adjust the levels to my personal taste; I'm just curious about this whole forward ear/forward tilt thing. I'm not trying to tell people what the right way is; I'm just wondering about the theory behind the "straight up" reasoning.

Savant, the mic on the analog Radio Shack SPL meter is not omnidirectional, as evidenced by MDS' response.

I'm not trying to push anything here; I'm just curious, really.

cheers,
supervij

 

[jonnythan]

I dont think so because there's no evidence for it whatsoever. Do whatever you want, it doesn't matter.

 

[MDS]

I don't want to speak for jonnythan but if I may I think what he is getting at when he said: 'Angling it forward slightly will make the measurement less representative of the actual sound pressure in empty space at that point' is this:

If you point the mic slightly forward (and angled downwards slightly from vertical) you will be measuring the SPL at the point slightly forward and downwards from your ears as opposed to holding it vertically at ear height where you would be measuring the SPL at you ear location (which would include both direct and indirect sound). That and you would be capturing more of the direct sound from the front speakers and slightly less of the indirect sound as I said previously. It's a small difference but could be meaningful and everything I have ever read with regard to the RS meter says to point it straight up.

I don't believe the mic of the RS meter is truly omni-directional but I could read the docs that came with it and see if it mentions anything about the design of the mic.

jonnnythan feel free to correct me if I misinterpreted what you said.

 

[jonnythan]

All I'm saying is that, assuming the measurement changes when you tilt the thing forward, there is no evidence that this change makes the measurement more accurately represent an in-ear measurement.

In other words, there is no evidence whatsoever other than pure conjecture that your totally arbitrary "tilt" of 10° will in any way make the measurement more accurate.

Let's assume that the measurement changes according to the tilt:

90° - 89dB
75° - 89.5dB
60° - 90dB
45° - 90.5dB
30° - 91.5dB

Which one of those is closest to what your ear really hears? Are you accounting for the fact that, while forward-oriented, the ears are situated horizontally and on opposite sides of the head?

If you want to compensate for something in a measurement, you have to have some idea what you're compensating for. You can't just alter the measurement or the way you take the measurement arbitrarily and just say that the "new" measurement is closer to "reality." It doesn't work that way.

 

[supervij]

Hey, I was just asking out of curiousity. You're right -- it was pure conjecture, and I have no evidence to back up anything I said. It was just a idea, and I wondered if it had any validity, that's all. Anyway, thanks for clarifying, guys.

cheers,
supervij

 

[Resident Loser]

As I recall...

...as per the Radio Shack (the meter I use) documentation, they say not to point it directly at the source but horizontally and at a right angle to it...and if you think about it it seems to make sense...each of your ears "hears" in a hemispheric pattern and placing the meter, whose mic is omnidirectional and mimics that hemi. response, at a right angle to the source is much more like your head facing forward...

Also holding the meter isn't a good idea, your body will affect the response to some degree...using a camera tripod (that's what the threaded opening on the SPL meters' bottom is for), elimimates that interaction...

jimHJJ(...just my 2 cents...)

 

[avliner]

Have read way back when, that the best bet is to point the mic. to the vertex and that's the way I do (with a help of a laser pointer, with the SPL mounted on a tripod, so I can get the right spot between the wall & roof).

Not sure whether this is the right procedure, but that's the way I always do.
Just my $0.2 worth...

Cheers.

 

[Resident Loser]

Does this...

Just my $0.2 worth...Cheers.

...reflect inflation?

jimHJJ(...color me curious...)

 

[Savant]

If we are talking about the RS digital meter, the mic is an omni. Whether the meter, as a whole, would be considered "completely" omni is unlikely - largely due to its not meeting ANSI standards for sound level meters - but I've used it enough that I would consider it a fair assumption.

For reference on RS' recommended measurement procedure, see page 11 of this PDF.

The same more or less holds for the RS analog meter. See page 12 of this PDF for RS' recommendations and a nice polar plot. Definitely omni below 1 kHz. {Page 5 of the same PDF specifically states the mic is omni, with (not atypical) frequency dependence.}

 

[Resident Loser]

I'll...

If we are talking about the RS digital meter, the mic is an omni. Whether the meter, as a whole, would be considered "completely" omni is unlikely - largely due to its not meeting ANSI standards for sound level meters - but I've used it enough that I would consider it a fair assumption.

For reference on RS' recommended measurement procedure, see page 11 of this PDF.

The same more or less holds for the RS analog meter. See page 12 of this PDF for RS' recommendations and a nice polar plot. Definitely omni below 1 kHz. {Page 5 of the same PDF specifically states the mic is omni, with (not atypical) frequency dependence.}

...have to check my paperwork at some point...I'd be willing to swear on a stack of bibles they rec'd the 90deg angle...perhps it's the vintage of the unit, mine was purchased back in '76 and was mfg. in Japan...

jimHJJ(...thanks for the update though...I'd hate to be promulgating out-dated info if vintage is an issue...)

 

[Savant]

Resident,

Perhaps you are thinking of pressure-response measurement mics? Those would have been more common on meters from the '70s (including, perhaps, old RS meters ) and their response characteristics require aligning them 90° to the incident sound. See page 15 of this PDF. As shown, even random response mics should be aligned 70-90° to the incident sound in a free field. This is only relevant for outdoor measurements for high frequencies, though. Indoors, like in home theaters, this is generally not applicable.

Most SLM mics these days - presumably including the RS meter mics - are of the random response variety (i.e., omnis) and can be aligned in any direction relative to the incident sound. (Bottom-right figure on page 15 of the referenced PDF above.) Probably stating the obvious (and perhaps repeating myself...): Once the size of the mic/meter becomes large relative to wavelength, the orientation of the mic/meter relative to the incident sound will become relevant. But this is usually at higher frequencies and is relatively insignificant for home theater-type sound level measurements.

 

[Resident Loser]

Well...

...at least I know my synapses are still firing and I'm not starting to ride off into the senior sunset...

FWIW, I rummaged past my collection of retired phono carts and the Shure stylus force gauge and pulled out the old Realistic SPL meter (Cat No. 42-3019) in it's original box complete w/owners manual...

Try as I might to find an online site to reproduce it's contents, I cant...I don't have a scanner, so you'll have to trust me on the following:

On page 6, Figure 3 shows the response plot as it appears in your .pdf link...adjacent to said plot is this caveat:

"Always position the Meter so sound is directed toward the mic from the side, as illustrated in Figure 4. Polar response is indicated in Figure 3."

Figure 4 (an overhead line drawing) on page 7 is similar to the one on pg. 15 of your link, with one significant difference...the meter is positioned so that it's 0 degree axis is perpendicular to the axis of the source, that 90 deg. reference I'd mentioned in my earlier post...Adjacent text, after some info re: the effects of bodily proximity, etc. continues:

"In addition, the Meter should not be pointed directly at the source; sound should be directed toward the mic from the side (see Figure 4). The same principles apply when the meter is mounted on a tripod."

S-o-o-o-o, I'd guess the best advice to supply from now on will be to follow the placement guidelines as indicated in the manual supplied with your specific meter.

jimHJJ(...one error I discovered, the paperwork was printed in Korea so I'll hazard a guess that's where the meter itself was produced...)