Radio Shack SPL Meter Placement
- Thread starter heeman
- Start date
soundhound
Well-Known Member
Technically, an omni directional instrumentation microphone should be pointed at the ceiling (90 degrees incidence) to avoid high frequency buildup when the audio wavelength is close to the diameter of the capsule, but in the case of the RS meter, it's quality isn't high enough to make a difference. Point it any way you wish.
heeman said:
Mount the SPL meter on a camera tripod, set the tripod on the sofa/chair at the listening position at ear height. Looking at the SPL display from behind the sofa/chair, begin calibration with full range pink noise (AVIA). After levels are accurately set, don't forget subwoofer phase.
Rope
Compliments of Soundhound!
Speaker Level Calibration
I thought I'd present some alternate thinking on level calibration of a home theater speaker
system. These issues are not really dealt with in any manuals or other papers associated with
home theater that I'm aware of. You will need the "Avia Guide to Home Theater" DVD test disk
which contains full bandwidth pink noise test signals for the main channels.
Some background:
On a movie dubbing stage (and in all professional work) full range pink noise is used for
speaker calibration. This is used because it exercises the entire audio spectrum from deep
bass to the highest treble at once, and provides a more accurate "average" sound energy
measurement of the speaker system. Analog SPL meters are used since generally an analog
meter movement is easier to interpret as an average level than the changing digits of a digital
display. The lowly RadioShack analog meter is used daily for checking the calibration of
dubbing stage systems, although the much more costly Bruel & Kjaer meters are used for
absolute calibration. The speaker systems on a dubbing stage are calibrated for 85dB SPL,
which is the average SPL of dialogue in a movie, rather than the 75dB used in consumer gear.
On the home theater front, manufacturers have standardized on a psudo-pink noise test tone
source. This tone is basically filtered pink noise with the center of the spectrum centered about
1Khz and falling off steeply at higher and lower frequencies. It does NOT exercise the bass or
treble at all, and as would be expected, it relies on the flatness of your speaker system in the
area of 1Khz for the accuracy of the calibration. Generally home speakers are pretty flat in this
region, but this is not always the case. If your front speakers are flat at 1Khz and your
surrounds have a peak of 3db at this frequency, after you calibrate your system, your
surrounds will end up 3db too low in level.
So, let's adapt the professional model and use it for home calibration.
For home theater use aside from increased accuracy, there is an important benefit of using full
range pink noise to calibrate your speakers. That is the fact that with this signal, TIMBRE
changes are very audible from speaker to speaker. By ear, you can readily hear any changes
in tone caused by room influences or non-identical speaker systems as the signal is switched
from speaker to speaker. Ideally, the pink noise should sound identical in every speaker
(except for the sub). This is not always achieveable, but it is the ideal situation. Make speaker
position or room adjustments until you get the most uniform timbre quality from each
speaker.
Here is the procedure:
First of all, turn off your subwoofer for calibration of the other speakers. This is because the
bass management in your receiver or pre/pro re-directs the bass from "small" speakers to the
sub, and makes measurements on the other speakers difficult. Full bandwidth pink noise
contains bass frequencies, and the bass coming from the sub will skew your measurements
and cause the meter to jump around needlessly.
As a compromise between absolute full range accuracy and practical ability to follow the
needle on your SPL meter, set the "weighting" to "A", rather than the more common "C", and
the meter response to "slow". "A" weighting makes the meter more sensitive to the mid range
frequencies and less so to the bass and treble, but nowhere near as extreme as the bandwidth
limited test tone built into receivers, pre/pros, and on most test DVDs.
Switch the pink noise to each main speaker, and set the output of each speaker to the same
level with the trim controls of your preamp or receiver, using the procedure you would use
with conventional test tones, i.e. with the SPL meter's microphone pointing upwards toward
the ceiling at the main listening position.
After you get all the main and surround speakers to the same level, swtich to "C" weighting
and note any differences: this will be mainly due to the greater sensitivity in the bass region of
the meter. If the needle is still relatively easy to read, you might want to do any final touchups
in the level of any speakers that give largely different readings from the "A" weighting
reading.
Finally, switch on your subwoofer and calibrate it with your SPL meter in the "C" weighting
position, using the dedicated subwoofer test tones from the Avia test DVD or your receiver.
You can use band limited test tones here since the signal the subwoofer is reproducing is
limited in bandwidth anyway.
The Avia home theater DVD test disc is unique in that it has 5.0 full range pink noise test
signals, and this is all you need to do the above calibrations. Note however that the pink noise
on the Avia disc is recroded at -20db below digital full scale verses -30db on other consumer
discs (and from receivers and pre/pros). You have two choices if you are calibrating to a
"reference" level on your receiver or pre/pro. You can either calibrate to 85db SPL verses 75db
with the volume control at "reference" level, or if the pink noise's volume is too bothersome,
reduce the volume control on your receiver or pre/pro to -10db and calibrate your system for
75db SPL.
soundhound
Well-Known Member
One thing I should make clear about that article I wrote: the RS meter is used to check sound pressure levels on dubbing stages, but it is not used to calibrate levels, and is not used to tune dubbing stage EQ for frequency response.
malsackj
Well-Known Member
http://www.prosoundweb.com/article/soun ... _more/live
This is an article that is presently published on Sound pressure meters.
This is an article that is presently published on Sound pressure meters.
