as promised, here is the corrospondance via email:
> 1. Since my PC running TrueRTA is in another room, I think I will use
> the pink noise tone from my Pio 1014. I dont imagine this will be a
My computer is also in another room. You just need long rca cables. Using pink
noise tone from your receiver is a problem. Usually (almost always) those tones
are band limited around 400Hz-2000Hz. They are used to calibrate channel
levels, not measure FR. AVIA has same tones. You need atleast 20Hz-20kHz pink
noise if you want to measure your speakers FR. You can get it from test disc
(AVIA etc.) or from TrueRTA's generator.
> 2. Lets say I did want to use TrueRTA as the generator as well...since
> the output is stereo, and the receiver is 5.1, how do I send that signal
> only to the LFE channel? I assume I will have to turn off all the other
> speakers. But since the truerta signal is not in the LFE channel, will
> it actuall play on the sub...this probably shows my lack of
> understanding how the LFE actually works
Yes, the receiver is 5.1 but you are sending stereo signal in, same as you were
listening normal cd's. You can't send signal only into LFE, because then the
signal would had to be digitally encoded and it's not, because it's just normal
analog stereo signal. So you have to set your receiver to normal stereo mode,
not PL2 or DD etc.
If your speakers are set to small (as they should), I assume that x-over is set
around 80Hz. The only speakers playing the noise are your mains and subwoofer.
If you want to measure for example your right surround, you have to hard wire
it so that it is the only speaker connected to your receiver at the time. If
you want to measure only your mains then just turn your sub off. If you want to
measure only your sub, then disconnect the minus wire from your speakers. And
LFE-track is an additional bass track. So it has to be in the disc before you
can even play it. You can't create it by yourself. Most DD and DTS encoded
movies have it. Often receivers let you choose where to direct it. Mains, sub,
maybe both. Often it's better to direct it only to sub. Then your mains don't
distress and the sub can those loud low frequencies better. Analog signal can't
> 3. I've gotten mixed reviews in regards to averaging the readings. Some
> say that it hides nulls and peaks that should be there. They are big
> proponents of using sine wave tones to test the response. Others have
> recommended averaging. So its a toss up. Let's assume I wanted to use
> discrete sine waves tones to compare agains the pink noise or quick
> sweep, how would i go about setting that up?
> Now as far as the RS meter is concerend...I found the following at the
> ETF site, which says the exact opposite of everyone else...ie they say
> the meter is very flat below 500hz and requires no corrections there,
> here is the quote:
If only low frequencies are to be measured, we recommend the Radio Shack
SPL meter. This unit is very flat - much more so than you will be able
to fix your room. It is therefore quite accurate for its intended
purpose. We supply a low frequency calibration file for this unit on our
support page on the web site under "Support". In this case, get a camera
stand from a photography store that will allow the mic to be mounted.
This stand may be expensive and the option below may be more desireable
in terms of total cost and performance. This microphone should be used
at levels below 95 dB for distortion measurements if the distortion
performance upgrade is used. Distortion measurements should be made with
a good high SPL musical instrument microphone when higher SPL's are to
Radio Shack SPL meter
ETF supplies a high performance calibrated electret condenser
microphonefor use in sensitive applications. The RS analogue or digital
SPL meter may be used for ETF as well. This will not supply the same
accuracy as the unit sold here but is recommended for applications where
critical adjustment of frequency response is already done or not necessary.
This unit has weighting curve adjustments that confuse a lot of users.
This adjustment does not control the output, it only damps the meter
Download the calibration file below and open it as a microphone
calibration file when using ETF with the RS unit. Please note that this
is not a true "calibration file" because it is not calibrated for your
unit. It will remove the gross response errors associated with this
device. Typical differences between this unit and any random unit are
within Â± 2 dB below 8 KHz, typically. This unit is virtually useless
for accurate high frequency measurements.
Other calibration files exist on the internet for this unit, these
calibration files apply when reading the meter movement, not when taking
data from its RCA output jack. These units are very constant gain below
500 Hz (flat frequency response) and do not require further low
I have to say that they are SO WRONG! Weighting does apply also to output jack.
I can easily prove that.
Look at that picture. It shows same system measured with weighting knob set to
c and a. If weighting would NOT affect output jack, those lines would be
identical. Are they? NO.
Also check this page:
This guy knows everything about this meter and he agrees with me. Scroll down
and there is a question about this problem.
Another proof is that I have calibrated my TrueRTA's SPL reading with 500Hz
tone, where meter does not have any error (or very small). As you know TrueRTA
can show SPL reading (total sound power). If weighting would NOT effect to
output jack, that reading and meter needle would have to show different values
below 100Hz (where meter has errors). I played 20Hz and 30Hz tones at 60dB and
70dB SPL's and checked the reading in TrueRTA and then in meter itself. They
were almost IDENTICAL (maximum Â±1dB). Those general correction factors show
that at 20Hz meter has 7.5dB error. So if weighting would not effect to output
jack, there would be that error or even close to that between those readings.
I don't believe that those general correction factors are accurate. I have made
my own correction file using that ETF file above 500Hz and general c-weighting
corrections below it. Those general CF's show that meter is almost flat to
20kHz. Everyone who knows something about microphones knows that it's not true.
ETF people know it too. Thats why there is almost 13dB correction at 20kHz in
their file. If the person who measured those general CF's did that big error at
higher end, how can we be sure that lower end is correct? I don't believe it
is. I believe that general c-weighting corrections are much more accurate.
Actually the difference is not so big.
At 20Hz only about 1.2dB. Below it a
little bit more. But above 1kHz that ETF file is much more accurate.
So use that correction file that I linked in that thread.
If you have any further question about TrueRTA or measurements don't hesitate