# How low freq can an RS SPL meter go?



## fight4yu (May 31, 2010)

Hi. I am wondering how low frequency I can output from REW and my RS SPL meter is still effective?
I have tried 15Hz, and it seems OK. But when I try 5Hz, it seems to give weird result (like 80-85dB at 5Hz)...
One possible explanation I can think of is because my RS SPL meter calibration file is down almost 30dB at 5Hz, and given my normal room ambient noise is at around 50dB.. even the SPL register nothing, it will still think it have 50dB + 30dB = 80dB at that point ? 
In that case, how can I get correct freq response down to 5 or 10Hz?


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## tonyvdb (Sep 5, 2007)

What SPL meter are you using? 5Hz is very low and in my opinion not a very useful reading. Below 8hz there is not very much if any usable information in movies anyhow. This is why even the best subs usually roll off at 10Hz.


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## fight4yu (May 31, 2010)

Just the regular Radio Shack Digital SPL meter. If not for 5Hz, how about 10Hz?


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## tonyvdb (Sep 5, 2007)

10Hz would still be fine, If your going to use it with REW make sure you load the proper calibration file into REW before using it. Generally SPL meters dont have issues with low frequencies its up high that they are much different.


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## Wayne A. Pflughaupt (Apr 13, 2006)

fight4yu said:


> One possible explanation I can think of is because my RS SPL meter calibration file is down almost 30dB at 5Hz,


Where did you get your calibration file? The one we provide only goes down to 10 Hz, and it’s -22 dB. Given the meter’s non-linear roll out below 40 Hz, I’ll bet 5 Hz something like -50 dB, not -30 dB. Either way, with compensation that severe, the meter can’t give an accurate reading that low.

Regards,
Wayne


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## fight4yu (May 31, 2010)

I got the calibration file from this forum 
THe one I used is RadioShack-33-2055-4050-CS.cal.
Yes, and I see from the text file that it goes down to 10Hz and it is -22dB, and when I look at the graph, it does say -32.7dB at around 5Hz.
My concern is that even at 10Hz, if it is compensated for 22dB. Then my "compensated" SPL with nothing at 10Hz will already be like 50+22=72dB. If I calibrate my REW at 75dB, will that even be meaningful, as the "floor" is only 3dB higher and will likely make the roll-off disappear?
In this case, should I be calibrating my REW at a higher SPL so I can see the rolloff without having the floor cutting it out? (or will the ECM8000 help because it might not have such a bigger compensation in such low frequency range?)

Thanks.


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## tonyvdb (Sep 5, 2007)

fight4yu said:


> My concern is that even at 10Hz, if it is compensated for 22dB. Then my "compensated" SPL with nothing at 10Hz will already be like 50+22=72dB. If I calibrate my REW at 75dB, will that even be meaningful, as the "floor" is only 3dB higher and will likely make the roll-off disappear?
> In this case, should I be calibrating my REW at a higher SPL so I can see the rolloff without having the floor cutting it out? (or will the ECM8000 help because it might not have such a bigger compensation in such low frequency range?)
> 
> Thanks.


My personal opinion is that you should be fine with the RS meter but thats an area that I am not as well versed as some others more knowledgeable. Hopefully they will chime in here to clarify.


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## Wayne A. Pflughaupt (Apr 13, 2006)

> In this case, should I be calibrating my REW at a higher SPL so I can see the rolloff without having the floor cutting it out?


The issue isn’t getting the in-room signal up high enough over your ambient noise floor. The problem is the meter. Its response is so far down at ultra-low frequencies that the signal is lost in the noise – i.e., it’s below the meter’s internal noise floor. IOW, the signal is so low it’s unusable.

Keep in mind that the RS meter is not really designed to be a frequency-response measuring instrument. It's designed to register sound pressure levels.


Regards,
Wayne


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## Wayne A. Pflughaupt (Apr 13, 2006)

To perhaps explain a little better, are you familiar with the popular Behringer 1124P that’s used as a subwoofer equalizer?

If you are, then you’re probably also familiar with the rear-panel switches that accommodate either professional +4 dBu or consumer -10 dBV nominal operating ranges. 

Both ranges can be thought of as an _internal gain structure._ Since there is roughly a 12 to 15 dB difference between the pro and consumer signal levels (depending on the reference source), one gain structure cannot be optimized for both ranges. In other words it’s impossible for the equalizer to deliver, at the same time, the best headroom for “hot” signals and the lowest noise floor from a single internal gain structure. The higher-level (professional) +4 dBu range will have the best headroom, while the lower (consumer) -10 dBV range delivers the quietest noise floor.

The range settings on a SPL meter operate the same way, providing a balance between the mic pre amp’s noise level and the expected volume of the signal being measured. The gain structure of each range selection on the RS meter has only about a 20 dB “distance” between the noise floor and the peak signal it can handle. 

That’s why increasing your in-room signal well above your room’s noise floor won’t make a difference. When you dial up the meter’s operating range to accommodate the high SPL signal, the meter’s noise floor is raised right along with it. Make sense?

Regards,
Wayne


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## fight4yu (May 31, 2010)

Thanks Wayne! Makes perfect sense in that case.
Now, if I really want to measure to around 10Hz, do I get a better Mic/Pre-amp like the ECM8000 (calibrated version)? Will that suffice?


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## Wayne A. Pflughaupt (Apr 13, 2006)

The RS meter is good enough down to 10 Hz, but we only provide a generic calibration file. Naturally you’d get readings that are more accurate with a calibrated mic and pre amp. And if you get the mic from Cross Spectrum, I believe they calibrate down to 5 Hz (but you might double check on that).

Regards,
Wayne


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## Philnick (Jul 9, 2008)

And here I am using the _analog _Radio Shack Sound Pressure Level meter! I'm not aware of any way to even input a calibration file - just don't use the "A" setting, which is designed to sniff out high frequency hiss, and totally ignores low frequencies.

Since I have a pre-HDMI 5.1 receiver I love, I bought the Panasonic BD-50, the first Blu-ray player to internally-decode all three lossless audio formats to a set of 5.1 RCA jacks (it can do 7.1 over HDMI by either bitstreaming the original or transcoding it to LPCM).

Once I realized that I had to redirect the bass from my surrounds (which have woofers but don't go all the way down) to the sub (the front trio goes low enough on their own) and manually calibrated the sub's output by 15db relative to the other channels using my SPL, the sound was astounding. When Neo flexes - and bends the world - near the end of _The Matrix_, the listening room bends as well. (And musical low bass is _felt_ as well as heard.)

I do have a gripe with both the DVE and Avia calibration disks: neither one includes a true .1 signal for calibrating the sub, relying instead on bass management to steer it from another channel. I ended up using the receiver's own test tones - I have one of the rare receivers (an old Yamaha) that actually applies channel trim and distance settings to the multichannel analog input, which makes setup a lot easier.


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