2011年3月16日星期三

有關用Sound meter 量度重低音時的修正

source from http://www.svsound.com/questions-faqs-rscomp.cfm

other discussion in AVS

Maybe you already know how powerful a tool a simple Radio Shack Sound Pressure Meter is. If you use some of the popular test disks out there to check your room response there are some things you need to know about the meter. Some of these disks, like the popular "Autosound 2000 Bass CD #101", have sweeps or even chapter stops that show how even (or not) the response of your system is down low, below 20Hz even.

The problem? The Radio Shack sound meter is not totally "linear" in its accuracy down there. All such meters are off by the same amount depending on the frequency however, which is good news! So that while its sensitivity below 20Hz is not what it is at 50Hz, there is a simple way to compensate for this deficiency. It's called a compensation chart. Using one is very simple, and a pencil and paper is all you need.

Simply look to the below frequencies and add the required decibels (dBs) at the frequency in question. So if your system seems to be measuring 70dBs with a 20Hz tone, then really it's putting out 77.5dBs. This is a VERY substantial difference, roughly TWICE the acoustical energy, requiring many times the amplifier to achieve. You can see then how critical the below chart is if you are using an equalizer to even out room response of your bass. Maybe you are not bothered by any such variances, but if you are desirous of getting the most from your system it's something well worth investigating. Adding a simple equalizer like the ART 351 or the more complex and powerful Rane PE17 allow you the flexibility to affect your system response in the low bass regions easily.

NOTE: Using sine waves at high SPLs can easily damage any subwoofer given their extreme stress on woofer voice coils.  Use only a few seconds for any given tone during measurements, allow a minute or so in between frequencies, and keep actual SPL's low, in the 75-85dB range.  Contact SVS if you have questions about room response measurements or proper use of sine waves in this process.

So find out what your subwoofer(s) are REALLY doing, and add the following numbers as appropriate to your SPL meter readings:

At 10Hz add 20dBs to the meter's readings and at ...

12Hz add 16.5dB
16Hz add 11.5dB
20Hz add 7.5dB
25Hz add 5dB
31.5Hz add 3dB
40Hz add 2.5dB
50Hz add 1.5dB
63Hz add 1.5dB
80Hz add 1.5dB
100Hz add 2dB
125Hz add .5dB

http://www.audioxpress.com/magsdirx/ax/addenda/media/koya2811.pdf

Subwoofer One" MS Excel worksheet

http://www.audioc.com/library1/testcd/freqnewwith.xls

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