From ???@0x000021D7 Thu Sep 23 12:22:28 1999 Path: pitt.edu!nntp.club.cc.cmu.edu!awabi.library.ucla.edu!128.32.206.55!newsfeed.berkeley.edu!newsfeed.axxsys.net!remarQ-easT!supernews.com!remarQ.com!news.mindspring.net!firehose.mindspring.com!not-for-mail From: Gary Sanford Newsgroups: rec.audio.tech Subject: Re: Please Help: Radioshack SPL meter Date: Wed, 22 Sep 1999 18:01:10 -0400 Organization: MindSpring Enterprises Lines: 227 Message-ID: References: <7rr383$gao$1@nnrp1.deja.com> <7sb081$bj2$1@usenet01.srv.cis.pitt.edu> Reply-To: sanford1@ibm.net NNTP-Posting-Host: d1.8a.31.93 Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Server-Date: 22 Sep 1999 22:01:44 GMT X-Newsreader: Forte Agent 1.6/32.525 Xref: pitt.edu rec.audio.tech:107898 Status: N On Wed, 22 Sep 1999 16:33:09 GMT, szekeres+@pitt.edu (greg szekeres) wrote: >In article , sanford1@ibm.net wrote: >>On Thu, 16 Sep 1999 15:44:07 GMT, gmarutz@my-deja.com wrote: > >Where did this come from? I find the 100 Hz reading suspicious as well as >the response above 10kHz. Was this some kind of sweep test or noise >averaging? >greg > Here's the complete citation: > I was all set to perform the mods myself, but it made more sense to use > it as is and then adjust the results using the numbers Michael Sims (I > think) provided a few months back. Maybe Michael could repost to save > you the work. Here it is! First, I would like to thank Eric Busch (The DUMAX technician) and Dave Clark from DLC Design for the low frequency data, Tom Nousaine for supervising the calibration, Bill Duddleston at Legacy Sound in Springfield, IL for letting PSACS meet at his business, and Paul Barton from PSB for furnishing the loudspeaker calibrated at the NRC anechoic chamber in Canada. The Radio Shack meter is a wonderful unit. About a two and a half years ago, I did a calibration curve for the RS meter using lab equipment, with Eric Busch from DLC Design adding the low bass down to 10 Hz. This was published in PSACS Sound Bytes in two issues. These are the corrections that should be added to the meter readout in order to achieve the correct SPL. These corrections are only valid for the meter set to C weighting, using 1/3 octave pink noise (easily available from various CDs), with the mic pointed at the speaker. Both my analog meters and my digital meter measured the same in October, 1996. These are corrections, they are to be added to the meter readout for the correct response in dB SPL. 10Hz +20.5 12.5Hz +16.5 16Hz +11.5 20Hz +7.5 25Hz +5 31.5Hz +3 40Hz +2.5 50Hz +1.5 63Hz +1.5 80Hz +1.5 100Hz +2 125Hz +0.5 160Hz -0.5 200Hz -0.5 250Hz +0.5 315Hz -0.5 400Hz 0 500Hz -0.5 630Hz 0 800Hz 0 1KHz 0 1.25Khz 0 1.6KHz -0.5 2Khz -1.5 2.5Khz -1.5 3.15Khz -1.5 4KHz -2 5KHz -2 6.3KHz -2 8KHz -2 10Khz -1 12.5KHz +0.5 16KHz 0 20KHz +1 To measure the in-room response of your speakers with the SLM, find a quiet and undistracted time, obtain a 1/3 octave pink noise CD, mic stand, and worksheet. With the speakers in their normal positions and using a mic stand for the SLM (I've had good results just holding the meter too, pink noise is forgiving), place its microphone where your ear would be at your favorite position or "sweet spot". Set the meter to "C" and "slow". Play the 1 Khz, 1/3 octave pink band and set the level on the amp or preamp, and the meter range, so the meter reads 80 dB at 0 dB on the meter. Higher levels might cause driver compression in the frequency extremes, rolling off the response. Now go back to the first ISO center 1/3 octave band on your CD (25 Hz on mine) and record the response: 3.5, or -1.0 , or -5.0, or whatever it is. Now step through the pink noise bands, recording the meter level each time. This is the raw data. Keeping the meter in the same measuring position, using an accurate CD, having a quiet room, repeating the measurements for accuracy, understanding what you are measuring, fresh batteries, not talking while measuring, having your meter calibrated for overall level (relative level comparison is unaffected), and doing only one speaker at a time if possible to avoid comb filtering (variations of plus or minus 2 dB are possible) are all important factors. Sometimes if I have to measure two speakers at a time, I move the meter around in a figure eight pattern, slowly, and try to obtain an average reading, as the microphone moves in and out of the combing peaks and troughs. Best accuracy would be obtained from taking several measurements at different listening positions, if one has the time and patience. Now take the raw data and make the corrections on the work sheet, entering the new values in the appropriate column. Time to either moan or marvel, since this is the actual in-room 1/3 octave pink noise frequency response of your speaker/system at that listening position. Plotting the results on graph paper in different colors for left, right, center, etc. makes it look cool. Yes, this way is tedious, but it is very inexpensive. And very accurate. For the third octave pink noise, I use Carver's Amazing Bytes CD, GRP Z-9907; other CD's with 1/3 octave pink noise ISO centers are: My Disc, Sheffield 10045-2-T Sheffield/Coustic Test and Demonstration Disc, Sheffield 10040-2 Autosound 2000 CD #103, $18, 800-795-1830 IASCA Setup and Test CD HI-FI News and Record Review "CD-II", $30 Japan Audio Society Audio Test CD-1, YDDS-2 --- these last two available from DB Systems, 603-899-6415. For the others: http://www.audioxpress.com/ http://www.mcmelectronics.com/main.html http://www.parts-express.com/ There are other CD's that have warble tones on them, but I am not comfortable using them. I also extensively use track 19, disc 2, from Delos "Surround Spectacular", DE 3179. This has a slow sine sweep from 160Hz to 20Hz with the voice of David Ranada announcing the frequencies as they descend. A fantastic help when used with the Radio Shack SLM, you can easily tell where the room peaks and dips are. http://www.delosmus.com/ You have permission to copy and distribute this information freely, as long as no commercial gain is involved. Radio Shack is your friend. Here is how I did my calibration: The $34.99 Radio Shack 33-2050 analog sound level meter has been around for over 25 years. Its predecessor, the 33-1028, was reviewed favorably in Stereo Review, (Julian Hirsch, "Equipment Test Reports", Stereo Review, August 1972). It has a much different curve than the ones I tested. To verify the accuracy of the newer version, I compared it to an Audio Control 3050 RTA, the same one Tom Nousaine used for years in his test reports for Car Stereo Review until he bought MLSSA. The overall SPL accuracy of all three of my Radio Shack sound level meters -- 2 analog and 1 digital 33- 2055-- were within 1 dB of 75 dBC SPL compared to the Audio Control. I then checked the frequency response, comparing it while set to C weighting and slow, with pink noise, 1/3 octave band by 1/3 octave band, to the Audio Control RTA in the SPL mode. Using the same official PSACS calibrated PSB loudspeaker and a pink noise CD, I made a calibration curve that can be subtracted from the results obtained by the Radio Shack in your living room to obtain accurate, repeatable measurements for about $60, including pink noise CD. Make sure your meter is set to C weighting. The digital meter and my second analog meter (6 years newer than the test unit in 1996) were the same as the test SLM. Response below 25 Hz done by Eric Busch with sine waves and B&K equipment at Dave Clark's DLC Design in Michigan. Michael Sims Prairie State Audio Construction Society ~~~~~~~~~~~~~~~~ Gary Sanford Email here only: sanford1@ibm.net