Martyo,
I have examined the values on the Radio Shack meter at the link below. They are quite acceptable. Even when the meter is 1.5 dB off at some frequencies in the upper end, it will not thwart the intention of this measurement. We are simply trying to find, within a few dB, the listening level for the purpose of selecting the right size amplifier.
http://www.subwoofer-builder.com/SPL-corrections.htm.
Nine times out of ten when a prospective customer calls and asks which amp is big enough for his needs he has no idea at what SPL he listens. "Not too loud" and "I like to turn it up when I get home from work" are not good answers. If you miss your SPL target by 6 db thats 4 times the power in watts. If you miss it by 10 dB thats 10 times and 20dB is 100 times. I am not kidding when I tell a customer who thinks he needs my 100 watt amp that he really only needs 35 watts or even 2.5. I will state without reservation that listener level differences have a wider range (65-105 = 40dB) than most speakers (82 to 102 = 20dB). There will be some exceptions in both those numbers but they are rare.
Below is a link that will show you the A, B and C weightings both by table and graph. C is what we use for this measurement. Don't sweat the roll-off at low frequencies. If the meter went down to 20 Hz flat air motion in the room could easily obscure the correct SPL at low levels. Even though the RS meter misses at the lowest frequencies that is not going to make any material difference is finding the listeners SPL.
http://www.sfu.ca/sonic-studio/handbook/Sound_Level_Meter.htmlWhat I imagine is going on in the reader's mind (given those quotes and questions) is the assumption that one can measure frequency response in their room with this meter. The simple answer is you can't. Standing waves and reflections will cause the meter to swing wildly with small changes in frequency and the numbers will be meaningless. The best way of testing a speaker's frequency response is a tone-burst generator and an oscilloscope. We did this at Beveridge and I do it for all my speaker tests. The burst is all over by the time any reflection comes back to the mic. The burst amplitude is recorded and, even more valuable, I get to see the shape of the burst. MLSSA can not do this and frankly satisfying MLSSA has resulted in some pretty bad sounding speakers. Harold Beveridge (we called him Bev) often said to speaker makers when shown their frequency response curves "Yea, that's great, now what does it look like when you move the microphone". He could move the mic all over the place and the response was virtually unaffected.
The bottom line of this topic is that we simply want to get in the ballpark as to power readings. Anyone who has an oscilloscope can skip the SPL measurement, go right to the speaker terminals and report the peak voltage at his highest listening level. That is the most accurate way to determine amplifier power, but I doubt many people have a scope. This is the sole reason I use have encouraged hundreds of listeners to get an SPL meter. To get your peak power simply take the SPL reading at 1 meter from the speaker while playing it at your listening level. Take that SPL subtract the 1 watt speaker sensitivity given in your speaker specs and you will have the dB above one watt that you need to produce that level.
Since math skills vary from person to person let's do an example. Say you measure 85 dB at one meter on axis and your speaker is 85 dB sensitivity then you need only one watt. Two watts will give you 3 dB of headroom and it will take 10 watts to give you 10 dB headroom. Still a rather small amp and you don't need more than 3 dB headroom for most music.
That one was easy. Say you measure the same 85 dB and your speaker is 95 dB for one watt. These are realistic numbers for the FE-103 I build and for many single cone speakers available today. Again subtract your SPL reading from the speaker sensitivity and you will get -10 dB (1 meter SPL - speaker sensitivity at one watt). Putting in the numbers: 85-95 = -10 dB. Minus 10 dB means you are listening 10 dB below one watt which is 1/10 of a watt. Again, -3 dB is half the power and every 10 dB is 1/10th. That makes 20 dB 1/100. I do plan to write an article on figuring dB's by an easy method I have developed that one can do in one's head.
For those of you who have peak reading meters that will hold a reading, you can use that in lieu of an oscilloscope. We are just looking for a peak voltage value. An amplifier that puts out 100 watts at 8 ohms will have a 40 volt maximum (peak voltage). A 100 watt 4 ohm amplifier will have a peak of 28.5 volts. To go about the numbers from the speaker side; take the peak voltage measured, multiply it by .707, square that number (multiply it by itself) and divide the result by the speaker impedance. That will be the power you are using and should be a few dB below the rated power of your amplifier.
As I write this last part I recall a reviewer who wanted to review my 300 watt monoblocks. I asked what speaker sensitivity at which he listened. He already had determined he used just a watt or two so I suggested the RM-10 (my SE amps came later). He told me he could tell the difference between a 100 watt and 300 watt amp even at those levels. I did not send him a unit for review.
Roger
