|   Measuring Frequency Response | newbster
Dec 8, 2002 6:52 AM | | I currently own a radio shack meter I use to calibrate all the channels of my HT. What I was wondering is how could I go about measuring the frequency response of my mains using the meter. How do the guys at Stereophile do it? Is the eqipment they use very expensive? Thanks. |
| re: Measuring Frequency Response | Dick Hertz
Dec 8, 2002 8:33 AM | | If you really want an accurate measurement of the frequency response, you'll need to shell out a few thou for a cheap Bruel&Kjaer mic and all the acouterments.
Radio Shack SPL meters aren't all that accurate for frequency response measurements, especially above 10khz. To get a rough idea, set your meter for "A" weighting and measure up to about 500Hz. Switch to "C" weighting and continue to about 10kHz. |
| re: Measuring Frequency Response | newbster
Dec 8, 2002 10:45 AM | | Thanks Mr. Hertz. Now where do I get test tones from? |
| re: Measuring Frequency Response | Dick Hertz
Dec 8, 2002 5:16 PM | | You can either order a CD from Parts Express, or you can download a free generator from NCH. Just do a search for NCH tone generator. |
| Use C- weighting, not A- weighting, for frequency response | Richard Greene
Dec 9, 2002 10:47 AM | | A weighting is used to measure for potential hearing damage.
It is deliberately down 50.5dB at 20Hz. (relative to 1000Hz.)
because deep bass has to be very loud before it will damage hearing
C weighting is deliberately down 6.2 dB at 20Hz. (relative to 1000Hz.) which compensates for the fact that the average person needs a 20Hz. signal to be 6.2dB louder than a 1000Hz. signal to sound equally loud (subjective measurements). C-weighting best correlates with what we hear at an average SPL of about 80dB -- U-weighting correlates better at 100dB. See link to understand how our ear's frequency response ("hearing ability") varies depending on how loud )SPL in dB) the music is:
http://www.webervst.com/fm.htm
U weighting (not available on Rat Shack meter) would be flat at 20Hz. (relative to 1000Hz.). A large portion of the published Rat Shack "corrections" are merely a standard adjustments from
C weighting to U weighting that would apply to any sound meter.
A small portion of the adjustments are actual Rat Shack errors using C weighting versus a "perfect meter" using C weighting.
The Rat Shack meter is reliable from 30 to 2000Hz., probably to within +/- 1dB versus a "perfect meter".
My own Rat Shack meter has a large peak (perhaps reaching +4dB between 3000 and 6000Hz. and then rolls off sharply above 10,000Hz.
No sound meter is very useful for measuring treble above 2000Hz. because treble preferences (partially due to treble hearing ability) varies so much among audiophiles (it would be difficult to correlate the treble measurements with your treble preferences).
Bass below 30Hz. is also difficult to measure with a sound meter as the measurement will include harmonic distortion at 40Hz. and higher along with wall/floors/objects rattling in the listening room ... and the wife hollering "turn it down". |
| Those are merely standard adjustments from C to U weighting | Richard Greene
Dec 9, 2002 11:07 AM | | They would apply to any sound meter.
In addition to those standard adjustments, the Rat Shack
meter will have small errors (using C-weighting the Rat Shack will probably be correct to within +/-1dB from 30 to 2000Hz., compared with a "perfect meter" using C-weighting.)
C-weighting is an adjustment to reflect the difficulty of hearing deep bass and high treble at normal volumes (roughly 75-85dB.)
U-weighting is the actual sound pressure measurement (unweighted) -- but not available with the Rat Shack meter. Your standard adjustments (not "corrections") will convert from C to U weighting but do not compensate for any measurements errors unique to your meter).
I recommend using C-weighting without adjustments to best correlate with what you hear, assuming you listen at an average SPL of 75-85dB, which most people do.
If you want to get more sophisticated, you can develop your own weighting curve as a substitute for C-weighting (which supposedly represents hearing of the "average person" ... and, of course, we audiophiles are all above average.)
In general, bass needs to measure (U weighting) louder than mid-range frequencies to sound equally loud to your ears .. unless the volume is really loud (100dB). C-weighting merely adjusts for that fact ... so that a flat frequency response using C-weighting will sound subjectively flat to the average person listening at an average SPL of roughly 75-85dB. The C-weighted measurement at 20Hz., for one example, plus 6.2dB will equal the U-weighted measurement at 20Hz.) |
| Hmm... | snickelfritz
Dec 10, 2002 12:44 AM | | Ideally then(according the C-wieghting theory), the low frequency response of a loudspeaker should rise smoothly to +6.2db at 20hz in order to sound subjectively accurate to most listeners.
What about live music and other natural sources of low frequency energy?
Wouldn't these sources constitute our perception of what is or is not accurate?
ie: a person that has a normal hearing deficiency below 50hz will perceive and accept as natural, live bass in a certain way(-6.2db at 20hz for example); compensating for it in reproduced music would always result in subjectively stronger bass than with live music, since live music is not(and cannot be with regard to acoustical instruments) filtered or equalized to compensate for hearing deficiencies.
I'm not sure under what circumstances an elevated lowbass sound would not be perceived as being louder than a live(real) bass sound.
The only way this will work is if the compensation is integrated directly with the hearing mechanism(as in a hearing aid), which would correct the perception of ALL low frequency sounds; not just those coming from the sound system. |
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