[cporada]

Hi everyone,
I was so excited about my new DIY supertweeters, I decided to go to Radio Shack and pick up an analog SPL meter to make some measurements with the stereophile test CD 2 warble tones and see if I could figure out why things sounded so much better.  Up to about 8kHz, all looks really good; no peaks/dips of more than 3-4db across the whole range of 20Hz to 8kHz, so my T-line subs have been well integrated with my Alpha's.  However, much to my surprise/confusion, when I try to measure the treble response, things are really weird.  Everything appears linear to about 8kHz, but then things appear to drop off sharply at 10kHz and above, with all readings being -8 to -10db (this is with the supertweeters attached crossed at somewhere around 18kHz with a 1uF cap, only a 6db/octave slope, so there should be significant output below that, output I can easily hear).  The strange thing is that I can hear all of the test tones very clearly (even the 20kHz one) at my listening position, which is where I set the meter to make the measurements.  Also, I can hear all of the tones clearly from my main tweeters, but the 20kHz I can only hear from the supertweeters.  Do you know if the radio shack meter is really inaccurate at high frequencies, or do I need to start figuring out what's "wrong" with my system to cause this tremendous roll-off?  The system sounds better than ever, so I'm thinking perhaps the meter is very poor.  I know for low bass it's no good, so I had to use some "fudge factors" I found at numerous sites on the net from people who sat down and figured out with much more expensive meters what the deficiencies of the radio shack one were, but most of these people seemed to think that it worked pretty well for higher frequencies.
Thanks for any input
Chris

[Mike Dzurko]

Chris:

This is an excellent question, and hopefully someone with experience with the RS meter will pipe in. I've never tried using it for more than relative or very quick measurements. I would think directionality would be one issue. Something is of course wrong, you shouldn't see readings -8--10 even without a supertweeter

[SET Man]

Hi,

    The RS SPL meter has been know to be not linear. I have the popular analog version. I remember that there was a post in Audio Asylum about this and there is a guy that have a chart of correction needed for the RS meter.
 
     As a DIYer and since I build my own speaker I got the meter just toconfirm what I hear. There is one easy way.... There is a test CD from Rives Audio, the "Test CD 2" This CD have 2 set of stable test tones, one is for flat and one is compensated for the analog RS meter. Go to http://www.rivesaudio.com and look for the Test CD 2.

     The other thing is that like you even with the Rives' CD I still get a steep roll off above 8K and I do have Fostex super tweeter FT17H that dose go up to 50K. Yes, I do hear the improvment, is just not on paper. This could cause by the way you measure. Are you mearsure your spekaer on axis? Meter on the hight as the tweeter? Even where you stand will effect the mesurement.

    I sloved the problem of where I should stand by using one tripod to mout the meter and another one behind the meter with digital SLR slightly above it pointing down to take the picture of the meter needle with long shutter release cable. This way I could get out of the way and take a picture for every Hz tracks. Than I just read the meter of the photos!

   Anyway, hope this help. And remember don't go crazy over the measurent with the RS meter and CD becaue this will never be as accurate as having more costly analyzer with calibrated mic. But it will give you enough info of what you are hearing.

Take care,
Buddy

[jeffreybehr]

"The RS SPL meter has been know to be not linear."

Yup; it's called 'weighting'.   The best you get with a RadShak meter is 'C' weighting, which deemphasizes the ends of the audible band a fair bit.  If you want to see your room response be REALLY nonflat, measure it with 'A' weighting.

I expect a Google search will quickly turn up corrections.

[cporada]

Hi everyone,
Thanks for the helpful responses.  I found the following website detailing modifications that can be made to the radio shack meter to make it linear:
http://cygnus.ipal.org/mirror/www.people.Virginia.EDU/~edw3g/rsmeter.htm
it turns out that the meter is down 4.4db at 10kHz, 8.5db at 16kHz and down a whopping 11.2db at 20kHz, at least according to the author of this website with the mod's, so perhaps I should learn to trust my ears more than the readings from the meter.  Interestingly, adding these adjustment factors to my readings, my speakers are completely linear in-room out to 20kHz, just like they sounded.  Hmm.  I guess there's a reason why people use more expensive test equipment to make critical measurements.
Chris

[ooheadsoo]

The specs actually say that the meter is only good out to about 10khz.  Same with the Nady meters that came out this year.  I have a Nady analog on the way - hope it will pan out.

[ctviggen]

I highly doubt your speakers are completely linear -- room modes are very hard to get rid of and will create nonlinearities.  If you want a better analysis tool, get ETF and a calibrated microphone.  (And this isn't a slam on this speaker -- any speaker placed in a room is going to "feel" the effects of the room.)

[cporada]

Hi Bob,
Thanks for your comments.  You're right, of course; they are not "completely linear" as I stated.  What I meant was that after correction for non-linearities of the RS meter, the in room response fell within +/- 4db over the range of 20Hz to 20kHz (except for a +5db bump between 80-125Hz, which is present both with and without the subwoofers, so I think this is a room problem that I need to fix), which I figured was probably as close as I would ever get to linear in my family room, especially since I don't have any kind of acoustic treatments up.
Chris

[cporada]

Just to follow up on my last post, does anyone know an effective way of treating a room to get rid of a bump in response in the 80-120Hz region?
Thanks
Chris

[Harry P]

Just to follow up on my last post, does anyone know an effective way of treating a room to get rid of a bump in response in the 80-120Hz region?
Thanks
Chris

Sounds like bass traps to me. Check out the Acoustics Circle here, I bet you can find lots of products or maybe even DIY plans to roll your own

[ScottMayo]

Just to follow up on my last post, does anyone know an effective way of treating a room to get rid of a bump in response in the 80-120Hz region?
Thanks
Chris

Sounds like bass traps to me. Check out the Acoustics Circle here, I bet you can find lots of products or maybe even DIY plans to roll your own

It's hard. Traps that go that low tend to be fairly large - there's no good way to trap bass without making a resonant chamber in the frequency range you need. If you want to make your own, there are things you can do with insulation and 6, 7, and 8' tall sonotubes, 2' diameter, which can help. You need several. If the absolutely only issue you have is this frequency hump - that it, imaging and liveness in the room are as you like them - then you might want to look at something like a TacT, to deal with it electronically. If there are other issues you want to go after too, you want to work up a room treatment plan and tackle them all at once.

But ask yourself if the hump is bothering you. Never mind the meter, what do your ears say?