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In Reply to: Measuring Speaker Output posted by Lucas Membrane on April 9, 2007 at 19:39:51:
I use a Behringer RTA and Behringer 1/2" condenser microphone. The RTA generates pink noise so it's a complete stand alone system for $350. You also need to get some books on how to make acoustical measurements. There are lots of pitfalls. The one by D'Appolito is very good.I do not recommend the Radio Shack meter. Two of them I checked against my RTA showed a frequency response peak at about 5k Hz making it useless for use with CD's that have 1/3 octave bands of noise.
Speaking of which, I also checked some CD's with pre-recorded 1/3 octave bands of pink noise. The band levels were quite accurate, but the bands were way too wide. The band centers were at the right frequencies, but the bandwidth was more like 2/3 to 1 octave wide, so you get misleading results. The Behringer RTA is 1/6 octave synthesized filters and they appear to be quite sharply cut off.
The PC based systems are good and the Behringer mike will work with them, but you need a mike preamp with phantom power. I was going to go that way, but found a Rolls microphone preamp was too noisy, so I returned it and got the RTA. You can do more with the PC based systems, but you also need to understand more to use them well and avoid artifacts.
I do several useful measurements with my rig. First, is in-room at my listening position. Takes into account the speaker, the speaker placement, mutual coupling between the two speakers, and room effects. Pretty sensitive to mike placement.
Second, I do the assembled speaker from a closer location. A lot less room effect and you do this with only one speaker. If you take it outside and lay the speaker on it's back you can get the true anechoic half space response in the bass. Indoors, you can use the Small in-box technique where the microphone is placed really close to the speaker. This works well enough to see the bass Q and roll off frequency independent of room effects.
Third, I do close miked individual drivers. You do need to mount the driver in something, particularly if it's open backed. Data taken this way agrees very well with published on-axis curves. Mids and tweeters you can do indoors.I found it very useful to see where I am. Listening is necessary, but your ears are not a very good spectrum analyzer. A couple of hours playing around with a good EQ and the RTA showed me a lot about what we perceive, Vs what we think is happening. Like "fast" bass.
Follow Ups:
vHi Jerry,I have often wondered about using the Behringer as a piece of test gear. You might be surprised to know that a lot of the SET (single ended triode) guys at the high efficency asylum use them for room/system correction and report good results. Do you use it in your system for room correction? For what you get, the price seems very resonable!
Actually, my chosen locations (done by ear and some help from the Infinity RABOS kit below 100 Hz) were pretty good. The Behringer gear just confirmed it for me. I did move my chair 6" to the right.In terms of what it actually did for me in an active way, it helped with port tuning and cabinet fill, selecting stand height, sorting out some mids I had, finding (and subsequently fixing) a misbehaving tweeter, finding a driver phase problem with the JBL's, setting the L-pads on the JBL's, measuring the response of some tweeters I thought I might want to use, and figuring out why amp A sounded better than amp B with the Advents. Since it's portable, I lugged it to a friend's house to help him get the setup and crossover right on his Bozak CG's.
The RTA is part of a much more eleaborate system and the box contains dual 1/3 octave EQ's. So I have used it to EQ my system to see what effect various regions have on the sound. That's kind of neat, you can set the EQ so the system measures flat, play music through the EQ to see what it sounds like, and if you like it, use the RTA to tweak the crossover and L-pads to get that net response.
It sound like it is quite useful. It is on my wish list now!
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