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Re: Measuring Speaker Output

Posted by Bold Eagle (A) on April 10, 2007 at 07:45:10

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.

Jerry

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