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In the Forum: Audio Discussions
In the Thread: Learning to interpret RTA

Post Subject: Verification of cheap RTA & thoughts on room sizePosted by jessie.dazzle on: 6/11/2010

Absolutely flat response will usually sound as you've described it; listen to your ears.

If you are hearing the opening bass line of "So What" via the sub, you must be running the sub pretty high in frequency to compensate for lack of mid-bass. From this one might guess that your RTA is telling the truth, and you may indeed be dealing with a mid/upper-bass suck-out.

You can confirm your RTA measurements as follows:

Find a means to play specific frequencies via your audio system; if you are able to play sound from your computer out through your audio system, downlaod a tone generator for the computer (I paid $25 a few years back; you can probably now find one for $0); if not, buy a CD with pre-recorded test tones.

Buy a cheap SPL meter of the sort Radio Shack sells:

http://www.radioshack.com/product/index.jsp?productId=2103667

Play a series of tones from 20-20KHz and compare the results with those of the RTA.

If this test reveals discrepancies, get a calibrated mic for the RTA (can be had for about $80):

http://www.cross-spectrum.com/measurement/calibrated_behringer.html

The mic will likely require a +48V Power supply:

http://www.behringer.de/EN/Products/MIC100.aspx

The above pre-amp/power supply will allow you to adjust gain and can be connected to your computer via its normal mic input (likely an 1/8th" mini-plug, so you might have to make up an adapter, as the pre-amp/power supply looks like it outputs via a 1/4" jack, of the sort you'd find on an electric guitar).

If there are still discrepancies, spend some cash on a more serious RTA.

Large rooms can be very difficult to load; you may want to wall off part of the space. This would almost certainly alter the situation in the event you really do have a 125-500Hz suck-out (though deficiencies and surplusses over limited frequency ranges are not a simple function of cubic volume).

My current "ideal" room would measure 23 x 20 x 8; such a space should in theory not "cramp" frequencies down to about 35Hz. At least that's what I'd like to think. A straight line from the lower front left corner to the upper rear right corner measures just under 32 ft, or about the dimension of a 35Hz wave, but in fact in the midst of writing this I'm thinking its ridiculous to assume the waves are going to orient themselves according to the longest dimension in the room (the diagonal) without some form of perhaps audible protest.

Here's a nice animation showing natural orientation of a wave as it leaves the source:

http://dev.physicslab.org/Document.aspx?doctype=3&filename=WavesSound_IntroSound.xml

Now take into account the following (from physicslab.org):

"...When two or more waves travel through the same medium at the same time, their waves actions will interfere with each other the same as any other types of waves..."

The only thing that can be said with any certainty is that large cubic volumes are more difficult to load. The best room may well be the one having just enough space to allow breathing room on all sides for both the speakers and listener, as well as adequate distance between speakers and listener.

jd*

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