Home General Asylum

General audio topics that don't fit into specific categories.

RE: What are "pseudo-anechoic measurements"?

Pseudo-anechoic measurements are time gated. The impulse response is windowed to exclude reflections. If measuring indoors, you place the speaker in the middle of the room, elevating it off the floor if possible, and set the time gate just prior to the arrival of the first reflection. That allows you to measure the frequency response of the loudspeaker only, free of room effects, but the data points will be about 200 Hz apart (typically), so it has poor resolution in the lower midrange.

To do better, you go outside and elevate the speaker as much as possible. I've measured small stand-mounted speakers on a makeshift wooden stand fixed to the top of an 8' stepladder. My goal was to get the microphone and tweeter about 12' up so I could get a clear 20ms time window. That gave me 50 Hz resolution in the frequency domain.

Amir uses a Klippel near-field holography measurement system. It moves the microphone around the loudspeaker and samples at points on a sphere. Then it uses an algorithm to separate the outgoing radiation from the incoming radiation. The math behind the field separation algorithm makes assumptions that are only satisfied in an approximate sense (to my understanding of it anyway). Sometimes the results obtained with this system match up well with anechoic measurements, and sometimes not as well.

For example, with the little KEF LS50 (which is nearly a point-source), his measurements are in good agreement with Soundstage's NRC chamber measurements.

On the other hand, his measurements of the larger Revel F208 seem to indicate a slightly elevated midrange. The other available measurements of the same speaker suggest a slightly dished upper midrange and lower treble instead. The latter agrees better with the way I hear them.



This post is made possible by the generous support of people like you and our sponsors:
  Schiit Audio  


Follow Ups Full Thread
Follow Ups

FAQ

Post a Message!

Forgot Password?
Moniker (Username):
Password (Optional):
  Remember my Moniker & Password  (What's this?)    Eat Me
E-Mail (Optional):
Subject:
Message:   (Posts are subject to Content Rules)
Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:  Automagically notify you when someone responds.