R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

Pages: [1]   Go Down

Author Topic: Measurement interference?  (Read 1333 times)

Sonovo

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 267
Measurement interference?
« on: December 28, 2006, 05:54:21 am »

This one was specifically for Ethan, but I'm sure there are others who can tell me what's going on.
?
I went through the LF pink noise test as well as the 1Hz stepped sine test found on www.realtraps.com and noted the values. My room seems unnaturally even, without big dips or bumps, which seems to go against what appears on most sites selling products. In general I have 3 or even 6dB of difference, but it's gradual, with no abrupt dropouts. I didn't find any modes or nodes at the listening position (although there were several outside the listening position).

However, while measuring, I noticed that when I got up to the mid 100's and up to mid 200Hz's or so, the measurement (C/slow on a Radio shack analogue meter) changed by up to 5dB as I moved around in the room (even though the meter is stationary...). The meter was placed on a stand to sit where my head usually is when working, I stood behind it and to the side a bit, but had to move in to note each reading. I noticed while working at these frequencies that the needle often moved (usually only about 2 dB, occasionally more) as I moved. By simply moving my head and/or changing my height (stooping over) I could hear nodes and modes, and the dramatic change of loudness, from almost non existent to exceedingly strong. According to the meter however, it was pretty even at the listening position itself.

Comments on what's going on here? Was my body functioning as an absorber and affecting the sound pressure around the meter?

Cheers,
Thor
Logged

Ethan Winer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 571
Re: Measurement interference?
« Reply #1 on: December 28, 2006, 03:35:06 pm »

Hi Thor,

Just to confirm, I did get your email and I passed it on to my associate Scott to answer. He's away on vacation until Saturday, and he'll address all of your other questions as soon as he gets back.

> My room seems unnaturally even, without big dips or bumps, which seems to go against what appears on most sites selling products. <

Yes, but if your room is very large that would account for some lack of peaks and deep nulls. You didn't mention the size of your room in your email to me, or in this post, so I can't comment further without knowing that.

> the measurement (C/slow on a Radio shack analogue meter) changed by up to 5dB as I moved around in the room (even though the meter is stationary...) <

Sure, that makes sense. The response at any given cubic centimeter in a room is the combination of the direct sound from the loudspeaker(s) plus many competing reflections. So when you move around your body is blocking at least one of the reflections compared to where you stood before. It also makes sense that this will be more pronounced at higher frequencies.

> Was my body functioning as an absorber and affecting the sound pressure around the meter? <

Probably not an absorber as much as a simple barricade.

> By simply moving my head and/or changing my height (stooping over) I could hear nodes and modes, and the dramatic change of loudness, from almost non existent to exceedingly strong. <

This too is very common. In fact, this is the main reason I put test tones on the RealTraps site - to hammer home how bad the LF response is in most rooms, and also how severely the response changes over even small distances. Here's another article, on my personal web site, that examines this even further:

http://www.ethanwiner.com/believe.html

The main purpose of this article is far removed from acoustics, but the research and measurements shown are relevant to your questions.

--Ethan

Sonovo

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 267
Re: Measurement interference?
« Reply #2 on: December 28, 2006, 08:27:41 pm »

Hi Ethan,

thanks for the informative reply.

The size of my room was mentioned in my other post, as well as in my email (and noted on the pdf diagram of the room). It's roughly 5.7m wide in the front, widening to approx 6.5m in the far rear. Length is somewhere around 9m all in all (but see further down). Ceiling is 2.5m (unfortunately, I'd love to have a few extra meters there). There is a physical barrier about 3/4 of the way back in the room (at 7.5m from the front wall), a row of cupbords that are 2m high and stretch almost all the way across the width of the room except for an opening the size of a doorway.

My listening position is 1.5m from the wall of cupbords, the speakers are approx 1 to 1.5m from front and side walls (not symmetrical, closer to side than front wall) and toed in somewhat. They are around 4-5m from the listening position and with 2-3m between them.

Do you have any practical suggestions as to how to measure while minimizing interactions? Or is it not that important in the larger picture?

Am I correct in thinking that I should be most concerned about the frequencies that did exhibit larger variation, and absorbing these? I.e. up to around 100-150Hz things looked really good and even, even while I moved around the room and listened. Should I be looking at absorbers in the 100-300Hz range, or is there still an advantage to absorbing down to say 40-50Hz or so?

Cheers,
Thor
Logged

Ethan Winer

  • Hero Member
  • *****
  • Offline Offline
  • Posts: 571
Re: Measurement interference?
« Reply #3 on: December 29, 2006, 01:13:37 pm »

Thor,

> Do you have any practical suggestions as to how to measure while minimizing interactions? Or is it not that important in the larger picture? <

It's probably not that important.

> Am I correct in thinking that I should be most concerned about the frequencies that did exhibit larger variation, and absorbing these? <

Of course! Very Happy

> Should I be looking at absorbers in the 100-300Hz range, or is there still an advantage to absorbing down to say 40-50Hz or so? <

To my way of thinking, what happens in the mid to high bass range is more important than down around 40 Hz. At least for a listening room or home theater. For a recording studio control room or mastering room of course you want the highest accuracy attainable.

--Ethan

Sonovo

  • Sr. Member
  • ****
  • Offline Offline
  • Posts: 267
Re: Measurement interference?
« Reply #4 on: December 29, 2006, 08:31:11 pm »

Well,

I tried plotting everything again to see things visually. There are a few major dips, but not as dramatic as on the website.

I have two deep notches at 60Hz and 168Hz, and clear notches at 120Hz, 153Hz, 263Hz and 280Hz. The one at 60Hz is broad and actually starts around 45Hz and slopes steeply down to 57, then curves up again with steep slope at 62Hz until it gets up to 66Hz. The one at 168Hz is pretty much just that one frequency. The rest seems not soo bad, except for perhaps a small bump/mode at between 220-230Hz. Quite fascinating to look at when you plot it out like that...

So maybe a little corner trapping on the front wall (behind moniotrs) to get the 60Hz point dealt with, and otherwise more extensive absorption higher up, focusing on the 120Hz - 300Hz range.

Thanks again for the tips, I'm looking forward to reading your email reply next year.

Cheers,
Thor


Ethan Winer wrote on Fri, 29 December 2006 15:13



> Should I be looking at absorbers in the 100-300Hz range, or is there still an advantage to absorbing down to say 40-50Hz or so? <

To my way of thinking, what happens in the mid to high bass range is more important than down around 40 Hz. At least for a listening room or home theater. For a recording studio control room or mastering room of course you want the highest accuracy attainable.

--Ethan

Logged
Pages: [1]   Go Up