Interpreting listening position measurements

Since people seem to be getting more interested in room treatments and measurements (they go hand in hand) I thought it would be great to have a thread that helps people interpret their listening position measurements.

It is pretty easy to get a measurement rig up and running (~$150-200) but it is much harder to interpret what the measurements are telling you. I want to be upfront and state that I am still very much a beginner when it comes to interpretting listening position measurements so this thread will be as much help to me as it will to others. I guess you could say I have an alterior motive in starting this thread as I actually want to learn more.

I posted up some measurements from the listening position after I had put up all of my room treatments and I recieved some PM's questioning me on a couple topics:

1). The wide dip from approximately 200-500Hz?

2). Why is there such a big difference between my left and right speakers?

3). Am I sure that my measurement rig is set up correctly (sound card calibration, mic calibration, no faulty cables, connections etc) and that the measurements are not a mistake?

Answer to question 1). I need to investigate further but my gut feeling is that it is room related.

Answer to question 2). was very easy to answer, my room is not a regular rectangle. It is an open plan living area with stairs to the back, dinning and kitchen to the right.

Answer to question 3). Again, I would need to investigate.

Well I did quite a bit of measureming this morning, moving the speakers, moving the listening position, near field, far field etc. etc. and I am certain that the measurement rig is set up correctly and that there are no faulty cables or connections. So that leaves me with the question of what is causing the wide dip ????

I systematically took measurements at the LP, 30cm above, below, in front, behind and combinations of... and this is what I got.

These are 1/3 octave smoothed.

Left speaker running full range

[ATTACH=CONFIG]39179[/ATTACH]

To demonstrate that the dip is caused by a room interaction I compared the response at the listening position vs 30cm in front and above the listening position

[ATTACH=CONFIG]39180[/ATTACH]

Interestingly you can see that most of the variations occur between 100Hz and 1000Hz.

My question to the more experienced measurers out there is.... what do you take from these measurements and what would you do about them (if anything)? I'm assuming you will want more measurements or different perspectives / types of measurements so feel free to ask. I have saved my suite of measurements from this morning.

Thanks in advance for any help.

Thanks A , this will come in handy when I finally get of my **** and run some tests.

Thanks for the info, I'm just starting to learn more about interpretation and measurements. Spent the morning with a fellow SNA-er, and was amazed to see that a change of just 10cm to the seating position made significant differences to the result.

I also found this site a great resource for how to get set up; what equipment you need and how to measure and interpret.

Thanks for the interest guys. Hopefully we get a few more to lend their experience / advice.

I'm another one really still needing to learn more about this, pretty much lost when it comes to those waterfall charts.

For measuring RT60 which is the better (stepped) signal, 1/3 Octave Sine or 1/3 Octave Noise? My measuring equipment used to automatically select the tracks from a test CD but I've not been able to program it with the Playback IR codes. Did some recent measurements using Noise but have just read that Sine is more accurate.

With my room-speakers response experience experience, I would like to make few general points (not necessary directly commenting on your graphs):

1. Measure each speaker separately. A graph resulting in measurement for both speaker usually does not reflect real nature of the problem. Why? Because its a SUM of two responses. Lets say one have a irregular dip around 200Hz in sum graph. Should one build a membrane panel or Helmholtz resonator tuned to that frequency? Well, once speakers are measured separately one might find one speaker has deep dip at lets say 220Hz and other has speaker and slight bump at 180Hz. In sum, graph for both would show dip at 200Hz, but using a bass trap tuned exactly to that frequency would not be as affective as a bass trap tuned to exactly 220Hz.

Look at my graph below. Surely there is only one speaker, which has a dib aroun 40Hz, not both. So isn't better to identify which speaker and which corner/wall cause the problem and fix it with one panel/bass trap rather then filling whole space with 40Hz bass traps?

2.Do not smooth your graph. 1/3 smoothing is cheating, hiding real problems. Yes, no smoothed graph looks bad, but thats the whole point of the exercise - to find a problem and fix it. No smoothing shows the whole magnitude of dip and bumps - 1/3 reduce them in amplitude.

3. Remember (a reader) you will never get perfect graph, never, not in your average residential room. But we do not aim at that. We want to have little dibs and bumps, preferably in +/- 5dB range (?). And make sure RT60 looks more or less smooth, without any obvious, more then 10-15% bumps/dibs to its neighborhoods and keep RT60 around 0.2-0.4s band. Unless one likes dead room - I dont really.

Here are my few advices, berun12

[ATTACH=CONFIG]39230[/ATTACH]

Hi Drizt, good to see you're onto this! You'll be overwhelmed once you realise the optimisation potential acoustic measurements offer.

- Is berun12 right in suggesting you measured both speakers at the same time? As he said - never do this. Treat each side fully independently, but work towards achieving identical transfer function at the LP from both sides. This normally means different treatment for both sides.

- Don't use 1/3 oct smoothing. You cover all important details. 1/20 oct is ok to remove noise from the FFT and retains all the detail. I tend to look at a FR at different smoothing widths to better understand details and global trends.

- Interesting is the comparison LP vs 30cm front/top offset. To interpret fully you should look at the impulse responses (especially the first 20ms), the energy-time curves and the waterfall plots.

- You showed the big dip is a room interaction - but it's definitely not good, you will perceive it as a coloration. The causes are manifold, but I'm confident that optimisation of speaker and LP position plus selective treatment of problems arising from reflections will allow improvement.

Although I'm not endorsing dead room designs such as yours, investigations are way easier. The following dot points are about how I would tackle a room such as yours. In a nutshell, I recommend that you start optimisation with the room mode dependencies and work out the placement of your main speakers and the LP:

- Take an inverse measurement approach - place the main speaker at the LP (midspan between tweeter and midrange driver at ear height) and measure the transfer function at the possible speaker positions with microphone at speaker midspan height. Take measurements at a grid of say 30cm over an area as wide as 1m and as deep as 2m for each speaker position (mark mic positions with tape on the floor).

- Then shift the LP forth and back in say 30cm increments and repeat the full set of measurements.

- Import all in Excel, discard all frequency bins above say 400Hz (that's all reflection dependent), calculate the magnitude, and normalise so that the mean is zero dB. Calculate the variance from zero mean for each response, and rank the responses based on the variance. Plot the 10..15 best responses.

You have now identified a range of positions resulting in the flattest frequency response in the modal frequency range. Inspection of the transfer functions and their waterfall plots should will reveal the Schroeder frequency. Optimise the LF response at the LP using narrow parametric EQs. Remember you are in the modal region - assuming that the speakers have constant power output with near monopole source directivity, any dips in the FR at the LP are due to fluctuations in the radiation impedance of the driver. The room-speaker system is thus minimum phase and EQ can be applied to correct!

Integrate your subs. I suggest inverse measurements again to position the subs. Asymmetric placement has higher chances of achieving near constant FR at the LP. Use a procedure such as Welti & Devantier propose to optimise the seat-to-seat variances.

Undertake meaningful nearfield measurements to achieve matching acoustic transfer functions for the subs and the main speakers. Otherwise you'll be struggling to achieve coherent summation. Pay attention to phase alignment - you'll need to delay the main speakers to achieve coherence with the subs. A word of warning - 80Hz crossover frequency means that the bandwidth of the subs is very small, i.e. the group delay and risk of poor transient response becomes very high (especially with sub designs of order higher than 2) . Acoustic filter design for the subs is something that makes all the difference in terms of LF perception. I suggest leaving that topic for a later discussion.

Your LF response should now be sorted. Shift focus to high frequencies:

Proceed investigating the effects of reflections on the transfer function at higher frequencies. Using ETC and trigonometry you can identify where strong reflections come from (simple in your case - dominant ones come from the ceiling with minor ones from the floor and rear). All important is the level difference between the first and subsequent peaks. Your ETC shouldn't have any dominant early reflections higher than -15dB compared to the initial peak. The ceiling one may require diffusion to reduce its level - it doesn't strongly contribute to spaciousness but may mess with the coloration of the reverberant signal (I assume you got a suspended plasterboard ceiling which absorbs LF). Floor reflections should be mitigated using heavy felt underlay and carpet.

I would then proceed to investigate the FR differences between the two speakers at the LR and use targeted EQ above say 300Hz to minimise these. You therewith increase the coherence between the two channels - you will find that phasing due to head movements at the LP is significantly reduced. Take care when interpreting your FRs - change smoothing from 1/20 to 1/6 to 1 oct to tackle large trends first and narrow deviations second. I know many people object EQs in the signal chain for audiophile reasons, but the gains outweigh potential signal degradation.

Your low reverberation room design should now show an ETC with no dominant reflections, waterfalls without dominant ringing, and FR that is essentially flat up to 14kHz. Bear in mind that 3dB sound pressure level difference corresponds to halving the acoustic power.

Given your low reverberation time and consequent absorption of all ER, I would investigate the IACC / ASW / LER to check whether you have enough reverberant energy left in the sound field to achieve the all important perception: immersion and source widening! Experiment with 1m2 reflector panels at the lateral reflection points and see how this affects your perception. And sit down in the room with the missus and a bottle of shiraz and talk with no background music. When you both are comfortable with the acoustic quality of the conversation (intelligibility, ease of communication, apparent liveliness, lack of background noise level) than your room design is good. If you feel slightly uncomfortable, you need to re-work.

Thanks for the thorough reply, ill read over it a few times.

To clarify what has been done to date and what can and cant be done.

Extensive measurements and subjective listening analysis has been done. The speakers, subs and listening position are set. All possible combinations have been tried and evaluated (even combinations that were not acceptable functionally or looks wise were evaluated). It is a difficult room but i have made the most of it. Only very small movements of the speakers and/or listening position are possible. Trust me, there are not many workable combinations in the room, I'm not just using that as an excuse.

The broadband absorbtion has worked a treat. The room feels completely natural, speech intelligibility is excellent (which was my main aim). As the titles in the graphs show only one speaker at a time was measured. The room is in no way dead. In fact I have never been in a room I would consider dead (except for a Qantas engine testing room that was an anoechic room. That was certainly an experience).

I hope that doesn't come across as too negative or dismissive. It is just the facts. I'm sure this advice will be very useful to others though. Thanks again for taking the time to write all of that up.

Edit: My sub integration discussion can be found here -> http://www.stereo.net.au/forums/showthread.php/42127-My-attempt-at-sub-integration

The speakers / subs can not easily be moved to the listening position and in the end it would not serve much purpose as there are only so many possible locations for the mains and listening position, I have tried them all (much to my wifes disgust, she is my helper).