An experiment anyone can do to perceive room modes

[normgr00]

How to experience room modes, and why your subjective audio experience as judgment of subtle audio differences in hardware may be irrelevant. Try my experiment below, see what you think and comment on your experience. The problem with using your ears as judgment is not that your ears are not quality instruments. They are excellent. It’s that room modes are such an extreme distortion that you can’t really pick out subtle differences in the presence of gross mode distortion. This effect is discounted if not ignored when evaluating sound. Anyone can use this sound experiment. The difference your ears hear is not just easy to sense, it is striking.

I have posted three sound files to play with. Try the 2.3K sound first. To set up the experiment mute the volume on all your speakers except one. This will eliminate phase distortions your ears may experience. We want to hear the sound from one point source. Then play the tone and sit somewhere and listen. As you listen turn your head and hear the differences in volume. Move your head to right and left and back and fourth. You will easily experience dramatic differences in sound volumes as you move your head. Plug one ear and sense that. I am curious how room treatment effects this, Please comment.

How I designed the experiment: The average distance between human ears is 14.9 cm. The wavelength for a sound tone of 14.9 cm is 2.3K Hz. At 4.6K Hz you will have two wave lengths between each ear. At 1.15K you get a half wave between your ears. I chose these frequencies because this would allow someone to experience the effect of room modes by moving their head a small amount. Modes can also make the sound loud in one ear and quiet in the other. With lower frequencies you would need to walk around the room and that would require memory to reference to the original sound. I wanted to eliminate memory effecting the experience. Your mind and your memory, even memory seconds old has a powerful effect on coloring your experience and so I want to eliminate that as well. At these frequencies you should be able to move your ear in and out of a sound mode by moving your head only an inch or two.

To visualize sound modes in a room think of this. Have you ever seen a flat surface with sand on it and the flat surface is vibrating at a specific frequency. The sand creates a symmetrical pattern. Where the sand is resting the board is not vibrating, and where there is no sand the board is vibrating. The board patterns represent a two dimensional example of room nodes. In a room the same thing happens only in three dimensions. Here is a video of this happening.

Keep in mind, as frequencies change, the three dimensional pattern of modes in the room changes. Each frequency produces a different pattern. Complex combinations of many frequencies like in music produce different mode patterns simultaneously and they are overlaid on top of each other, so different frequencies drop in and out out at different points in the room making it difficult for the mind to realize what’s going on. I can only imagine how difficult it would be to evaluate something like a cable or any other audio component vs another with so much variation in frequency volumes. Your ears position in the room has an influence that is orders of magnitude greater than any variation in audio quality you are attempting to evaluate. You may think one component sounds better than the other then move your head and the judgment is now totally different. Now your mind takes over and you are subjected to the powers of the many subconscious biases, https://ventureharbour.com/cognitive-biases-influence-buyer-decisions/.

 

[dasdoing]

well. You can identify the modes in REW and then use the generator to produce a sine-wave at the frequencies. You then walk through the room, and you will be stunned by how the tone changes, not only in loudness, but also in perceived direction.

 

[kemmler3D]

Agree, I think everyone should try this at some point. You can also use a tone generator, if you sweep slowly eventually you will probably find a frequency where it appears to disappear if you move your head a tiny bit... And suddenly acoustic treatment in your room will seem much more urgent...

 

[DVDdoug]

Usually when we say "room modes" we are talking about lower frequencies below the Schroeder frequency.

At higher frequencies things "get crazy" and more random so we usually try to get a flattish overall response rather than correcting every "wiggle".

It's kind-of amazing to me that with regular music we don't notice the drastic changes when we move our head or walk around the room. But it does give us clues to the size of the room or if we are outdoors (if we were blindfolded. ) The low frequency effects ARE usually noticeable but they don't happen with slight changes in position.

I am curious how room treatment effects this, Please comment.

With a single speaker in a dead room (or anechoic chamber), you wouldn't get any reflected waves so you'd only hear changes related to changing the angle or distance of your ear. With stereo you'd still get interaction & interference from the two different sound sources.

Most people don't like a very-dead room, but some do... A dead room could be ideal with a surround mix containing the large-room reflections/reverb from a concert hall. (I have a "typical" untreated room and with stereo I like to use a "hall" or theater" effect on my AVR.)

...With headphones we also get weirdness (lots of wiggles) in the higher frequencies from the short reflected wavelengths. All of Amir's headphone measurements show it and he talks about it in his Headphone Measurements video. But again, we don't really notice it.

Somewhere I read (either Floyd Tool's book or Ethan Winer's book) that the distance between the ears creates a dip at about 2kHz with regular stereo that affects intelligibility. Of course the mixing engineer is hearing the same thing and he (unconsciously) compensates for it. BUT when you play surround sound in stereo, of course the mixing engineer had a center speaker so when the center is downmixed to left & right you get a loss of intelligibility.

Also, early reflections improve intelligibility so you usually don't want to eliminate them completely with room treatment.

I can only imagine how difficult it would be to evaluate something like a cable or any other audio component vs another

Properly working cables (and lots of other stuff) don't make any difference but that's one reason people think they hear a difference. The solution is repeated, statistically valid Controlled Blind Listening Tests. You'll get the same random variations with "A" and "B" so if there is no real difference, you won't be able to reliably hear a difference or identify "X" in an ABX Test.


...Somewhat related - I was once doing some experiments with high frequencies and an SPL meter in a microphone stand. I wasn't surprised by the variations I was hearing or that the measurements changed when I moved the mic stand. But it DID surprise me that moving around behind the SPL meter changed the measurements by several dB. We don't normally hear differences when someone else is walking around in the room (not blocking the speakers) but it's probably happening!