Questions about sound

[Jens]

I have a couple of questions about sound, not really triggered by the thread on wind blowing through leaves, but perhaps it inspired me. The first thing I was wondering is, a few years back we were making a video, and we were scouting for locations, and when we went into a certain location, a foyer, the director said it seemed great, but said, "we have to stop that noise." And I didn't really know what he meant, but when I listened carefully there was a fan sound. He said it would come out in the recording. Now in the same way, I remember once I was recording samples (of footsteps) on my iPad, and when I walked through the automatic door the microphone clearly picked up the whir of the motor opening the door, but I couldn't hear it at all.

What I was wondering is, does that happen because the microphone is more sensitive to those sound frequencies, or it is because of a filtering effect, that we are actually hearing those sounds but somehow filter them out?

[Jens]

I hope that at least Grant Hutchison will know the answer, because anyone who knows the word "psithurism" must know that.

[Delvo]

Sounds (or effects on sound such as reverberation) that seem to belong in the environment we're in at the moment don't usually get our attention because we focus. Playback of a recording occurs in some other environment where the same background noise or effects would not be expected and don't seem to fit in, so then they do get our attention. Experience at recording in various locations and then listening to those recordings can make one more aware of subtle environmental things that one wouldn't have been aware of otherwise.

Also, microphones are not typically much more sensitive to some frequencies in our hearing range than to others, but they are often very subject to directionality, which is something that our hearing systems are set up to minimize. So a microphone could pick up a sound coming from the particular direction it's pointed at even if that sound doesn't really contribute much to the general omni-directional ambience that you perceive when you're there.

[Jens]

Thanks a lot for the information. I'm thinking of giving a short presentation about sound synthesis (since I play synthesizers), and one of the things I wanted to talk about was how incredible it is that our brain can make out all the sounds in the world around us even though we are really only hearing one complex waveform (different sounds with overtones of their own), (well actually two, I guess, since we have two ears), and then are figuring out what the sounds are and where they are coming from. It seems pretty incredible to me that we can do that.

[profloater]

There is no doubt we filter out continuous sounds but hear them when played back in a different sound environment. A sound engineer friend invited me to a session laying down background sounds for a film, including ticking clocks and footsteps outside with other ambient sounds. It took all day to agree on five minutes of a scene. The result was amazing as an almost unconscious audio setting on top of which would be a separate speech track from actors. It seems to be like our sense of smell. Professionally I seek silence and the feeling of silence from just a quiet room to a well insulated space including ground vibrations is more of a feeling than an audible consciousness. These tiny sounds we ignore but we notice the ambience of a still forest or indeed a cave.

[trinitree88]

Humans can hear from about 20 Hz up to about 20,000 Hz. Elephants communicate in infrarsound, below that, and cat, bats and mice communicate and navigate at ultrasound frquencies.... ( the cat can hear the mice talking).
As I don't know your age, in your ( my) late forties, you begin to lose the high end of the auditory spectrum....Doctors call it "high frequency hearing loss" and when they test kids in school for hearing disorders...sometimes staff discover they are losing the upper end, around 19,500, then 19,000, then 18,5000 hz. Normal age - related attenuation. It is pretty independent of your musical inclinations. I had a kid's cellphone go off in class, at 60 years old and everybody heard it but me.....much to their amusement. You can find a website that generates sounds at incremental frequencies, easily achieved with your computer, or cellphone and check yourself out. A bigger loss is vision.
pete

https://www.googleadservices.com/pag...SwQ0Qx6BAgQEAE

[Jens]

Now I have another question, that to me is more difficult to ponder. So when we listen to an orchestra, we are listening to a large number of instruments, all with multiple overtones. And in some cases, like percussion and chiff sounds, they are not even harmonic. And then we hear other frequencies from reverb. So how do we tease all those into a sound that we can make sense of? I guess it’s like a Fourier transform.


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[Jens]

I also wanted to ask how we tell where a sound is coming from, particularly when it’s either directly in front or in back of us. But I figured that one out. Apparently we instinctively turn our head slightly.


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[Jens]

As I don't know your age, in your ( my) late forties, you begin to lose the high end of the auditory spectrum....Doctors call it "high frequency hearing loss" and when they test kids in school for hearing disorders...sometimes staff discover they are losing the upper end, around 19,500, then 19,000, then 18,5000 hz. Normal age - related attenuation. It is pretty independent of your musical inclinations. I had a kid's cellphone go off in class, at 60 years old and everybody heard it but me.....much to their amusement. You can find a website that generates sounds at incremental frequencies, easily achieved with your computer, or cellphone and check yourself out. A bigger loss is vision.
pete

I do know about that hearing loss--I've also tested my hearing with my kids, and there were high sounds they could hear that I couldn't. But in this case that's not the issue. In the case of the fan, I wasn't that clear perhaps, but I could hear it. Once the director pointed out that there was a hum, and I listened, I could definitely hear it. It's just that I hadn't noticed it before being told that it was there. So I guess it was the "environmental noise" thing, where my brain was filtering it out as unnecessary information.

[grant hutchison]

Yes, it's got nothing to do with real hearing loss. It's "attention deafness"--in technical language, selective auditory attention. Part of that is tuning out continuous sounds, which is mediated through novelty detector neurones in the midbrain, and part of it is because a lot of our sensory experience is actually modelled by our brains, and we are only alerted if the sensory input doesn't match the model. So we tune out fan noises (but suddenly become aware of them if they stop); and we are simply unaware of all the little routine noises our environment makes (but will be alerted if these noises are unusual or absent).
So producing an appropriate soundscape in a video or sound recording involves getting rid of background hums and drones, but putting in the little sounds our brain expects to accompany the events portrayed, which may not have been picked up by the recording equipment.

With regard to the Fourier transform, this is a feature of how the sounds are delivered to our brains. The cochlea contains a range of hair cells with different resonant frequencies, so a complex sound entering the ear is converted to a set of nerve impulses corresponding to a range of frequency bands. What is remarkable is how we can analyze the sound of an orchestra into the contributions of different musical instruments. I think that requires initial experience of hearing each instrument played individually, so that we can pick out these learned sound spectra from the wealth of frequencies arriving at our ears.

On sound localization, that's certainly helped by tilting the head, but there's also a degree of filtering imposed by the shape of the external ears, which imposes different "spectral notches" on sounds arriving from different directions. In her television series Sound Waves: The Symphony of Physics, Helen Czerski visited a sound laboratory and listened to sounds delivered through headphones which were artifically and progressively "notched" so as to make the apparent sound move around her head. It's coarse and error-prone, but it's probably why our external ears are the shape they are.

ETA: If memory serves, Czerski needed to have the shape of her external ears modelled so that the sound spectrum could be notched in the correct way to exploit her brain's learned directional cues, which of course are specific to the shape of her ears.

Grant Hutchison

[Jens]

Grant, that's all cool to hear. I knew I could get some interesting insights. I think you're probably right about the orchestra. It may be that in the case of an orchestra, we don't really hear the individual instrument sounds, except when they are standing out, like when the woodwinds are playing a different line than the strings or something like that. Well, the percussion definitely stands out, a single timpani is easily resolvable. But on synthesizers I have sampled sounds from orchestra hits, for example, and you really can't make out the individual sounds.

[Trebuchet]

Yes, it's got nothing to do with real hearing loss. It's "attention deafness"--in technical language, selective auditory attention.

As I sometimes have to acknowledge to my wife, my hearing is fine, it's my listening that's the problem.

[profloater]

It is interesting how our brain processing of sound differs from vision. In vision we build a model of edges and colours with no separation of frequencies, while hearing performs that Fourier style analysis with some culturally acquired judgements. Hearing is also not shut off in sleep while both can be radically adjusted by hypnosis. It may not seem obvious but our brain ability to time things is central to frequency identification, ability to speak and other short term behaviours. It is an ultra short term memory function which I believe, from conversations with a brain researcher, are still poorly understood. There must be some physical tuning by the hair cells in the ears which fire at a frequency rather than sending up down data signals, but that still leaves a lot of processing to be aware of an orchestra or interpret speech, calling on longer term memory to identify what is going on.