lets talk about sound and its directionality

[elbandito]

I find myself fascinated by the concepts of sound directionality and its relation to the inverse square rule. Now, I'm fairly described as a beginner in the field of audio, so please correct me where I'm wrong and let's have a geeky conversation!

As i understand, sound is at its most directional in the high frequencies. To me, this means that when placing mics for recording, we must pay closest attention to things like cymbals when trying to mic a room - especially when using mics with severe off-axis coloration.

Further, the inverse square rule says that in ideal conditions, sound intensity drops 6 dB (half the volume) every time the distance is doubled, and conversely, the volume doubles as the distance is halved. This implies to me that, as relates to directionality, frequencies will become more or less directional based on their distance from the mic. This would be why close micing techniques become so important sometimes - especially in bad sounding rooms, where reflections can bounce back into a microphone.

I don't really know much more than this, unfortunately and i don't have any leading questions ATM to further this discussion but I'm hoping that somebody will pick this topic up and share some info while I'm off reading about this topic some more. There's got to be some closet geeks lurking about this place!

 

[mixsit]

I find myself fascinated by the concepts of sound directionality and its relation to the inverse square rule. Now, I'm fairly described as a beginner in the field of audio, so please correct me where I'm wrong and let's have a geeky conversation!

Oh tay!

As i understand, sound is at its most directional in the high frequencies. To me, this means that when placing mics for recording, we must pay closest attention to things like cymbals when trying to mic a room - especially when using mics with severe off-axis coloration.

I think, actually not more directional. Higher frequencies are more easily blocked (shaded) and in lots of cases only get generated in narrower or more specific directions. That's not to say they aren't or can't be radiated in wide patterns. Gun shot? Ok, snare's better.

Further, the inverse square rule says that in ideal conditions, sound intensity drops 6 dB (half the volume) every time the distance is doubled, and conversely, the volume doubles as the distance is halved.

Up to the point where you've backed out into the free field (is that what its called? ) and the reflection level about matches the direct sound. Then it stays about the same anywhere in the room. (We in a room?

This implies to me that, as relates to directionality, frequencies will become more or less directional based on their distance from the mic. This would be why close micing techniques become so important sometimes - especially in bad sounding rooms, where reflections can bounce back into a microphone.

I don't know, (or I doubt) 'more or less directional by distance'. I can think of getting closer for two reasons; One is the tone'/style, maybe size' there, the other is better s/n' where you want to keep a room tone down that isn't helping the cause, or just as likely simply a wet/dry mix choice.

I don't really know much more than this, unfortunately and i don't have any leading questions ATM to further this discussion but I'm hoping that somebody will pick this topic up and share some info while I'm off reading about this topic some more. There's got to be some closet geeks lurking about this place!

Yeah.
Well I've been called worse

 

[John Willett]

A good book to read is "Acoustics and the Performance of Music" by Jürgen Meyer.

It includes diagrams of musical instruments and how they radiate sound at different frequencies.

A superb book and well worth having - I have both the first edition and also the latest (5th) edition.

Here is a diagram of a cello from the 1st edition to give you an idea:-

 

[moresound]

Sound waves are ever spreading out from it's source until it has diminished, unlike light waves that duplicate themselves and travel for ever.

Be kinda cool if sound waves acted like that, but we would all be deaf within a few minutes of birth.

If sound could travel within the vacuum of space our own star *Sol* would be so loud to us that again we would have no ears!

 

[mixsit]

A good book to read is "Acoustics and the Performance of Music" by Jürgen Meyer.

It includes diagrams of musical instruments and how they radiate sound at different frequencies.

A superb book and well worth having - I have both the first edition and also the latest (5th) edition.

Here is a diagram of a cello from the 1st edition to give you an idea:-

View attachment 68912

The shaded areas are what, most prominant, by some amount or..? If so then Wow.

 

[c7sus]

I like Leslies.

They send sound in every direction.

 

[Bobbsy]

Good topic.

A small but significant distinction is that it's not so much that sound is less directional at higher frequencies; rather it's that the human hearing system can't as easily work out the direction at lower frequencies.

It's down to the relationship between the wavelengths of various frequencies compared to size of the human head. The brain calculates direction from the slight differences in the sound reaching the two ears. It's the slight phase differences that the brain can process to work out directional information.

At frequencies above about 1600 Hz, the distance between the ears is greater than the wavelength of the sound making it easy to detect the slight timing/phase differences. As frequencies go below 1600 Hz, the wavelength becomes less than the average head size, with around 800 Hz the distance being around half the wavelength. As frequencies fall below 800 Hz, it becomes increasingly difficult to detect any directional information--and by the time you get down to frequencies normally used by subs, there's not really any directional info at all--hence the way subs are usually run in mono.

....and that's just the basics. After that you can get into the difference in directionality between point sources and arrays when designing speaker systems, the effects of reflections in closed rooms (one frequency from my TV always seems to come from the other side of the room) etc. etc. and you can study for years and still only scratch the surface.

 

[Hakea]

Fascinating post Bobbsy. Very interesting info.

You can easily tell that I'm a rank newbie to the wonderful world of audio theory. When I read this paragraph...

As frequencies fall below 800 Hz, it becomes increasingly difficult to detect any directional information--and by the time you get down to frequencies normally used by subs, there's not really any directional info at all--hence the way subs are usually run in mono.

....my imagination immediately went underwater and started going "ping, ping, ping...." like in the old movies. For just a split second I did wonder why submarines would be in mono? Wrong sort of sub I guess...

Chris

 

[brother rat]

Thanks Bobbsy.

One of the biggest oversights in discussions like these is that the important thing here is HUMAN PERCEPTION of sound. Our ears are highly colored, strangely directional omni mics with built-in compression and Eq that feed into a brain packed with various other processng - and we can't turn any of that off. I read a boring/fascinating article that covered some of this (I'm almost positive it was on Sound on Sound, but I can't find it unfortunately).

But the directionality and scattering of various frequencies is a very interesting topic, even in the electromagnetic spectrum (like blue dot tailights on hot rods, and the blue appearance of the sky).

 

[Steve Henningsgard]

I can't be certain that something like this doesn't exist, but could you imagine an infrared-type camera that displays sound frequencies?! I'd about shit myself.

 

[mixsit]

A good book to read is "Acoustics and the Performance of Music" by Jürgen Meyer.

It includes diagrams of musical instruments and how they radiate sound at different frequencies.

A superb book and well worth having - I have both the first edition and also the latest (5th) edition.

Here is a diagram of a cello from the 1st edition to give you an idea:-

View attachment 68912

Does anyone know if that book has plots like that for other instruments? (I'd love to see one for ac guitar.
Did some google and about all I came up with was that same diagram.

 

[brother rat]

I can't be certain that something like this doesn't exist, but could you imagine an infrared-type camera that displays sound frequencies?! I'd about shit myself.

I saw a band once that had stacks of televisions on stage with them. They had the outputs of their amplifiers going to some frequency converter and then into the televisions. (At least, this is what I believe the setup was.) It made some strange displays and was a pretty cool gimmick.

 

[Bobbsy]

The trouble with frequency analysis is that, unless your music is composed of a pure sine wave, the display looks like a mish mash most of the time.

Here's a picture of a fraction of a second of some actual music:

(It was just to illustrate how MP3 lops off higher frequencies btw.)

 

[John Willett]

Does anyone know if that book has plots like that for other instruments? (I'd love to see one for ac guitar.
Did some google and about all I came up with was that same diagram.

I can't lay my hands on my copy of the latest edition at the moment.

I *think* the latest edition includes guitar, it is expanded over the 1st edition.

But the 1st edition includes diagrams for:-
Violin
Cello
Double-bass
Flute
Oboe
Clarinet
Bassoon
French Horn
Trumpet
Trombone
Tuba

 

[mixsit]

I can't lay my hands on my copy of the latest edition at the moment.

I *think* the latest edition includes guitar, it is expanded over the 1st edition.

But the 1st edition includes diagrams for:-
Violin
Cello
Double-bass
Flute
Oboe
Clarinet
Bassoon
French Horn
Trumpet
Trombone
Tuba

Thank you.