Physics of Frequencies

[CharlieH]

Hello again BR,

I think this may be more of a physics question than a sound question, but I'll ask it anyway.

From my understanding of waves, the higher the frequency of a note, the more energy it has. Why then is it bass you hear outside a club or venue instead of treble?

 

I have thought about this for a while, and the only thing I can think of is that venues just turn up the bass really loud. Another theory is than I know subs are omni-directional, so I can assume that they will bleed away from the intended direction more.

 

First off, are the facts I have said correct?

And why is it bass not treble you hear outside a venue?

 

Thanks a lot everyone,

Charlie

[J Pearce]

You hear bass outside a venue because bass is less easily blocked/absorbed than treble.

[Chris L]

Apologies if this is not correct, but I think:

 

Bass propagates more freely in air - the attenuation per unit distance is lower. Also, bass will penetrate surfaces more easily.

According to this article - http://srikant.org/core/node6.html - energy expended in wave propagation (per unit time (as speed is constant, so also per unit distance)) is proportional to the square of the frequency ie higher frequencies have higher energy...

Interestingly in speakers, it tends to be the case that higher frequency drivers have a higher efficiency and so require less amplifier power than LF drivers. I don't know the theory behind this as (mathematically) I'm an EM person, rather than an acoustic person.

This is the same as with EM waves.

 

Let me know if there's an error above...

 

HTH

C

[Simon Lewis]

Charlie,

 

To answer the first part, the higher the velocity that an air particle vibrates at, the greater the variation in pressure which is perceived as being loouder.

However, there are many other factors at work....

 

In terms of why do you hear bass outside a venue, it could be because of air absorption, (if sound has travelled long distances), it could be because sound has been absorbed by porous absorbers (which work better on higher frequencies) or it could be due to the fact that the Mass Law (which indicates an increase in sound reduction index with increasing frequency and that that every doubling of the mass of a partition will result in a 6 dB reduction in the level of sound transmitted through it) does not hold for low frequencies where resonance and co-incidence effects occur.

 

The upshot is, mid to high frequencies get attenuated, bass frequencies don't, and that's what you hear outside the venue.

 

Simon

[JimWebber]

And just to add to that, I believe that Higher Frequencies are much more directional....

 

In other words, if you come away from line of sight from the treble source, the levels drop significantly.

 

Jim

[CharlieH]

Brilliant, thanks for all the really detailed replies. I don't quite get some of the more scientific ones, and if anyone could dumb them down then that would be great, but if you don't have the time then I will work it out, or ask my teacher to explain.

 

Many thanks for your time,

Charlie

[revbobuk]

And your first statement 'the higher the frequency of a note, the more energy it has' isn't really accurate. There is a link between frequency and energy density with a pressure wave, but it isn't quite that simple. Lower frequencies move the particles of the air more slowly, but they move them further!

 

The wikipedia page on sound energy density is helpful. Bottom line? If there is 1kW going in to the bass, and 1kW going in to the high/mid, then, if both systems have the same efficiency in coupling electrical to acoustical power, then the energy density will be the same for each - and then all the factors other people have mentioned above come into play.

 

Mind you, my physics degree is a bit old now, and I guess I would have to add IANAP! (work it out!)

[CharlieH]

And your first statement 'the higher the frequency of a note, the more energy it has' isn't really accurate. There is a link between frequency and energy density with a pressure wave, but it isn't quite that simple. Lower frequencies move the particles of the air more slowly, but they move them further!

 

The wikipedia page on sound energy density is helpful. Bottom line? If there is 1kW going in to the bass, and 1kW going in to the high/mid, then, if both systems have the same efficiency in coupling electrical to acoustical power, then the energy density will be the same for each - and then all the factors other people have mentioned above come into play.

 

Mind you, my physics degree is a bit old now, and I guess I would have to add IANAP! (work it out!)

 

OK, so would I be wrong in saying that the mid has the lowest energy then? Bass & Treble can deliver higher energy waves (with the same input) than mid? If I'm wrong then please correct me! This has been bugging me for a while now!

[Jivemaster]

As regards the "sound outside" it's called noise nuisance by the neighbours and most venues try to keep the noise outside right down to pacify the neighbours and satisfy the licensing authority! It is relatively easy to attenuate mid and top frequency acoustic energy BUT LF ie BASS is not easy to attenuate and it is transmitted easily through walls and other structural parts of buildings.

[Chris L]

Just a quick summary:

 

The answer to the question you asked "Why then is it bass you hear outside a club or venue instead of treble?" is due to a number of other factors.

At lower frequencies (bass), walls and objects are less of an obstacle than at high frequencies for a couple of reasons:

1: The lower a frequency, the longer the wavelength. When the wavelength of a wave is large compared to an obstacle, it is much easier for it to bend (diffract) around it. So bass frequencies can spread out through doorways and windows more easily.

2: It is easier for a wall or other surface to absorb high frequencies as it is harder for them to vibrate quickly than slowly. Ie if you have a high frequency, to reflect, the wall would need to vibrate more quickly than to reflect at a low frequency. (that's a bit of a crude explaination, but it should let you visualise what is going on)

 

Point 2 also applies to air. In air, bass frequencies are absorbed much less than high/treble frequencies. For example, a 30Hz very low bass note in air (20 deg C, 10% relative humidity) will become 0.02dB quieter every 100m due to absorbtion. Whereas a very high treble note at 15000Hz will attenuate (become quieter) 20dB every 100m due to air absorbtion.

 

HTH

C

[paulears]

OK, so would I be wrong in saying that the mid has the lowest energy then? Bass & Treble can deliver higher energy waves (with the same input) than mid? If I'm wrong then please correct me! This has been bugging me for a while now!

No - that's not what they're saying at all. You're just trying to simplify something that is quite a complex subject.

 

What they're actually saying is that there are many different things at work here.

 

HF drivers in a PA system need less real energy to produce a loud SPL - lots of reasons, but mainly that LF speaker drivers are pretty inefficient. All that cone movement in and out, trying to shift lots of air - vs a tiny, lightweight driver that has a higher conversion rate in terms of volume out for power in. Higher frequencies get absorbed more easily than lower frequencies, and as a consequence, by the time they have travelled through air, people, brickwork etc they are much weaker. Mid frequencies in the audio spectrum are a bit of both - more resilient than the HF, but less so than LF.

 

It doesn't need to bug you at all. The other thing that can happen with LF is that it gets artificially 'boosted' by room resonances - many parts of a structure could have a natural resonance at a particular frequency - if this happens to be in the bass area, then this can ring in sympathy - and if the thing ringing happens to be a ceiling panel, or some of the structural steelwork, then this can travel, via the fabric of the building into other areas that are out of the direct sound path. It's even possible for the concrete floor slab to pass on this LF energy - and some acoustic sound sound designers/architects float these large slabs on springs or rubber to isolate them. Think about this one - the bass actually moves the solid floor. Putting a hand in front of an HF driver really close to might just tickle? You can blow out a candle with a sub connected to a kick drum source - that's serious energy we're talking about. Put a naked flame in front of an HF source and you can just see a vibration - a twinkle around the edges. Try this and see.

 

If you want to know where the 'energy' is - it's down at the bottom - but in a very limited range. How do you equate lots of energy spread over maybe just 8 white notes on a piano keyboard, vs a smaller amount spread over the rest?

 

Going back to your first post

the higher the frequency of a note, the more energy it has

This is the snag, for all practical purposes, that's not a correct statement without additional clarification. Consider 1000W of electrical or acoustic energy dispersed over 360 degrees in the horizontal and vertical planes against 40W dispersed in a narrow cone. Try out some maths and see what results you get. See how complicated such a simple question really can be?

[CharlieH]

Ok, first off thanks to everyone who have posted, especially Chris & Paul. Chris, thanks for your time, I appreciate the effort you have put in, and I'm starting to understand it now, especially the bit about attenuation.

 

Paul, thankyou (as always) for a great explanation. I apologise for trying to over-simplify this, I was ignorant at how complex it actually was! I get the buildig resonance bit, that makes sense. I also get te attenuation bit. The point about subs needing more power to move the amount of air I understand, and the directional bit I also get.

 

So thanks to everyone for your time, it Is starting to make sense now, and will hopefully stop playing on my mind.

 

Charlie