pavelcherepan

Reflection of sound waves by different materials

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Hi everyone! Today I was watching a performance by Tokyo Philharmonic Orchestra at NHK Symphony Concert Hall building and got this question. As you can see in the image below the concert hall looks pretty fantastic and what you can notice straight away almost everything is either wooden or covered with wood.

 

I sort of understand the idea behind this: since for many if not most of orchestral instruments sound is created amplified within a wooden chamber, if afterwards it is reflected by a non-wooden surface if may change its characteristics somewhat compared to the original. Hence in order to preserve the proper sound its wood all around.

 

But anyway, does this have any physics basis? Would sound characteristics change if reflected by different surfaces?

 

post-105906-0-31517400-1432732773_thumb.jpg

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I sort of understand the idea behind this: since for many if not most of orchestral instruments sound is created amplified within a wooden chamber, if afterwards it is reflected by a non-wooden surface if may change its characteristics somewhat compared to the original. Hence in order to preserve the proper sound its wood all around.

That is an interesting point. I have never thought about that before.

 

However, I suspect a more significant point is that wood is not as reflective as, say, ceramic tiles so you do not get as many distracting echoes. Also notice that there are very few flat, parallel surfaces. This breaks up standing waves and minimizes any unpleasant resonances. (And looks great, too. I am disappointed I have never been there.)

 

The Royal Albert Hall in London, had famously bad acoustics because of its large glass dome. This was fixed by diffusers hanging from the ceiling.

190px-AcousticDiscsRoyalAlbertHall.JPG

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I think the wood panelling is purely for architectural purposes.

 

The triangular objects attached to the ceiling are probably an attempt to condition the auditorium characteristics by controlling sound reflections.

 

 

The issue is that reflected sound can lead to strong undesirable standing waves between opposite faces/walls of a rectangular enclosure.

 

You cannot eliminate these completely, but the triagular pitch of the roof helps.

 

High quality loudspeakers often contain internal panels, deliberately set at odd angles to obviate this problem with rectangular boxes.

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Thanks for your responses guys!

 

I think the wood panelling is purely for architectural purposes.

 

Probably you're right. I was just making some wild guesses :)

 

 

 

Also notice that there are very few flat, parallel surfaces. This breaks up standing waves and minimizes any unpleasant resonances.

 

 

The issue is that reflected sound can lead to strong undesirable standing waves between opposite faces/walls of a rectangular enclosure. You cannot eliminate these completely, but the triagular pitch of the roof helps.

 

This makes a lot of sense. Thanks!

Edited by pavelcherepan

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But anyway, does this have any physics basis?

Check speed of sound in different materials:

http://www.engineeringtoolbox.com/sound-speed-solids-d_713.html

 

When one object hits other, they're starting vibrating, and moving air around them.

If we will have same length sticks, made of f.e. Copper and Steel, and hitting one end of them, their other end will "learn" about hitting, with different delay.

Other end is starting vibrating and moving air and becoming source of sound in air (and whole their body).

Edited by Sensei

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If you compute one reflexion from the acoustic impedance (density*velocity) or air compared with a solid, you find that sound should be completely reflected except from the lightest foams. The impedance mismatch is like 104.

 

Though, nothing is simple in acoustics. If the wave arrives at a flat angle, it can pass more easily to the other material - both if the transition is long or short as compared with the quarter wavelength. This is the idea behind pyramids in anechoic chambers.

 

Then, you have multiple reflections, which cumulate losses. As this hall looks like, it has parallel panels at the top, which favour multiple reflections locally and reduce the reflected wave.

 

Finally, our ears and brains are accustomed to echo and reverberation, and we do make a difference between 10 reflections and 100 - corresponding to low losses in both cases.

 

So I wouldn't be surprised if wood - with that particular shape - lets the hall sound differently from ceramic. Just think of an empty room: it does sound very hard with naked plaster or stone walls but far smoother with wood walls.

 

Does it relate with the instruments' material? I suppose not. We can tell where an instrument was played (empty room, room with carpets, room filled with people), be it made of metal or wood.

Edited by Enthalpy

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...

High quality loudspeakers often contain internal panels, deliberately set at odd angles to obviate this problem with rectangular boxes.

They use different thicknessess for the panels as well.

Edited by StringJunky

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Many effects combine in loudspeakers...

The designer wants to avoid mechanical resonances by the walls. Odd angles can improve that.

Fibrous material is put to attenuate the resonances and does its job. It works through a thermal process rather than on the flow.

Not least: acoustics isn't so well established nor understood, the designer does what he believes to be useful, the customer buys what he believes to be best, the sales dept recommends what the buyers are ready to believe - so not every design detail is sensible.

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