What Are Acoustic Holes In Glass?

acoustic holes building soundproof windows coincidence effect coincidence frequency mass mass air mass resonance room acoustics soundproof soundproof windows soundproofing standing waves Jan 30, 2023

If you have ever wondered why soundproof windows are designed the way they are then this lesson is for you. This is also a great article on what acoustic holes are in windows.

1) Mass-Air-Mass Resonance

Mass-air-mass resonance is when sound hits the outside pane of glass which then translates to the airspace between the two panes of glass. The air space acts like a spring and translates that sound to the second pane of glass. This transmission of sound waves causes a dip in the transmission loss of the window at a certain frequency.

The frequency where that dip occurs depends on the thickness of the glass and the airspace between the two panes of glass.

In the diagram below from The Master Handbook Of Acoustics, you can see how increasing glass thickness and increasing the airspace between the glass panes will decrease the frequency where the mass-air-mass resonance occurs. The goal is to get that frequency as low as possible for your recording studio design.

In the next diagram of my own recording studio windows you can see that I used thick pieces of glass with a large air gap to reduce the frequency at which the Mass-Air-Mass resonance occurs.

2) Coincidence Resonance

The second type of acoustic hole that happens with glass is known as the coincidence effect. This phenomenon occurs when the speed of the sound wave moving through air matches the speed of the sound wave already in the glass. This will cause an acoustic hole at that specific sound wave speed also known as the frequency. The acoustic hole will cause a dip in transmission loss letting that specific frequency through at a greater volume than other frequencies near by.

The figure below from the Master Handbook Of Acoustics shows that with two panes of 1/8" thick glass with a 1/4" air-gap the coincidence frequency happens at 4k. This means sound at 4k will travel more easily through the window than frequencies above and below 4k.

The formula for estimating the coincidence frequency is:

f=500/t

f= the coincidence frequency

t= the thickness of the glass

If we look at my two studio windows the coincidence frequency of the 1/2" pane is 1000Hz and the coincidence frequency of the 3/8" pane is 1333Hz.

The two dissimilar panes of glass make it so the coincidence frequency of one pane of glass cannot make it through the other and vice versa. This is a solution to preventing an acoustic hole in your soundproof window.

3) Standing Waves

Standing waves occur when two wave forms collide with each other head on either increasing the amplitude of the two waves, decreasing the amplitude or completely cancelling the wave out. Standing waves occur in the airspace between to panes of glass and can increase the resonance at certain frequencies thus causing acoustic holes.

To combat standing waves, we put glass fiber on the inside our windows in the air space to absorb the sound waves that bounce around inside the airspace. This will help with some of the standing waves inside your window airspace.

Conclusion

The greater the window mass and airspace between the panes of glass the better your soundproofing.
Two different thickness of glass will reduce the coincidence effect and improve your soundproofing.
Adding glass fiber around the inside of your window in the airspace will reduce standing waves and improve your soundproofing.

I hope this article has helped you understand why we build the soundproof windows the way we do. If you would like to learn how to build a soundproof window like I did in my studio you can check out this video: https://youtu.be/JQiozaQcTkk

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