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The Quarter Wavelength Rule And Why It’s Important To You

Dennis Foley April 16, 2014 2 Comments

There is a rule that acoustical engineers follow in creating sound absorbing technologies. It is called the quarter wavelength rule. Without getting too technical, you can define the rule in its impact upon a lower frequency wavelength. Lets use 30 Hz. as our wavelength in this example. A 30 Hz. wave is approximately 37′ long. In order to absorb this long wavelength, we need to create a sound absorbing device that can absorb completely, at least 25% of this wavelength. That would be a distance of a little over 9′. This is the heart concept of the quarter wavelength rule.

How To Apply The Quarter Wavelength Rule

This methodology pertains to limp mass absorbing materials. A limp mass absorber is a box filled with building insulation or a bundle or bale of building insulation. It could also be a box filled with cotton material or any other sound absorbing material. It is the 9′ length that we must focus upon. It is that quarter wavelength rule of having enough space to absorb at least 25 % of the design frequency we are going after to absorb. If we take a 40 Hz. wavelength at 28′ in total length and divide that number by 4, we get a quarter wavelength of around 7′. This quarter wavelength rule has been around in acoustics for many years but has been forgotten in the sales literature of most companies that market “bass traps”.

Bass Trap

The term bass trap is a misnomer to say the least. Bass or what people should say is low frequency energy or wavelengths can not be trapped by anything. They do not stop at any material selection including concrete and even steel. We have built rooms for clients that have two 8″ thick walls of poured concrete separated by another 6″ of air space and can still measure 30 Hz. wave activity on the outside of the wall. Trapping “bass” does not exist. Reducing the amount of the energy created by low frequency wavelengths, however, can be achieved if you have the technology or lacking that the space to apply the quarter wavelength rule.

I explain this concept a little further in this video:

“Bass Traps”

The quarter wavelength rule shows us that in today’s marketplace that what is called a low frequency absorber or “bass trap” can not be any such thing. First, as we discussed earlier, there really is no such thing as a “bass trap”. Secondly, the amount of space required in order to absorb low frequencies using the quarter wavelength rule goes beyond what any manufacturer of low frequency absorbers uses in any of their product lines. Foam wedges that are 24″ thick can only go down to 90 – 100 cycles using our quarter wavelength rule. How can these be called bass absorbers? They can not but as you know, they are. Special technology is required if you are going to use a small amount of space to absorb energy below 100 Hz.

Helmholtz vs. Diaphragmatic

There are two technologies that you can use to absorb energy below 100 Hz. You can use diaphragmatic absorption or Helmholtz resonators. Helmholtz resonators require a cabinet or tube with a slot that has a calculated width and length. The cabinet itself must have a predetermined and calculated depth and volume. A diaphragmatic absorber is a sealed unit with a cabinet similar to a Helmholtz resonator but the cabinet has an internal fill material that also contributes to the overall unit’s performance. The cabinet’s depth contributes to the level of absorption and the cabinet’s internal fill material affects the rate of absorption from that designed for resonant frequency.

Bending The Laws

A limp mass absorber which is foam or boxes filled with building insulation must correspond to the quarter wavelength rule and provide the necessary space requirements in order to achieve their lower frequency performance parameters. If they do not, and most don’t, then they won’t work. Only specially designed technology can bend the laws of physics and absorb a large amount of energy in a small amount of space.

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In Summary
So I hope that helps you. If you have any questions at any time I am always on hand to help answer them. Leave them in the comments section or email me at info@acousticfields.com. If you would like to learn more about room acoustics please sign up for my free videos and ebook by joining the mailing list here. I send room tuning tips and things for you to test in your room every Wednesday. They are easy to follow and really help you enjoy more of your music.

Thanks and speak soon

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Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others.

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2 thoughts on “The Quarter Wavelength Rule And Why It’s Important To You

  1. Thanks for this article, which I have read with great interest.
    I am planning to build a small studio in my garden this spring and I am wondering if there is any sensible application of the quarter wavelength rule when considering the size of the cavity between the exterior and inner stud walls?

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