Hi all,

I would like to cancel out / attenuate almost completely the following frequencies:

0.5-20 hz

25-90 hz

100-500 Hz

from my understanding this can be done in 2 main ways, using multifrequency Helmholtz resonators similar to this one here: https://www.mdpi.com/1996-1944/15/18/6450.

And through utilizing active noise cancellation.

My questions are; is it possible to fully absorb the frequencies listed above in the first place? and if so could I get more info than what I have listed above on how to achieve this?

I know that as part of the active noise cancellation, that eminent technology have the TRW 17 rotary subwoofer which goes down to 1 Hz. It is quite pricey though, if I have a professional make it for me could the price go down (along with an amp)? Would it be as reliable? Or should use an array of multifrequency Helmholtz resonators to achieve my goal?

 

hi Tim,
As A Student, is this a College or Homework assignment?

Also, can you describe the application for these filters?

Moderation

 

hi Tim,
As A Student, is this a College or Homework assignment?

Also, can you describe the application for these filters?

Moderation

Hi, thanks for the reply.


This isn’t a homework or school project; I’m working on a personal research project involving low-frequency noise control in a small indoor environment.

I’m exploring two parallel approaches; passive (multifrequency Helmholtz resonators) and active (low-frequency transducers); and I’m trying to understand whether complete attenuation in these ranges is realistic, and what the practical design limitations might be.

 

Welcome to AAC.

I have only a general comment, not from directly experience with low frequency acoustic mitigation (my experience in this area is from practical noise and echo reduction in recording studios).

The successful HRs (Helmholtz Resonators) that are cited in the paper as achieving "perfect absorption" do so at a singular frequency. This (lack of) bandwidth (analogous to high Q in electronic filters) makes achieving this much simpler.

The broadband attempts are targeting 85% absorption rather than "perfect". Your question is framed as "fully absorb" and if by that you mean unmeasurable residual then the answer is "no". While it might be possible in principle, it is definitely impossible in practice.

You probably don't mean this, so the question becomes, what exactly do you mean. The best answer to this is an explanation of the application rather than a very limited requirements list and a reference to small spaces.

 

Welcome to AAC.

I have only a general comment, not from directly experience with low frequency acoustic mitigation (my experience in this area is from practical noise and echo reduction in recording studios).

The successful HRs (Helmholtz Resonators) that are cited in the paper as achieving "perfect absorption" do so at a singular frequency. This (lack of) bandwidth (analogous to high Q in electronic filters) makes achieving this much simpler.

The broadband attempts are targeting 85% absorption rather than "perfect". Your question is framed as "fully absorb" and if by that you mean unmeasurable residual then the answer is "no". While it might be possible in principle, it is definitely impossible in practice.

You probably don't mean this, so the question becomes, what exactly do you mean. The best answer to this is an explanation of the application rather than a very limited requirements list and a reference to small spaces.

Thanks for the response, I appreciate the clarification on “perfect absorption.” You’re right, I don’t mean literally zero measurable energy.

My goal is to reduce the targeted frequency bands to a level where they’re no longer perceptible or impactful in a small indoor space (~X m³). In practice, that probably means aiming for the maximum achievable attenuation given realistic construction constraints, with the understanding that some residual will remain.

The application is both minimizing low-frequency disturbance in a quiet workspace and research into infrasound perception thresholds. I’m exploring whether a combination of tuned passive elements (multi-frequency HRs) and active cancellation can together yield a large enough reduction across those bands to be subjectively effective.

Without necessarily fully absorbing all those frequencies mentioned, I’d be very interested in your thoughts on:

• How steeply the absorption curve can realistically drop off outside a narrow HR tuning, and whether staggered tunings can meaningfully cover, say, a 20–30 Hz bandwidth.
• Practical Q values and trade-offs for low-frequency HRs in small volumes.
• Where active elements tend to be more effective than passive ones for the ranges I’ve listed.

 

Noise cancellation and noise attenuation are two different things.

Filters are used to attenuate electrical signal noise. This means that the noise can be reduced somewhat but never at 100%.

You can use a high-pass filter to attenuate noise below 20 Hz. A 1st-order high-pass filter with a cut-off frequency at 20 Hz will attenuate the amplitude to 70% and the power to 50% at 20 Hz. The attenuation gets better the further away you are from the cutoff frequency.

One problem is allowing signals from 20 Hz to 25 Hz. This means that you have to live with some attenuation in the pass band and some signal leakage in the stop band,

Noise cancellation is possible if you have an exact model or replica of the noise. Cancellation has been somewhat successful using both electrical and acoustic cancellation. Achieving 100% cancellation is difficult to achieve in most circumstances.

 

Hi, thanks for the reply.


This isn’t a homework or school project; I’m working on a personal research project involving low-frequency noise control in a small indoor environment.

I’m exploring two parallel approaches; passive (multifrequency
Helmholtz resonators) and active (low-frequency transducers); and I’m trying to understand whether complete attenuation in these ranges is realistic, and what the practical design limitations might be.

Can I assume that the "noise" that you are trying to control is acoustic sound waves and not electronic signals? It makes a big difference to the solution. Electronic signals are relatively easy to control using frequency selective filters to attenuate or cancel parts of the signal's acoustic frequency spectrum. On the other hand, attenuating or cancelling selected frequencies in actual sounds over a specific area is difficult. This is because of the properties of sound waves.
The amplitude and phase of a signal that can cancel sound at a particular frequency would depend on the location of the receptor. If the receptor is a person who could move around in the space, their exact location in that space must be used to modify the amplitude and phase of the cancellation signal. Just rotating the head of the receptor would change the properties of the required cancellation signal.
Helmholtz resonators lined with acoustic absorption material could be used to reduce the perceived amplitude of sound waves over relatively narrow frequency bandwidths but they would by no means eliminate them. Good luck with your project. I will follow this thread with interest.

 

I was assuming it was acoustical sound to be absorbed r cancelled. Cancelling requires a frequency and phase match. Easy to describe, almost impossible to implement in the normal world. Quite possible in a good college acoustic lab. Sometimes. Active adaptive noise cancellation??? Possible in theory, but rather expensive.

Absorbing sound with just plain lossy materials can certainly be done, and it works well, for sections of the spectrum, not for narrow bands.The others are correct about that part.

 

Absorbing sound with just plain lossy materials can certainly be done, and it works well, for sections of the spectrum, not for narrow bands.The others are correct about that part.

Helmholtz resonators only work over very narrow acoustic bandwidths.

 

Can you tell us what is the source of the noise to be reduced? Is it external noise, noise generated from a fixed source in the workspace, or from a device that can move around the workspace? It makes a big difference to to how you could use noise cancellation.