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DIY Tube Bass Trap and frequency tuning

abdo123

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@sarumbear @voodooless

Thank you both for the informative exchange, it seems logical that @sarumbear went for a tube / cylinder shape since -if my memory serves me well- such a shape with one dimension being more than double the other (two) dimensions will provide the highest Q resonance.

but if we consider the material to be infinitely rigid (doesn't flex or let sound energy out), wouldn't there be two strong resonances in this case? one for the (largest) dimension of the shape and one that corresponds to the volume of air in the object?
 

voodooless

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but if we consider the material to be infinitely rigid (doesn't flex or let sound energy out), wouldn't there be two strong resonances in this case? one for the (largest) dimension of the shape and one that corresponds to the volume of air in the object?
My guess is: infinitely rigid would mean that sound would bounce eternally in the box. No energy would be lost, and eventually, it would come out where it came in. Different from a speaker, you don't want very stiff walls. You want to convert sound energy to the movement of materials other than air and eventually heat. In fact for a speaker, that would be the ideal inside as well, just the outside should be rigid so that only the drivers radiate sound.
 

sarumbear

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@sarumbear @voodooless

Thank you both for the informative exchange, it seems logical that @sarumbear went for a tube / cylinder shape since -if my memory serves me well- such a shape with one dimension being more than double the other (two) dimensions will provide the highest Q resonance.

but if we consider the material to be infinitely rigid (doesn't flex or let sound energy out), wouldn't there be two strong resonances in this case? one for the (largest) dimension of the shape and one that corresponds to the volume of air in the object?
I don't think in a Helmholtz resonator the chamber shape makes much difference. However, I doubt the Q of the resonance as at such low frequencies matters much. After all even an octave is just 20-30 Hz wide.

I have though made a test unit where the chamber was a cylindrical oil tank. At the time I had not observed much difference in the effective Q compared to final units.
 

sarumbear

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My guess is: infinitely rigid would mean that sound would bounce eternally in the box. No energy would be lost, and eventually, it would come out where it came in. Different from a speaker, you don't want very stiff walls. You want to convert sound energy to the movement of materials other than air and eventually heat. In fact for a speaker, that would be the ideal inside as well, just the outside should be rigid so that only the drivers radiate sound.
The workings of a sound absorbing resonator is in the name, sound should be absorbed. In acoustics, absorbing often means converting to heath.
 

Jack B

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Hi. Hoping someone will enlighten me. I have a nasty room resonance at 75.5 Hz. Using the "Helmholtz Resonator Calculator" referenced by SarumBear, I built a cylindrical resonator calculated to resonate a bit below my target frequency, with the idea that I would later fine-tune the resonance to 75.5 Hz by slightly enlarging the diameter of the neck/hole in the round 3/4" thick top. But, how to accurately measure the resonance?

I grabbed a woofer that was not in service, connected it to my receiver, dialed up REW to do a sweep from 60 to 85 Hz, and aimed it at my crude resonator (all outdoors).

I am clearly in the ballpark, but how to accurately determine the center frequency? Do I measure in the middle of the "neck", even with the top, or toward one side, or a few inches above the "neck", or what? I get slightly different frequencies depending on microphone location. Unfortunately I had put all the apparatus away before it occurred to me that I might stuff the microphone down inside the resonator to measure. So before dragging all that stuff outdoors again, I am asking for sage (or any) advice. Thanks in advance. Jack
 

sarumbear

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Hi. Hoping someone will enlighten me. I have a nasty room resonance at 75.5 Hz. Using the "Helmholtz Resonator Calculator" referenced by SarumBear, I built a cylindrical resonator calculated to resonate a bit below my target frequency, with the idea that I would later fine-tune the resonance to 75.5 Hz by slightly enlarging the diameter of the neck/hole in the round 3/4" thick top. But, how to accurately measure the resonance?

I grabbed a woofer that was not in service, connected it to my receiver, dialed up REW to do a sweep from 60 to 85 Hz, and aimed it at my crude resonator (all outdoors).

I am clearly in the ballpark, but how to accurately determine the center frequency? Do I measure in the middle of the "neck", even with the top, or toward one side, or a few inches above the "neck", or what? I get slightly different frequencies depending on microphone location. Unfortunately I had put all the apparatus away before it occurred to me that I might stuff the microphone down inside the resonator to measure. So before dragging all that stuff outdoors again, I am asking for sage (or any) advice. Thanks in advance. Jack
Place the microphone inside the unit and measure indoors, in situ. However, if the resonator is resonating near the frequency of the room mode than measure the room and adjust it according to the final room response.
 

voodooless

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Place the microphone inside the unit and measure indoors, in situ.
Am I right in assuming that this way you can also figure out the ideal place for the thing? Move it around while measuring and place it where it's resonating the most?
 

sarumbear

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The location of the resonator is as near as possible to where your microphone was when you measured the room mode.
 

sarumbear

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Am I right in assuming that this way you can also figure out the ideal place for the thing? Move it around while measuring and place it where it's resonating the most?
Yes but a large enough resonator will suck the room resonance wherever it is located. The room resonance is fixed hence a resonator tuned to that resonance will work anywhere in the room. We are talking about the efficiency.
 

ryanosaur

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I suggest reading general physics tutorials where sound waves are explained. Here is a good resource. It has a section on resonance.

Good physical knowledge is fundamental in understanding how speakers work and how sound behaves within spaces (rooms). I urge any H-Fi enthusiast to spend time to learn or refresh their knowledge on vibrations & waves.
This!
I don't know how many times I have suggested similar advice.
*thumbs up
 

Jack B

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Place the microphone inside the unit and measure indoors, in situ. However, if the resonator is resonating near the frequency of the room mode than measure the room and adjust it according to the final room response.
Thank you very much for your advice. Just so I can better understand (I am NOT trying to argue); if I already know that the center of the room resonance is exactly 75.5 Hz, what would be wrong with independently tuning the resonator to that exact frequency, in free air, before bringing it back into the room? Will the presence of the resonator in the room, alter the room resonance frequency that it is trying to suppress?

I must be very careful as it is easy to raise the resonator frequency by enlarging the hole diameter in the top/neck, but very difficult to add material to reduce the diameter of the opening. Thank you for your help. Jack
 

sarumbear

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Thank you very much for your advice. Just so I can better understand (I am NOT trying to argue); if I already know that the center of the room resonance is exactly 75.5 Hz, what would be wrong with independently tuning the resonator to that exact frequency, in free air, before bringing it back into the room? Will the presence of the resonator in the room, alter the room resonance frequency that it is trying to suppress?

I must be very careful as it is easy to raise the resonator frequency by enlarging the hole diameter in the top/neck, but very difficult to add material to reduce the diameter of the opening. Thank you for your help. Jack
Nothing wrong with that. It’s more complicated (in my eyes) to have non-adjustable resonator, that’s all.

I suggest using a single hole with a a simple slider to cover as it’s needed to tune the frequency. The shape of the hole is not important. The mechanism for tuning a resonator should be made easy and firmly fixable. That way you should be able to tune the resonator by ear using a sine wave signal of the resonant frequency while in the room.
 

Jack B

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Nothing wrong with that. It’s more complicated (in my eyes) to have non-adjustable resonator, that’s all.

I suggest using a single hole with a a simple slider to cover as it’s needed to tune the frequency. The shape of the hole is not important. The mechanism for tuning a resonator should be made easy and firmly fixable. That way you should be able to tune the resonator by ear using a sine wave signal of the resonant frequency while in the room.
Thank you very much, sir. You are a gentleman and a scholar!
 

sarumbear

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Thank you very much, sir. You are a gentleman and a scholar!
You are very welcome. We all aim to increase our collective knowledge. Good luck with your project.
 
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