[SML] 125Hz room resonance

[mouse at fieldmousepro.com]

A couple of thoughts:

- I'm not sure how a resonance in your room (especially at such a low 
frequency) would give you Tinnitus at 8kHz. Are you sure the 8k isn't 
being generated by a CRT TV, electronic device, or switch-mode power 
supply (which are in just about everything these days)? You can use your 
RTA (or even a free RTA app on your phone) to listen close to devices 
and find the source of errant noise.

- There are acoustical room mode calculators online that can give you 
the frequency peaks for a room of any size. Just enter the dimensions 
and it will calculate the resonances. Studio designers use these all the 
time to play with different room sizes before the walls go up.

- I'm not surprised that a blanket had little effect on the 
low-frequency issues in the room. Low frequencies have long wavelengths 
and tend to be unaffected by materials that are thin.

A good rule of thumb to use when designing wall treatment and absorber 
panels is the 1/4 wavelength principle. The material will start to lose 
effectiveness at wavelengths which are longer than the thickness of the 
material x 4. As an example, a 1kHz frequency has a wavelength of around 
a foot. If you cover a wall with fiberglass, you'd want it to be at 
least 3 inches thick to fully absorb frequencies from 1kHz and above.

A 100Hz frequency has a wavelength of about 10 feet, so things start to 
get challenging. There are a couple of things you can do to cheat the 
system, though. One is that the most effective slice of absorber 
material is the section right at the face, so if you need 2.5 feet of 
thickness, you can get most of the same results by getting a panel 
that's just a few inches thick and spacing it away from the wall by 2.5 
feet. Another cheat is that small rooms (relative to the size of the 
wavelength) can often get away with far less material if strategically 
placed. The corners of a room are a good way to get distance away from a 
wall with less sacrifice of the room's useable space. Try hanging the 
same blanket across the corner of your room and see if that makes any 
difference.

Stephen Lee

On 2026-03-25 1:54 pm, Tim Catlett via Stagecraft wrote:

> Hey all,
> 
> I've just encountered a problem that I think this group is well 
> equipped to assist me with. While not directly stagecraft-related, 
> stagecraft (and audio) knowledge is applicable. I deeply appreciate any 
> advice in advance.
> 
> My bedroom seems to have developed a 125Hz resonance (along with one 
> more significant one at about 33Hz), which seems to have prompted my 
> brain to give me "observational(?)" tinnitus, which is consistent tone 
> with a perceived value of about 8KHz. It's very annoying.
> 
> I tracked the tone around the room with an RTA and was able to identify 
> a "hotspot", but it seems that even hanging a heavy soft-surface (woven 
> wool blanket) has not dampened the problem. I'll also try putting foam 
> behind the television when I get home today.
> 
> Any advice?
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