Sanity check: Wall assembly options for backyard drum studio (targeting 70 dB at property line)

[MisterBuildMan]

Hello all!

I'm planning a detached backyard drum studio (new construction, room-within-a-room design) and want to sanity-check my wall assembly options before committing. I've done some research but the theoretical transmission loss calculations I'm running seem pessimistic compared to what I see in successful builds online. Some background on my project:

Project Goals
- Primary use: Acoustic drum practice and recording
- Location: Residential backyard in Texas USA, ~30 ft from property line
- Foundation: Concrete slab (no floating floor planned)
- Roof/ceiling: TBD - haven't finalized assembly yet
- Noise constraint: City ordinance limits:
- Daytime (7 AM - 10 PM): 70 dB(A) / 80 dB(C) at property line
- Nighttime (10 PM - 7 AM): 50 dB(A) / 60 dB(C) at property line
- I plan to play during daytime only, so targeting the 70 dB(A) / 80 dB(C) limit
- Source level: Acoustic drums typically produce 105-115 dBC (I'm using 110 dBC as my reference)
- Required isolation: ~40 dB minimum to meet daytime limit, targeting 55-60 dB for safety margin

The Concern
I'm particularly worried about kick drum frequencies (40-80 Hz). My understanding is that double-wall systems have a mass-spring-mass resonance frequency where isolation drops, and I want to make sure whichever assembly I choose doesn't have its resonance frequency right in the kick drum range.

Options I'm Considering

Approach A: Decoupled Double Stud Wood Frame (most common approach I've found in the US)
- Outer leaf: 2×4 studs + 2× 5/8" drywall (~21.4 kg/m² / 4.4 lbs/ft²)
- Inner leaf: 2×4 studs + 2× 5/8" drywall (~21.4 kg/m² / 4.4 lbs/ft²)
- Cavity: 8" total (3.5" outer stud + 1" gap + 3.5" inner stud), filled with mineral wool
- Estimated resonance: ~41 Hz

Approach B: CMU (concrete block) Outer Shell + Decoupled Wood Inner Frame
- Outer leaf: 8" CMU solid grouted (~420 kg/m² / 86 lbs/ft²)
- Inner leaf: 2×4 studs + 2× 5/8" drywall (~21.4 kg/m² / 4.4 lbs/ft²)
- Cavity: 4" air gap with mineral wool
- Estimated resonance: ~42 Hz

Approach C: Decoupled Double Brick Shells
- Outer leaf: Single wythe brick (~195 kg/m² / 40 lbs/ft²)
- Inner leaf: Single wythe brick (~195 kg/m² / 40 lbs/ft²)
- Cavity: 4" air gap
- Estimated resonance: ~19 Hz

My Calculations

I built a simple transmission loss calculator using the London/Sharp mass-air-mass formulas. The graph attached shows estimated TL across the frequency spectrum for each approach.

According to the graph, Approach A (double-stud wood) shows a resonance dip right around 40 Hz, while B performs similarly but with more mass (thus more isolation across the board), and C's resonance is well below the kick drum range.

But here's my confusion: Countless drum studios have been built successfully with double-stud wood construction (Approach A), and Rod Gervais specifically recommends it. What is missing from these theoretical calculations?

It seems that the isolation right at the kick drum frequencies for approach A are basically negligible. It doesn't look like it would drop that specific frequency to my target.

Questions
1. For those who've built studios with double-stud wood: did you have issues with kick drum isolation, or does it work fine in practice?
2. Given my 70 dB daytime limit, would Approach A be sufficient, or should I consider the heavier masonry options?
3. What am I missing here? There has to be something here given all the studios I've seen built with this approach with great results.

Thanks for any insights!

[MisterBuildMan]

Oops, I think I may have forgotten to attach the image in my original post. Here's the graph comparing TL across the frequency spectrum for the three approaches I mentioned above.

[Soundman2020]

Hi there, and Welcome to the Forum!

Congratulations on your project! It sounds VERY nice. So here's just a few comments, in no particular order:

- Foundation: Concrete slab (no floating floor planned)

Great! You probably don't need a floating floor. Here's why: Floating your floor: How and why... and why not.

- Noise constraint: City ordinance limits:
- Daytime (7 AM - 10 PM): 70 dB(A) / 80 dB(C) at property line

Cool! Lucky you! Those are unusually high levels, so it should not be too hard to get the isolation you need.

- Required isolation: ~40 dB minimum to meet daytime limit, targeting 55-60 dB for safety margin

Sounds about right. To put that in perspective, typical house walls give around 30 dB, which isn't very much. 40 dB is about twice as good (subjectively: half as loud). 60 dB is about as much as you can hope for in a typical home studio, unless you have very deep pockets. 50 to 55 is reasonable and achievable on a typical good budget.

I'm particularly worried about kick drum frequencies (40-80 Hz). My understanding is that double-wall systems have a mass-spring-mass resonance frequency where isolation drops, and I want to make sure whichever assembly I choose doesn't have its resonance frequency right in the kick drum range.

All isolation barriers have some type of natural resonance. The advantage of a double wall MSM system is that you can tune it to your needs. Ideally, the tuned frequency should be half of the lowest frequency that you need to isolate. So if you need to isolate the typical kick at around 80 Hz, the tuned frequency of your wall should be below 40 Hz. If you need to isolate 6-string bass guitar, which goes down to around 32 Hz, then you'd need a tuned frequency of 16 Hz or lower. Etc.

I built a simple transmission loss calculator using the London/Sharp mass-air-mass formulas. The graph attached shows estimated TL across the frequency spectrum for each approach.

Nice work! Those graphs look realistic, and doing a calculator like that isn't easy. Good job! If you'd care to share that tool, I'd be happy to post it here on the form for everyone to use, if they wish, obviously giving credit to you. Of course, if you don't want to share, that's fine too! That took a lot of work to get right, I'm sure.

According to the graph, Approach A (double-stud wood) shows a resonance dip right around 40 Hz, while B performs similarly but with more mass (thus more isolation across the board), and C's resonance is well below the kick drum range.

Right! And note that you have two dips in the response for that. Not exactly sure what that second dip is (around 300 Hz, it seems): it could be any of several things. My guess would be a coincidence dip. But that reduces your overall isolation. I would not go with double-stud walls using that construction layout where drums are involved, unless you beef up both leaves considerably.

But here's my confusion: Countless drum studios have been built successfully with double-stud wood construction (Approach A), and Rod Gervais specifically recommends it. What is missing from these theoretical calculations?

I think you'll find that Rod uses more mass and greater air gaps on his drum studios. You also don't seem to be taking into account the insulation in the gap: that's a major part of the MSM equation. Maybe you are, but you didn't mention it in the graphs. That's important to know. The key metrics there are the GFR (Gas Flow Resistivity) of the specific insulation material, and the thickness of that material. You get maximum isolation when the cavity is 100% filled with suitable insulation.

3. What am I missing here? There has to be something here given all the studios I've seen built with this approach with great results.

In addition to the above (insulation), you are probably not considering Messrs. Fletcher and Munson. They did a number of experiments on human hearing way back, and discovered that we are not as sensitive to low frequency sounds as we are to mids and highs, and the sensitivity varies based on the level of the sound. They developed a set of curves that show this sensitivity, and those are know a the "Fletch-Munson curves". Later, other folks did additional work, and revised those curves. They are now referred to as the "equal loudness curves". Basically, we just aren't very good at hearing low frequency sound at a low dB level. We can hear mids very well at the same dB level. So, even though your isolation might not look so good for kicks and floor toms, as measured in our numbers, when you take into account the equal loudness curves, they probably won't be heard be actual people, as long as they are reasonably quiet.

In fact, the "A" scale on your sound level meter very roughly approximates the 40-phon Equal Loudness Curve, while the "C" scale very roughly approximates the 100-phon Equal Loudness Curve. Take a look at those curves, think about your frequencies and levels, and you'll likely find that either of your "B" and "C" construction options would work just fine, and maybe even "A" if you beef up the mass some more and put plenty of suitable insulation in the air gap. Green Glue can be your fried here: more effective than extra layers of drywall, and especially at low frequencies. Here's why: The truth about Green Glue

You might find that option "B" would be really good, especially if you increase the cavity depth to 6" and put a layer of Green Glue between your two layers of drywall.

Hope that helps a bit!

- Stuart -