Performance
How Sound Gets Through an Unsealed Mail Slot
A door's ability to block sound is usually described with a single number: its Sound Transmission Class, or STC. That number is measured on a sealed, gap-free assembly in a lab, so it doesn't reflect how much a single unsealed opening can undo in practice. A mail slot is exactly that kind of gap, closed by nothing more than a light flap, the same kind of unsealed opening that also lets cold air into a home.
What STC Actually Measures
Sound Transmission Class is a single number rating derived from a door or wall assembly's transmission loss, the amount of sound energy it blocks, tested across a range of frequencies in a laboratory. The lab test method is ASTM E90; the results are converted into an STC rating using ASTM E413.1 A higher STC number means more sound is blocked.
A related rating, Outdoor-Indoor Transmission Class (OITC), uses the same ASTM E90 lab method but classifies the results under ASTM E1332 instead, weighted toward the lower frequencies typical of outdoor noise such as traffic rather than the speech frequencies STC is weighted toward.2 OITC values usually run 5 to 10 points lower than STC for the same assembly. OITC was developed specifically for exterior envelope assemblies exposed to outdoor noise, and it appears mainly in noise-sensitive siting situations, such as buildings near airports or highways, rather than in typical residential door specification, where STC remains the more established and widely referenced standard.3
STC ratings are calculated for a sealed, tested assembly. They describe what a door can do at its best, not necessarily what it does once it is hung, gapped, and used every day. For context, a perfectly sealed solid-core wood door typically rates in the high 20s STC, and a perfectly sealed hollow metal door typically reaches the low 30s.4
| STC Range | Performance | What You'd Typically Hear |
|---|---|---|
| 20 to 25 | Very poor | Normal speech is easily understood through the door |
| 25 to 30 | Poor | Loud speech is understood fairly well |
| 30 to 35 | Fair | Loud speech is audible but hard to make out clearly |
| 35 to 40 | Good | Loud speech is only faintly heard |
| 40 to 50 | Very good | Loud speech is not audible |
| 50 and above | Excellent | Most sounds are inaudible |
General STC performance ranges commonly used to describe real-world listening experience at each rating level.5
Why a Small Gap Causes a Large Loss
Sound takes the path of least resistance. A gap does not just add a small amount of leakage proportional to its size, it can eliminate most of a door's rated performance outright. Unlike the narrow perimeter gap around a properly adjusted door, a mail slot is an intentional opening cut completely through the door panel itself. Sound doesn't have to pass through the door's insulating materials. It can travel directly through the opening if it isn't effectively sealed. That's why even well-built doors can allow noticeable noise through an unsealed mail slot.
Industry testing shows that a clearance as small as one eighth of an inch around a door's edges can reduce the effective performance of an STC 52 door to roughly STC 21.6 This example illustrates how sensitive acoustic performance is to unsealed openings. It should not be interpreted as an STC estimate for a mail slot, which has a different geometry and has not been evaluated using this comparison.
Why This Matters Beyond Comfort
Unwanted noise is not only an annoyance. A 2014 review published in The Lancet linked environmental noise exposure to measurable health effects, including sleep disturbance and cardiovascular strain.7 An entry door facing a street, a shared hallway, or a porch is one of the more direct paths street and foot traffic noise has into a home, and a mail slot is a gap in that path.
Where BOTA Products Fit
Neither the Brass-Seal Forged Brass Mail Slot nor the BOTA Mail Slot Insulator carries a published STC or OITC rating. The Brass-Seal's ASTM E283 testing measures air leakage under pressure, not sound transmission, and the Mail Slot Insulator has no independent acoustic test data published. What both products do is address the underlying cause described above: an open or loosely sealed cavity in the door, the same cavity behind water and pest intrusion concerns as well. The Brass-Seal replaces the mail slot hardware itself with a gasketed closure; the Mail Slot Insulator retrofits a magnetic seal into an existing cutout, using the same acoustic-lined flap material as the Brass-Seal's gasket. Closing the gap is the mechanism behind any noise reduction either product provides, not a tested acoustic value.
Key facts on mail slots and sound:
- STC ratings are based on ASTM E90 laboratory testing, classified using ASTM E413.
- OITC is a related rating for lower-frequency outdoor noise, classified under ASTM E1332, and typically runs 5 to 10 points lower than STC for the same assembly.
- A perfectly sealed solid-core wood door typically rates in the high 20s STC; a perfectly sealed hollow metal door reaches the low 30s.
- A door's rated STC assumes a sealed assembly; unsealed gaps can eliminate most of that rating in practice.
- A gap as small as one eighth of an inch around a door's edge can reduce an STC 52 door's effective performance to roughly STC 21.
- A mail slot is a larger, more direct opening than a typical edge clearance.
- Environmental noise exposure has documented links to sleep disruption and cardiovascular effects.
- Neither BOTA product carries a published STC or OITC rating; both work by closing the gap rather than by a tested acoustic value.
Frequently Asked Questions
Does BOTA publish an STC rating for the Brass-Seal or the Mail Slot Insulator?
No. ASTM E283 testing, which applies to the Brass-Seal only, measures air leakage, not sound transmission. Neither product has independent acoustic test data published at this time.
Will sealing my mail slot reduce street noise?
It can reduce one direct path for outdoor noise to enter the home, but the overall noise level also depends on the door, surrounding walls, windows, weatherstripping, and other openings.
Will sealing my mail slot make my door as quiet as a solid wall?
No single fix does that. Closing the mail slot removes one specific, direct sound path through the door. The door's overall STC still depends on its material, thickness, and how well it is sealed around its own frame.
What's a good STC rating for an exterior door?
A perfectly sealed solid-core wood door typically rates in the high 20s STC, and a perfectly sealed hollow metal door reaches the low 30s. Ratings meaningfully above that generally require added mass or specialty acoustic construction, not just a tighter-sealing mail slot or door hardware.
Sources
- ASTM International. ASTM E90: Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements; ASTM E413: Classification for Rating Sound Insulation.
- ASTM International. ASTM E1332: Standard Classification for Rating Outdoor-Indoor Sound Attenuation.
- Commercial Acoustics, "OITC Rating 101: Outdoor-Indoor Transmission Class."
- Kuhn, M. "STC Doors In the Wild." iDigHardware.
- Riverbank Acoustical Laboratories, "Understanding Sound Transmission Class (STC) Ratings and Testing."
- Trustile Doors, "Understanding STC Ratings: Sound & Acoustic Rating Chart for Doors," Technical Information.
- Basner, M., Babisch, W., Davis, A., et al. "Auditory and Non-Auditory Effects of Noise on Health." The Lancet, 2014.