It is how much quieter a sound gets after it passes through a barrier like a wall or door.
Loud sound hits a barrier: most reflects, a quiet trickle passes through, and TL (dB) is the difference between the two sides.
What it is
How much quieter a sound gets after passing through a barrier like a wall, door, or window.
Key facts
Transmission Loss (TL) is in decibels: TL = 10 x log10(1/tau), where tau is the transmission coefficient (sound power through divided by sound power hitting it)
Mass Law: doubling a wall's surface mass (kg/m2) adds about +6 dB TL; doubling frequency also adds about +6 dB TL
Mass Law formula: TL is roughly 20 x log10(mass per area x frequency) minus 47 dB
Single-number ratings: STC (USA) and Rw (Australia/ISO) squash the TL-vs-frequency curve into one number; higher = better
Low frequencies (bass/kick/sub, 30-120 Hz) have the LEAST transmission loss, so they leak most and cause noise complaints
Speed of sound is 343 m/s at 20 C; from a point source level drops -6 dB each time distance doubles (inverse-square law); -10 dB sounds about half as loud
A 1% open area (cracks, gaps under doors) can cap a wall at about 20 dB no matter how heavy it is; the weakest panel/gap sets the limit, not the mass
How it works
Sound waves in the air hit the barrier and push on its surface.
The barrier's mass resists moving, so most energy bounces back (reflected) instead of through.
A small slice of energy vibrates the barrier and re-radiates as sound on the far side.
Heavier, denser, thicker, well-sealed barriers vibrate less, so less sound gets through.
TL in dB = the level hitting the wall minus the level coming out the other side.
Any air gap, crack, or weak panel lets sound flank around the mass and slashes the TL.
Real examples
Party next door: a thin gyprock wall passes the bass thump (low TL) while double-brick mostly blocks it (high TL).
Marquee gig in a field: canvas walls give almost zero TL, so the whole street hears your subs.
Recording in a venue: a propped fire door (gap = low TL) lets foyer chatter bleed onto your tracks.
Two-stage festival: a heavy timber-and-mass-loaded-vinyl barrier between stages adds TL to cut bleed.
Pub gig: swapping single 6mm glass (Rw ~30) for double glazing (Rw ~38) keeps neighbours happy.
How it helps in live sound
Chase low-frequency leaks first: subs (30-120 Hz) have the worst TL and cause most complaints.
Seal every gap before adding mass: door brushes, threshold seals and acoustic sealant beat a heavier wall with a hole in it.
Aim subs and tops away from boundaries; orientation cuts what hits the weak wall.
Use mass + air gap + insulation (decoupled double-leaf) for real TL, not one thick panel.
Add a sub high-pass and pull sub level after midnight; -6 dB at the desk = a quieter street.
Carry an SPL meter (C-weighted, slow) and log levels at the boundary to prove compliance for the council.
Everyday analogy
It's like a fishing net catching a fast ball: a flimsy net barely slows it, but a thick heavy net catches almost all of it, with only a trickle slipping through.
Watch out
Myth: 'just use a heavier wall.' Truth: a 1% air gap caps TL at about 20 dB, so sealing leaks matters more than mass, and bass always leaks worst because TL drops with falling frequency.
Fun fact
A wall can have a 'coincidence dip' frequency where it suddenly becomes nearly transparent to sound, so a panel that blocks 45 dB elsewhere might block only 25 dB at that exact pitch.
Key takeaways
TL = how many dB a barrier strips off sound passing through it.
Mass Law: double the mass OR double the frequency = about +6 dB TL.
Bass leaks most; low frequencies always have the lowest TL.
Gaps and the weakest panel set the limit, not the heaviest part.
Rw / Ctr (AU) and STC (US) are the single-number TL ratings to quote.
Decouple + add mass + seal + insulate for real-world blocking.