Energy Conservation

Sound energy hitting a surface always splits into reflected + absorbed (heat) + transmitted, and the three add back to 100%.

What it is

Sound energy never vanishes; it only converts to heat, transmits through stuff, or reflects back.

Key facts

• Sound energy splits 3 ways at any surface: Reflected + Absorbed + Transmitted = 100% of incident energy.
• Energy balance law: rho (reflection) + alpha (absorption) + tau (transmission) = 1, each a fraction of incident energy.
• Speed of sound in air = 343 m/s at 20 degrees C (about 1235 km/h).
• Double the distance from a point source = -6 dB SPL (inverse-square law: intensity drops to 1/4).
• +3 dB = double the acoustic power (energy); -3 dB = half the power. +6 dB = double the pressure = 4x the power.
• Absorption coefficient alpha runs 0 (perfect reflector, concrete ~0.02) to 1.0 (total absorber, open window = 1.0).
• Acoustic foam alpha ~0.6-0.9 at highs but near 0 below 250 Hz; it converts mid/high energy to heat, not bass.
• Sabine RT60 = 0.161 x V / A, where V = room volume (m3), A = total absorption (surface area x alpha, in sabins); RT60 = time to decay 60 dB.
• Mass law: doubling a wall's mass per area adds ~6 dB transmission loss; blocking needs MASS, absorbing needs POROSITY.
• A loudspeaker is only ~1-5% efficient; the other 95%+ of amp energy becomes heat in the voice coil, not sound.

How it works

1. Sound wave (energy as moving air pressure) hits a surface.
2. Part REFLECTS back into the room (hard surfaces, echo).
3. Part is ABSORBED, friction in porous material converts it to tiny heat.
4. Part TRANSMITS straight through the wall to the neighbours.
5. Add the three together and you always get 100% of what arrived.
6. To kill unwanted sound you must redirect its energy: absorb (heat) or block (mass), never just hope.

Real examples

• Clap in an empty hall: long echo because energy keeps reflecting with nowhere to be absorbed.
• Same hall with a crowd + curtains: clap dies fast because bodies and fabric soak the energy into heat.
• Bass thumping through a nightclub wall to the street: low-frequency energy transmitted, not absorbed by thin foam.
• A speaker's voice coil getting hot after a loud set: electrical energy that did not become sound became heat.
• Carpet on a marquee floor cutting harsh slap-back: high-frequency energy converted to heat instead of bouncing.

How it helps in live sound

• Echoey marquee or hall? Add porous soft goods (drapes, pipe-and-drape, carpet) to convert reflections to heat and tame RT60.
• Foam panels fix flutter echo and harshness (mids/highs) but do almost nothing for boomy bass below 250 Hz.
• For bass control bring bass traps or thick (100mm+) rockwool; thin foam will not trap low-frequency energy.
• Noise complaint from neighbours = transmitted energy: you need MASS/barrier (plywood, mass-loaded vinyl), not acoustic foam.
• Move the PA closer to the audience: -6 dB per distance doubling means a near source needs far less power than a distant one.
• Outdoor gig with no walls = near-zero reflected energy, so it sounds dead; deploy delay fills rather than cranking the mains.