A vibration becomes audible sound when it causes air pressure fluctuations in (roughly) the 20 Hz–20 kHz band at a level your ear can detect. In practice, this happens when at least one of these is true:

1. The vibration itself is in the audible band, and it couples efficiently to air.
2. The vibration creates higher-frequency components (harmonics, impacts, turbulence).
3. The vibration excites a structure or cavity resonance that radiates efficiently.

A useful engineering phrase is: “Vibration must couple to the air as pressure waves.”
Not all vibration couples well; some stays in the structure and is mostly felt, not heard.

---

1) What kinds of sound can a large floor-mounted compressor create?

A. Low-frequency rumble and “throb” (structure-borne dominant)

What it sounds like: deep rumble, pulsing “whoom-whoom,” sometimes felt as vibration in floors/sofas/bed frames.
Typical cause: the compressor’s cyclic forces couple into the slab. Even if the compressor room seems “not that loud,” residents may still experience low-frequency sound because the building acts as a transmission path.

Where it comes from (mechanisms):

• Rotational imbalance and cyclic loading
• Reciprocating compression pulses (for piston compressors)
• Torque ripple from motor drive
• Mount stiffness (rigid mounting injects more vibration)

Frequency clue (rule of thumb):

• If the motor runs around 1500 rpm, the rotation rate is: $$1500\ \text{rpm} \div 60 \approx 25\ \text{Hz}$$That 25 Hz may be felt more than heard, but its harmonics at 50, 75, 100 Hz can be clearly audible.

---

B. Tonal hum (50/60 Hz family) and harmonics

What it sounds like: steady “hum,” sometimes with a “buzz” quality.
Typical cause: electromagnetic forces from the motor and power system; also mechanical components excited at harmonics.

• Common tonal lines: 50 Hz (Europe) and 100 Hz, 150 Hz, etc.
• If a variable speed drive (VFD) is used, you can also get distinct tones that move with speed (and sometimes a high-frequency “whine,” see below).

Why it can be prominent in apartments: tonal noise is very noticeable to humans, and structure-borne coupling can make these tones appear strongly even if airborne levels in the compressor room are moderate.

---

C. Mid-frequency “growl” or broadband noise (airborne dominant)

What it sounds like: continuous “machine noise,” sometimes like a rough hiss/roar.
Typical cause: turbulent airflow, cooling fans, gas flow through valves, and general mechanical radiation from housings.

• Often strongest from a few hundred Hz up to several kHz.
• This component is typically more controllable with enclosures, absorption, and airborne isolation, but it can still leak through doors, vents, and shafts.

---

D. High-frequency whine (often VFD-related) or “screech”

What it sounds like: sharp whine, sometimes 2–10 kHz or higher, occasionally very irritating.
Typical cause:

• VFD switching-related tonal components, motor magnetostriction, or inverter-driven effects
• Smaller fans also can generate tonal blade-pass components

This is usually airborne, but it can also couple into structure if components excite panels or brackets.

---

E. Pulsation/“chuffing” and pressure-related noise in connected piping

What it sounds like: rhythmic “chuff,” “puff,” or low-frequency “pumping,” sometimes audible far away through walls.
Typical cause: pressure pulsations in suction/discharge lines, especially for reciprocating machines or poorly damped systems.

Transmission path: this often becomes pipe-borne vibration, which can:

• travel along pipes through the building,
• excite pipe clamps and wall penetrations,
• and re-radiate as sound inside apartments.

---

F. Impulsive events: clicks, bangs, start/stop thumps

What it sounds like: occasional “clunk,” “bang,” “click,” or sudden thud—especially at start/stop or load changes.
Typical cause: contactors/relays, check valves, pressure relief events, mechanical looseness, or mounts bottoming out.

These events can be disproportionately annoying because they are intermittent and attention-grabbing.

---

2) Why can residents hear it even if the compressor room is not extremely loud?

Because the building structure can act like a distributed loudspeaker:

1. Compressor vibration enters the slab.
2. Vibration travels through concrete/steel with relatively low attenuation at low frequencies.
3. In apartments, floors/walls/ceilings vibrate slightly and re-radiate sound into the air.

This is why complaints often describe:

• “I hear it in my bedroom at night,”
• “I feel it more than I hear it,”
• “It’s a constant hum with a pulse.”

---

3) A concrete “example spectrum” you might expect (illustrative)

Assume a compressor motor near 1500 rpm (25 Hz rotation rate) and mains at 50 Hz:

• ~25 Hz: rotational fundamental / low-frequency structural vibration (often felt)
• 50 Hz, 100 Hz, 150 Hz: strong tonal hum family (often clearly audible)
• 200–800 Hz: structural resonances, casing radiation (“growl”)
• 1–5 kHz: airflow/fan noise, possible tonal components
• 2–10 kHz: possible VFD/motor whine (if present)

The dominant bands depend heavily on compressor type (reciprocating vs screw), mounting, slab type, and piping layout.

---

4) What determines which of these dominates?

Key practical factors:

Building structure: continuous slabs and rigid connections transmit low frequencies efficiently.

Mounting and isolation: rigid mounts transmit structure-borne energy; resilient isolators reduce it (if correctly sized).

Operating mode: constant speed vs VFD; load/unload cycles; start/stop frequency.

Piping supports: hard clamps and wall penetrations can inject vibration into the structure.

Room acoustics: hard rooms amplify airborne noise; panels and cavities can resonate.

---

Summary: A Practical Decision Rule

Vibration becomes audible sound when it produces measurable pressure fluctuations in the audible band. This usually happens via one of these paths:

1. Direct radiation (vibration already in 20 Hz–20 kHz and couples well)
2. Harmonics (non-sinusoidal periodic motion creates audible multiples)
3. Resonance (structures/cavities amplify and radiate at audible frequencies)
4. Nonlinear impacts/friction (create broadband high-frequency sound)
5. Sharp pulses/turbulence (wideband content even if repetition is low)