Vibration Isolation Pads and Where the Hum Comes From

They stop mechanical movement entering the structure, which is the low hum in a distant room — and they do almost nothing about the noise you hear standing beside the unit.

Two Different Noises, One of Them Fixable This Way

Complaints about heat pump noise are almost always about one of two things,
and they need completely different answers.

Airborne noise is what you hear standing next to the outdoor unit: fan
noise, air moving across the coil, and the compressor’s own sound travelling
through the air. Structure-borne noise is different. It is the machine’s
mechanical vibration passing into whatever it is bolted to, travelling through
the building’s structure, and re-radiating as sound somewhere else entirely —
usually as a low hum in a room that is not near the unit.

Isolation pads address the second one. They do very little about the first,
and expecting them to is the most common disappointment here.

How the isolation actually works

An elastic element between a vibrating machine and a rigid structure acts as
a spring. Above a certain frequency, that spring cannot follow the machine’s
motion fast enough, so it absorbs the movement instead of transmitting it. Below
that frequency it transmits, and near it, it can amplify.

Two practical consequences follow, and both explain failures people find
mysterious.

• The isolator has to be soft enough for the load it carries.
  A pad barely compressed by the weight on it is behaving as a rigid spacer, and
  a rigid spacer transmits everything. This is why an isolator sized for a heavy
  machine does nothing under a light one.
• Too soft is also a failure. An over-soft mount lets the
  equipment move, which stresses the refrigerant lines and the electrical
  connection, and it can put the assembly’s natural frequency near the
  compressor’s running speed, which makes the noise worse rather than better.

Matching the isolator to the weight it carries is the whole skill, and it is
why isolators are specified by load rather than sold as one product.

What each material is for

The path people forget

An outdoor unit isolated perfectly at its feet can still hum through the
house, because the feet are not the only thing touching the building. The
refrigerant lines are a rigid metal path from the compressor to the indoor
equipment, and a line set clamped hard to a joist is a very effective
transmitter.

Where a unit is bracket-mounted on a wall, the problem is worse again,
because the bracket bolts through the structure and the wall is a large
radiating surface. Wall brackets are attractive in a snowy climate and they are
a structure-borne noise decision as much as a snow one.

So the checklist is the whole path, not the pads:

• Isolators under the unit, sized for its weight.
• Isolated clamps or resilient hangers on the line set, especially at the
  first fixing after it leaves the unit.
• A flexible sleeve and a resilient seal where the line set penetrates the
  wall, rather than the pipe packed rigidly into the hole.
• The electrical whip left with a loop in it rather than pulled taut.
• Condensate piping isolated too, since it is another continuous solid
  connection.

Where the unit sits matters more than what it sits on

Isolation reduces transmission. Distance and geometry reduce the problem
before it exists, and they cost nothing at design time:

• Not against a bedroom wall, and not below a bedroom
  window. Obvious, and routinely done anyway because it is the shortest line set
  run.
• Not in a corner or a narrow side yard if it can be
  avoided. Two hard surfaces meeting behind the unit reflect airborne noise back
  and toward the neighbour, which is a separate mechanism from vibration but the
  same complaint.
• On its own pad rather than on the house structure where
  the ground allows it. A free-standing pad shares nothing with the building.
• Consider the neighbour’s wall as well as yours. Many New
  York municipalities have measurable limits, and the measurement is taken at a
  property line rather than at your window — the subject of
  heat pump noise rules in New York towns.

Placement is also constrained by the equipment’s own limits, so it is not a
free choice: line set length and rise bound how far the
unit can move, and where indoor heads go is its own question, covered in
wall mounted head placement.

What modern equipment changed

A single-stage compressor starts at full speed and stops, so its noise is
intermittent and its worst moment is the start. An
inverter-driven compressor runs continuously at varying
speed, which is quieter overall and changes the character of the complaint: a
continuous low hum is easier to notice at night than an intermittent one, even
when it is objectively quieter.

Variable speed also means the machine passes through a range of frequencies
rather than sitting at one, so a resonance in the structure that only exists at
one particular speed will announce itself at some point in the season. That is
the source of the classic report that the unit hums only sometimes, for no
apparent reason.

The same reasoning applies to other equipment that now lives in or beside
the house — a battery’s cooling fans are a documented source of complaint in
their own right, covered in
what a home battery really sounds like.

Diagnosing a hum you already have

Before buying anything, establish which noise you have. Stand outside next
to the unit and listen, then go to the room where the complaint is. If it is
loud outside and quiet inside, it is airborne and pads will not help. If it is
modest outside and clearly present inside as a low hum, it is structure-borne
and the path is worth tracing.

Trace it by touch. With the unit running, put a hand on the pad, the
brackets, the line set at its first clamp, and the joist or wall nearest the
penetration. The place where you can feel it is the place to isolate. That is a
five-minute exercise that saves buying the wrong product.

Retrofitting isolators under an installed unit means lifting it, which
means supporting the line set so it is not strained — work for the installer
rather than the homeowner, and a reasonable thing to ask them to return for if
the noise was there from commissioning. It is also worth doing at the same time
as anything else that requires lifting the cabinet, such as correcting the
height and drainage described in the notes on
keeping meltwater moving under the unit. In a dense
setting where the neighbour is a few feet away, the whole question gets more
serious, which is the situation described in
putting a heat pump in a Brooklyn brownstone.

Frequently Asked Questions

What do vibration isolation pads actually do?

They put an elastic element between a vibrating machine and the rigid structure it sits on, so the machine’s mechanical movement is absorbed rather than transmitted into the building. That addresses structure-borne noise — the low hum that appears in a room away from the unit. It does very little about airborne noise, which is what you hear standing next to the equipment.

Will pads make my outdoor unit quieter outside?

Barely. Fan noise, air across the coil and the compressor’s own sound travelling through the air are airborne, and isolation at the feet does not touch them. Airborne noise is addressed by placement, distance, and avoiding hard reflecting surfaces behind and beside the unit, not by what it stands on.

Can an isolation pad be too stiff or too soft?

Both, and each fails differently. A pad barely compressed by the weight on it behaves as a rigid spacer and transmits everything. One that is too soft lets the equipment move, which stresses the refrigerant lines and the electrical connection, and can put the assembly’s natural frequency near the compressor’s running speed, making the noise worse. Isolators are specified by the load they carry for this reason.

I have pads and it still hums indoors. Why?

Almost certainly another path. The refrigerant line set is a rigid metal connection from the compressor into the building, and clamped hard to a joist it transmits very effectively. The wall penetration packed rigidly around the pipe does the same, as does a taut electrical whip and rigid condensate piping. Isolate the whole path, not just the feet.

How do I tell which kind of noise I have?

Listen next to the unit, then in the room where the complaint is. Loud outside and quiet inside is airborne, and pads will not help. Modest outside and clearly present inside as a low hum is structure-borne. Then trace it by touch with the unit running: the pad, the brackets, the line set at its first clamp, and the framing nearest the penetration. Where you feel it is where to isolate.

Is a wall bracket worse than a ground stand for noise?

Generally yes for structure-borne noise, because the bracket bolts into the structure and the wall is a large radiating surface. A free-standing pad on the ground shares nothing with the building. Brackets are attractive in a snowy climate for getting the unit above the drift line, so it is a trade rather than a mistake — but it is a noise decision as well as a snow one.

Why does my variable speed unit only hum sometimes?

Because it runs at varying speed rather than one fixed speed, so it passes through a range of frequencies over a season. A resonance in the structure that only exists at one particular compressor speed will announce itself whenever the machine happens to run there, which is exactly the intermittent, apparently random hum people report.

Can I add isolators to a unit that is already installed?

Yes, but it means lifting the cabinet, which means supporting the refrigerant line set so it is not strained. That is installer work rather than a homeowner job, and it is reasonable to ask the installer to return for it if the noise was present from commissioning. It is also worth combining with any other work that requires the unit to be lifted.