Compare parking AC noise at the same position and under the same operating conditions. A number measured beside an outdoor housing cannot be compared directly with one measured at a sleeper's pillow. The installation itself also influences what the driver hears.

Make the test a purchase agreement

Before comparing units, agree on the vehicle, mounting arrangement, cabin condition and relevant operating modes. Write down the measurement position and the distance to nearby surfaces. Keep doors, windows, curtains and bedding in consistent positions. Record background sound with the equipment off.

This is a suggested procurement protocol, not a claim of compliance with a particular acoustic standard. For a formal certified rating, use the applicable method and qualified test provider.

Capture more than the quietest mode

Observe initial cooldown, temperature maintenance and mode changes. A quiet low-output setting may be comfortable after the cabin is cool but may not meet the requirement at the beginning of a hot rest period. Record cabin temperature alongside sound so that a reduced sound level is not mistaken for equivalent cooling performance.

Include the sound of startup and repeated cycling. Ask the driver about drafts, tonal sounds, rattles and noticeable changes rather than reducing the assessment to a single subjective “quiet” score.

Separate equipment sound from installation vibration

If a rattle appears, document where it is heard and under which conditions. Have the installer inspect the installation using the manufacturer's requirements. Loose trim, contact between components or an unsuitable mounting arrangement can affect the installed result. Do not prescribe a universal bolt torque or improvised isolation material.

For a comparative purchasing test, use an appropriate sound meter and document its setup. A phone recording can help communicate an intermittent sound, but it should not be presented as a certified acoustic measurement.

Report a useful result

The report should identify the exact model, vehicle, mode, temperature conditions, background sound, measurement location and observed interruptions. Include the driver assessment separately from the instrument readings.

Manufacturers increasingly emphasize quiet rest-period operation, as in the Cooltronic G3 announcement. Buyers can respond by requesting comparable evidence. The question is whether the installed system meets the fleet's agreed comfort requirement while providing adequate cooling, rather than which brochure displays the smallest number.

Fix the microphone position before the first reading

Put the meter where comfort is actually judged: at pillow height, on the side of the bunk where the driver sleeps, with mattress and bedding in their normal positions. A reading taken in the aisle, at the window or beside the outlet answers a different question. Fix the exact spot once, then reuse it for every unit and every session in the comparison.

Keep the position identical across vehicles. If truck A is measured at the pillow and truck B in the center of the cab, the two numbers do not describe the same listening task. Park all test vehicles the same way relative to walls, fences or other trucks, because reflected sound changes what both the meter and the driver hear, and note the surroundings in the sheet.

Photograph each setup. A picture of the meter position, the closed curtains and the bedding arrangement lets a colleague repeat the measurement weeks later, or lets a supplier understand precisely what was measured when a result is disputed. Small shifts in position can move the reading noticeably, so the photo is part of the evidence rather than a formality.

Write the position rule down before the first unit arrives: where the meter sits, how long the reading settles, and who records it. A rule fixed in advance prevents the quietest corner from being chosen after the fact, and when two people take turns measuring, both follow the same written procedure so their sessions stay interchangeable.

  • Height: meter at pillow height, roughly level with the ear of a driver lying down.
  • Horizontal position: the side of the bunk actually used for sleeping, never the aisle or the foot of the bed.
  • Surfaces: curtains closed, bedding arranged as normal, nothing stacked between the meter and the unit.
  • Settling time: one full minute in the mode before the first reading is taken.
  • Clearance: keep the meter at least a hand's width from pillows, walls and the cab lining so reflections do not skew the figure.
  • Orientation: the same microphone direction in every session, because the meter reads differently facing the unit than facing away.
  • Recording: mode, position photo, background reading and observer initials on the same line as the number.
  • Repeat policy: the same position for every unit, every day, for the whole comparison.

Build a mode-by-mode comparison sheet

A single number per unit hides the differences that matter at night. Build a sheet with a row for each documented operating mode — cooldown, low, medium, fan-only and any labeled night or sleep mode — and fill one row at a time with the meter in the fixed position. Wait for the reading to settle before writing it down.

  • Startup: the first minute after switching on, while the compressor and fans spin up.
  • Cooldown: output near maximum while the cabin is still warm from the day.
  • Maintenance: steady running once the target temperature has been reached.
  • Cycling: the compressor stopping and restarting, including the click or thump of each transition.
  • Fan-only: ventilation without cooling, which shows how much of the sound belongs to the fans rather than the refrigerant circuit.
  • Night or sleep mode: whatever the manufacturer labels as reduced output, measured exactly as the driver would use it.
  • Auto mode: the unit choosing its own fan and compressor steps, recorded over a full cycle rather than a single instant.
  • Restart: the mode re-entered after a brief power interruption, which sometimes produces a louder or longer start than the first switch-on.
  • Setpoint changes: one deliberate step up and down, to hear how the unit responds when a driver adjusts temperature during the night.

One sheet per candidate unit, saved with the same file naming, keeps the eventual side-by-side review honest. When three suppliers each send a polished brochure number, the fleet's own rows answer a question the brochures never do: what each unit actually did in this truck, in this bunk, in the modes the drivers select on a summer night.

Fill the sheet in one session per truck, at the same time of day where possible. A mode recorded at noon on hot asphalt and the same mode recorded at midnight on grass are not comparable, because the unit works against a different heat load and the background sound differs between the two sessions. Note the parking surface and the weather in the same row as the reading.

Add a column for what the meter cannot catch: a smell from the outlet, a shift in airflow direction, condensation around the housing, a rattle from the cab trim. These notes cost nothing during the session and frequently explain a number afterward. One complete sheet per truck is worth more than three perfect readings taken under conditions nobody recorded.

Repeat each reading and log the background

Take at least three readings per mode, a minute or two apart, and write down all of them instead of keeping only the best. Meter readings move with fan steps and compressor cycling, and a single number can land on a quiet or loud moment by chance. If one reading differs sharply from the other two, note what the unit was doing at that moment.

Record the background level with the unit switched off, at the same position, before and after the session. A rest area beside a highway and a quiet depot yard differ enormously, and a unit measured against a loud background can look deceptively unremarkable by comparison. The off reading anchors every other number in the sheet, so treat it as data, not paperwork.

Account for the vehicle's own sounds as well. A DC-DC converter, an inverter, a refrigerator compressor or a battery cooling fan each add noise of their own, and part of a unit's apparent loudness may belong to another component. Switch off or isolate whatever can safely be isolated, and record what remained running so the comparison credits or blames the right part.

Keep the raw readings with the date, time, mode and outside conditions in one place — a spreadsheet, a logbook page or the fleet maintenance system. Numbers that survive only in memory cannot settle a dispute six months later, and the complete record is what turns a noise complaint into a decision a supplier can actually act on.

  • Unit off, one minute settled, reading taken at the fixed position — before the session starts.
  • Unit off again after the last mode — a second background reading catches changing surroundings.
  • Parking surface, nearby walls or passing traffic, and the weather noted in the same row.
  • Every auxiliary load listed: inverter, DC-DC converter, refrigerator, battery fan, telematics box.
  • Auxiliaries switched off where it is safe, with anything left running written down explicitly.
  • All three raw readings per mode kept, including the outliers, with a note of what the unit was doing.
  • Date, time and cabin temperature logged beside each row so any session can be reconstructed later.
  • Battery voltage noted at the start and end of each session, since supply voltage can change fan and compressor behavior.
  • Any interruption — a fault code, a breaker trip, a manual stop — written into the row where it happened.

Judge tonal sounds separately from overall level

Two units with identical meter readings can feel entirely different at night. A broadband rush of air masks conversation and fades into the background, while a narrow whine or a rhythmic thump cuts through silence and keeps a light sleeper awake. Ask the level question and the tone question separately, and write down any tone you can hum or any pattern you can count.

Tonal sound usually traces to a component: a compressor whine, a fan blade passing its shroud at a fixed rate, a control box hum or a bracket resonating with the roof. Describe the pitch as low, medium or high and the pattern as steady, pulsed or intermittent. That description lets the supplier point to the likely source far faster than one averaged figure.

Separate the vehicle from the unit when a rattle appears. Trim panels, curtain rings, loose straps and items in overhead storage all respond to vibration, and they will follow the truck to the next unit if the housing is changed. Quiet the cabin contents first and retest before attributing the sound to the air conditioner itself.

When a tone is confirmed, retest once after the installation has been inspected. Mounting stiffness, panel contact and duct joints all shape what reaches the pillow, and a modest unit in a careful installation can beat a better unit in a careless one. The comparison should reward the combination the fleet will actually receive, not the housing alone.

  • Whine: a steady, narrow tone that rises slightly with compressor speed.
  • Rush: broadband airflow noise that changes with fan step rather than with compressor state.
  • Click or thump: a short event at compressor start or stop, sometimes with a brief vibration afterwards.
  • Hum: a low constant tone from a control box, converter or transformer rather than the refrigerant circuit.
  • Rattle: an irregular buzz from trim, brackets or stored items reacting to vibration.
  • Pulse: a repeating pattern with a rhythm a listener could count, often from a fan blade passing its shroud.
  • Hiss: refrigerant or airflow through the vents, usually noticed during mode changes rather than steady running.
  • Creak: a slow structural noise from roof or brackets as panels warm, cool and settle during the night.
  • Beat: two similar tones drifting in and out of step, often from two fans running at nearly the same speed.

Give the driver a structured listening script

A driver's impression is evidence when it is collected the same way every time. Ask the same questions after the same exposure, in the same order, and record the answers beside the meter readings. An unstructured comment such as “it seems fine” cannot be compared across units or across weeks, and it cannot defend a purchase decision later.

  1. Lie down in the usual sleeping position for ten minutes with the unit in maintenance mode.
  2. Rate how easy falling asleep tonight would be, on the fleet's own simple scale.
  3. Name every sound that can be identified: whine, rush, click, thump, rattle or hum.
  4. Say whether a draft reached the face or neck and whether it was unpleasant.
  5. Repeat the same answers after thirty minutes, not only after the first minute.
  6. State whether the sound would wake you once, repeatedly or not at all during a full rest period.

Treat the script as part of the test record, not a conversation. Collect it before drivers compare notes with each other, because a first impression told out loud is worth more than an opinion formed after hearing a colleague's verdict. File the answers with the meter sheet for the same session so the numbers and the experience stay attached to one unit and one set of conditions.

Keep comfort judgments separate from temperature judgments. A bunk that is too warm or too cold will be reported as uncomfortable regardless of the sound level, and the noise evidence becomes contaminated. Confirm the driver is satisfied with the temperature before recording the sound impression, and write the cabin temperature beside each answer so the two records stay apart.

Rotate drivers across the comparison only between complete unit sessions, never inside one. Different people hear differently, and mixing listeners within a single unit's record blurs the result. Record who rode in which truck, so a later reviewer can see whether a favorable score followed a particularly tolerant listener rather than a genuinely quiet installation.

Compare rooftop and split systems in their own layouts

A rooftop unit puts compressor and condenser fan directly above the bunk, separated from the sleeper by the roof structure; a split system leaves only the evaporator and its fans indoors and moves the noisy parts outside. The same comfort claim therefore reaches the pillow by different paths in the two layouts, and the numbers should be read with that in mind.

Compare like with like. A rooftop unit measured on one truck and a split system measured on another differ in roof construction, insulation, lining and mounting, so no conclusion about the hardware survives the comparison. If the fleet runs both layouts, keep two comparison sheets and judge each unit against its own group before any purchase is made.

Installation quality dominates the final part of the path. The same rooftop unit transmits noticeably more vibration through a stiff mounting than through a compliant one, and the noise reduction guide lists which design features to ask suppliers about. Judge the installed result first, then ask what a different installation would change.

Finish every session with both units off and the cab silent, then listen for what remains: a relay clicking, a control box humming, condensate dripping into its gutter. Small constant sounds matter more across an eight-hour rest than a loud number for thirty seconds at startup, and they only become audible when everything else has stopped.

  • Rooftop: compressor and condenser fan above the bunk; judge the roof structure, the mounting and the lining between housing and pillow.
  • Split: only the evaporator indoors; judge the indoor fans, the duct path and the routing of refrigerant lines and wiring to the outside unit.
  • Mounting points: the same positions and hardware type on every test vehicle wherever the layout allows.
  • Roof openings: no test unit installed beside a sunroof, antenna or vent that can reflect or channel sound into the sleeper.
  • Interior condition: the same curtains, bedding and storage load in every session, because soft furnishings absorb sound.
  • Separate sheets: one comparison table per layout type, never mixed into a single ranking.
  • Condensate routing: the drain path checked on every unit, since dripping water onto a step or tank adds its own noise at night.
  • Windows and curtains: the same closing arrangement in every session, because glass reflects sound back into the cab.
  • Engine-off condition: all sessions with the engine off, since idling noise changes both the background and the unit's load.

Next step

Compare the available configurations in the Vethy product catalog, then request the current model-specific documentation before selecting a unit.

Sources and method

Frequently asked questions

How many readings per mode do I need before the numbers mean anything?

Three readings per mode, taken a minute or two apart, is a practical minimum for a purchasing comparison. Write down all three rather than only the lowest, and if the spread is wide, take a fourth and note what the unit was doing, because a compressor restart or a fan step explains most outliers. A certified acoustic rating belongs in a qualified laboratory; this method ranks installed units against each other under identical conditions.

Can I compare noise results between two different trucks?

Only with caution. Trucks differ in roof construction, insulation, interior lining and parking surroundings, and every one of those differences changes what the driver hears. If the trucks are the same model from the same batch, a direct comparison is reasonable. If they differ, treat the results as separate case records and compare each truck only against itself, before and after any change to the unit or its installation.

What sound level is quiet enough for sleeping?

No single figure fits every driver, which is why the fleet should set its own target before testing starts. Light sleepers tolerate far less than an average suggests, and a tonal whine can disturb at a reading that looks harmless on paper. Agree on a maximum reading at pillow height, confirm it with the structured driver questions, and reject any unit whose tone keeps the test driver awake even when its number passes.

Can a phone app replace a sound meter?

A phone is genuinely useful for recording the sound so a supplier can hear the tone, the cycling and the rattle, and for spotting gross differences between modes. It is not a calibrated instrument, and readings vary between phones. If no proper meter is available, use one dedicated phone for every session, keep it in the fixed position, and present its figures as comparative notes rather than measurements.