Noise Control in High-FFU Density ISO 5 Cleanrooms: Designing for Worker Safety

Introduction

In an iso 5 semiconductor cleanroom, dozens, or even hundreds of Fan Filter Units (FFUs), are located in the ceiling. Although each FFU might individually appear harmless, cumulatively they create a persistent, broadband background hum due to combined fan noise, airflow turbulence and structural vibration.

This problem of noise in clean rooms is compounded by the discomfort it causes. Workers need to yell to make themselves heard. Fatigue and concentration deterioration are typical after long shifts, and alarms and equipment anomalies are often overlooked. Maintenance workers experience even worse localized sound levels when they work under the FFUs.

This guide provides a systematic approach to the challenges posed by FFU sound levels, focusing on practical, noise control, engineering informed by an understanding of the pertinent regulatory environment.

Figure 1: High-density FFU arrangement — multiple noise sources combine to raise the room's background sound level.

What Is High-FFU Density Cleanroom

FFU Basics

A Fan Filter Unit includes a fan, HEPA/ULPA filter, housing, control module and inlet/outlet plenums. Its main purpose is to provide filtered air to the clean space and to do so, maintain air uniformity and cleanliness.

Why iso 5 environments are more susceptible to noise problems

ISO 5 clean rooms are more susceptible to noise issues for multiple reasons. In order to accommodate uniform air flow and specific air changes, many terminal units must be utilized.

• The noise level for a single fan-filter unit (FFU) does not apply to the entire clean room.
• When multiple units are required, this results in an elevated background noise level.
• The FFU noise will only increase as the filters load and the FFU works harder to compensate for the increase.
• Poor acoustic design becomes noticeable and costly once the clean room is operational.

Common Uses

• Manufacturing semiconductors
• Making display panels
• Assembly of electronics
• Manufacturing of biopharmaceuticals
• Manufacturing of medical devices
• High quality production of food and laboratories
• Manufacturing in the field of aerospace

Common sources of FFU Noise

Noise reduction for clean rooms should begin with an understanding of the sources of noise. Noise caused by ffus is never a single noise, as it's caused by a combination of mechanical, aerodynamics and structural sources.

Motor and Fan Mechanical Noise

• Electromagnetic hum and bearing noise from the motor
• Flying impeller due to tolerances and imbalances
• Vibrations that travel from the rotating assemblies to the casing
• Structural resonance of the fan casing

Aerodynamic Noise

• Blade passing frequency with tonal noise
• High velocity air flow through the filter media
• Turbulence at the inlet or outlet grills
• Turbulence due to an abrupt change in the duct cross-section or the position of a damper

Structure-Borne Sound and Vibration

• Vibration on the ceiling caused by the mounting of an FFU
• Resonance of the mounting or support system
• Transmission through duct either tray or cable trays
• Coupling of adjacent units through the building structure

Filter Resistance and Maintenance State

Noise management cannot only depend on the nameplate sound level of the fan and the change of pressure in the filter directly effects the operating point of the fan and the sound emitted.

Paths of noise:

• Motor / impeller →
• Housing vibration and air turbulence →
• Ceiling, plenum, and duct →
• Room reflections & superposition →
• Worker ear level

Figure 2: FFU noise generation and how it works

Single Noise FFU vs Total Room Sound

Single FFU Sound Level vs Room Sound Level

Product data sheets include sound pressure levels measured a set distance away (e.g. 1.5 m) from the unit in free-field cases. In practice, there are many variables in the room that affect and change the sound level (e.g., existing reflections, many sound sources, room reverberation).

• Tested sound level (lab conditions)
• Installed sound level (in-situ)
• Room average sound pressure level
• Ear level exposure
• Local sound level directly under the FFU
• A-weighted and C-weighted sound levels

Basic Idea of Sound Level Summation

When similar sound sources are present, the total room sound pressure level is not a linear summation. It is a logarithmic summation and is dependent on a number of variables including the number of sources, their distance, sound absorption of the room, and the sound source.

For n identical uncorrelated sound level sources, the increased sound pressure is about 10 × log₁₀(n) dB. In the case of 4 FFUs, the total sound level is 6 dB higher; 16 total FFUs would be 12 dB higher than a single FFU.

Noise Criterion (NC) Curves – Why dBA Is Not Sufficient

In semiconductor fabrication facilities, total dBA is not the only number of concern. Tools such as Wafer Steppers and Lithography Scanners are very sensitive to low-frequency vibrations. When it comes to low-frequency noise, the NC (Noise Criterion) and NR (Noise Rating) curves look at octave bands of noise, particularly the 63 Hz and 125 Hz bands, where A-weighting underrepresent the noise energy in those bands.

For iso 5 cleanrooms, design benchmarks are typically NC-55 for normal production and NC-60 for areas with higher airflow (i.e. air change) activities. For areas with low-frequency producers, NC-50 or lower may be applicable. Vibration may be the real concern, not airborne noise in these cases. It is important to note that low-frequency noise may be significant even if a noise source is classified as dBA 65.

* NC values per ASHRAE Handbook. Always validate with on-site octave-band measurements.

Reasons Why Low-Frequency Noise Is Ignored

• Low-frequency noise is less directional and impossible to localize precisely.
• Low-frequency sound in the environment is lower due to A-weighting.
• Low-frequency sounds quickly transmit through the structure of buildings.
• Building occupants may present symptoms of fatigue but are unable to pinpoint the cause.

Consider octave band or 1/3-octave band analysis. The use of a singular dB(A) assessment is highly insufficient.

Noise Compliance with OSHA and Worker Safety

OSHA's noise compliance (29 CFR 1910.95) is an exposure assessment of noise, so where, how loud, and for how long in each case a duty is performed comes into play for compliance.

Single Decibel Assessments with OSHA

• dBA and exposure time,
• location of the worker with respect to the noise source,
• presence of protective hearing devices and their attenuation,
• the presence of peak or impulsive noise,
• the necessity of a hearing conservation program,
• audiometric assessment and training,
• the potential for the exposure to noise to cause injury.

Best Practices for On-Site Noise Measurement

• Classify noise measurement and worker engagement zones.
• Validate the operation and the air control settings for the fan filter units (FFUs) in use.
• A type I or type II calibrated sound level meter should be used for measurement.
• Poll A-weighting and octave band data.
• Analyze noise under the FFUs were in the idle, normal operation, and peak load modes.
• Use the timing of the load cycles to provide an estimate of noise exposure to the worker for the full operational day.
• Use a profile noise measurement prior to and subsequent to the modifications made to the facility.

Recommended Testing Areas

• Position of Operators' Ears
• Directly under FFU clusters
• Aisles among equipment rows
• Return air corridor regions
• Maintenance corridors
• Breakroom and control room

Noise-control in cleanroom design

Noise-control in cleanrooms emphasizes a hierarchy consisting of source control, path interruption, and the shielding of receivers, all of which must be compatible with cleanroom integrity.

Source Control

• Design selections must include low noise level frequencies, use of high efficiency EC motors that control internal speed, improved design of impellers, and reduction of excess fan speeds. Vent filters should be replaced once the filter begins to exhibit a flow resistance.

Path Control

• Vibration isolators should be used in the mounting of fan units, flexible connections used in the ducting for air handling units and fan units, panels should be coated to reduce their noise and their resonance. Where appropriate, air pathways should be designed to allow sound absorption. The clean room should be separated from the mechanical room with sound rated walls.

Receiver Protection

• High noise level clusters should be shielded and relocated. Design selections should provide quiet break areas. A barrier should be designed to provide protection to the receiver and should be located in such a way that the barrier does not impede or interrupt airflow.

Why Ordinary Acoustic Foam Is Not Suitable

• Ordinary acoustic foam is neither suitable for cleanrooms nor controlled environments because of its low particle generation, its inability to clean or be compatible with cleanroom chemicals, its maintenance burden and associated costs, its potential to alter pressure differentials, and its potential to impede airflow. Acoustic treatments in ISO 5 spaces must be certified for low particle generation, cleanability, fire resistance, and chemical compatibility.

Choosing Acoustic Panels for Clean Rooms

When you pick a panel for a clean room, you need to assess their capability to control sound, their cleanability, their safety in the event of a fire, the affect they would have on the flow of air, and other factors. The chart below presents basics of what you need to know for each.

Material Specifications for clean room iso 5

• Micro-perforated 316L stainless steel Panels – Micro-perforated 316L stainless steel panels are cleanable, and resist corrosion. Perforation rate greater than 20% is required for sound absorption.
• PTFE-coated Acoustic Batting – PTFE-coated acoustic batting sites at the ISO 14644-14. This makes them ideal for areas with high moisture.
• Aluminum Honeycomb Sandwich Panels – These panels are rigid, lightweight, and fire rated to be safe. Containing foam to help absorption of sound, these panels do not shed fibers.
• These panels should have the necessary certifications for GMP and ISO 14644-14 compliance with respect to particle generation and chemical compatibility.

Where These Panels Can be Placed

• Where lower wall panel sections are not in contact with critical airflow
• In the equipment plenum and air return corridors
• In air handling unit rooms and the fan decks
• In localized zones near the work stations (with airflow verification)

Deiiang™ Technical Solutions

Deiiang™ sees the control of cleanroom noise as an integrated system. This perspective encompasses numerous aspects ranging from air handling and static pressure control, to the choice of FFUs and on-site acoustic assessments.

EC/DC Motor Control

• Deiiang™ has pioneered an innovative technology that allows for the control of fans based on the needs of the system at any point in time. This results in decreased noise levels and increased energy savings.

Low-Pressure Drop Airway Design

• Deiiang™ has developed an innovative design that decreases control system resistance. The design relies upon strategically layered filters, grilles, and coils. This design results in decreased energy savings and lowers the level of aerodynamic noise.

Intelligent Control Systems

• Aerodynamically, sound, and airflow act in accordance to one another. Noise varies based on airflow. With a Controlled Demand Ventilation Design (CDVD) intervention, Deiiang™ is able to mitigate noise. Deiiang™ is able to reduce noise through aerodynamically controlled demand ventilation and pressure control, along with alarms upon the differential pressure levels of the filters. Deiiang also provides data for planning maintenance.

CFD & CAA Coupled Analysis

This brings together conventional Computational Fluid Dynamics (CFD) with Computational Aeroacoustics (CAA) to model the interaction of airflow velocity with sound. This ensures that any of the planned solutions such as panels, baffles, or absorbers, do not create recirculation zones or dead spots that could compromise cleanliness or pressure differentials. Our simulations verify that the added acoustic components do not significantly affect the flow structure required to meet ISO 5 requirements.

AHU–FFU Co-Design

AHUs are used to treat bulk air, while FFUs are utilized for final distribution. Mismatched static pressure or airflow setpoints can create over-pressurization, wasted energy, and elevated noise. Deiiang™ offers coordinated design and commissioning to mitigate the above concerns.

Deiiang™ Case Study

Deiiang™ collaborates with facility teams to identify and reduce FFU noise in high-density cleanrooms. The following case study provides an overview of the selected methodology, and specific project details can be provided upon request.

Subject: Noise Control in an ISO 5 Semiconductor Cleanroom

• Sector: Semiconductor manufacturing
• Cleanroom area: ~800 m²
• FFU count: 286
• AHU: 4 primary units with VFDs.
• Objective: Alleviate operator complaints of fatigue and challenges in perceiving alarm signals.

Diagnostic Phase

• Mapping out how sound levels change across 12 positions
• Logging FFU speed vs filter pressure drop
• Identifying the most prominent frequencies using Octave band analysis
• measuring vibration on ceiling grid and FFU housings

Intervention

• Balancing AHU static pressure to mitigate over-supply
• Utilizing real-time data to set FFU speed demand
• Install cleanroom-compatible acoustic panels in non-critical areas
• Upper walls and return plenum
• FFU mounts received vibration isolators
• Targeted solution: Mitigated T-grid gaps with custom EPDM gaskets

Results

Fig. 3. On-site noise mapping and verification after acoustic treatment

Key takeaway: Creating the best solution involves optimization, balancing static pressure and airflow, combined with tactical FFU noise solution. The problem identified was the FFUs, so addressing just FFU noise was inadequate.

Typical User Scenario

David — Manager, Semiconductor Fab Facility Engineering

• Located in a high-tech manufacture area
• Cleans 5 ISO with multiple FFUs
• Accountable for cleanliness and pressure differentials, EHS and employee satisfaction

David's Pain Points

• Post hoc line extension caused a 40 % increase in FFU count
• Employee complaints of continual low-frequency rumbling
• Widely variable measurements throughout the room
• Filter changes halted increasing noise levels for only a short duration.
• Because of cleanroom constraints, the standard office acoustic products cannot be used.
• Retrofitting should be done with the utmost care to maintain ISO 5 certification.

Common Pricing Errors

Micro-Glossary

FFU (Fan Filter Unit)

An integrated terminal unit that has a fan and either a HEPA or ULPA filter used to provide clean air to cleanrooms.

ISO 5 Cleanroom

A Cleanroom that has ≤ 3,520 particles (≥ 0.5 µm) per m³, as per ISO 14644-1. Such a Cleanroom must have high rates of airflow and uniformity.

NC (Noise Criterion) Curve

A set of curves used to determine the maximum allowable sound pressure level in octave band measurements. These curves are used for evaluating low frequency ranges of noise in various types of sensitive situations.

CAA (Computational Aeroacoustics)

A technique for simulating the noise that is generated within a flow system by using propogation of sound waves in connection with the flow simulation.

A-weighted Sound Level

Accounts for human ear response to varying frequency sound levels; used for assessing workplace noise.

Sound Pressure Level (L_p)

Determined at an individual measurement point; used for exposure assessment of employees.

Static Pressure

The air system creates opposition that a fan needs to work against. The higher the fan moves, the air system's resistance, and noise increases.

Sound Power Level (L_W)

Emitted sound from a source regardless of its surrounding that is fully characterized; a tool for evaluating and comparing products.

Recommended Tools and Resources

FFU Noise Self-Diagnosis Tool

Answer a few questions to get a preliminary diagnostic direction. This tool is based on Deiiang™ field experience and helps you identify the most probable root cause of your cleanroom noise issue.

Additional Resources

• Cleanroom Noise Survey Checklist — test point planning, FFU status, calibration, exposure time, before/after comparison. Download PDF (placeholder)
• FFU Noise Risk Assessment Template — FFU count, model, airflow, static pressure, filter ΔP, measured levels, risk rating, recommended actions. Download PDF (placeholder)
• Cleanroom Acoustic Design Consultation — Deiiang™ offers engineering support for noise assessment and mitigation.

External References

• OSHA Noise and Hearing Conservation
• NIOSH Occupational Noise
• ISO 14644-1 Cleanrooms and Controlled Environments
• IEST Recommended Practices (including RP-CC002.3)
• ANSI/ASA Acoustic Standards

Deiiang™ · Cleanroom Acoustic Engineering · Product Designer: Jason Peng

Information provided for general reference. Always verify performance data with official Deiiang documentation and project-specific measurements.