Wednesday, December 11, 2019

Acoustical Effects of Liners in Terminal Units

We take lots of questions from designers and acoustical consultants regarding how terminal unit liners affect room sound levels. As you are probably aware, our published sound levels are based on ½” thick dual density fiberglass or EcoShield unless otherwise stated. Our TEAMS selection software adjusts the acoustical performance based on the lining material, but still many people question these results. So let’s look at the various liners.

Standard Liners – Most people consider these to be ½” dual density fiberglass or EcoShield. These materials perform almost identically with respect to both thermal insulation and acoustical performance. We therefore do not distinguish between the two when estimating sound levels.

Heavier Liners – When additional protection against condensation is desired, most people opt to go with 1” dual density fiberglass or EcoShield. For applications involving attenuators and/or fan-powered units, the additional thickness may provide reduce radiated and discharge sound levels.

Foil Liners – For critical environment applications, most people choose either SteriLoc or foil-faced EcoShield. These liners are intended to prevent insulation fibers from reaching the air stream. For this reason, these liners are installed using galvanized Z-brackets and foil tape to seal all cut edges. The foil covering reduces the sound absorption while slightly increasing the transmission loss of the casing. This could result in slightly higher discharge sound levels and slightly lower radiated sound levels.

Dual Wall – For specifications calling for dual wall construction, we offer UltraLoc. UltraLoc is solid 22g. galvanized steel over 1” dual density fiberglass. This liner is considered a heavy-duty alternative to the foil-faced liners. It is intended to prevent insulation fibers from reaching the air stream, but it is less susceptible to damage in the field. The solid inner wall prevents any sound absorption and greatly increases the transmission loss of the casing. This generally results in higher discharge sound levels and lower radiated sound levels.

Engineered Polymer Foam Insulation – For specifications calling for non-fiberglass liners, we offer FibreFree. This is a polymer foam insulation. It provides a reasonable amount of sound absorption while its density provides a slight increase in transmission loss to the casing. For most customers, the sound difference between this liner and exposed fiberglass or EcoShield would be negligible. It should be noted that EcoShield is a lower cost option when meeting non-fiberglass spec requirements.

There are many reasons why people question sound performance with regard to liners. For instance, switching the liner from ½” EcoShield to 1” EcoShield doesn’t change the sound levels of a DESV without an integral attenuator. This is surprising, but it doesn’t have any effect because the damper is at the discharge end of the casing. This means that the air stream never really comes in contact with the liner, so there’s no effect on discharge sound levels. And the extra thickness doesn’t increase the transmission loss of the casing enough to reduce the radiated sound level by any measurable amount.

If we look at that same DESV with an integral attenuator, going from ½” EcoShield to 1” EcoShield still doesn’t change the radiated sound level, but the additional casing length allows the choice of liner to reduce discharge sound levels. Sometimes people question why a particular liner doesn’t seem to provide the expected drop in sound level. That often occurs only looking at sound levels in terms of noise criteria (NC) levels. Very often sound reductions may be occurring in so-called non-critical octave band frequencies that do not change the overall room sound level. This can be deceptive because although the NC may remain the same, the overall resulting room sound quality may be improved by removing annoying high frequency tones or sound in the speech interference bands (500, 1000, 2000 Hz).

You might wonder about the effects of heavier critical environment liners like foil-faced EcoShield, SteriLoc, and UltraLoc. These liners are thicker and have a hard facing, so they tend to block or reflect sound. They don’t absorb sound energy, so they often increase discharge sound levels and render optional attenuators ineffective. They do however add enough transmission loss to the casing to lower radiated sound levels. The same is true to lesser extent for our FibreFree material. It provides sound absorption to a lesser extent than a soft liner, but the density provides some reduction in radiated sound levels.

Dual duct terminals are available with integral mixer/attenuators. The main purpose of this feature is to mix hot and cold airstreams together. Any resulting sound attenuation really only serves to reduce the amount of noise generated by the mixing process, so bear this in mind. These are really air mixers and not attenuators.

Fan-powered terminals, both series and parallel-flow, are affected differently by liner selections. Soft liners like fiberglass and EcoShield allow sound to radiate in all directions with a little additional sound coming from the induction port. Increasing the thickness of a soft liner will tend to provide a slight decrease (1-2 NC points) in radiated sound.

Estimating the effects of a high transmission loss liner like UltraLoc in a fan-powered terminal is very difficult for several reasons. First of all, this type of liner tends to block sound radiating from the top and side panels. That can reduce the chance of having low frequency noise coming out the bottom of the unit, but it tends to create a very directional and concentrated noise from the induction port. If this directional noise travels across the ceiling and is absorbed by the ceiling plenum, room sound levels could be very low. If this sound reflects off nearby ductwork or is contained by a constricted plenum space, the room sound levels could be higher.

Lab testing in accordance with ASHRAE Standard 130 is of little use when predicting the effect of these liners, because these products are tested in reverberant chambers. This type of sound room removes all directionality from the sound source, so it isn’t very useful when dealing with a directional sound source. Therefore the best way to determine the performance of a fan-powered terminal with a high transmission loss casing is through mock-up room testing. Titus offers this service to our customers whenever it is required.

Hopefully this information will help improve your understanding of terminal unit liner options and how they may impact the acoustical performance when making product selections.

For information on this topic, please contact Randy Zimmerman at or Titus Communications at