ISTIQ Noise Control Weblog

Room acoustics describes how sound behaves in an enclosed space.

The way that sounds behave in a room can be broken up into roughly four different frequency zones:

• The first zone is below the frequency that has a wavelength of twice the longest length of the room. In this zone sound behaves very much like changes in static air pressure.
• Above that zone, until the frequency is approximately 11,250(RT60/V)^1/2, wavelengths are comparable to the dimensions of the room, and so room resonances dominate.
• The third region which extends approximately 2 octaves is a transition to the fourth zone.
• In the fourth zone, sounds behave like rays of light bouncing around the room.

Natural Modes

Computer plot of the application of criteria to a room with correct dimensions

Computer plot of the application of criteria to a room with bad dimensions

Typical acoustical absorption of the standard panels developed by Oscar Bonello

The sound wave has reflections at the walls, floor and ceiling of the room. The incident wave then has interference with the reflected one. This action creates standing waves (described at Standing wave) that generate nodes and high pressure zones. The mode spacing is a very important matter especially in small and medium size rooms like recording studios, home theaters, broadcasting studios and concert halls. Some methods were created from 1940 - 1981 to obtain the best mode spacing to avoid sound coloration, but none of them succeed, because the mode spacing is not only a geometric problem  An incorrect mode spacing in a room determinates a bad frequency response from a loudspeaker or a music player

In order to solve this problem, Oscar Bonello, professor at the University of Buenos Aires, created in 1981 the Modal Density concept that solves the problem introducing concepts from Psychoacoustics  This new Bonello’s Criterion as it was named, analyzes the first 48 room modes and plot the numer of modes in each one-third of octave. The curve must increase monotonically (each one-third of octave must have more modes than the preceding one). This Criterion is now the standard method of designing the room dimensions.

Reverberation of the room

After determining the best dimensions of the room, using Modes Density criteria, the next step is finding the correct reverberation time. A good explanation of the theory can be found at Reverberation. The reverberation time depend on the use of the room. Times about 1.5 to 2 seconds are needed for Opera Theaters and Concert Halls. For Broadcasting & Recording studios and Conference rooms, values under one second are frequently used. The recommended Rev Time is always functioning of the volume of the room. Several authors give their recommendations A good approximation for Broadcasting Studios and Conference Rooms is: TR[1khz] = [0,4 log (V+62)] – 0,38 TR in seconds and V=volume of the room in m3  The ideal RT must have the same value at all frequencies from 30 to 12.000 Hz Or at least, is acceptable to have a linear rising from 100% at 500 Hz to 150 % down to 62 Hz

In order to get the calculated RT in a room, several acoustics materials can be uses as described in several books. A valuable simplification of the task was proposed by Oscar Bonello in 1979. It consists of using standard acoustic panels of 1 m2 hanged from the walls of the room. These panels use a combination of three Helmholtz resonators and wooden resonant panel. This system gives a large acoustic absorption at low frequencies (under 500 Hz) and reduces at high frequencies to compensate the typical absorption of people, lateral surfaces, ceilings, etc

GENERATOR SETS ACOUSTIC TREATMENT

Introduction

Generator Sets in general, produce an overall noise level in the range of 100 to 115 dB(A), depending on the rating and manufacturer. The higher the rating; the higher the noise level produces by the genset.

A typical design of Acoustic treatment for genset  room normally will reduce the noise level down to 75 to 85 dB(A) measured at 1 meter away from the genset house. However, the noise can be designed to achieve better if the requirement call for it.

In Malaysia, the Department of Environment (DOE) guidelines on the Acoustic Treatment is to control the noise level to 65 dB(A) during daytime and 55 dB(A) during nighttime, measured at the boundary of premises.

Control At Source

The best way to treat noise is always to do it at the source and for Generator Sets, there are three major noise sources produce by the gensets: -

• Noise emitted by the Radiator Fan
• Noise emitted by the Exhaust System
• Noise emitted by mechanical engine itself

Acoustic Equipment for Noise Control treatment in the generator sets room mainly consists of the following: -

• Fan Discharge Silencers
• Intake Silencers
• Exhaust Silencers
• Acoustic Doors

Except for the door, the rest of the equipment are in static condition where there is no moving part in the inside or outside of the Acoustic Equipment. 

FAN DISCHARGE SILENCERS

The silencer is normally installed in front of the radiator fan in order to control the noise emitted by the fan. How much noise to be reduced will depend on the length of the silencer; the longer the silencer, the better the attenuation and thus, more noise will be reduced. However, it will increase the pressure drop across the silencer.

The width and height of the silencer will depend on the airflow of the fan. Higher airflow require the size of the silencer to be bigger.

Discharge Silencers should be not be installed too close to the fan to avoid noise regenerated by the fan blades. Flexible coupling should be used between the silencers and adjoining radiator where there is a possibility of vibration transmission.

INTAKE SILENCERS

Since the generator sets required air ventilation for cooling and combustion purposes, an opening at the generator set house is required. However, this will let the noise escape out from the room. Intake Silencers, consequently is used to serve these ends.

It is highly recommended that the Intake Silencers are not to be installed with the same side with the Discharge Silencer. This is to prevent the “short circuit” of the ventilation.  

Intake Silencer is normally position at the high level of the room.

EXHAUST SILENCERS & SYSTEM

Exhaust system of the generator set produce the highest noise level of all the three sources. The noise level for the exhaust is typically around 110 - 115 dB(A).

When designing an exhaust system there are a number of factors that should be considered to maximize the design in terms of performance and cost. A few of the items that should be taken into consideration include: -

Acoustic Environment into which exhaust is discharging

• Engine Rating, i.e. - exhaust flow
• allowable engine back pressure
• engine exhaust outlet noise level

Engine Exhaust Outlet Design, i.e. single or dual outlets

• Exhaust Piping Arrangement within engine room
• Exhaust Piping Support, Anchoring, and Guiding
• Exhaust Stack Height
• Configuration of Exhaust Silencers, i.e. single or dual inlet

ISTIQ Exhaust Silencers come in standard, stock silencers for general-purpose applications and custom models to fit most OEM engines. These include: -

Model IXP - Multi-Chamber Reactive Type Attenuation - 30 dB     Model IXS - Straight Through Absorptive Type Attenuation - 20 dB

Model IXC - Supercritical or Combination Attenuation - 35 dB

ACOUSTIC DOORS

The requirement of Acoustic Door to be incorporated at the generator set room is obviously to prevent the noise from transmitting through the doorways.

ISTIQ Acoustic Doors used for generator sets are all steel doors utilising the absorptive edge principle. They are used to contain the noise within noisy areas or to prevent noise transmission to sensitive areas.

DUCT SILENCER

Whenever ventilation is required in the enclosed compartment and at the same time noise is to be controlled, installing duct silencers or attenuator are the best way to solve the problem. The silencers permit air to flow through while reduce the noise by absorbing it.

In selecting a silencer, a balance between noise attenuations and pressure drop need to be considered carefully. By using internally developed software, ISTIQ Noise Control can help to make the right choice quickly and most importantly, accurately.

Introduction

ISTIQ Duct Silencers are designed to efficiently reduce noise by generator sets, air handling systems, compressor, blower system and all situations requiring air intake and outlet for these equipments to operate.

A duct silencer should always positioned in the plant room and if possible as close to the noise source as possible. Normally for a single equipment, two units of duct silencers - intake and discharge is required to effectively reduce the noise to a desired level.

Construction

ISTIQ Duct Silencers consist of a galvanised sheet metal case containing two side panels and one or more splitters. Normal low pressure silencers uses Pittsburgh  seams with attached angle frames step welded to the case. Duct sealer is then used to seal to ensure that the silencer is airtight. For high pressure system, the silencer has fully welded seams. The angle frame are protected by a paint finish.

Inside the splitters, rockwool or fibre glass is used as sound absorptive infill and is held in the splitters inside a perforated galvanised steel skin. Different  acoustic infill is used depending on the application to ensure excellent performance and its effectiveness.

For heavier industrial or marine application, enhanced construction and corrosion resistance are required.  ISTIQ Engineers will be glad to discuss and advice your specific needs regarding the design of the structural, aerodynamic and corrosion protection requirement.

Features

ISTIQ Duct Silencers are available in two different models, which basically have two different thicknesses of splitters - 200 mm and 300 mm. The thicker the splitter the better performance at lower frequencies. Each of the models have six length sizes from  900 to 3000 mm. The cross sectional  area has no limited size. The performance or the insertion loss of each model is shown in the table 1.

ISTIQ Duct Silencers will be supplied in sections whenever the silencer dimension exceeded 2100W x 2100H x 2100mmL. This will make the transportation and installation easier. The assembly of sectionalized silencers to be done at site.

SELECTION GUIDELINES

The right selection of silencer should gives the essential attenuation at a permissible amount of pressure drop of a given air requirement. The selection method therefore can be simplified as follows :-

• Establish the noise target. Noise target could be in noise overall value or in NC criteria as recommended in Table 1.
• Obtain the noise level of the source,
• Calculate the difference of the level between the noise target and the noise source. Should there is any distance involve or any condition which noise will reduce naturally, it should be considered.
• Match up the difference of the level with the insertion loss of ISTIQ silencer and select the model as shown in Table 2.
• The silencer can be sized up by knowing the airflow requirement and the allowable pressure drop across the silencer by using the formulae below :

                Pressure Drop   = kv²

                where v                = Air Flow in m³/s

                                                   Width x Height in m²

                And k as listed in Table 3.

For detailed procedures of calculating the required attenuation, ISTIQ Engineers will be glad to help.