Semi-Anechoic Chambers

Semi Anechoic Chambers

Much like full anechoic facilities, semi-anechoic chambers (sometimes referred to as Hemi-Anechoic Chambers) are lined with absorbent materials, however the floor is left as a flat finish with no absorption. Semi-anechoic chambers are more common and still allow very precise measurements to be taken, but the free-field area is only half a sphere (or hemisphere, hence the name Hemi-Anechoic). Hemi-anechoic chambers offer an ideal solution to testing large or heavy items as there is no need for a specialised floor to cover any wedges found in full anechoic laboratories.

Although some design issues are overcome with semi-anechoic chambers, facilities are usually large in size and have complex requirements to suit the product being examined. In the case of taking acoustic measurements for a large generator as an example, it is not simply a case of placing the item in the middle of the lab and taking the necessary measurements. The following are just some of the elements which enable such a facility to operate effectively:

Silent ventilation system capable of passing large volumes of air into the hemi-anechoic chamber when an engine is running

Fuel system for powering the generator whilst on test

A silenced extraction system for removing all of the noxious fumes from the enclosure

Heating / cooling system to ensure that a constant temperature and humidity are kept inside the chamber to get the most consistent results

A control and monitoring system for engineers to use to effectively run the hemi-anechoic chamber

All the above (and many more possible options) need to be carefully considered at the planning stage of the hemi-anechoic enclosure in order to get the best possible results for the end-user.

More About Semi-Anechoic Chambers and Applications

Semi-anechoic chambers are used very widely in automotive testing – noise, vibration and harshness (NVH) testing by both car and truck manufacturers and third party test houses. Also termed vehicle semi-anechoic chambers (VSACs), these are a standard laboratory for pinpointing sources of noise on a vehicle being operated on a dynamometer or rolling road.

Hemi-anechoic chambers are also used for Pass-By road tests, with exterior noise Pass-By standard ISO 362 for examining on an outside track, to measure the noise radiated from a vehicle to a stationary bystander on the street during an urban driving cycle.

Measurements in various indoor acoustic analysis rooms show good correlation to evaluations performed in open-air tracks. The principal criteria is that assessments be performed in an acoustic free field (semi-anechoic chamber) reproducing external free-field sound propagation within a controlled environment. This removes the risk of other extraneous noise affecting the result and enables repeatable and comparative measurements to be taken.

The hemi-anechoic space is achieved by lining a sufficiently large sound proof chamber with sound absorbing wedges for the lowest frequency of measurement interest. A Dynamometer test bench is then used to simulate the road operation of the vehicle. The vehicle’s radiated noise is measured using a roving microphone array, which collects time-based acoustic data. Movement of the vehicle past static microphones, as in the open-air test, is achieved by ‘pulsing’ the microphones, positioned along either side in the chamber, past the vehicle on the centrally located dynamometer.

For an industry standard low cut-off frequency of 50Hz, with a measuring envelope of 15.0m wide x 20.0m long x 4.5m high, we normally suggest a minimum room size of 18.0m wide x 23.0m long x 6.0m high, wedge tip to wedge tip. This roughly equates to an inside concrete chamber size of approximately 21.0m wide x 26.0m long x 7.5m high.

For lower cut-off frequencies, however, based on a 4-cylinder engine being examined having a lower engine speed of 1,000rpm then the lowest firing frequency is approximately 34Hz. To design a hemi-anechoic room with a lower frequency cut-off of 34Hz, the wedge depth should be ¼ of that wavelength at 20OC which equates to 2.6m. For this example, the inner dimensions of the hemi-anechoic chamber, wedge tip to wedge tip, should increase to 20.2m wide x 25.2m long x 7.6m high. The building shell dimensions would also increase accordingly.

For a high performance chamber, the walls, floor and ceiling would typically be of a double skin construction and utilise anti vibration mounts to de-couple the structure from the host building. Access is typically by back to back high performance acoustic doors lined with wedges on the internal face. Given the nature of the facilities in sealing the inside of the chamber from the outside world, a ventilation system is added to all anechoic chambers giving a silent source of fresh air at all times when in use.

Body in white, trimmed body, cabin cavity and car components modal properties determination (the frequencies on which there is a tendency for the structure to vibrate and the shapes into which the structure is, at the same time, deforming), design changes verifications, hot spot reasons recognition, correlation of passengers’ vibration comfort with the vehicle’s modal response, noise ratio identification transferred through the structure and through the air, vibration and noise path localisation from the sources (powertrain, wheels, aerodynamic noise) to the driver and passengers ears, finding the area of most effective intervention to prevent noise propagation, local body sensitivity measurements, panel contribution analysis, simulation model tuning and verification and scanning laser vibrometer measurement.

Shake & rattle or damping material labs are used for body leakproof and damping analysis, ultrasonic detector, smoke and endoscope tightness evaluation, insulation, damping and absorbing material application, acoustic treatment material absorption and insulation properties measurement, standalone (impedance tube) and apply to the car plus squeak & rattle analysis. Nominally 10.0m long x 7.0m wide x 3.5m high clear.

Quiet rooms or components labs are designed for NVH examinations of the substructures and components (HVAC, seats, wipers, windows, starter motors, alternators and the like), door slam noise measurement, ultrasonic detection, smoke and endoscope tightness assessment etc.

These acoustic rooms are ideal for sound quality analysis, jury evaluations, artificial head psychoacoustic measurement, subjective listening acoustic test preparation performance and evaluation. Nominally 8.0m x 5.0m x 3.0m high isolated structures.

QuietStar design and supply the ventilation systems so as to keep the full acoustic responsibility with one supplier.

Designed for use within a limited space, where control of noise, volume, temperature and pressure drop is a very specialised skill. With our ‘turnkey’ expertise, we are able to co-ordinate this with the Chassis Dynamometer supplier and user.

To fully simulate the open-air vehicle noise as described in ISO 362, the vehicle should be tested with the exhaust fully exposed and dumping into the anechoic space. This type of examination can lead to the dangerous collection of high levels of carbon monoxide and other harmful gases. For this reason, the hemi-anechoic chamber should be sealed to prevent leakage to the surrounding occupied areas. In addition, the facility should include a ventilation system capable of moving sufficient fresh air through the chamber to remove exhaust fumes, whilst maintaining the acoustic integrity and ambient noise requirements specific to the facility.

To prevent exhaust fumes re-circulating and affecting the driver’s health, it is essential to sweep the chamber’s entire cross-section with a flow great enough to take harmful fumes with it. The Pass-By chamber is approximately 18.0m wide x 6.0m high (108m2) at even at only 0.5m/sec velocity, the required volume equates to over 50m3/sec. Normal VSAC facilities are specified at approximately 40,000m3/hour (only 11.1m3/sec) and Pass-By’s are increased to 54,000m3/hour (15.0m3/sec).

There are normally five operating modes for chamber ventilation plus additional plant and controls for exhaust, dynamometer cooling, 24 hour pit extract, dyno pit ventilation and localised spot cooling systems. Also in the Control is an emergency shut-down mode to suit the Client’s requirements. The systems also include sensors for temperature, pressure plus feedback on damper and valve positions on both the air and mechanical systems.

The system allows different levels of access and control dependant on the user’s experience – more experienced test engineers can select a manual mode (via password override) that gives access to a ‘’Manual Mode’’ where the user can adjust volumes and temperature set points. The system will still prevent miss-use via safety protocols.

The standard 5 modes are:
General Ventilation
Minimum Forced Air Cooling with recirculation
Forced air cooling with recirculation
Maximum forced air cooling with recirculation
A control and monitoring system for engineers to use to effectively run the hemi-anechoic chamber
Tailpipe examination no recirculation (this can also be used as Purge)
Controls – Modulating valves, temperature controllers, pumps, strainers, orifice plates, buffer vessels and the like are budgetary at the moment until concept details are agreed with the client about the central energy plant.

All the necessary items to allow a smooth change in operation from general ventilation to FACD mode is included in our scope of supply. This operation uses motorised dampers and hard wiring to the variable speed fan drive with inverter control. The frequency inverter cabinet is used as a marshalling box for external control of operating mode, emergency shutdown, fire mode all being hard wired switch feeds (by others).

Exhaust extract – For the purpose of our quotation and technical review, we have used a 250kW power vehicle with an estimated exhaust mass flow of 0.25kg/s at 400O C. Based on this, a single inlet centrifugal fan is used whose duty is 1.67m3/s at a mixed gas temperature of 90O C. This condition is also based on 25O C air entrainment into the hot exhaust path to ensure a low operating temperature for the exhaust fan.

Diluted exhaust air is discharged to atmosphere at an appropriate height above ground.

Low level, high temperature ducting in the pit will be stainless steel and after the point where cool air is entrained the ducting shall be stainless steel. Low level ducting is laid to fall with drain points at the lowest level.

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