THE QUIET HOOD DESIGN
Key to designing the ultimate kitchen range hood and ventilation system is understanding the delicate balance of quietness and efficiency.
To get started, we will look at:
- Measuring Sound
- Things that effect the noise level
- Products to quiet the sound
- Manufacturer's Noise Ratings
Decibels or dB's as the stereo system aficionada would call them come from Bell Labs. Back in the 20's, engineers at Bell Telephone Laboratories developed a method to quantify the reduction of audio over a telephone cable detectable to the average listener. These units of measurement were adopted by international standards bodies and given the name decibel in honor of the telecommunications pioneer Alexander Graham Bell. Decibels or db's measure pressure; when used in acoustics to measure power levels, they become dBA's. The "zero" point on the scale is set at the lowest level where sound is detectable
Examples of Decibel Levels
|dB || ||Level Source of Sound |
|1 || ||Threshold of acute hearing|
|10 || ||Whisper, calm breathing - just audible|
|20 || ||Light rainfall, rustle of leaves - average person's hearing threshold|
|30 || ||Dripping faucet|
|40 || ||Quiet Office|
|45 || ||Household refrigerator|
|50 || ||Normal conversation, average office|
|55 || ||Average automobile idling|
|60 || ||Singing bird, TV at normal listening level, window air-conditioner|
|65 || ||Quiet Typewriter|
|70 || ||Automobile at 50 mph, 50 feet away|
|75 || ||Average factory vacuum cleaner|
|80 || ||Crowded restaurant, garbage disposal|
|89-90 || ||Major highway at 35 feet away|
|100 || ||Jack hammer at 3 feet away|
In 1936, American psychologist, Stanley Smith Stevens proposed the sone as a psycho-acoustical measurement of sound. Generally, the idea was to establish a unit of measurement for loudness. A group of folks were played tones starting at the lowest level that can be heard (frequency of 1000 hertz and a sound pressure level of 40 decibels). People were tested individually and they judged the relative "loudness" of each tone. So for example a tone at 4 sones was perceived twice as loud as a tone at 2 sones.
Key to this description is that this measurement is subjective. Imagine your stereo volume sound control set at 4 (on an indicator level of 0-10). If you turn the treble say from 5 up to 10, without touching the volume knob, you'll find the music to be louder as a result of higher frequency.
Examples of Sone Levels
|Sone || ||Level Source of Sound |
|0 || ||Threshold of acute hearing|
|.02 || ||Leaves rustling, calm breathing|
|.15-0.4 || ||Very Calm room|
|1-4 || ||Normal Talking at a distance of 3 feet|
|4 || ||TV set normal volume at a distance of 3 feet|
|4-16 || ||Passenger car at a distance of 33 feet|
|16-32 || ||Major highway at a distance of 33 feet|
FACTORS THAT EFFECT THE SOUND LEVEL
The operating sound level of a kitchen range hood is dramatically affected by the size of the duct through which the air is being exhausted, the total amount of filter area in the hood, the type/design of the grease filter in the hood, the number and proximity of elbows (turns) in the ducting to the hood, and the loudness of the blower motor
- Duct Size - generally, the smaller the duct diameter, the greater the sound. Always use the manufacturer's recommended minimum duct size.
- Air Velocity (CFM's) Vs. Duct Size - Cubic feet per minute (CFM) measure the amount of air moving through the ventilation system each minute. As the amount of CFM is increased, the louder the sound of the air moving. The CFM capacity of the hood should be matched to the size of the hood. Generally there should be from 125 CFM per square foot of the bottom area of the hood (a hood measuring 4' wide x 2' deep should have 1000 CFM). Always use the manufacturer's recommended duct size.
- Filter Area & Size - the greater the area covered by filters in the hood bottom, the quieter the operating sound. Filler or spacer panels between filters reduce the total filter area and increase the sound level.
- Filter Design - the smaller the openings in the filters, the higher the sound level.
- Duct Work - ducting should be round and any turns in the ductwork should be with smooth round elbows. Square elbows or elbows set too closely together, not only increase the noise level, but reduce the efficiency of the ventilation.
- Ventilator Motor Design & Control - There is a delicate balance between the ventilator motor, the fan blade it drives and the housing that holds the components. A 400 CFM fan can be louder than a 600 CFM fan. The speed control of a fan can range from on/off, to a 2 -, 3 - , or 4 - speed control, to an infinitely variable speed control. The higher the speed of the fan, the louder.
NOISE REDUCTION ACCESSORIES
There are products that can be included in the ventilation hood design to reduce the operating noise level of the system.
- Duct Mufflers - Sound attenuating mufflers can be added to the ducting that can reduce the operating noise of a remote ventilator (motor mounted on the roof or in an attic or similar space) by more than 50%.
- Sound Deadening Pads - self-adhesive sound absorption pads can be applied the interior of a hood's liner or hood body, or ductwork to reduce harmonic resonance caused by ventilator motors, air movement, operating vibrations.
- Coatings - paint on sound absorbent materials are an alternate to the sound deadening pads
Sones are used by many kitchen range hood manufacturers as a measure to compare the relative loudness of their product. ABBAKA specifically uses decibels to avoid the bias of perception. One might also note that dishwasher manufacturers (where the battle for quiet took on new dimensions) use decibels exclusively.
BY KITCHEN RANGE HOOD MANUFACTURERS
ORDERING THE ULTIMATE IN QUIET HOODS BY ABBAKA
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- Send us an e-mail to email@example.com, and please put ABBAKA Quiet Hood Design in the subject line
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