Do you HEAR what I HEAR? HVAC Acoustics 101 - Getting Back To Basics...

In this podcast, we're going to explore the importance of Acoustics in HVAC and how it relates to air distribution in the system. This is the audio version of a LIVE online event with our partners at BOSCH Thermotechnologies. 

Watch Full Video Recoding on our YouTube Channel: https://youtu.be/iUWsKqY_zPo

Noise is a topic that plays a major role in any HVAC system yet it is a topic that is either misunderstood or not very well understood in the HVAC world. People constantly confuse key terms, definitions, and commonly rated values. 

Some examples of those terms are sound unit dB vs. dB(A); HVAC Sound Pressure vs. HVAC Sound Power; AHRI 260.2011 vs. AHRI 260.2017, A-Weighted Sound, to name a few.

So, in this course, we will go back to the basics of sound where we will define commonly used key terms, review the concepts of acoustics and frequency, understand the difference between sound pressure and sound power, and review how sound is measured in the HVAC units. We will wrap up with some case studies which will help to determine how room construction and unit construction can affect the NC (noise criteria) rating of a given room.

We will also show real-life examples of mitigating HVAC Equipment Noise. 

Questions? Email Tony Mormino at [email protected]

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What is a Decibel

A pressure variation in a sound wave can be measured in micro Pascal's to determine its loudness. The greater the pressure variation there is in a sound wave, the louder it'll be. The micro Pascal is an actual physical measurement of the pressure variation in a sound wave. 

the weakest sound detectable by the human ear is 20 micro Pascal's. 

To put this into perspective a pressure change at this level causes the eardrum to displace less than the diameter of a hydrogen molecule. At the same time, the ear can also withstand pressure changes more than a million times this lower threshold. This type of measurement would result in a rather large and awkward scale. To avoid this another scale is used the decibel or DB scale. 

A-weighted sound in HVAC System Acoustics

A-weighted sound criteria are commonly used in outdoor sound evaluations - and are often incorporated into city building codes when referencing the maximum acceptable sound pressure levels at the property line. It is popular because it is a single number that most sound meters include.

The Use of NC Criteria in HVAC Units

Noise Criteria, or NC, is used as a standard describing or expressing the complexity of relative loudness of a space within a range of frequencies, where the acoustical environment greatly influences the amplitudes and frequencies heard by the occupant. 

So, simply stated, NC is the Sound Pressure at each Octave Band - with an NC of 35 referenced as the most common noise level specified for HVAC equipment.

HVAC Sound and Frequency

The frequency which is measured in hertz is the number of pressure variations that occur per second in a sound wave the sounds frequency produces a distinctive tone. This is why a flute that has a high frequency and a bass drum that has a low-frequency sound is so different. 

What is the difference between Sound Power vs. Sound Pressure in HVAC Systems?

SOUND POWER and SOUND PRESSURE are often presented when discussing Acoustics for buildings. Engineers must clearly distinguish and understand the difference between SOUND POWER and SOUND PRESSURE levels.

AHRI defines SOUND POWER as ACOUSTIC POWER, which is, in a specific frequency band, the rate at which the total airborne sound energy is radiated by a source, measured in watts.

Simply stated, this means SOUND POWER is the rate at which total Sound Energy is emitted, radiated, reflected, or transmitted by a sound source, per unit time - or basically, WHAT SOUND TH