Noise Floor in a Room: A case study advocating the need for Air-Conditioning while Enjoying High-Quality Music or Movie at Home

I have written blog on Dynamic Range (https://avhead.com/dynamic-range/) earlier. As far as the systems are concerned, low-level linearity, extremely low noise levels and high resolution are the factors that will dig loads of details. These are not easy traits and not easy to achieve while system can do it equally well at louder volume levels too. To mess up things further, not many systems can do it simultaneously in a complex piece of music.

However, maybe all this is achieved by spending loads of money and carefully choosing the equipment; but you may still not hear things that you herd when you auditioned the stuff. And the culprit could be, not the noise floor of the electronics but the noise floor of the room!

Today we examine two contrasting situations where the use a ceiling fan at full speed and contrastingly use the AC at low fan-speed. We shall also examine other situations between these two extremes, however, these two will be explained in detail as they represent the extreme situations.

Before we began, a disclaimer here is important that this article is not a representative by taking a large sample-data from many rooms but a snapshot of what is happening in my living room specifically.

All the readings are taken between 2 and 3am to minimise the effect of unwanted noise and isolate the case where the sole difference between the readings is HVAC system. Following graphs show the two extremes with three line plots. In the first graph, you can see difference between quiet room (black trace) and AC on at lowest fan speed. Next is the graph where fan is blaring on fastest speed against the same quiet room graph.

AC at Min Fan Speed is adding almost nothing to the quiet room

Very significant noise of about 20dB SPL added to the quiet room when Ceiling Fan is blaring at its max speed. This is more than enough to mask important micro details which are the forte of good high-res system and recordings.

What are the PNC Curves? Preferred Noise Criterion Curves are an amendment over older NC curves that are more non-forgiving and tighter on low frequencies (useful for music and HT applications as we are not just measuring speech and ambient noise in public spaces but measuring it for specific full-range indoor applications) and specifically used for measuring indoor static noise such are HVAC and forced ventilation noise.

To simplify the stuff further the curves yield a single digit noise index (mentioned in the upper right corner). You can see that the quiet room (All Fans Off) and the AC Fan Low reading both yield PNC of 50dB SPL and it is evident that form the graph that the lines are almost trudging each other as if they were identical twins!

On the other hand, the lines are divorced and somewhat running helter-skelter with ceiling fan being full. PNC index is 60dB compared to 50dB SPL – a significant rise. Noteworthy is the significant rise of up to 20dB SPL from 500Hz to 1000Hz – a very important speech region. And in speech intelligibility zone of 1kHz to 4kHz the noise added by the ceiling fan is still significantly high (it must be noted that the room was completely closed during this reading – which will not be the case in real life scenario. With windows open, the external noise added will only worsen the situation further).

What will this do to our music and movie reproduction? Well, my All Fans Off curve itself is pretty noisy for a very high-res music playback and some of the modern movies dubs (such as Tenet). However, some of the important sounds made by the loudspeakers will fall into the zone between black and blue trace with the fan on. It will be masked completely or at times partly by the drone of the fan and you’ll miss out on that finer experience for which you bought the system in the first place.

I often tell my clients that the most basic acoustic treatment that you can do is to add a good AC to your room!

Forget just movie/music experience, but 60dB SPL is a huge noise by itself for daily smooth functioning of the household. Long term exposure to continuous noise such as this will have bad health effects for sure.

On a side-note, it is better to have larger fans running at lower speeds than the smaller high-speed mosquitoes! It is also better to have more fans in a bigger room and facilitate the air circulation more smoothly.

However, while having a critical music listening session or a interesting movie watching switching on the AC is imperative. It is also important to calibrate the reference levels in the “AC ON” situation so that the calibration curves are properly representing the listening SPL level.

Other curves with AC Fan Mid and High and Ceiling Fan at Half speed are given here just for understanding purposes of the transition of the situation.

Somehow my AC at medium speeds is adding low-frequency noise to the room. I later realised that it is happening at the master-seating position particularly!

High Speed of AC Fan is adding lot of noise in the critical band. Thus it becomes important to choose a silent AC model and keep AC well-oiled and serviced. It is advisable to use. slightly higher install capacity to instantly chill the room and then allowing the lower fan speed to have comfortable listening environment.

Even Ceiling Fan at Medium speed (though not comfortable to sit throughout the listening session in hot weather) is still contributing lot of noise to the general environment.