Sound Measurements And Observations

Introduction:

In three notably different acoustic places, this report will showcase and discuss some sound level measurements and the observations that revolve around the experiment. The measurements that will be broken down in this elaborative report have recently been performed concretely utilising an instrument that determines the sound levels, particularly in decibels (dB), which is called a sound pressure level meter (SPL Meter).

The measurements of the quietest place:

• Room dimensions (cm) 425 x 375 x 260
• Weighting A-weighting
• Time weighting Fast
• Minimum (dBSPL) 28.7
• Maximum (dbSPL) 34.2
• Average (dBSPL) 23.4
• Temperature 18 °C
• Sound sources ambient noise, traffic noise of the cars outside the room, breathing, wind, the clock ticking, footsteps,

Description:

In the night of the 14th of November 2019 between 10:20 and 10:25 sound measurements were carried out using an SPL meter in combination with a condenser microphone and spectrum analyzer plug-in. The measurements above indicated that the room is a typical one in terms of size. And due to the fact that the environment is considerably quiet, A-weighting was used specifically. As the human ear, A-weighting effectively eliminates the lower and higher frequencies that the average person can not hear. In addition to that, fast setting was utilised as long as the environment has significantly fewer impulsive signals compared to other atmospheres with dissimilar surroundings and with multiple directional fluctuation movements from the sounds’ sources; for instance, a busy road, factory, and a raucous restaurant.

In a room that contains a bed, a large couch, cupboard, disk, two monitors speakers, acoustic panels on the walls, bass traps and a human being holding the SPL meter.

Breathing definitely is one of the main sound sources of that environment. Extremely quiet sounds such as rumbling or gurgling that are the sounds of air and fluids as they move by the muscles through the digestive system. Other faint sounds include the clock ticking, footsteps and ambient noise (background noise). The sounds that can be heard from outside the room are mainly from two ventilation holes, two windows in the walls and the door of the room. These distinctive sounds consist of ambient noise, noise of the cars outside the room and wind sigh.

Observation:

Sound divide into two main categories, which are audible and inaudible sounds to the human ears. Although it was fairly quiet environment, yet still the SPL meter captured a decent number of sound between 28.7 and 34.2 dBSPL. And the whys and wherefores behind that absolute silence is an impossible thing to be achieved because that would require absolutely no vibrational energy, no present phonons. Consequently, the use of the appropriate frequency weighting, commonly A-weighting, is extremely important for measuring the sound level pressure. For instance, if C-weighting is utilized instead of A-weighing, a measurement of a tonal noise around 31 Hz could result in an error of 40 dB (NoiseMeters Australia, 2018)

The measurements of 2 metres away from a busy:

• Weighting A-weighting
• Time weighting Slow
• Minimum (dBSPL) 72.4
• Maximum (dbSPL) 81.3
• Average (dBSPL) 74
• Temperature 22 °C
• Sound sources ambient noise, busy traffic, cars, large trucks, sirens, motorbikes, ,road tram, people talking, wind, birds, footsteps

Description:

For five minutes period on the 12th of November 2019 for between 12:20 and 12:25 in the afternoon, sound measurements were calculated using an SPL meter placed two meters away from the busy traffic of Clarendon Street, in Southbank. The environment is open and it contains dozens of sound sources. Whether it is the people that are walking and talking on the sidewalks or the moving cars, motors and trucks on the road. Trams also pass by Clarendon Street. Emergency car passed by and its sirens made the meter go up high to 81.3dBSPL. How and the which way the wind is blowing also had a significant effect on the reading on the meter. The birds were flying here and there making all their own noise such as tweeting and cawing. Although the fact that the environment is noticeably bustling, A-weighting was used specifically. And the reason for that is even though lots of noise occurred, it didn’t go exceed the general using of A-weighting which is usually below 100dB (NoiseMeters Australia, 2018). The other reason for not using C-weighted settings is fact that C-weighting used to measure peak sudden impulses (Pulsar Instruments, 2019). As mentioned above A-weighting effectively eliminates the lower and higher frequencies that the average person can not hear. Additionally, slow time weighting was utilised because the environment has remarkably way more impulsive signals compared to quiet atmospheres with different surroundings and with less directional fluctuation movements from the sounds’ source.

Observation:

A variety of multiple sounds from numerous sources has been captured. Once again there are sounds that we can hear and there are sounds simply we can not hear. In that specific place there were many sounds occurred from different sources. Some of them were loud that quiet noises couldn’t be noticed but still exist.

The measurements of a constant sound source of around 85dBSPL (Pink Noise):

Description:

In a bed room studio that is acoustically treated, the SPL meter was placed in front of a large monitor speaker connected to a computer with a DAW that generates pink noise at 85dBSPL. Slow time weighting utilised. The readings are:

• 1 meter away from the sound source
• A-weighting 85.2
• C-weighting 86.7

Observation:

A-weighting is almost 1.5 decibels less than C-weighting. And the reason comes down to how A-weighting and C-weighting are different. A-Weighting covers the full frequency range of 20Hz all the way up to 20 kHz. Whereas C-weighting is basically flat or linear between 31.5Hz and 8kHz.

• 4 meters away from the sound source
• A-weighting 73.1
• C-weighting 74.5

Expected drop in SPL:

According to the formula:

2 x (double) SPL = +6 dB

Then: SPL drops 6 dB for each doubling of distance.

Doubling 1 metres, which is 2 metres away from the source will result a drop of 6 decibels.

And doubling the 2 metres to 4 metres will result a drop of another 6 decibels.

4 metres away from the sound source = -12dB drop

The measurements correlate because the first reading was at 85 and when moved 4 metres away, the number decreased by 12 decibels at 73.

Conclusion:

After calculating many sounds from multiple sources and different places and atmospheres, from the quiet room, Clarendon Street to the constant sound of 85dB, using the appropriate settings is essential to getting the correct readings. It is also important to note that there are sounds the human ear cannot hear. For example infrasound (low frequencies below 20 Hz) and ultrasound (high frequencies above 20 kHz). However, having the knowledge and recognising the technical aspects of how sound and acoustics work will help calibrate a suitable procedure when it comes to decision making regarding any measurements.