Why you can’t hear the shopping list: sound level and distance from the source

Sound gets softer as you move further from the source.

The table below has been adapted from one published by Wind Energy Development Corp. (Rhode Island). It’s a particularly good table, because it uses two distinct sorts of noise – one you may be able to control, and one you can’t. It also has distance measures in. The decibel level is reduced by about 6 dB, every time you double the distance between the listener and the sound source. This is because intensity is reduced to ¼ of the original.

dB(A) Overall Level Examples in this range Subjective Loudness Relative to 70 dB
120 Uncomfortably Loud Military jet aircraft take-off from aircraft carrier with afterburner at 15 metres (130 dB)
Oxygen Torch (121 dB)
32 times as loud
110 Turbo-fan aircraft at take -off power at 61 metres (118 dBA)
Riveting machine (110 dBA)
Rock band (108 – 114 dBA)
16 times as loud
100

Very loud

Boeing 707 or DC-8 aircraft at one nautical mile (185 m) before landing (106 dBA)
Jet flyover at 300 m (103 dBA)
Bell J-2A helicopter at 100 ft (100 dBA)
Night club music, inside (97-104 dBA)
8 times as loud
90 Boeing 737 or DC-9 aircraft at one nautical mile (185 m) before landing (97 dBA)
Power mower (96 dBA)
Motorcycle at 7m (90 dBA)
Newspaper press (97 dBA)
4 times as loud
80 Propeller plane flyover at 300m (88 dBA)
Diesel truck 64 kph at 15m (84 dBA)
Diesel train 72 kph at 30m 9 83 dBA)
Food blender (88 dBA)
Milling machine (85 dBA)
Hair dryer near ear (80 dBA)
2 times as loud
70 Moderately loud High urban ambient sound (80 dBA)
Passenger car 105 kph at 8 m (77 dBA)
Busy free-way at 15 m from pavement edge (76 dBA)
Living room music (76 dBA)
Radio or TV-audio. vacuum cleaner (70 dBA)
>70 dB(A)
60 Air conditioning unit at 30 m (60 dBA)
Truck @ 30 mph @ 328’ (65 dBA)
Conversation (60 dBA)
Busy general office (60dBA)
1/2 as loud
55 Car at 64 kph (55 dBA)
50 Quiet Large transformers at 30 m (50 dBA) 1/4 as loud
40 Bird calls (44 dBA)
Lowest limit of urban ambient sound (40 dBA)
Refrigerator (44dBA)
Ceiling Fan (45dBA)
35 Background noise in a quiet office
Bedroom…35 dB
20 Large auditoria (NCB Rating system requirement)
10 Just audible
0

Threshold of Hearing

The practical implication of the inverse square law and the relationship of sound level to distance is that as you recite the shopping list and your partner walks away, every time you double the distance the sound drops by 6dB.  This means that if you move from one metre from the speaker to 3 metres from the speaker, the sound is more than half as loud.  Maybe you have turned round too, so it may be half as loud when you only move an extra metre further away.

Voice level in  dB A

 

Distance (metres) normal raised loud shouting
0.3 70 76 82 88
0.9 60 66 72 78
1.8 54 60 66 76
3.7 48 54 60 66

Imagine you are going shopping and you and your friend are in the kitchen. Your friend is reciting the shopping list to you as you walk away. You move away by half a meter and the voice level is half as loud. That’s a lot! The listener is probably not facing the speaker any more either. Consider that many consonants are less than half as loud as vowels. If the later items in the list contain consonants, you may be doomed to be reprimanded for not listening properly and making mistakes. The sound picture of cheese and peas are near enough that chopping the loudness in half and taking away the visual cue could lead you to a platter of peas and biscuits at dinner.

Blamey Saunders hearing aids keep sound audible and comfortable, so can help. However, my advice is to stay standing still and listen to the whole list before you move away – even with your hearing aids on.

2 Comments

  1. This is good information, thanks Elaine. Perhaps I can crib it for my book (yes, I am writing it) The Edge of Silence.
    I also learned recently that within about 1 metre, the sound of voice is dominated by that directly transmitted from the speaker. Beyond that, it becomes dominated by reverberated sound – ie, it arrives via an indirect route.
    This would mean that ‘direct’ sound has fewer opportunities to be modified by being echoed, absorbed, amplifed…generally, mushed by the surroundings.

    Rod Taylor

Leave a Reply

Your email address will not be published. Required fields are marked *