Sensitivity, Power, Distance Lost, Time Delay........
4th January 2004
Discussion of typical car audio application scenario using horn loudspeakers to replace conventional dome tweeter. This topic also covers power (watts), distance energy lost / drop (as distance increase, dB reduces) and finally time delay due to distance length.
Please bare in mind some knowledge of room acoustics and human brains' psychology and psychoacoustics is required. Please read the followings before hand: -
Flat Frequency Response
Why most tweeters are crap
Typical high powered, high efficiency horn loudspeaker as tweeter - 109dB SPL/w/m capable of 150watts high-passed at 2kHz using 24dB/oct.
Typical 6.5" true mid-basses - 89dB SPL/w/m 100watts RMS
Typical 15" woofer - 89dB SPL/w/m 200 watts RMS
The difference between the horns and the mid and woofer is 109 dB - 89 dB = 20 dB
Using inverse power rule 10^(20/10) = 100, therefore 100 times the difference, not units but multiplication.
eg: - for regular speakers 89dB SPL /w /m + 20 log(100) = 109 dB /100w /m
i.e. when horn plays 1 watt, the rest of the speaker will need to play 100 watts to be equal.
If the horn plays 4 watts, the rest of the speaker will need to play 400 watts to be equal.
If the horn plays 10 watts, the rest of the speaker will need to play 1000 watts to be equal.
100 times difference - don't forget. - no mids and woofer can sustain even 400 watts. .................................................................... start thinking yet?
However all speakers will be at 108dB SPL. Two horns, two mid-basses and a woofer is allot more sound than one speaker, therefore the human hearing psychological impression is estimated to be 10^(108/20)x2e-5 times 5 speakers then 20log(ans/2e-5) = 121.9 dB SPL. Using simple assumption.
However a horn and a mid-bass does not share the same frequency, therefore the above formula is not strictly correct, however only provide the estimated loudness impression level.
The furthest horn to the furthest ear is around 1.65 meters, while the closest horn to the closest ear is 1.37 meters. The difference is 0.28 meters. Therefore the dB SPL difference due to distance lost is 20 log (1.65/1.37) = -1.61 dB SPL.
i.e. left horn to driver's right ear versus right horn to driver's right ear.
However in car acoustics' reflections plus assumption of 0.5 seconds decay sustained left over acoustical energy will contribute and hence not loose 6dB.
Finally time delay. The time difference between 1.65 meters and 1.37 meters is: -
Speed of sound is estimated as 347 meters per second Malaysia day time non air-conditioned condition.
1.65 / 347 = 0.00475 seconds = 4.75 milliseconds
1.37 / 347 = 0.00394 seconds = 3.94 milliseconds
So the difference between the furthest speaker to the closest ear and closest speaker to the furthest ear is only a mere 4.75 - 3.94 = 0. 86 milliseconds
My god - 0.86 milliseconds. And we listen to sound almost non stop. I wonder what is 0.86 milliseconds of difference is going to make ???
acoustic wave phase are shifted 0.86 ms !!!
What else??? can't think of any now..... except
human brains process sound in 35 ms segment window - analogue - hence continuous movement change of the 35 ms window.
0.86 / 35 ms = 0.02457 occupation time ........
There is a constant 0.02457 time segment which is not very right - according to the brain ...... this is acceptable and ignored for most of the day - as the human ears are "ONLY" a secondary awareness system - priority has been given to the eyes :) to look as pretty girls :)
So, what are all these Sensitivity, Power, Distance Lost, Time Delay........ if without the understanding of at least the basic of human brains, human psychology and psychoacoustics.............
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