GrimSurfer
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A recent discussion on the Benchmark's AHB2 discussed the consequences of an amp with measuring THD of -100 vs one with -120 dB. Although unstated, I perceived that the real issue was the audibility spurious sound energy. This got me thinking about how sound levels work across the audio chain...
The decibel is a standard measure of sound pressure. It can be measured as a number of weighted or unweighted values. Setting aside weighting, the two key issues are that:
(1) two or more proximate sound sources of exactly equal value are louder than one source on its own. A loud source and a softer source add up to an incrementally higher spl. Said another way, sound adds up. It doesn't "average out"; and
(2) incremental increases of spl are more significant than simple numbers suggest. Increases of 0.5 dB are audible. Increases of 1 dB are noticeable. Sound intensity appears to double in intensity every 3 dB and doubles in volume every 10 dB. (Hopefully I got that the right way around!)
THD and Noise are caused by different things that don't necessarily factor into the scope of this post. This doesn't mean that these causes are insignificant. Nor does it mean that one isn't necessarily more audible than the other. It is just a way of keeping things simple by looking at the way that the product of both can be heard (or not).
The human auditory system can be fooled that THD is naturally occurring resonance, particularly when the artifacts are even-order (doubling, quadrupling of the primary frequency, etc). Sometimes the ear cannot be fooled, such as happens when encountering audible uneven order harmonics (tripling, quintupling etc.) to which the ear can be quite sensitive.
Regardless of whether the auditory system is fooled or not, sensitive or not, THD is sound extraneous to the intended signal.
Noise is random sound that doesn't necessarily correspond to a fundamental frequency. It too is extraneous to the intended signal but, as is often the case, is spread across a broader frequency range.
Setting aside root causes and the ear's ability to be fooled, THD and Noise is, well, just "noise" to the listener. We may not hear it as such. To some noise is fatiguing. To others it adds certain characteristics to sound, which is sometimes referred to as colour. There are likely more imaginative terms out there to describe the effect Noise has on sound. As an objectivist, many of these fly in one ear and out the other
So what are the implications of THD, Noise, or THD+N on the audio chain? The first thing is that the source of the "noise" doesn't matter. A source player, DAC, pre-amp, or amp can (and do) introduce noise at each step of the audio chain. This noise adds up, resulting in a level that is both significant and audible.
Let's look at a practical example of a source player, DAC, pre-amp, and amp -- each of which have THD+N ratings of -100 dB. Together, they have the potential to produce a noise floor of -94 dB. When played at reference levels, they'd theoretically be audible against a human hearing range of 118 dB but probably masked or inaudible except when dynamic peaks rise above reference level.
That same combo with ratings of -120 dB have the potential to produce -114 dB. When played at reference level, they'd theoretically be audible. The levels would be so low, however, that they'd be mostly masked.
I base my probability statements on the Fletcher Munson curves which show that human hearing acuity varies with frequency, with ear being most sensitive to the middle frequencies.
If the components all had noise characteristics in the -124 dB range, then the combined noise of all four devices (-118 dB) would be impossible for the human ear to detect at reference levels. This isn't a matter of opinion. It is based on math and human performance data.
If just one of those components had a spec of -100 dB, the THD+N of the entire audio chain would rise by 18 dB to -99.5 dB.
But what if you could offset that one pedestrian (or to the Aussie vernacular, ordinary) component measuring -100 dB with a really exceptional one measuring -140 dB? Would it average out?
It might not do as much good as you would think. If the other three components were -124 dB, -124 dB, and -140 dB (!) you'd still be looking at a combined THD+N of -99.96 dB. Such is the logarithmic nature of sound pressure levels.
In other words, the overall distortion and noise of an otherwise superb system can be significantly (read: audibly) increased by the introduction of one pedestrian component. This is not a matter of opinion, but an issue of math.
The big take-away is that ALL of the noise, from all of the components in the audio chain, matters.
When playback is much lower than reference levels (say 70-75 dB "easy listening), a certain amount of noise can indeed be inaudible. But anyone seeking exceptional audio performance would likely take greater comfort in knowing that system noise is "inaudible" up to the point of amplifier clipping than having to exercise extreme caution in component matching or volume settings.
I make no claim to having anything but ordinary gear. This doesn't blind me to science. Rather, it sharpens my appreciation of the rarity of exceptional sound and makes me skeptical when people make bold claims against that which math and human physiology says otherwise.
Thoughts?
The decibel is a standard measure of sound pressure. It can be measured as a number of weighted or unweighted values. Setting aside weighting, the two key issues are that:
(1) two or more proximate sound sources of exactly equal value are louder than one source on its own. A loud source and a softer source add up to an incrementally higher spl. Said another way, sound adds up. It doesn't "average out"; and
(2) incremental increases of spl are more significant than simple numbers suggest. Increases of 0.5 dB are audible. Increases of 1 dB are noticeable. Sound intensity appears to double in intensity every 3 dB and doubles in volume every 10 dB. (Hopefully I got that the right way around!)
THD and Noise are caused by different things that don't necessarily factor into the scope of this post. This doesn't mean that these causes are insignificant. Nor does it mean that one isn't necessarily more audible than the other. It is just a way of keeping things simple by looking at the way that the product of both can be heard (or not).
The human auditory system can be fooled that THD is naturally occurring resonance, particularly when the artifacts are even-order (doubling, quadrupling of the primary frequency, etc). Sometimes the ear cannot be fooled, such as happens when encountering audible uneven order harmonics (tripling, quintupling etc.) to which the ear can be quite sensitive.
Regardless of whether the auditory system is fooled or not, sensitive or not, THD is sound extraneous to the intended signal.
Noise is random sound that doesn't necessarily correspond to a fundamental frequency. It too is extraneous to the intended signal but, as is often the case, is spread across a broader frequency range.
Setting aside root causes and the ear's ability to be fooled, THD and Noise is, well, just "noise" to the listener. We may not hear it as such. To some noise is fatiguing. To others it adds certain characteristics to sound, which is sometimes referred to as colour. There are likely more imaginative terms out there to describe the effect Noise has on sound. As an objectivist, many of these fly in one ear and out the other
So what are the implications of THD, Noise, or THD+N on the audio chain? The first thing is that the source of the "noise" doesn't matter. A source player, DAC, pre-amp, or amp can (and do) introduce noise at each step of the audio chain. This noise adds up, resulting in a level that is both significant and audible.
Let's look at a practical example of a source player, DAC, pre-amp, and amp -- each of which have THD+N ratings of -100 dB. Together, they have the potential to produce a noise floor of -94 dB. When played at reference levels, they'd theoretically be audible against a human hearing range of 118 dB but probably masked or inaudible except when dynamic peaks rise above reference level.
That same combo with ratings of -120 dB have the potential to produce -114 dB. When played at reference level, they'd theoretically be audible. The levels would be so low, however, that they'd be mostly masked.
I base my probability statements on the Fletcher Munson curves which show that human hearing acuity varies with frequency, with ear being most sensitive to the middle frequencies.
If the components all had noise characteristics in the -124 dB range, then the combined noise of all four devices (-118 dB) would be impossible for the human ear to detect at reference levels. This isn't a matter of opinion. It is based on math and human performance data.
If just one of those components had a spec of -100 dB, the THD+N of the entire audio chain would rise by 18 dB to -99.5 dB.
But what if you could offset that one pedestrian (or to the Aussie vernacular, ordinary) component measuring -100 dB with a really exceptional one measuring -140 dB? Would it average out?
It might not do as much good as you would think. If the other three components were -124 dB, -124 dB, and -140 dB (!) you'd still be looking at a combined THD+N of -99.96 dB. Such is the logarithmic nature of sound pressure levels.
In other words, the overall distortion and noise of an otherwise superb system can be significantly (read: audibly) increased by the introduction of one pedestrian component. This is not a matter of opinion, but an issue of math.
The big take-away is that ALL of the noise, from all of the components in the audio chain, matters.
When playback is much lower than reference levels (say 70-75 dB "easy listening), a certain amount of noise can indeed be inaudible. But anyone seeking exceptional audio performance would likely take greater comfort in knowing that system noise is "inaudible" up to the point of amplifier clipping than having to exercise extreme caution in component matching or volume settings.
I make no claim to having anything but ordinary gear. This doesn't blind me to science. Rather, it sharpens my appreciation of the rarity of exceptional sound and makes me skeptical when people make bold claims against that which math and human physiology says otherwise.
Thoughts?