Technical meanings

We see a lot of abbreviations and terms used in the specifications of hifi components, but what do they mean?

Harmonics: Direct multiples of a frequency. For example, a 1kHz signal will have harmonics at 2kHz, 3kHz, 4kHz, etc as it is periodic at these frequencies just as it is at 1kHz.

Even-order: Harmonics that are even number multiples of the fundamental. For 1kHz, even order harmonics are 2kHz, 4kHz, 6kHz, etc. Usually associated with valve equipment, and described as "sweet"

Odd order: The odd number multiples. Usually associated with solid-state equipment and described as "harsh".

THD: Total Harmonic Distortion. This describes how closely the output of a component matches the input. 0.1% distortion means that the output is 99.9% the same as the input (scaled by any appropriate gain of course). This is measured by measuring the size of the harmonics as described previously.

THD+N: Total Harmonic Distortion + Noise. The same as THD but with the amount of noise (random additional signal components) also measured. These two measurements are very closely related and often not differentiated between.

RMS: Root Mean Square. This is used in description of amplifier powers usually (although it has other uses in engineering, but we're looking at a specific context). This describes power more accurately in the context of an AC waveform (like music). The P=VI or VV/R equations work fine for DC, but an AC waveform is constantly changing. For a simple sinewave of +/-50V amplitude, the RMS voltage is 50x((root 2)/2), or around 35V. Thus peak power might be 50x50/8 = 312.5W, but RMS power is 153W.

IMD: InterModulation Distortion. This is where two different frequencies in the audio signal combine and produce various harmonics at other frequencies. A standard test for this is to introduce 19kHz and 20kHz sine waves at the same amplitude and measure the resultant 1kHz (difference) amplitude. High levels of IMD may be associated with a lack of "clarity"

Crosstalk: If you apply a large signal to one channel only, can you hear it in the other channel? Crosstalk describes how much the signal spills over into the other channel(s). There are many ways that it can happen. Dual-mono designs minimise it. High levels of crosstalk will cause a compressed stereo image (where everything seems to be in the middle), while low crosstalk will give a wide sound stage with clear positioning.

SNR: Signal/Noise Ratio: How much noise occurs compared to the size of the signal.

DNR: Dynamic Range: How loud the loudest sound is compared to the quietest sound. In a digital system this is the amplitude of a single LSB compared to a full-scale deflection. In an analogue system, the smallest signal is limited by the noise floor of the system (see SNR).

Please add anymore you feel confident to describe, or ask about any I've missed. Please don't specify descriptive terms (like bright, open, etc) unless it is characteristic of a technical specification.

 

So, if THD "describes how closely the output of a component matches the input" does it also include IMD? ie, is IMD a subset of THD?

A "null test" (which has been discussed here a bit recently) would appear to be a more accurate way of measuring how closely the output of a component matches the input. What's more it can be done in real-world conditions and with music as opposed to test tones. Would be great if there was a way to standardise the null test so the results could be converted to a percentage number just like THD. I guess you'd have to use a standard "test suite" of music also.

Michael.

 

No, they are not the same. A THD test is done with a single frequency, usually 1kHz or a frequency sweep (20-20k). Thus it can not measure IMD as there aren't two frequencies input to measure the interaction between.

I am not familiar with the null test. I'll look into it.

 

See here (bottom 2 paragraphs) for some info on the null test. Also discussed at length on this thread.

So, in relation to THD, it doesn't strictly measure how closely the output matches in the input, or, it only does so when fed with a single frequency test tone.

Michael.

 

Right, I've looked into it a little.

The null test as described on this forum is similar in operation to a test system such as those sold by Miller Audio Research and Audio Precision (the latter is essentially industry standard) although somewhat cruder. The THD test is done as a null test for a single tone input, with the amplitude of the remenant quantified and related to the amplifier output level. IMD does the same for two tones. Tones are used rather than music because music will give a different result with different tracks, or different recordings of the same track, etc.

 

Presumably a significant difference is that the null test is a time domain test, whilst a measurement of THD (at least as I'm used to the term being employed) is a measure of based purely on the amplitude spectrum (i.e. not phase spectrum as well).

 

Isaac,

I'd be interested in a brief explanation of the "damping factor".

Thanks

 

There are two meanings:
#1 - that warm feeling you get when you arrive home with your Shiny New Toy (SNT factor being the inversed ratio of package size to wallet depletion)
#2 - The product of a common engineering error induced by the introduction of random SWMBO distortion into the the SNT signal path. Is usually counted in negative "boules"
hth

 

Then what would you call the feeling in my back I got after carrying my 30-kg plus SNT upstairs the other day (you may include credit card depletion, as that "really" occurs a few days/weeks later) ?

 

Damping factor is the ratio of output impedance of the amplifier to the load impedance (usually quoted for 8 ohms). Thus if an amplifier has an output impedance of 0.08 ohms into a load of 8 ohms, the damping factor is 8/0.08 which is 100. This is quoted for a specific frequency, usually 20Hz. The effect on sound is usually described as "tight bass" for an amplifier with high damping factor.

 

So the higher, the better ?

 

In the same way that THD being lower is "better". Damping factor doesn't tell the whole story, and there are plenty of amplifiers with relatively poor damping factors (switching amplifiers tend to offer damping factors of 10-40 or so) which sound very good. On paper, an infinite damping factor is desirable (ie no output impedance on the amplifier).

 

Just found this thread, and very interesting it is too. Should it be somewhere more visible? Would be nice to think that it could expand but remain factual.

 

There's a common belief that high damping factor results in the amp having more 'control' of the speakers leading to better-defined decay and less overhang. I'm led to believe that this is in fact a myth - unless your damping factor is in single figures, it's not an issue at all (essentially the speaker design is all that actually matters here). See here for an explanation with equations and things.

 

Unfortunately, it's not quite that simple.

A speaker isn't a nice flat 8ohm resistance.



That is the impedance trace of the humble B&W DM302, which retailed around £150. Nominally it's an 8 ohm speaker but we can clearly see a 3ohm dip at 200-300 Hz, and a 4 ohm dip at 50Hz, with a 17 ohm peak in between at 100Hz. Thus if the output impedance of the amplifier is fixed such as to provide a damping factor of 10 into 8 ohms, when you hit this 3 ohm dip it's not 10 any more... It's less than 4.

The next step of complexity comes in the amplifier, where you will find that the output impedance also varies with frequency. The 200Hz part of the DM302's impedance plot is not actually a problem because almost any amplifier will have an extremely low output impedance at that frequency. That is not the case, however, at 50Hz or 20Hz, where the speaker's impedance is again low.

This interaction between speaker impedance, amplifier output impedance and frequency results in much of the difference heard in the bass region, where two amplifiers with "flat" frequency responses might have quite different subjective response, one sounding "thin" and the other sounding "full".

 

Which will be an issue, consistent with

As I understand it, the take-home message is that there's no audible gain in having a (nominal wrt 8 ohms) DF of over say 50 even at a rather conservative estimate with most speakers. Hence the likes of the claimed damping factor of 1000 of the Rotel RB-1090 is completely irrelevant, unless you're using Scintillas or something. Or have I missed the point?

 

No, you've not missed the point. DF is one of those specs, like any other, that can be overplayed.

For example, NAD claim a -3dB response for the S300 of DC-250kHz. This has absolutely no possible useful application, but it's bigger than any other spec you've read...

 

A prime example of taking for granted also those prefect specifications being some what no so

 

Ok, here is my 2 p worth. Damping factor is the ability of an amplifier to hold the speaker cone still in a no signal situation - let me explain.
A speaker is a coil of wire wound inside a magnet ie an electricity generator. When a signal from thje amp moves the coil in and out the speaker acts like a generator sending voltage back to the amplifier - called back emf. Now if the amp has a really low effective output impedance it will do a really good job of shorting out that back emf.
To demonstrate the principle - If you get one of the old style VU meters or an analogue volt meter and shake it the needle will move about freely. Take a bit of wire, short the terminals and shake it again the needle will hardly move. this is exactly the same science going on. An amp with a high damping factor will stop the speaker generating back emf and will therefore give much better control over the movement of the cone. The effect is largest at bass frequencies as that is where the excursions are biggest and therefore where the largest back emf is developed.
And that's why bass always sounds tighter on an amp with a high damping factor.
Dave.