Talk:Loudspeaker measurement

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I think the following quotation from the article is slightly misleading:

"Specifications like 'Frequency response 40Hz to 18kHz', which are all too common, are in fact meaningless. The situation is even worse for headphones, with manufacturers quoting figures like '4Hz to 22kHz' for headphones that are far from flat and often as much as 20 to 30dB down at 4Hz".

This short quote explains why:

"The ideal frequency response for headphones is a controversial question. For other devices in the transmission chain..., such as microphones, amplifiers and loudspeakers, a flat response is usually the design objective, with easily definable departures from this response in special cases. A loudspeaker is required to produce a flat sound-pressure level response at a distance of typically 1 m. The free-field SPL at this point reproduces the SPL at the microphone location in the live field. A listener in front of the loudspeaker distorts the SPL by diffraction, so that his ear signals no longer show a flat response. However, this need not concern the loudspeaker manufacturer, since the listener would also have disturbed the sound field at the live performance in the same manner. The headphone manufacturer, on the other hand, is directly concerned with producing these ear signals. The requirements laid down in the standards have led to the free-field calibrated headphone, whose frequency response replicates the ear signals for a loudspeaker in front... However, since frontal localization is not achieved, this spectrum shows unnatural colouring. Therefore, a compromise between this and a flat response is made, the exact response being largely a matter of taste".

- Loudspeaker and Headphone Handbook, 2nd edn, John Borwick ed., Butterworth-Heinemann Ltd 1994. Chapter 12 'Headphones', by C A Poldy of Philips Speaker Systems, p493.

[edit] Loudspeaker coloration

"color: the timbre of a musical sound; "the recording fails to capture the true color of the original music". [1]

I dispute the factual accuracy of the section 'Colouration analysis'. While resonances and such do color the reproduced sound, coloration can (and does) occur through other mechanisms. The term coloration comes from optics where white light can be 'colored' by passing it through a filter. Similarly, white noise can be 'colored' by passing it through a filter, e.g., pink noise. Alfred Centauri 21:21, 5 April 2007 (UTC)

[edit] Loudspeaker colouration: described in 1968 by the British Broadcasting Corporation

Since at least 1968 in both the consumer audio trade and professionally amongst speaker engineers the term colouration as applied to loudspeakers has referred to properties of a loudspeaker, measurable or not, that change the character of the audio being reproduced by the loudspeaker relative to the source applied to the speaker's terminals. There is no reliable correlation between the measurement of the loudspeaker in either the frequency or time domain that conclusively explains or identifies what the listener may perceive as colouration nor is there universal agreement amongst listeners as to the change in subjective quality when colouration is detected by listening exposure. Colouration in a loudspeaker system may even increase the listener's enjoyment depending on the frequencies and amplitudes of the colourations.

For this reason it is not possible to say that 'all colouration is bad' nor is it possible to conclude with certainty from an examination of the technical measurements that a loudspeaker is/is not coloured. It is a matter of listening preferences. There are, accordingly, no known quantitative index rankings for colouration reflected in the large number of speaker brands and models available to the consumer representing a wide technical spectrum.

Colouration is mentioned in the following paper: "New B.B.C. Monitoring Loudspeaker" by H.D Harwood/BBC Research Dept. published in Wireless World Magazine March 1968 in 'Tests' "... and in particular comments have been made regarding the freedom from colouration of the bass response ..." and also in the second article in April 1968 under 'Details of Units' "Further experience with the unit revealed a slight colouration in the 1.5kHz region" and "The effect on colouration can easily be demonstrated by applying Pink Noise ..."

Alanshawuk 18:54, 21 April 2007 (UTC) HAL

I agree (as the prime originator of the article) with a lot of the above comments. However, they do not point to errors, so much as the desirability of expansion and qualification.

With regard to headphones, I have myself written almost exactly as quoted, that they should be not designed to have a flat response. I would avoid the use of the term 'ear signals' though, because there is a common misconception that headphones should produce a flat response at the eardrum, which is very far from true. The earphone should replicate as far as possible the sound field at the entrance to the ear canal, the resonance of the canal being present at all times and therefore expected by the brain. The article is specifically referring to low frequency response though, and at frequencies below a few hundred Hz head and pinna effects are negligible, so that a good earphone should have a flat pressure response at the ear canal entrance (or anywhere within the sealed earphone for that matter). Measurements show this to be far from true, many headphones that claim a frequency response down to 4Hz being well down at 20Hz, with negligible output at 4Hz (see Lindos Electronics test sheet database for evidence).

I do not agree generally on colouration, and I think the BBC text must be considered out of date, as it was written before the measurement of colouration due to delayed resonance became common, and when speakers that achieved a flat basic frequency response were rare. Harwood's research and designs were seminal work back in 1968, and led to speaker designs referred to as LS3 LS7 etc which are still held in high regard, though they did not acheive the precision response, low colouration, and accurate time-coherence of modern designs. Nowadays I think most people regard the main cause of colouration as delayed resonance, though I would add one exception, which is the effect of phasing at crossover between drivers, which can cause colouration despite a flat net response. Again, the BBC conclusion that colouration is not necessarily undesirable, and can be a matter of listener preference, is probably outdated. Subjectively, people may prefer different sounds on specific pieces, but preferring the sound over the original is not what reproduction is about. A good speaker, in a good listening room reproduces the original sound - if you want something else, use tone controls. This is a complicated subject though, and it could be argued (as I know all too well) that it is the listening room that is the major problem, often masking or overwhelming speaker defects. The only perfect listening room is an anechoic one, which does not add any reflected sound, but listening to stereo (without surround) in such a room is said to be unbearable. In practice, most rooms add massive resonances at low frequencies. Speakers in an anechoic listening room sound terrible - not because of any defect in response or colouration, but because the only room acoustics heard are then those of the recording studio, but reproduced from two points at the front of the listener, which is all wrong. The brain expects reflections from behind and all around, and would much rather hear false reflections from a listening room than non at all. This leads to the modern quest for true spacial surround sound, in which great progress has been made in the last few years, with a trend to one or two channels of sound encoded using perceptual coding, along with spacial parameters for the spectral components carried on a low-bandwidth encoded channel. Such systems will, ideally, use many low colouration speakers in a near-anechoic (or digitally compensated) room. --Lindosland 00:30, 5 August 2007 (UTC)