Speaker Principles

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When indigenous people were first shown a photograph they could not at first recognise a picture.   Over time the mind unconsciously learns to interpret information (no matter how abstract) and associate it with experience.   This illusion is so great that only by being attentive we can hear that a speaker is not and cannot be a guitar, saxophone, violin or an orchestra.

Speakers and musical instruments share similar properties including the laws governing their behaviour.   It is a miracle a speaker system can be made to sound real.   But the fact is most don’t.   Loudspeakers have principally remained unchanged for approx 50 years.   There have been few improvements since their invention.   One example is epoxy compounds for bonding voice coils to the cone to improve power handling.

The greatest advances have been in marketing where illusions of hearing differences are mostly generated by descriptions.   As with musical instruments it is possible to make a speaker system come alive.   All information on this site is directed to the understanding of bringing sound systems alive by application of four way active technology.

6 specifications

  1. Frequency response
  2. Polar response
  3. Spectral energy response
  4. Efficiency and power
  5. Inter-modulation
  6. Dynamic power response

 

Specifications are a guide similar to a road map.   No matter how detailed it cannot show you what you will actually see until you get there.   Another example is a list of ingredients for a recipe, again it cannot tell you how it will actually taste until you make it.   Specifications that are similar for the same size musical instruments or speakers do not tell you which sounds best.   You actually have to listen.   Large speakers and musical instruments suit low frequencies and vice versa.

A single loud-speaker and most musical instruments can only function effectively and efficiently within 3 octaves (octave is ratio 1:2) or 1 decade, ratio 1:10.

Speaker measurement

1. Frequency response measurement.   All electronics today has a frequency response capability that extends well beyond the audio range.   The measured sound spectrum is 20Hz to 20kHz.   A speaker and most musical instruments have a limited frequency response and only accurate within 3 octaves (octave is ratio 1:2) or 1 decade (decade is ratio 1:10).   3.2 octaves is 1 decade.   The standard way of measuring a speaker or speaker system is with a microphone, at one point at 1 meter on axis and a tone sweep across the frequency spectrum and plotted.   dB/mW   This single measurement provides vital information for detailed research.   But, frequency response measurement with a single sweep note (alone) does not provide sufficient information for how a speaker will sound when reproducing music.

Speaker bandwidth

2. polar Response Dispersion.   Measurement of sound dispersion around the sound system, plotted at different frequencies.   Small speaker systems (passive) are the majority and have wide dispersion at low frequencies but narrow beaming dispersion at high frequencies.   An ideal sound system will have even dispersion at all frequencies.   To achieve this the diameters of speakers must change each 2-3 octaves, this requires 4 different size speakers to cover the sound spectrum.   90deg dispersion is ideal.

Polar response

Theoretically a single speaker would have to change diameter from (1in – 24ft) or (20mm – 8m) to maintain similar level and polar dispersion over the frequency spectrum.

3. Spectral Energy Response.   Includes Energy response, Frequency response, polar response and Efficiency for the whole acoustic energy delivered from a speaker system giving a closer understanding of how a system behaves.

Speaker spectral energy

(A)   The on axis frequency response appears flat.   The high frequency energy is small but highly directional.   The low frequency energy is larger but spread around the box.   Off axis, there is little to no high frequency and the sound will be muggy.   (B)   On axis the frequency response is trebly.   Both high and low frequency energy is equal but the high frequency energy is highly directional and the low frequencies are spread around the box.   (C)   On axis frequency response is flat.   Both high and low frequencies have a similar polar response and their energies are equal.   Off axis the sound will remain balanced and the level will evenly decrease.

To achieve (C) an ideal system would have 4 speakers each covering 2-3 octaves.
(12-15/18in for Sub-bass) .   (8-10in for Lower voice) .   (4-5in for Upper voice) .   (1in for the Harmonics)

4 way polar response

4. Efficiency and power.   Cone Speakers are approx 2% efficient.   Power and efficiency is dependent on magnet and voice coil size.   (BL) B is total magnetic flux in the gap, and L is length of voice coil wire.   Also mass and area of cone, suspension compliance, damping and frequency.

The fine art of musical instrument and speaker making, is combining efficiency and power, responding with clarity and evenness, complying within the 3 octave rule, 1 decade.   A musical instrument and speaker may be efficient but uncontrolled which means the sound is colored the notes uneven without clarity.   Many cheap musical instruments and speakers behave this way.   An instrument or speaker may be in-efficient having a flat response the notes even but lacking dynamic expression and responsiveness, requiring to be played harder to be heard.

dB/mW

A 3dB efficiency difference between speakers is double or half the power, but 3dB is only heard as a slight difference in loudness.   A 10dB efficiency difference between speakers is equal to x10 or 1/10 the power. 10dB is only heard as double or half as loud.   It is easier and less expensive for manufacturers to make low efficient speakers have a flat response.   With small domestic sound systems this may not be a problem as modern high powered amps are inexpensive.   With large professional sound systems speaker efficiency is very important.

5. Inter-modulation   Linearity and Intelligibility.   Muddle-ness and inter-cluttering within the music which makes it difficult to discern detail.   With continued listening this becomes fatiguing.   This is caused by interference within and between speaker components, compounding as the power increases.   Primary inter-modulation is where a single speaker is used beyond 3 octaves or full range.

The large cone movement (excursion) for low-frequencies, modulates the middle and higher frequencies, causing them to sound dirty.   Lobe and node distortion is caused by high-frequencies creating secondary vibrations and chaotic resonances within the speaker cone, causing it to sound harsh and screechy.

Speaker linearity

This problem is most noticeable at venues and Live concerts where not a single word can be understood.   We have become so accustomed to hearing inter-modulated sound from distorted sound systems that we have accepted it as being normal.

6. Dynamic power response   Is the ability for the sound fidelity to remain intact between low and high power.   It is not possible for one amplifier driving different speakers (passive) or one speaker used beyond 3 octaves to achieve this accurately.   The majority of sound systems are passive due to cost and the fashion for systems to be small.   For a system to sound consistent and accurate at all power levels it must be active.   Each speaker driven by its own amplifier and matched in efficiency, power and dispersion.   This also eliminates cross interference within and between the speakers (inter-modulation).

Spectral ballance
Spectral ballance 2

Professional sound systems use horns and compression drivers for extra power and efficiency.
More information in Horn Systems page.

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© 2013 Lenard Audio Institute
High Engineering