Oestex ©
GUITAR
AMPS PAGE
SECTION 6:
THE LOUDSPEAKER
IMPORTANT NOTICE:
Contrary
to popular belief, there is no limitation on the number of loudspeakers that
can be connected to an amplifier – in fact the more speakers the better the
system works because the sound is disbursed over a larger area.
A
common misconception that large loudspeakers drain more power from an amplifier
than small loudspeakers. This is not true.
Another
misconception is that only a small number of loudspeakers can be connected to
an amplifier. This is not true.
This is because the Amplifier is a GENERATOR and the
loudspeaker is the LOAD on the Amplifier. When more than one loudspeaker is
driven, the power is shared between them so there is no power loss – in fact
because the cone of each loudspeaker does not need to travel as far, less
energy is required so it becomes more efficient. The frequency response will
also improve and with less distortion.
The system power handling capacity will increase according to
the rating and load sharing of each loudspeaker. Load is shared in proportion
to the rated impedance of each loudspeaker in the system.
Note: Tube Amplifiers requires accurate matching of the
ACTUAL loudspeaker load to the nearest RATED or NOMINAL Output Transformer
secondary winding impedance. Do the maths.
Loudspeakers are rated for both power handling capacity and
SPL. The diameter of the loudspeaker does not affect the load on the amplifier,
because the loudspeaker voice coil
impedance will automatically respond to the power available to it from
the Amplifier. A 12 inch driver will not sound louder than a 10 inch driver
having the same SPL efficiency – but it will sound different because the cone
is thicker and heavier.
Loudspeaker magnet size is not an indicator of performance –
refer to the selected speaker specs.
Loudspeakers are air pumps so it follows the larger the
diameter the more air volume is pumped.
To prevent acoustic energy short-circuiting from the front to
rear of a loudspeaker the loudspeaker MUST be mounted on a suitable baffle
board or mounted in a cabinet designed to match the loudspeaker characteristics.
A loudspeaker has a limited cone excursion capability.
Over-driving a loudspeaker will destroy it mechanically.
A loudspeaker has a limited electrical current capability.
Over-driving a loudspeaker will destroy it electrically.
Each loudspeaker diameter has an optimum voice coil diameter
v frequency response. The larger the voice coil diameter the better the bass
response and lesser the high frequency response. Compare the specs.
The way it works is that the
amplifier is an electrical generator that supplies AC power to its load, in
this case a loudspeaker. Thus the amplifier is a supplier of electrical power and the loudspeaker is a consumer of electrical power.
This works in the same
way that a stove or heater or lamp draws power from the mains supply. The
amount of power consumed is proportionate to the rated device power consumption
– e.g. a 40 watt lamp will consume twice the power as a 20 watt lamp on the
same circuit.
So how does the amplifier
know how much power to supply
to the loudspeaker?
The amount of power supplied by the amplifier is
determined by the voltage it
delivers to its loudspeaker load
impedance. In our case for
guitar amplifier applications the load is a loudspeaker having a typical impedance of 2, 4, 8, 16 or 32
Ohms.
The voltage across the load impedance terminals in turn
determines the current drawn
by the load because a speaker presents a notionally fixed load impedance. See Ohm’s Law
Voltage output from an amplifier can be anywhere from zero at zero
signal input to its maximum voltage
output capability at full signal input.
The actual voltage output is determined by
either the Amplifier’s Gain Control or the Guitar’s on-board “Volume” (gain)
control.
WATTS = Volts x Amperes.
(Amps) AC WATTS = DC WATTS
So if the amplifier
supplies the Volts what then determines the amount of current (Amperes) supplied?
The answer is the LOAD,
which is the loudspeaker impedance – usually 4 or 8 Ohms.
The formula to calculate
this is AC Watts = voltage (E) squared divided by the load impedance (R).
e.g. a 40 watt amplifier
driving a 4 Ohm speaker will deliver 12.65 Volts (V) AC to the loudspeaker.
That is 12.65 squared = 160/4 = 40
e.g. a 40 watt amplifier
driving an 8 Ohm speaker will deliver 17.89 Volts (V) AC to the loudspeaker.
That is 17.89 squared = 320/8 = 40
The current will be
Current (I) squared x load impedance (R)
e.g. a 40 watt amplifier
driving a 4 Ohm speaker will deliver 3.16 Amps (I) AC to the loudspeaker. That
is 3.16 squared = 10 x 4 = 40
e.g. a 40 watt amplifier
driving an 8 Ohm speaker will deliver 2.24 Amps (I) AC to the loudspeaker. That
is 2.24 squared = 5 x 8 = 40
Another common
misconception is that too many loudspeakers will harm the amplifier. This is
not true.
It follows that because
the actual load is the sum of its parts there is no limit to the number of
loudspeakers that can be driven by one amplifier. So long as the load presented
to the amplifier is the correct load to suit the Output Transformer setting/tap
then the amplifier does not know if it is driving one speaker or ten.
The power output is
simply spread/divided across the number of speakers connected to it.
The only proviso is that
when speakers having unequal rated impedances are mixed together then the power
supplied to each will not be identical – e.g. In the case of a 4 Ohm and an 8
Ohm speaker connected together in series,
the 8 Ohm unit will receive the highest share of the supplied voltage so will attract the
greater power. The 8 Ohms unit will
receive twice the voltage of the 4 Ohm unit and therefore four times the power.
That is the 4 Ohms unit will receive only 1/3 the total power. In this situation
speaker power handling capacity becomes an important consideration.
Conversely, in the case
of a 4 Ohm and an 8 Ohm speaker connected together in parallel, the applied voltage
is the same for both speakers but the 4 Ohm unit will consume the highest share
of the supplied current so
will attract the greater power. In this case the voltage supplied from the
amplifier is the same for both loudspeakers.
An alternative if the Output
Transformer is multi-tapped, is to supply the 4 Ohm unit from the 4 Ohm tap and
the 8 Ohms unit from the 8 Ohms tap.
It is not wise to mix
speakers of differing impedance or efficiency in the one cabinet, because the
more efficient speakers will tend to acoustically drive the weak units. When
the speakers are different it is better to use separate cabinets.
The easiest way around
this is to use identical speakers for multiple cabinet systems.
SELECT THE LOUDSPEAKER
In traditional full range – 30 Hz to 20 kHz –
home hi-fi systems, the bass loudspeaker (woofer) intended to reproduce down to
about 30 Hz requires a large enclosure of at least 8 cubic feet volume, made
from heavy timber and suitably padded internally with damping material.
However this is just not practicable for the
professional musician because the box is too big and bulky, too heavy and will
not readily fit into our motor vehicle or onto a stage.
Large, heavy enclosures are not likely to fit
comfortably onto a standard venue stage after the other musicians have taken
their share of available space. Large, heavy enclosures can also present a
safety hazard if their centre of gravity is too high, or their height to base
dimensions ratio is out of proportion (read tall, narrow or thin
enclosures).
Graphic equalisers and tone controls may be
used to compensate for bass rolloff in the lower register - if at all
What we need for the professional guitarist
musician is a small enclosure having a high power handling capacity across the
guitar frequency range – nominally 80 Hz to at least 1kHz to capture the
fundamental tone for each note.
But it is the harmonics that give sound its
distinctive tone and unique identity, so to capture the harmonics of the upper
register the system frequency response needs to extend to at least about 3 kHz
to capture the fourth harmonic across the first 12 frets.
Most importantly it must have an even
frequency response so that as each note is played moving up or down the musical
scale, the sound loudness will be constant.
That is a big ask.
However, all is not lost because buried in
the annals of textbooks and papers, is a wealth of information to help us in
our quest to build our ideal guitar amplification system.
SPL RATING
One useful indicator of power needs is the
loudspeaker's SPL (sound pressure level) rating. This rating is determined by
measuring sound pressure (acoustic) energy under standard defined conditions -
usually at 1 metre from the radiating surface with an electrical input power of
1 watt RMS at 400 Hz (cps).
The SPL rating enables a quick reliable
comparison to be made between different choices of loudspeaker. For example a
speaker having an SPL of 87 db will require TWICE as much electrical
input power (RMS Watts) as a similar size loudspeaker having an SPL of 90 db to
reproduce the same sound pressure energy level (i.e.
"loudness") in the listening room.
In other words a 10W amplifier/90 db
loudspeaker combination will sound just as loud as a 20W amplifier/87 db
loudspeaker combination!!! Now there is some food for thought!!
If you can afford electrically efficient
loudspeakers like JBL, Altec, Electrovoice, or Eminence, having a sensitivity
of 105 db SPL or more, then amplifier power needs will be hugely less - unless
you like very loud music!! Believe it or not, such a loudspeaker will
need only 1/63 rd the power of that needed to drive an 87 db efficient
loudspeaker. That is, your JBL, Altec, Electrovoice, or Eminence, will be
effectively 63 times more powerful with the same amplifier!!
Another way of saying it is that a 105 db
loudspeaker driven by a 1 watt amplifier will be just as loud as an 87 db
loudspeaker driven by a 63 watt amplifier. Time to throw away those old
inefficient drivers!!
Standard lab tests on tube amps for power
output are always performed using resistive loads (which present a constant
load impedance to the amplifier) at typically 400Hz. This frequency coincides
with the standard test frequency for loudspeakers. It also approximates the
frequency range where rated loudspeaker impedance is expressed in its rated
value in Ohms. – e.g. Performance at 400 Hz is well above the lower range -
where we need strong performance.
For further details of the relationship
between power and decibels please refer to the Decibel Chart
provided.
A word of warning though - increased
loudspeaker efficiency means higher output levels for background hum, noise and
hiss. More attention will be needed to the amplifier's design if an acceptable
result is sought. Ultimately there will be a trade-off between the competing
needs. There is some practical justification then in using a low-cost amplifier
to drive a low cost loudspeaker!!
When comparing SPL ratings ensure they have been
measured using the same method. Some manufacturers quote SPL ratings at full
rated output, which will obviously look better than the SPL produced from 1
watt at 1 metre.
NOTE: Although some specific brands of
loudspeakers have been mentioned above as being (relatively) "high
efficiency", not all of the models produced by these manufacturers are in
that category.
SPL ratings for specific brands and types of
loudspeakers vary widely so it is recommended that the manufacturer's catalogue
data sheet be studied before forming conclusions.
POWER LEVELS
It is wise to limit power output to a level
that the loudspeakers can withstand comfortably, noting that loudspeaker power
ratings are generally set at 400 Hz and assume the loudspeaker is installed in
a properly designed enclosure that provides adequate cone loading and damping
to limit system resonance and prevent cone overshoot.
As a rule of thumb, it is a wise precaution
to halve the loudspeaker manufacturer's ratings, particularly if they are
published as "peak" watts.
Music power, IHMF and PMP etc. ratings should
be appropriately derated to RMS equivalent.
Remember, our SPL rating is measured at 400
cps. The actual frequency response over the full frequency range - particularly
in the lower register - needs to be evaluated.
It has been observed that some manufacturers
slightly exaggerate the power ratings of their loudspeakers - computer speakers
are a current example - e.g. 1200 W from a 4" loudspeaker - so look for documented
verification or test reports of quoted or claimed ratings.
Always compare speaker ratings on RMS rated
power handling capacity. RMS is an internationally recognised unit of
measurement and provides a standard way of rating and comparing.
On the other hand, the commonly used rating
of Peak Power has different meanings in different places. It can be
anywhere from 1.414 x RMS to 2 x RMS.
PMPO, or Peak Music Power Output, means the instantaneous power the
unit can handle. It essentially relates to the current the unit can handle for
a very short interval of time before it fuses or the speaker mechanically
self-destructs. PMPO is totally useless and meaningless rating for BASS
guitars.
The short-time current a conductor can carry
during a short-circuit - such that the conductor heats to a predetermined
temperature during the duration of the short-circuit, is defined as I squared x
T (current squared x time = a constant). The value of the constant is
irrelevant - except to determine time.
The International Standard time for determining
short-circuit ratings (which require the conductor to remain intact and not
suffer visible damage) is ONE second, so for practical purposes the temperature
rise of a conductor is determined by the square of the current - which is also
the formula for determining power.
We can thus say that the power handling
capacity of a loudspeaker (or any wire based device such as a transformer) is
directly proportional to the electrical current in - up to its maximum
capability, when the conductor will fuse (in a loudspeaker this is the voice
coil winding)
PMPO ratings simply take an RMS value and
divide it into a very short time interval.
If we use that one second nominal time as a
basis for determining a PMPO rating, then for a 10 watt RMS rated loudspeaker at
400 Hz (which is the standard rating frequency), we can calculate a PMPO rating
for a time interval of one half of an RMS cycle (1/800th of a second for a 400
Hz signal) by the formula:
1 second multiplied by 800 = 800 x 10 watts =
8000 Watts
This is how 10 watts increases to 8000
watts!!
Obviously the rating will vary dramatically
with frequency.
If we increase the frequency to 1,000 Hz the
PMPO will increase to 20,000 watts.
Remember ancient Australian saying:
"BULLSHIT BAFFLES BRAINS" !!
LOUDSPEAKER DESIGN AND CONSTRUCTION
Many modern loudspeakers have huge magnets
and rock hard cones made from heavy plastic, metal or carbon fibre materials.
Unfortunately none of this indicates
efficiency and it may well be that a loudspeaker that looks rugged and chunky
with huge magnet actually has a low SPL performance resulting in a severe
efficiency loss matching the lower price tag.
Often it will be observed that in
loudspeakers up to 15 inches in diameter, those having small magnets and paper
cones actually outperform those with large magnets and plasticised carbon fibre
cones - particularly in the lower bass region. This is because the heavy cone
requires more power to drive it. More power input means more electrical current
through the voice-coil. More current means larger diameter wire. Larger
diameter wire is heavier and produces a lower ampere/turns ratio in the
magnetic field. Heavier wire means more mass to shift. More mass to shift
requires a stronger magnetic field. A stronger magnetic field requires a
larger, stronger magnet. Because there are physical and cost limitations on all
of these parameters, the usual approach is to simply use larger wire, which
translates into less electrical efficiency in the electromotive energy
available to drive the cone.
Another relevant design feature with modern
high-excursion cone drivers is that when the cone is driven to its extremity,
the voice coil must still be within the magnetic field of the pole piece to
electro-magnetically drive and/or control cone movement. It is obvious that the
more the cone travels in and out, the longer the voice coil must be, which
means there will be fewer turns of wire actually in the magnetic field of the
driver at any instant of time. This translates into lower efficiency - unless
design techniques are incorporated to maintain the electro-mechanical
efficiency of the transducer.
So our modern inefficient 100 W RMS
loudspeaker may need 100 W to drive it - but not to actually deliver more sound
pressure output than the 20 W loudspeaker of yesteryear.
Cone suspension systems vary markedly but, as
a rule of thumb, modern rubber surround suspensions used in sub-woofers and car
(automobile) audio speakers are not as reliable as the traditional
"accordian" suspension. Cellular foam rubber suspensions tend to
literally fall apart after a relatively short period of time - particularly if
exposed to sunlight, however solid neoprene rubber suspensions offer good
performance and a long life.
Treated linen surrounds are best for most musical
instrument applications.
When selecting a loudspeaker, carefully check to ensure the
cone can move reasonably freely - but not "floppy" - and the spider
suspension (usually yellow treated fabric around the voice coil at the back of
the cone) is of large diameter and capable of free travel. To check, gently and
carefully grasp the cone on opposite sides using the thumb and forefinger then
check movement. The coil should not rub or scrape.
The spider should sit flat to position the
voice coil centrally in its magnetic field.
It will be observed by the very critical
listener that paper-coned loudspeaker sound qualities vary throughout the
seasons. In summer, when cones dry out, they tend to give brighter, cleaner
sound, and sound louder. But in winter, or wet weather, the moisture content in
the cone increases, making it heavier and therefore less sensitive to transient
peaks, producing a 'duller" sound.
Bass response will tend to improve slightly
and resonance reduce from a heavier cone. This feature is incorporated into
heavy duty professional bass speakers, which have heavy cones, heavy voice coil
wire and large magnets to maintain efficiency.
Note however that a well designed loudspeaker
of modest power rating - e.g. 25-30 W RMS - does not need a large diameter
voice coil or large magnet to be an effective transducer.
For high fidelity reproduction, it is far
better to have a light cone move little than a heavy cone move a lot - the
further the cone travels the less linear the sound reproduction will be.
It follows that for a given level of power
output, a large diameter loudspeaker will be more linear than a small diameter
loudspeaker.
2.4 LOUDSPEAKER
FREQUENCY RESPONSE
The Guitar has a useable frequency range of
about 3 octaves.
Bottom E is at about 87 Hz and top E at the G
string is about 329 Hz.
When the instrument is tuned to A440 pitch of
440 Hz:
The 6th
string Bottom E is set to 82 Hz
The 1st string
Top E is set to 329 Hz
Using the above
chart, the harmonics of 6th string Bottom E (82Hz) occur at approximately 165 (2nd), 248 (3rd), 330
(4th), 413 (5th), 495 (6th), 578 (7th), 660 (8th), 743 (9th), 825 (10th) Hz.
Using the above
chart, the harmonics of 1st string Top E (329Hz) occur at approximately 658 (2nd), 987 (3rd), 1316
(4th), 1645 (5th), 1974 (6th), 2303 (7th), 2632 (8th), 2961 (9th), 3290 (10th)
Hz.
However, allowing for a top fundamental
frequency of say 659 Hz at the 12th fret and harmonics to the 10th
harmonic of that fret to 6,590 Hz, we need a wide range speaker to accurately
reproduce the full for guitar.
A good quality cone speaker will do the job
well. In fact a good PA single or twin cone speaker will be ideal.
But guitarists do not think that way and want
a speaker designed specifically for them – i.e. they prefer to see the label
say “Guitar Speaker”.
In fact, as is shown below in the frequency
response curve graphs for some popular respected guitar speakers, the useable
range extends only to 5 to 6kHz at best. The result is loss of all harmonics
above that limiting frequency. Some compensation can be achieved with the use
of a treble tone control but this has limited effect.
However there is more to guitar tone than
fundamental frequencies and some degree of midrange is desirable for
colouration or “brightness”.
From the above and the frequency response
curves below, we can assume that because the typical guitarist plays mostly
around the middle of the neck between the 5th and 12th
frets (Key of A) he does not need to hear the higher harmonics and is satisfied
with midrange performance to about 3 kHz.
That translates into a simple single-cone
speaker system supplied from an amplifier having a flat response to less than
10 KHz.
Those requirements give us a good guide to
the quality and performance of the Output Transformer requirements.
Note: If higher harmonics are sought it is
essential to use a wide-range speaker delivering good top-end response.
Separate tweeters are not viable so a solution is in twin-cone speakers.
2.5 FREQUENCY RESPONSE
AND POWER RESPONSE
The POWER response, or sensitivity expressed
in SPL (Sound Pressure Level), of a loudspeaker system is critical because a difference
of +- 3 db equates to a power change of double or half the electrical watts
from the datum at any point upon the frequency response curve.
A difference in SPL
of +- 10 db equates to an electrical
power input change of ten times increase or 1/10 the electrical watts from the
datum.
A difference in SPL
of +- 20 db equates to an electrical
power input change of one hundred times increase or 1/100 the electrical watts
from the datum.
The effect of this generic performance in
loudspeakers is that a speaker having a rated SPL of 90 db at 1 watt electrical
power input will require an increase of ten times that power – or 10 watts – to
achieve the same acoustic loudness (SPL) as a speaker rated at 100 db at 1
watt.
Theoretically, if a band plays at 100 db
average SPL the guitar will need only a 1 watt amplifier when using a speaker
rated at 100 db at 1 watt.
But the 1 watt is measured at 1 metre from
the speaker itself.
Unfortunately SPL drops away very quickly
with distance, people and furnishings soak up sound, and rooms often have sound
absorbing ceilings and/or carpeted floors.
Typical stages have carpeted floors to dampen
vibration and for comfort. These also soak up acoustic energy.
Now if the background noise level in a venue
is say 60 db, then for the band to play at a typical comfortable SPL of 100 db,
the amplifiers need to drive the speakers to maybe 130 db – (extremely loud).
30 db increase equates to a power increase of
1,000 times.
That means our 100 db sensitive speaker
requires 1,000 watts input - impossible to attain because the loudspeaker
will not be able to handle that much power.
In the case of the 90 db SPL speaker it does
not have a chance because the power input will need to be 10,000 watts for the
same SPL of 130 db.
Similarly, a frequency response difference of
3 db equates to a need for twice the electrical power to attain the same
"loudness" - refer Decibel Chart for
power ratios.
So a better (i.e. 'flatter") frequency
response curve means cost-free power output. It also means more natural sound
where individual notes across the musical scale are reproduced with more equal
loudness.
SYSTEM 1:
An example of this phenomenon is shown in the
following typical frequency response graph for a high-quality low-frequency
reproducer - i.e. "Woofer" - SYSTEM - i.e. Woofer loudspeaker
mounted in an enclosure.
IMPORTANT NOTE: This graph does not show the
frequency response of the loudspeaker by itself, and include enclosure resonances
and enclosure design impact upon loudspeaker performance - which is what the
listener hears in any case.
Note the average
SPL of 100 db across most of the usable frequency range indicates unusually
efficient performance over a very flat response curve - by any measure this is
a "top of the range" LF Woofer driver unit.
Bottom E at 40
Hz is about 15 db down, which can be easily offset with a tone control or
Graphic EQ
Loudspeaker
System 1
In this case, the loudspeaker is mounted in a
medium sized vented/ported enclosure, resulting in two peaks at 33 Hz and 66 Hz
about the natural resonance of about 43 Hz.
Note: Although HF response rapidly falls away
above 3,500 Hz, the frequency response needs described above indicate this
loudspeaker is suitable for bass guitar.
It may also be suitable for lead guitar
because the useable high frequency performance extends to over 3 kHz however it
may not sound bright and clean because the cone will be thickened for heavy
bass work.
The important feature to note with this
enclosure configuration is that even when using a very high-quality transducer
offering an average SPL of 100 db, the efficiency at system resonance (43 Hz)
drops to 87 db - a reduction of 13 db on the average. Although this may appear
to be an alarming performance, it is very typical of vented-enclosure
loudspeaker systems.
If the lower register is equalised to flat
response, supported by adequate amplifier power, then this system shows
remarkable flatness across the entire bass guitar range of fundamentals and
harmonics.
The slight increase in SPL in the mid-range
is also of great benefit in speech applications.
It is also noteworthy to comment that the
enclosure used for the above test is relatively large by volume. Typical small
volume vented enclosures will demonstrate substantially higher peaks above and
below the natural resonance - see paper on Loudspeaker Enclosures
design.
Patchy LF response is definitely noticeable
to the musician and should be avoided where practicable.
SYSTEM 2:
When mounted in an infinite baffle
enclosure (i.e. sealed box) of generous proportions, or in a wall, the
response curve will change to something like the following:
Loudspeaker
System 2
Note: This is not the same driver unit as per
the above Loudspeaker System 1 but the effect is well demonstrated.
The useable +- 3 db frequency range is
nominally 50-1600 Hz.
It is also of interest to note the average
"flat" section of the frequency response is about 3 db down on the
first graph - System 1 - this system thus requiring twice the input driving
power for the same loudness.
This loudspeaker may not be suitable for lead
guitar because the midrange is peaky and falls away rapidly above 1500 Hz,
where guitar harmonics are rich..
SYSTEM 3:
This system is included to demonstrate a
different low-frequency and high-frequency roll-off characteristics to System
2.
Loudspeaker
System 3
The HF response of System 3
rolls off rapidly from 1,000 Hz.
Consequently, this loudspeaker is NOT suitable for lead guitar because
there is no definition of harmonics. It may be OK for rhythm guitar.
GUITAR
SPEAKERS.
Major manufacturers produce a range of
discrete guitar specific loudspeakers, tailored for individual characteristics.
The general characteristic is that they are
loud, clean and bright.
Here are a few examples:
The published frequency response for the
popular Celestion Blue 12 inch driver shows a steady rolloff in all open
strings (82-329 Hz). It is -7db at 82 Hz (open bottom E).
Using 1 kHz as a reference, the useable
frequency range is about 100-6kHz – nicely matching a guitar fundamental tones
plus harmonics. Due to the 5-10db rise in midrange SPL the sound will be
bright.
The gradual bass rolloff from 500 Hz down and
boost in the 2k-5kHz range are deliberate. As noted above, a 10 db increase in
SPL is huge in power terms.
Eminence
Professional Series Delta Pro
This 12 inch driver offers 97-107 db SPL
across the useable range, is relatively flat but with an accent towards the
mid-range and highs.
Eminence recommends this driver for
professional audio in both sealed and vented enclosures. Ideal for full-range,
mid/hi, and monitor wedges.
In other words a good general purpose
all-round loudspeaker.
The Eminence
Patriot Copperhead 10 inch driver offers a quite different response curve,
with a reasonably progressive upwards slope on the full guitar frequency range
– ideal for bright and clean lead guitar
Eminence says the Copperhead guitar speaker
combines the best of two tones, balancing country honk with a touch of classic
blues. Extremely balanced vintage tone for smooth, driven leads and clean
rhythm.
The Celestion G10 Creamback 10
inch driver offers a smoother, more even response, relatively flat across
the guitar frequency range. It is -5db at 82 Hz (open bottom E).
Using 1 kHz as a reference, the useable
frequency range is about 80-6kHz – nicely matching a guitar fundamental tones
plus harmonics. Due to the 5-7db rise in midrange SPL the sound will be bright.
Average SPL of this driver is nominally 5db
lower than the Celestion Blue 12 inch driver shown above, resulting in the
amplifier needing to deliver nominally
four times the power output to produce the same loudness as the Celestion Blue
12 inch unit. Food for thought.
This speaker would be suitable for either
lead or rhythm application.
2.6 TWEETERS
It is customary in two or three way
loudspeaker systems to rate the whole system and not the individual components.
Hence a tweeter from a 100 W RMS hi-fi system may actually only have a 10-20 W
RMS rating on its own (within the restricted frequency response of the
tweeter), because in typical music the power output is substantially more in
the lower register than in the HF range.
It is common to see lower priced drivers
rated in "Music Power" in an attempt to match low and high frequency
power ratings when processing a "music" signal.
The tweeter and/or midrange unit in fact may
have an even lower power rating if it is supplied through a system crossover
network, which typically soak up 50% of power input (-3 db insertion loss).
In the case of the three examples above, for
an even full-range frequency response, the Tweeter driver would need to have an
efficiency matching the Woofer driver unit. In all three cases
illustrated, the Tweeter will need to be of very high quality - i.e. SPL 95-100
db - to match the Woofer performance.
WARNING: In the case of guitar
amplification, Tweeters cannot be successfully used in the same way as in hi-fi
systems. The guitar pumps out high frequency signal at much the same power
level as midrange or low frequency signals. This means that Tweeters must have
a similar power rating as woofer units in the same system.
PIEZO tweeters usually have very high power
ratings but exhibit very patchy frequency response characteristics, rendering
them unsuited for high quality systems.
One way around this is to use twin-cone
speakers, however these are generally intended for PA applications and not made
with power ratings high enough for BASS. Of course multiple twin-cone units may
be practically viable for some users.
There are however exceptions designed for
musical instrument applications.
The P Audio
10 inch models HP10W and HP10T illustrate the
difference between identical professional musical instrument loudspeakers with
and without a "whizzer" tweeter cone. This loudspeaker spec. sheet
has been chosen as an example because it directly compares identical
designs which are typical of this class of loudspeaker.
Note the improvement in HF performance above
3,000 Hz in the case of the twin cone unit HP10T. This characteristic defines
the advantage of a "whizzer" cone, which enables the loudspeaker to
be driven at full power across all frequencies within its range.
IMHO twin-cone speakers are the very best
option in lead/rythm guitar applications, where strong harmonics are desirable.
In the case of BASS guitar, twin-cone loudspeakers can be used
satisfactorily but are not necessary - unless strong harmonics are required by
the musician.
Note: As with all cone tweeters,
Whizzer" cones are directional, so as the listener moves away from the
frontal axis the SPL reduces, however in the case of bass reproduction this is
not likely to be an issue. However if the amplifier is to be used as a backup
for other instruments or voice, then twin cone loudspeakers are a good
investment.
.
.
2.7 MULTIPLE LOUDSPEAKERS
Contrary
to popular belief, there is no limitation on the number of loudspeakers that
can be connected to an amplifier – in fact the more speakers the better the
system works because the sound is disbursed over a larger area.
Refer
to the IMPORTANT
NOTICE”
at the top of this page.
Multiple loudspeakers - preferably of
identical design and construction (although satisfactory performance can be
obtained with dissimilar but tonally compatible units) - connected in series or
parallel as needed - are a very efficient means of adding more power handling
capability. It is usually cheaper to add an extra loudspeaker than to double
amplifier power. Conversely it is usually cheaper to add an extra low-cost
loudspeaker to double power handling capacity than to instal a single high cost
higher performing unit. Do the sums!!
There is a cutoff point though - at about 100
W RMS continuous, where the available options for loudspeakers
disappears very rapidly.
Few manufacturers make loudspeakers capable
of handling both the electrical and physical stressors created at this power
level. Usually they come in giant sizes – i.e. 18, 21 and 24 inch drivers,
which are beyond the bounds of practicality for most users. However a true 100
W RMS can easily be handled continuously by four 12 inch drivers, mounted in a
single large enclosure, or in multiple enclosures.
15 inch drivers are of course superior to 12
inch for bass.
Generally speaking, the bigger the better.
Serious consideration must be given to the
loudspeaker behaviour at the power level intended to be reproduced. The
validity (genuineness) of manufacturer's ratings needs to be carefully
researched because it is too late to discover that a manufacturer has
exaggerated ratings after the loudspeakers have been purchased and installed.
A little headroom can save an expensive
loudspeaker from self-destruction resulting from electrical or physical damage.
.
.
2.8 DISTORTION
Loudspeakers are notorious for generating
distortion - of many kinds - so the more headroom the better. It follows that
multiple speakers will require less cone travel for a given level of acoustic
energy (loudness).
Twin-cone loudspeakers incorporating a whizzer
cone, tend to change their frequency response with increasing cone travel.
At low power outputs the cone and voice coil are very much controlled by the
magnetic circuit, but at high cone excursions the cone may over-travel, be less
damped, and lose relative drive to the high frequency whizzer cone -
resulting in diminished high-frequency response and increased intermodulation
distortion. However twin cone loudspeakers offer simplicity of enclosure
design, high power output, predictable response and are a preferred low-cost
option for public address systems, guitar amplifiers, car audio and home sound systems
where full-range reproduction is sought. There is no loss of power into
crossover networks.
In twin cone units it is preferable for the
"whizzer" cone to be of curvilinear, exponential or logarithmic curve
shape - ie trumpet like form. Plain simple cones do not provide linearity in
frequency response or ideal tonal properties or SPL.
Some researchers have suggested that mixing
twin-cone loudspeakers with tweeters does not work well, however it has for me
so I am converted to accepting mixed cone systems.
.
.
2.9 CONE EXCURSION
Remember that the more the cone excursion,
the greater the cone over-travel and the greater the back EMF into the
amplifier.
Cone travel is a product of its kinetic
energy so basically the heavier the cone the harder it is to stop it and
reverse its direction - in response to the driving signal power.
If the amplifier is configured with negative
loop feedback, the higher back-EMF will cause the feedback system to work
harder, which in turn will cause the tonal properties of the sound to change as
the amplifier tries to correct the effects of back-EMF from the loudspeaker
system.
Another feature of excessive cone travel is
substantially increased intermodulation distortion - not so much of a problem
for bass guitar or electronic organ, but a definite challenge for hi-fi.
A simple solution to this effect is to use
multiple loudspeakers in a totally enclosed or suitably loaded enclosure. The
internal air pressure dampens cone travel in both directions, resulting in
substantially higher efficiency and reduced back-EMF.
Research has demonstrated that the more cones
included in an array of loudspeakers, the greater the low-frequency response
from the system.
Watch cone movement - in and out. All
loudspeakers have finite physical limitations.
To protect loudspeakers from damage and to
extend useful service life and reliability, it is better to use multiple
drivers with cones travelling less than a single driver with cone travelling
more.
Damping of the cone movement can be
electrical via amplifier feedback systems and acoustic via cabinet design,
however since we want to pump air at low frequencies, damping can be
counterproductive.
.
.
2.10 CROSSOVER NETWORKS
Another design element to consider is that
commonly used crossover networks soak up power so it is an essential
pre-requisite to determine how much audio power will be lost in the crossover
network and add that to total needs.
Commonly seen losses in crossover networks
are in the region of 3 db, which is another 50% of power gone down the drain,
never to be heard of again!!
Some crossover networks are worse than that.
Crossover networks also suffer roll-off at
the crossover frequency – i.e. roll-off in both upwards and downwards
responses, so there tends to be a hole or dip at the crossover frequency.
To prevent power loss for BASS guitar we do
not want a crossover network in the loudspeaker system at all.
If top end is wanted, then it is better to
bi-amp with an electronic crossover up front.
.
.
2.11 ENCLOSURES
Horn systems are significantly more
acoustically efficient that direct radiator systems - in the order of 30%
compared to 5% - i.e. up to six times more efficient. Accordingly, the
amplifier drive requirements for a horn system may be significantly less.
However horn systems have different "sound" or tonal qualities to
direct radiator systems and that may not be acceptable to all listeners.
All loudspeaker systems require adequately
damped cabinets to minimise resonance from the cabinet material and shape. This
can occur at any audible frequency - not just in the bass register.
In the 1950's it was the norm for enclosures
to be as large as was practicable. Totally enclosed loudspeaker enclosures
(infinite baffle) require large volume for low resonance - 10-20 cubic feet
volume was considered the minimum essential for resonance free solid bass
response down to 30 Hz with a 12" speaker (it still is).
Guitar speaker enclosures need not be so huge
and with the right choice of driver unit an open-backed cabinet can give good
results.
Vented/Ported enclosures (with or without
tunnel or passive radiator) offer substantial reduction in enclosure size but
produce two resonance peaks – one above and one below the natural loudspeaker
resonant frequency. These system peaks also cause similar rises in loudspeaker
impedance, but the good news is that the peaks have less amplitude than for an
unloaded cone, resulting in a flatter power response from the amplifier. This
is important for a guitarist or bass guitarist because we want all notes on the
scale to have equal loudness.
Finally, in my opinion it is wiser to spend
more on the loudspeaker than the amplifier, because the loudspeaker will have
greater effect on the final sound. As explained elsewhere on this site a 3 db
improvement in loudspeaker efficiency (SPL) saves half the amplifier power. In
other words, a 100 db SPL speaker requires only half the power input as a 97 db
SPL speaker. It is far more economical to buy a higher performing speaker than
an amplifier having twice the power.
Note: On the same scale SPL/power ratio a 99
db SPL speaker requires four times the amplifier power than a 105 db SPL
speaker to achieve the same SPL loudness level to the listener in the room.
Note: In the current global trading
environment - price and performance are not always directly connected.
For an informative study into loudspeaker
enclosures for guitar amplifiers see Jim Lill’s videos at https://www.youtube.com/watch?v=-eeC1XyZxYs
and https://www.youtube.com/watch?v=IC96P3icayc
IMPORTANT:
1. It is advisable to mount large enclosures
on wheels or castors to facilitate loading, unloading and transportation to a
venue, as well as moving around and positioning on stage.
2. It
is advisable to instal retractable or external handles on each side of a
cabinet to facilitate lifting. The joint between a handle and the cabinet
should be sealed to prevent air leakage from inside to outside of the cabinet
because this will affect speaker performance dramatically.
3. To
protect the speaker cone from impacts or tears it is essential to cover the
front baffle with either grille cloth or wire mesh.
EXTREMELY IMPORTANT:
A cone loudspeaker is a
simple air pump. Cone loudspeakers rely upon a tight seal between the front and
rear of the cone to prevent the acoustic energy short-circuiting around the rim
of the frame. The free air power handling of a loudspeaker is only a small
fraction of the rated power handling capacity when properly mounted and loaded
in an enclosure because the lack of acoustic loading on the cone can result in
over-excursion of the cone and potential physical damage at quite low power
levels - particularly at the resonant frequency.
In all applications where
power levels and cone excursions may be high, it is desirable to seal the metal
rim of the frame to the cabinet with a sealant such as black mastic compound -
particularly when the speaker is mounted on the front of the baffle - i.e. on the outside of the cabinet.
.
.
2.12 TONE
Tone controls and/or graphic equaliser are
essential for a guitar system to customize the sound sought.
IMPORTANT: The low frequency boost should
occur at 80 Hz or less. A lower frequency is preferable to avoid over-accentuating
the low notes.
If the boost frequency is too high, the bass
is likely to sound boomy or hollow. When coupled with a poor quality speaker
system, the result will be hollow bass devoid of depth of tone.
REMEMBER - ALWAYS TAKE CARE WHEN WORKING WITH
HIGH-VOLTAGE - DEATH IS PERMANENT!!
Contact:
"electron"
Email: contact
This page is located at
http://www.oestex.com/tubes/guitaramps/6_speakers.html
This page last modified 13 July 2023