The 75's should be plenty loud.
Unless you're playing bass, the cab should not be ported and when it is ported,
the port needs to be tuned so the back wave is in tune with the front wave.
You likely have phase cancellation occuring due of the port. Try closing it up
or you have to tune the port with the proper tube length so the resonant frequency
of the bass signal is in phase with the front of the speaker. There are several ways of doing this.
You can google up the details if you dont know how already. One is to use a multimeter, punp
the resonant frequency into the cab while monitoring the signal, and adjust the tube length
till the meter gives you maximum voltage. The other would method which is better is to use
two mics, one on the port and one on the speaker. Run both mics into either a dual scope or dual
channel audio analyzer and visually match the two waveforms as you pan through the audio frequencies.
You would then tune the tube length so the two waves go positive and negative at the same time.
If the port length is too long of short, the waves will be shifted and cancel eachother out.
How well this will work depends on whether the cab has the right air volume to begin with for the speakers resonant frequency.
Since this is a critical factor in ported cabinet building/tuning, you may or may not be able to tune the cab for optimal frequency response.
If the cab is too small, even with the port in phase, you can wind up having too much upper lows making the speakers sound muddy for guitar.
If the cab is too big the speakers wind up sounding farty and lifeless.
This is why I suggest closing the port first. Sealed cabs do have to have the right air volume too but you wont have to deal
with te phase cancellation issue. The air inside a sealed cab does act as a shock absorber for the speaker piston and it will
bounce back quickly if the air is right. This gives the guitar strings a snappy responce. You can however pack the cab with
additional insulation to reduce the air volume if the cab is larger and sounds too bassy.
A cab size of 15"H x 28"W X 14"D is just about perfect for a 2X12" cab and should get you the proper bass response.
If its smaller/shallower than 14" then your lower guitar strings wont have as much thump on the low notes.
You'll have to use the amps EQ to boost the frequencies it lacks. If its bigger simply pack the cab tightly with
additional fiberglass insulation against the sides to reduce the air volume which reduces the bass response.
The ideal amount of insulation lets you run the amps bass EQ setting at 12:00
If you have the cabs interior dimensions I can calculate what you're getting now by plugging the dimensions
and speaker specs into an online calculator that calculates the proper cab resonance. For guitar its isnt a hyper
critical calculation. Guitar is a midrange instrument and unless you are into playing Jazz Guitar you want most
fundamental frequencies of the guitar strings that go as low as 80hz to be tapered off at below 150hz.
to be Its mainly when you port a cab and want to achieve maximum bass volume in HiFi, Bass guitar
A normal electric guitar range is between 150~6Khz. Everything below is rolled off by both the amp head
and speakers.
What you hear as a nice thump from the strings is actually the strings first harmonic which is an octave
above the strings fundamental tone which begins at about 165Hz. Having a port, even when its tuned, produces
too much fundamental frequencies, which overpowers the speakers abilities to produce the secondary harmonics.
This makes the strings sound like muffeled crap. Reduce the fundamental bass responce and more direct midrange
from the speakers will be heard and those speakers will clean right up and come alive.
Originally Posted by Kazinator
Help Desk