(Submitted Article) – Bass Amplifier Isolation

By P.H. Townsend
TecPadz LLC

Anyhow, between sound check and the first set, I ran across the street to the big box store, and grabbed a pair of the foam kneeling pads from the gardening section. Back on the stage, I put the pads beneath the Bergantino HD112 cabinet, hoping that they might reduce the vibrations that the bass was inducing in the stage. After the first set, the sound guy gave me the thumbs up; which I took to mean that my solution with the pads helped. In fact, I now believe that he was merely indicating he had zeroed-out the bass vibrations at the board. I still have those kneeling pads and the Bergantino cabinet, and as will be shown later, it is unlikely that my solution was any solution at all.

I’ve spent some time the past few years trying to solve some of these problems with amplifying our instrument. Most recently, I’ve put together a measurement system and undertaken a first series of measurements to see how some of these solutions actually either work, or don’t work. Measurements were performed to look at two interactions: stage-coupling between the bass cabinet and a hollow stage platform; and between the bass cabinet and the bass amplifier sitting on top of the cabinet. The use of an accelerometer sensor allows an accurate look at several aspects of the problems, which are not always as we perceive them.

A Practical Measurement System

 Instrumentation and Controls

The details of the gear used to make measurements are a bit dry (and not nearly as interesting as bass gear!). But they are described below so that others can test the results that will be discussed later in the article. The equipment used for the measurements is described as follows:

The “bass signal” is provided by a Leader LAG-120B Audio Signal Generator. The generator puts out a single selectable frequency, which is musically rather dull, but useful for knowing what you are measuring. 200Hz was chosen, because that lower-mid frequency is pretty representative of where good bass tones happen. The signal from the generator is routed in parallel to an oscilloscope for reference and to a bass amp as the drive signal.

The oscilloscope was a Tektronix TDS-320, which is a dual-trace oscilloscope, and both the drive signal amplitude and vibration sensor response can be viewed at the same time. Both signals read out in V_RMS.

Figure 1: Leader LAG-120B Audio Signal Generator and Tektronix TDS-320 oscilloscope

A TE Connectivity Accelerometer 2G Analog 4030-002-120 was used as a vibration sensor. The sensor was attached to a 0.5” aluminum plate to provide improved inertial accuracy in the motion transfer from the vibrating surface (or, more plainly, to keep it from bouncing around). Accelerometers actually measure changes in the rate of displacements; but the amplitudes are relatively indicative of the magnitude of the displacements that are happening due to the vibrations.

Figure 2: TE Connectivity Accelerometer 2G Analog 4030-002-120 on 0.5” aluminum plate

A final part of making reproducible measurements is making sure that a controlled level of energy is going out of the bass cabinet. A BAFX3370 Digital Sound Level meter was used for this purpose. The sound meter was positioned six feet in front of the “stage” for stage coupling measurements, and positioned six feet in front of the bass cabinet for amplifier isolation measurements.

The background noise level during measurements was 42 dB to 47 dB, due to birds chirping, cars going by, and the neighbors yelling at me; depending. The reference drive level for all measurements was 95 +/- 1 dB. Proper ear protection was used, because that level of monotonic signal projection can be rather annoying.

Experimental Deployment

Mobile risers, such as the stage where I had my adventure with the big band, use 5/8” thick 4’x8’ sheets of plywood laid on top of a metal frame. So the test stage used in this work was a 5/8” 4’x8’ sheet of plywood set on top of a frame made up of 2”x4” boards. The plywood sheet was not screwed down to the frame, because the mobile risers just have the plywood sheets laying in the frame. The behavior of the stage may likely be different if it were secured along the edges and in the middle; and if you are gigging at your local biker bar on a hollow stage, that is the more likely construction.

A Genz-Benz Shuttle 6.0 bass amplifier was used for the drive signal. Three different cabinets were studied: a Bergantino HD112, a Genz-Benz 410XB, and a homemade cabinet using ¾” plywood construction loaded with a JBL K-140 driver.

For stage coupling measurements, the sensor was screwed directly to the plywood sheet in a location representative of an adjacent microphone stand. Figure 3 shows the general location of the sensor on the stage plate adjacent to the bass speaker cabinet.

Figure 3: Accelerometer deployed adjacent to Bergantino HD112 bass cabinet on stage during stage coupling measurements.

Figure 4: Accelerometer deployed on amplifier top surface on Genz-Benz 410XB cabinet.

For the amplifier isolation measurements, the sensor was attached to the amplifier top surface with tape. Figure 4 shows the location of the sensor during measurement of amplifier isolation on the Genz-Benz 410XB cabinet. Both attachment methods allowed the sensor to track the vibrations of the respective surfaces with reasonable accuracy.

Stage Coupling Measurements

Measurement Details

A bass cabinet projecting a 200Hz signal at 95 dB causes significant vibrations in the stage sheet. Accordingly, reproducibility of the measurements required screwing the sensor directly to the stage surface. Otherwise, the sensor inaccurately tracked the stage surface motion.

Also, as will be discussed later, significant acoustic coupling between the cabinet and the stage occurred. So, the measurements were performed outside on a driveway apron as a cost-effective method to create an infinite half-space environment in which the acoustic coupling boundary conditions would be simplest.

Materials

(b)

(d)

Figure 5: Mechanical isolation devices used to investigate stage coupling interactions; (a) TecPadz Cab Pad, (b) detail view of TecPadz Cab Pad element, (c) Auralex Baby Gramma Pad, and (d) foam kneeling pads.

Four different vibration isolators were examined to reduce the mechanical coupling between the cabinets and the stage. Data shown below were gathered with the Bergantino HD112; but, both the Genz-Benz 410XB and JBL cabinet were also evaluated, yielding similar results.

Figure 5 shows the vibration isolation devices used to make stage coupling measurements; (a) a TecPadz Cab pad, (c) an Auralex Baby Gramma Pad, and (d) a pair of foam kneeling pads from a big box store. Figure 5(b) shows a close up of the construction of the TecPadz Cab Pad element, consisting of cross-linked polyethylene foam, Sorbothane, and mole skin from bottom to top. Cab Pads with both 70 durometer Sorbothane and 50 durometer Sorbothane were evaluated. Sorbothane is a urethane copolymer rubber, which has particularly strong vibration and shock-absorption properties. The durometer values indicate the pliability of the rubber formulation, with the 50 durometer rubber being more pliable than the 70 durometer material. More pliability generally indicates more vibration absorption.

Measurements

Table 1 shows the measured stage plate vibrations using a Bergantino HD112 cabinet both without and with vibration isolators placed between the cabinet and the stage plate.

Table 1: Accelerometer response amplitudes for stage vibrations with several vibration isolating pads between the speaker cabinet and the stage.

Discussion

The observed stage plate vibrations were least with no vibration isolation pad and larger for all of the isolation devices examined. This result suggests that mechanical coupling between the cabinet and the stage plate is not the dominant mechanism for energy transfer from the cabinet to the stage. So, mechanical vibration-isolating elements do not reduce the energy transfer from the cabinet to the stage.

The other mechanism for energy transfer from the cabinet to the stage is via the pressure waves in the air created by the bass driver(s); and it appears such acoustic coupling may be the dominant transfer mechanism to the stage plate.

The variation in the transfer amplitudes observed between the different isolation devices may be due to a variety of causes; but, one candidate would be the height of the cabinet above the stage plate when the cabinet is resting on the isolation device. The Auralex Baby Gramma – which is a quite excellent mechanical isolation device – supports the cabinet higher than the other isolation devices investigated, and also leads to the highest stage vibration amplitudes observed.

The wavelength of a 200Hz signal in air at 25 C is about 1.73 m. It is reasonable to expect that, as the height of the cab above the stage approaches an integral half wavelength, the acoustic coupling interaction would increase; even though such interaction may at first glance seem counter-intuitive. Such interaction may explain the differences observed between the different isolation devices.

To test potential methods to mitigate acoustic coupling, the two foam kneeling pads were positioned immediately in front of the cabinet on the stage, instead of beneath the cabinet; the hypothesis being that the acoustic pressure waves emanating from the front of the cabinet could be at least partly absorbed by the foam before making contact with the stage plate surface.

Figure 6 shows the arrangement with the kneeling pads acting as dampeners for the acoustic waves projected from the cabinet.

Figure 6: Foam kneeling pads positioned in front of the cabinet as acoustic pressure wave dampeners.

Figure 7 shows the oscilloscope traces for (a) the bass cabinet directly on the stage and (b) with the “dampener” foam kneeling pads positioned in front of the cabinet. The “dampener” kneeling pads reduced the stage vibration amplitude from 111.7 mV to 41.0 mV. This result suggests that one method to reduce stage vibrations from on stage bass cabinets would be to place acoustically absorbing mats in front of the bass cabinet. The required geometry of such mats, thickness, and materials to be practical would need to be investigated. Further work is needed to optimize such devices.

(b)

Figure 7: Oscilloscope traces for (a) the bass cabinet directly on the stage and (b) with the “dampener” foam kneeling pads positioned in front of the cabinet.

The way that players perceive bass sound quality when the cabinet is interacting with the stage is complex. In fact, some players may prefer the sound of the instrument when the cabinet is interacting with the stage to a larger extent; and in such case, the Auralex Gramma Pad could produce a preferable local bass tone in the vicinity of the stage, even though its function in such use is to raise the cabinet above the stage surface, rather than absorbing the mechanical vibrations from the cabinet.

Stage coupling from the bass cabinet may be more problematic from the perspective of the sound engineer mixing the various signals on the stage for a front-of-house (FOH) sound, since induced stage vibrations are strong enough to disturb microphones, which are prominent in the FOH mix. It is possible that vibration-isolating pads attached to the feet of microphone stands may be an effective method to isolate the microphone signals, in particular; though such isolation boots on microphone stand feet are uncommon.

Finally, an increasing number of large stage gigs are moving towards ampless configurations – wherein the bass cabinet is eliminated from the stage, and the player monitors their instrumental part along with the rest of the ensemble using an in-ear monitoring (IEM) system. From the results described in this section, it becomes apparent why bass cabinets are problematic for sound engineers trying to create an optimal house mix; and why the trend of removing bass speaker cabinets entirely from the stage may increase in coming years. We, as bass players, rightfully love the tones coming from our cabinets. But those same vibes can cause major issues for sound engineers trying to optimize the ensemble performance sound for the audience.

Amplifier Isolation Measurements

Bass amplifiers have been placed on top of the bass speaker cabinet at gigs since the first separate amplifier and speaker cabinet rigs for bass appeared in the late 1950’s. More than one player at the club gig has made the mistake of putting an open beverage on top of the amplifier, only to have it fall off, and douse the surrounding area in the liquid of choice. Or, perhaps a tuner, which rattles off the top of the amp, smashing on the floor; thus, requiring part of the pay for the evening to go towards a new tuner. Or, had the tubes in the amplifier become microphonic as a result of the shaking they receive. Or, with the more recent 5-pound 900w amplifiers, had the entire amplifier fall to the ground. I’ve seen just about all of those at one time or another.

The vibrational characteristics between different bass cabinets do vary, mainly due to the differing internal bracing methods used by various builders. But, even the most well-braced cabinets vibrate when significant sound pressure is projected from the drivers; Newton’s Second Law. Unfortunately, the top surface of the bass cabinet does not simply vibrate in a uniform up and down motion across its surface.

The vibration amplitude varies across the top surface of the bass cabinet, due to the bracing details of the individual cabinet. This variation suggests a vibrational profile survey prior to assessment of the performance of the isolation devices. Otherwise, random placement of isolating devices adversely affects reproducibility of individual measurements, and thus, quantitative comparisons are not otherwise possible.

As will be demonstrated below, vibrational transfer to the bass amplifier is dominated by mechanical coupling, in contrast to the stage coupling effects discussed earlier; and isolation devices can be effectively used to reduce the transfer of mechanical energy to the amplifier. Results are shown for isolation of a Genz-Benz Shuttle 6.0 micro bass amplifier. Investigation of isolation for larger amplifiers is left for future work.

Measurement Details

The sensor was simply placed on the cabinet top surface during measurement of the cabinet top surface profile.

During amplifier isolation measurements, the sensor was attached to the amplifier top surface with tape to improve the inertial tracking of the amplifier vibration.

Acoustic coupling between the cabinet and the amplifier was not observed. All measurements were performed with the cabinet positioned on a concrete slab.

Materials

Four different vibration isolators were examined to reduce the mechanical coupling between the cabinets and the amplifier. Data shown below were measured with the Genz-Benz 410XB, for reasons explained below. The Berganitino HD112 and JBL cabinet were also evaluated, yielding similar results.

Figure 8 shows the basic vibration isolation devices used to make amplifier isolation measurements; (a) a TecPadz Amp Pad F, (c) a TecPadz Amp Pad S, and (e) 10” x 20” swatch of foam mesh tool drawer liner. A fourth device using only the foam component of the Amp Pad S completed the group.

(b)

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Figure 8: Mechanical isolation devices used to investigate amplifier isolation performance; (a) TecPadz Amp Pad F, (b) detail view of TecPadz Amp Pad F element, (c) TecPadz Amp Pad S, (d) detail view of TecPadz Amp Pad S element, (e) 10” x 20” piece of foam mesh tool box liner, and (f) detail view of foam mesh tool box liner.

Figure 8(b) shows the Amp Pad F (foam) element, a cross-linked polyethylene foam disc 1.5” diameter x 0.75” thick.

Figure 8(d) shows the Amp Pad S (small) element, a composite disc construction with a cross-linked polyethylene foam disc 1.25” diameter x 0.5” thick, a 50 durometer Sorbothane disc 1.0” diameter x 0.25” thick, and moleskin from bottom to top.

Figure 8(f) shows a detail of the foam mesh tool drawer liner.

Tool drawer liner is a foam mesh material that is readily available at many hardware stores and departments. This material may be readily available to players, which is one reason why it was included in these measurements.

Measurements – Cabinet Surface Variations

Table 2 shows accelerometer signal values at three locations the top surfaces of the Bergantino HD112 cabinet and the Genz-Benz 410XB cabinet.

Table 2: Accelerometer response amplitudes for cabinet top surface vibrations at several positions on the cabinet top, for Bergantino HD112 cabinet and Genz-Benz 410XB cabinet.

The Bergantino HD112, being an extremely well-braced cabinet, has a pronounced low in the center of the top surface, increasing to larger values at the cabinet edges. Even though the cabinet has high-quality bracing, the cabinet does vibrate in response to the force of the acoustic energy projection, but most strongly toward the cabinet edges. Unfortunately, this favorable quality also leads to a steep gradient in the response amplitude across the top surface from center to edge.

When placing an amplifier on top of the cabinet, this vibration gradient causes significant data scatter and high standard deviations of the measured amplitudes of the amplifier vibration, due to the variation of position of the actual support points for the amplifier from measurement-to-measurement and with the different isolation devices. For this reason, measurements with the HD112 are not reported.

The Genz-Benz 410XB showed a much more uniform vibration profile across the cabinet top surface, with a low at the back center of the cabinet. So, measurements on the 410XB were much more uniform than observed on the Bergantino HD112, and are reported below.

Measurements – Amplifier Isolation

Table 3 shows accelerometer amplitude values for the Genz-Benz Shuttle 6.0 bass amplifier positioned on the (center) top surface of the Genz-Benz 410XB cabinet, with no isolation device (amplifier placed directly on top surface of the cabinet), and with four different vibration isolation devices between the cabinet and the amplifier.

Table 3: Accelerometer response amplitudes for top surface of Genz-Benz Shuttle 6.0 amplifier positioned on Genz-Benz 410XB cabinet for several vibration isolation devices.

Four measurements were made through the series of different isolation devices, and average values and standard deviations for each set are shown. A Decoupling Factor (DCF) defined as the ratio of the signal amplitude with the isolation device to the amplitude without an isolation device is indicated for each series; and this derived value permits quantitative comparison between the different isolation devices.

Several relevant comparisons are discussed below.

Discussion

Referring to Table 3, vibration amplitudes observed on the amplifier surface with no vibration isolation device are significantly larger than vibration amplitudes observed with any of the isolation devices examined. So, the use of a vibration isolation device is effective at reducing vibration transfer from the cabinet to the amplifier.

The lowest vibration amplitudes were measured with the TecPadz AP-S isolation device beneath the amplifier. So, the TecPadz AP-S was the most effective isolation device examined in this work, with a DCF value of 0.1. The foam element TecPadz AP-F was slightly less effective, with a DCF value of 0.16. But, the AP-F element is simpler and more economical.

The foam element designated “AP-S, No S,” is the same foam material as the foam element AP-F; but the thickness of the element is only 0.5” vs the 0.75” of the AP-F element. The DCF value of 0.30 for the smaller foam disc vs 0.16 for the AP-F elements shows that thicker foam isolation elements are more effective at damping the vibrations.

The lowest value of DCF observed with the TecPadz AP-S shows that the Sorbothane disc is more effective at vibration damping than the foam base.

The foam mesh tool drawer liner is also effective at reducing vibration transfer to the amplifier. A single layer reduces the DCF to 0.44, or a little less than half of the vibration amplitude observed without an isolation device. Doubling the thickness of the tool drawer liner beneath the amplifier further decreases the DCF value to 0.17, similar to the value observed with the TecPadz AP-F. If using tool drawer liner to isolate the amplifier, a double layer is a good idea to increase the vibration decoupling.

Summary

Bass amplifier speaker cabinets are a significant source of vibrations, which can be transferred by either acoustic or mechanical coupling to adjacent equipment on the sound stage. A system to directly observe vibrations was described. The system uses a Tektronix TDS-320 dual-trace oscilloscope, a Leader LAG-120B Audio Signal Generator, and a TE Connectivity Accelerometer 2G Analog 4030-002-120.

Stage coupling measurements with three different bass guitar cabinets demonstrated that the primary vibration coupling between the cabinet and the stage was through the projection of acoustic wave energy from the cabinet drivers, and not from mechanical contact between the bass cabinet and the stage plate. A method to absorb the projected acoustic energy using absorbing mats placed on the stage plate in front of the cabinet was demonstrated.

Vibration transfer was observed between the bass guitar cabinet and an amplifier resting on top of the cabinet. The vibration transfer from the cabinet to the amplifier is reduced by inserting an isolation device between the amplifier and the cabinet. The most effective vibration isolation was observed using isolating elements containing both a polyethylene foam and Sorbothane rubber. This device demonstrated a decoupling factor of 0.1.