Subwoofer isolation pads – Do they make any sense?

In my previous article on my experiments with my hybrid Sonos setup I alluded to trying out isolation pads for subwoofers. These are typically a dense foam base which has a soft fabric covered plywood sheet on the top to hold the subwoofer. The idea is to absorb speaker vibrations and prevent them from transferring into the floor and causing resonances.

Question is do they make any sense or are just snake oil just like so many other stuff in the audio world. The only way for me to determine this was to get a couple of these, test and measure and return if they made no difference. So, did they make a difference for me eventually? Yes, I am keeping those. Should you get one ? It depends!

In my hybrid Sonos setup I have two Subs. One a Sonos Sub and another an OSD Trevoce 10” Sub carefully DSP tuned to complement the Sonos sub. Even after lots of tweaking of settings and DSP, furniture and speaker adjustments there was a slight nagging boominess to the bass. Costly room treatments were not an option, especially in a rented apartment. After researching a bit, I ordered two Auralex Subdude II isolation pads which subjectively, made the boominess disappear with tighter bass. However, I wanted to measure with REW. REW is not subject to placebo effects so I hoped to get an objective picture.

Note here that the Sonos Sub has two opposing force-canceling drivers resulting in negligible vibration of the subwoofer cabinet but some energy still transfers to the floor from direct contact. The OSD Sub has passive radiators but they are not force-canceling, so it does vibrate. Note also that I have a carpeted floor so carpet should absorb some vibrations from the subs. I was interested in the Decay graphs from REW to understand the extent of resonance/ringing per frequency range. I decided to use the spectrogram graph type as it shows decay times independent of peak volume levels. My measurements were all volume matched to 75Db SPL for Pink Noise. The spectrogram is a view of the waterfall 3D graph from above.

I took two measurements, one without the Auralex pads on both subs and one with them. My first measurements showed reduced decay times, albeit small, but across the entire frequency range! This was very surprising to me. I was only tweaking the subwoofer setup, so how can it impact the higher frequencies. I could not believe those results and was confused. I could not clearly recollect if I had done something odd or maybe even changed the settings in the middle, or it could be differing background noise. So, I used the setup for a few days and took another measurement on another day at a time when things were uber-quiet and there was only me at home. I took two measurements in quick succession with and without the pads and was very careful to not affect anything and ensured background noise was minimum. I got the same result! So, I measured yet again and once again got the same result!

The following is a spectrogram overlay that compares the decay times. Click on the image for a bigger clearer picture. All measurements were taken from the Main Listening Position or MLP for short. I used a UMIK-1 and it was pointing straight. I did not use the 90 degree config. I will try that another day. Maybe, it will pick up the reflected sound better and show a bigger difference.

Spectrogram boundary in white = measurement without Auralex pads. Inner rainbow colored spectrogram = measurement with the pads.

The vertical axis is decay time. It is clear that the inner spectrogram has smaller decay times across the frequency response. The benefit is small, but it is present even with my carpeted floor and subjectively audible. Human hearing is very sensitive to timing changes, reverb, echoes etc. So how is this working? I suspect a few things:

• The Auralex are doing their job at the lower frequencies.
• REW does a frequency sweep so, I suspect the previous frequency’s decaying tone is affecting the next in the range. But this still does not explain the benefit at the higher frequencies much beyond the time of the lower frequency decays.
• I live in an apartment, on the second floor and the building is made of wood and synthetics (drywall for eg). There is no concrete. So vibrations, even minor ones are transmitted easily.
• The subwoofers are kept just next to my bookshelf speaker stands that have metal spikes into the carpet. So, the isolation pad foam are likely absorbing higher-frequency vibrations through the carpet transmitted to the floor via the stand.

So, finally the question, should you get an isolation pad? Considering the following questions can help:

• Do you live in an apartment? If yes then it can make some sense to avoid trouble with your neighbors.
• Is the building made of concrete or wood? Obviously it makes more sense in a wooden construction.
• Even in a concrete building do you have a wooden floor?
• Do you have a carpet? Wooden floor with no carpet and it makes more sense to get an isolation pad for your sub.
• Are you living in your own house, possibly concrete construction and have a carpet with audio setup on the ground floor? Though it will make hardly any difference to the sound, you can still get one for mental peace, especially if you have OCD like me!!
• Do you have room treatments like acoustic panels and bass traps? Please still buy these isolation pads because they are another fancy item to talk about and show off your deep knowledge of room acoustics to friends and family…

Caveats

Well, the last two jokes apart, there is some sense in getting these pads based on your situation as is clear from the remaining points and the earlier discussion. However, one thing I could not measure is how much floor and wall vibrations were present before and after. Measuring that accurately with enough sensitivity requires expensive equipment. The free smartphone based apps are likely not sufficient here. Subjectively, listening to music from outside my apartment, the low frequency effects did sound lower at the same volume. Also, my measurement microphone was horizontal, pointed towards the speakers. I later learned that it is recommended to point it upwards and use the 90 degree calibration file to better assess room effects.

My experiments with Sonos – 3

Obsessed with quality

When I was in my early teens, I remember developing a taste for the sound of music. Theaters and concert halls would mesmerize me not by the large screens or stages but by the sound and the large speakers! I was more interested in the way audio was being produced and delivered. If it was audio, it had to be well-presented. I would balk at the tinny or boomy sound of small cassette players and two-in-ones. It was the same thing with my father. If he did not have the right equipment, he would not listen to music, period!

My first experience with hi-fi was amplifiers and speaker enclosures designed and built by my dad. Electronics was his hobby and I would watch intently, and sometimes help as he painstakingly soldered components, screwed boards, cut and drilled plywood etc. He ended up making several mono amps and speaker boxes for friends and family. Unbeknownst to me he was saving up all this while for something bigger. It was an exciting evening when he walked in with three big boxes and unboxed the Sonodyne Uranus. A top-tier component hi-fi system of that time.

It had a discrete cassette deck, 12-band graphic equalizer (+/- 12db), AM/FM tuner, turntable and a power amplifier employing the STK 4131-II chips. It also came with two or three way bass reflex speakers. That audio obsession has been with me to this day. I am unable listen to music unless the presentation reaches some level of finesse. I have gone years without listening to music when situation did not permit me to invest in proper audio equipment. A cursed obsession you might say! That also means then even when I acquire equipment, I spend a lot of effort in tweaking the systems and the rooms in search of qualitative improvements. While the effort is large, the rewards are large to my ears as well. I find myself listening to more music as I get to that level of desired perfection. It is an inflexion point of quality and diminishing returns given a certain budget.

Measure – tune, measure – tune, measure – tune

Coming back to my hybrid Sonos system of the present, it was great but there seemed to be something missing. Playing back music from 90’s era movies, it seemed the drum beats were weak. The punch was missing. I cannot listen to music like that. At that point I realized that I had to measure it. A bit of recollection and research led to the following:

• Minidsp UMIK – 1 calibrated microphone
• Amazon Basics adjustable boom microphone stand
• Open source Room EQ Wizard software
• This excellent beginner’s guide: https://www.youtube.com/watch?v=Ev1bSSL8tRA
• A Laptop

I needed some additional stuff since I had to connect my laptop to the Sonos system. I could connect it to either the Arc or the Amp, but in either case those needed HDMI ARC. Sonos provides an optical to HDMI ARC converter cable. So, I had to figure a way to convert laptop USB to digital optical. Such a thing was hard to find but I eventually found a USB DAC that replicated the USB digital signal onto optical as well as line-out. So these were the additional things I had to get:

• Sonos optical to HDMI adapter – Included with Sonos Arc
• Signstek USB DAC
• A USB C to USB B cable
• An HDMI cable

So, I was able to connect the DAC to my laptop’s USB, connect the Sonos optical to HDMI cable to the DAC’s optical input on one end and the HDMI cable on the other end. The DAC replicates the USB input signal to the optical input as well making the optical input also behave as an output. Finally, I plugged in the HDMI cable into the Amp’s HDMI port. Playing youtube confirmed that it works. Sound was coming through the Amp’s speakers and there were no lip-sync delay issues.

The first hiccup came when I grouped Amp with active HDMI-in with the Arc. The Arc sound was delayed resulting in a bad reverb! This is the Sonos 75ms group audio delay. Interestingly, the group delay is not present when connecting via the Arc’s HDMI port and grouping the Amp. After a bit of fidgeting I got rid of the delay by adjusting the Amp’s “TV Dialog Sync” setting. I could’ve just used the Arc’s HDMI port but messing around leads to more learnings.

Anyway, I stepped through the video and made my first test measurements to understand the process and verify that everything works. Now, I was ready to do some real work. First I had to get my surroundings as quiet as possible:

• Close all doors and blinds
• Shut off the water dispenser and refrigerator
• Somehow get unhappy wife and kid into a bedroom and get them to be absolutely quiet
• Pray the neighbors do not make sudden noise like closing a door loudly
• Pray no vehicles enter or leave the apartment parking
• Pray neighbor’s dogs do not bark
• Pray …

My first measurement was only with the Arc + Ones + Sub setup. The Amp was not grouped and not active. This setup was tuned with Trueplay:

z

As you can see. I have a bad living room in terms of acoustics and Trueplay room correction DSP is no magic. There is a lot to be desired in the Bass frequencies below 200Hz. The peak at around 50Hz looks like a room mode. Now here, I have to mention that I am not looking for a flat frequency response. There is some controversy around this topic but to me a flat frequency response is incredibly boring. It is more desirable to go for a Harman Curve. While Harman curves were initially designed for headphones we also have them for loudspeakers. Example:

Next I measured the Arc+Amp grouped setup with Amp crossover set at 96Hz.

Not much difference in the bass as the Amp does not have a sub. It only has the Q Acoustics 3030i bookshelf speakers. But 100Hz – 200Hz is a little more fuller and the midrange from 1KHz to 4KHz is boosted resulting in clearer dialog in movies. I figured maybe letting the Q Acoustics handle a bit more bass would help with the dip at 60Hz to 100Hz. They are rated till 46Hz. Reducing the crossover to 70Hz yielded this.

x

Notice the little bump from 70Hz to 90Hz. So the Q Acoustics were helping a bit in that range. Also the spike at around 50Hz has reduced a bit. It is looking more like a Harman but still quite jagged. At this point I was out of options to tweak further with the existing system as Sonos has limited EQ capabilities. Two further next steps were possible:

• Adjust room setup – my living room was congested and needed some cleanup and re-alignment.
• Add a subwoofer to the Amp, preferably one with a built-in parametric EQ so I can adjust the response.

Eventually, I did both. The sofa was better aligned with more frontal space. Got rid of some old stuff, repositioned some other furniture. I saved up and splurged on the OSD Trevoce 10” EQ DSP Subwoofer. This subwoofer comes with some advanced DSP features including a 25-band parametric equalizer. Optimal Speaker Design or OSD for short, typically designs outdoor speakers but recently have been getting into home theater audio. While SVS is a more popular and respected subwoofer brand, I read some reviews of this less-known sub, especially the sound and vision review and grabbed it for its DSP features. I was not disappointed. SVS subs only have a 3-band parametric EQ and in the end I needed to use 12 bands on the OSD to get some results.

In the meantime my wife was a little upset with a second sub in a small apartment and thought I am losing my sanity with the repeated microphone setup and crazy measurement sessions for hours. That the speakers made all sorts of booming and squealing sounds during the frequency sweeps did not help! To make a long story short after days of tweaking, measuring, tweaking and countless parametric EQ curves later I had something which looked better on the graph and sounded better. Towards the end of the measurements, for the first time I could hear some glass items rattle. I was getting better bass extension down to 25Hz, thanks to the OSD Trevoce. The next graph shows the dual-sub setup with earlier response – deep green and new response – light green.

x

As can be seen, bass response is fuller and meatier with extension down to 25Hz. I had to adjust the Sonos sub level down by -1 to avoid a bad down-spike null at 46Hz. It seems at some volume 46Hz frequencies from both the subs coupled with room resonance were canceling each other out and losing energy. Just a bit of level adjustment gets rid of that effect. But I still had boomy sound.

The graph in light green is still not great however, even though it is closer to my desired room response curve. The peaks that 55Hz and 106Hz are too forceful. Also, while listening, it sounded quite boomy and reverberating along with too much sound pressure even at lower volumes. At that point I realized that I had the “Loudness” feature enabled on the Sonos Amp. This loudness is a DSP that boosts certain frequency ranges to improve listening at lower levels. In this case though my Subwoofers DSP was already doing quite a bit of work, so adding “Loudness” was an overload. Maybe okay for an all-singing, all-dancing party mode but not for general listening. Turning that off yielded a much better result with boominess significantly reduced. The next graph shows same setup with “loudness” off and Sonos amp crossover at 110Hz.

The bass is better controlled but we do have two drops at 73Hz and 128Hz. The next graph is a comparison, loudness on – violet, loudness off – deep blue/indigo.

You can see which frequency ranges are boosted by the Sonos “Loudness” feature. At this point getting rid of the two drops at 73Hz and 128Hz while still keeping bass under control will require room treatment with sound absorbing panels and bass traps to control resonance. I am not inclined to do that in a rented apartment, especially when I am going to move in future.

A better view of the boominess or sound ringing in a room can be seen via a 3D waterfall graph which shows the frequency tone decay over time in ms. I generated a Waterfall Overlay: White – Loudness OFF, Orange – Loudness ON

We want to have a decay time (Z-axis) to be as low as possible, less than 300ms ideally and at least less than 450ms. In this image you can see that the Orange overlay with loudness ON had much more forceful bass and worse decay times compared to loudness OFF in white. We still have some bad decay around 40Hz and nearby in the white graph. It is just my living room resonances at certain frequencies that can only be dealt with by bass traps/absorbent panels.

There is another thing here. Most subwoofers vibrate. Some like the Sonos Sub and some KEF subs have force canceling opposing drivers that have minimal vibration, but still it is not nil. The OSD sub that I got does vibrate quite a bit. All that vibration passes to the floor and walls especially in an apartment and especially in one which is constructed of wood and synthetic materials. That contributes to room resonances/ringing and annoys neighbors. So, I plan to get subwoofer isolation bases that absorb vibrations using an acoustic foam layer and make some measurements using those.

Needless to mention, I learned a lot during this exercise and thoroughly enjoyed it. It is surprising how little changes can have a big impact and how it can be tricky sometimes to balance dual-subwoofers especially if they are of different makes. However, the real enjoyment is the resultant audio quality when you start to re-discover your existing music collection and start to discern sound effects and instruments you have not noticed before.