When measuring a loudspeaker in a room, it’s important to interpret the results accurately. More often the case than not, a pesky dip in a certain area is either a result of a reflection caused by the wavelength distance relationship between the microphone and floor or the relationship between the distance of the speaker’s woofers and nearby boundaries, such as the wall behind it.

Some guidelines in interpreting what is real and what isn’t include:

• Does the measurement anomaly go away when you move the microphone?
• Does the measurement anomaly coincide with impedance curve of the system?
• Does the measurement anomaly coincide with 1/2 wavelength of mic position relative to the ground?
• Does the measurement anomaly persist even after you try boosting that frequency range?

If you answered YES to #1and #3 and #4 and NO to #2, you are likely dealing with a measurement anomaly. If you answered YES to #1 and #4, and NO to #2 & 3 it is likely you are dealing with a room node.

If you have seen evidence of impedance curve anomalies at 50 or 120 Hz, you may be looking at some sort of system-sourced pathology. It is assumed, of course, the system wasn’t designed with major dips at 50 & 120 Hz!

It’s not unusual to see odd or unexpected glitches show up in the impedance curves of loudspeaker systems. That they do is no surprise, as what occurs in the acoustical or mechanical system is also reflected in the electrical system, producing small impedance peaks & dips. On those occasions when these oddities occur, identifying their magnitude, frequency and\or Q can provide information useful in tracking down the cause, whether it be systemic or arising from the interaction of the system with its environment.

In the figure here, we see a classic example of an acoustic event (one of my youngsters thumping around directly above the area where I happened to be doing an impedance sweep on a totally enclosed box system). The blue curve is the sweep done when all was quiet. The red curve was done when youngest daughter was doing cartwheels on the floor above, producing a momentarily noisy measurement environment.

If you have a case where you suspect your cabinet is causing anomalies in either your dB SPL curves or impedance curves, think in terms of wavelength, λ.

Focusing now on dB spl response curves, unintended acoustic response curve anomalies often occur as a result of reflections, standing waves, or other such mechanical or acoustical resonance phenomena which directly relate to things such as panel dimensions, edges, and distance to nearest room boundaries.

First, find the wavelength of your observed dips (in this case 50Hz and 120Hz), by simply dividing the velocity of sound (c ≈ 1130 ft/s) by the frequency. 1130 ft/ s divided by 120Hz gives 9.4 Feet or about 113 Inches. A half-wavelength would be about 57 inches. At 50 Hz, the half-wavelength would be about 136 inches. Neither of those figures relates to any cabinet dimensions in a meaningful way and nothing appears to be out of sorts or otherwise unusual with the system impedance curve, so the cabinet isn’t the culprit.

Measurement Notes by Gene DellaSala

• 57 inches is about the distance between the mic and floor.
• 136 inches is about the distance between the speakers and the couch where the measurement was being conducted and is also about the distance between the subwoofer and an opening to a hallway where the two guest rooms and bathroom reside.

Conclusion

The Velodyne SMS-1 is a fantastic tool for dialing in subwoofers and adding those final touches to tweaking your system for optimal performance. It also offers an incredible “real-time” look at exactly how your room interacts with the subwoofer.   All in all, the SMS-1 response graph was quite a useful tool in helping to fine tune system response.  For those without the benefit of an Audio Analyzer or LMS measurement system, you should be able to achieve satisfactory results using the Velodyne OSD measurement display.  This system works and, depending on your room conditions and subwoofer system, it can be that little helping hand to tweak out your system response. In some systems it could even make a world of difference to combat room issues while opening up more placement options for your subwoofer(s) previously unattainable without a minimum phase adjustable PEQ system.

With the SMS-1 you may never have to Crawl for Bass again. One thing to remember is that this is a tool. And like all tools it must be used properly. Consumers who think they can toss their subwoofer anywhere and use the SMS-1 to sort out the pieces will be disappointed with the results. Those who understand that the SMS-1 will allow them to make adjustments to subwoofer placement, room treatments and then dial in the subwoofer for those final touches are sure to be very pleased indeed.

If you haven’t dealt with subwoofer to room integration part of your system yet, you will be amazed at how much more articulate and natural sounding your subwoofer(s) will sound when properly equalized.  As always, do the best job you can passively treating your room for the most balanced and consistent sound possible.  Then, throw in an SMS-1 or two to fine tune your systems bass response and further push the envelope of performance.

We give a hearty recommendation to this tool and think that everyone who is serious about bass could stand to gain from integrating an SMS-1 into their listening room.