In this app note, we will show you how to tune a multisub setup (i.e. multiple subwoofers) using a miniDSP.

Why use multiple subwoofers? [Top]

The primary reason for using multiple subwoofers is to improve evenness of bass response across the whole listening area. While EQ can correct the response to be flat in one location in the room, it cannot correct for spatial variation. For example, if the level of 40 Hz in one seat is 10 dB different to the level in the next seat, the difference between the two seats will always be 10 dB, no matter how much EQ is applied. The only solutions are to find a better position for the (single) subwoofer, or to use multiple subwoofers.

Multiple subwoofers can also help with other problems. For example, a persistent null in the response can be filled in by using multiple subwoofers. (A null is not just an area of low output but an area of zero output. No amount of EQ can properly correct for a null.) Because real listening rooms are limited to where subwoofers can be placed (due to furniture, doors, and aesthetics), it may not be possible to locate a single subwoofer in the optimum position. Use of more than one subwoofer provides a greater degree of freedom with subwoofer location.

Even two subwoofers can deliver an improvement over a single (see graphs below), but three or four is generally considered optimal.

Use of multiple subwoofers is not a reason to use low-quality subwoofers! Low distortion, sufficient frequency extension and output, minimal cabinet vibration and driver self-noise, high quality electronics (if powered), and low port noise (if ported) are necessary attributes for good results, regardless of the number of subwoofers used.

Choice of DSP [Top]

We will assume in this app note that the subwoofers are being used for home theater, driving two to four subs from the AVR's subwoofer output. If purchasing a dedicated DSP for subwoofer tuning, the four-output miniDSP units are ideal. They are summarized in Table 1.

Table 1. miniDSP four-output unit summary

miniDSPInput levelOutput levelSample rate#PEQsOptimum sub amplification*1
2x4 0.9V/2V SE 0.9V*2 SE 48 kHz 6 Inbuilt plate amps
2x4 Balanced 2V/4V SE or Bal 2V/4V Bal, 1V/2V SE 48 kHz 6 Pro amps
2x4 HD 2V/4V SE 2V SE 96 kHz 10 Normal stereo amps or inbuilt plate amps

(*1) This is a suggested optimum "fit" for each unit. The best match will depend on the input sensitivity specifications of your own amps/subs.

(*2) This level is not high enough to drive some amplifiers to full output power. Check the input sensitivity of your subs or amplifiers. If in doubt, choose a model with higher output level.

While this app note is written for the case where a four-output unit is dedicated to subwoofer management, any miniDSP product with enough outputs can be used in this application. Units with a higher number of outputs may be suited for applications with more subwoofers, combined active crossover/multisub systems, and multiroom installations. See the full range. If in doubt or if you have any questions, please contact our tech support team.

Getting connected[Top]

An example setup is shown in Figure 1 below. The subwoofer output from the AVR (or AVP) is connected to the miniDSP input, and up to four powered subwoofers are connected to the miniDSP outputs.

Typical multisub setup using powered subwoofers

Figure 1. Typical multisub setup using powered subwoofers

Figure 2 shows another option, where "pro" amps are used to obtain higher power levels (i.e. the subwoofers themselves do not have amplification built in).

Multisub setup using passive subwoofers and external amplification

Figure 2. Multisub setup using passive subwoofers and external amplification

Either of the above or a combination is a fine choice. If you have a simple system and don't need a full-featured AVR, you can use a miniDSP nanoAVR HDA instead of the AVR.

For a miniDSP 2x4 or miniDSP 2x4 Balanced, the best choice of plugin is the 4Way Advanced. For a miniDSP 2x4 HD, use the 2x4 HD1 plugin and set the routing matrix like this:

Routing matrix for miniDSP 2x4 HD for multiple subwoofers

Multiple subs can mean long audio cables and electrical equipment connected to different power outlets. This situation is a prime candidate for problems with noise and hum. While noise pickup is not usually an issue in a domestic environment, balanced cabling and connections can reduce the chances of problems. More likely are ground loop problems. Running a long electrical lead so that a "far away" subwoofer is plugged into the same power board as the other electronics may be better than simply plugging that subwoofer into the nearest power outlet. The shield of a balanced cable can be disconnected at one end (e.g. disconnect "S" on the miniDSP Balanced end of the cable.) In extreme cases, audio isolation transformers may be necessary.

Warning: never ever disconnect the safety ground of any equipment or use "cheater plugs." Doing so is extremely hazardous and may cause injury or death. If you do run into a ground loop problem, tackle it safely!

The procedure[Top]

The response of subwoofers in rooms is unpredictable and there are various approaches to obtaining the best response when using multiple subwoofers. Acoustic measurements are an essential part of this task. In this app note we will describe three methods in order of increasing difficulty and provide examples of measurements in our test room.

The overall procedure is:

  1. Take multiple measurements of the sub or subs around the listening area.

  2. Change parameters like delay and gain to reduce spatial variation. (You can also try changing subwoofer location.)

  3. Run a global EQ to improve smoothness.

Note the general approach: multiple subs to reduce spatial variation, global EQ to improve smoothness. We will use three measurement locations – one in the center of the listening area and two in different locations. You can use more measurement locations but at some point it will be easier to switch to using Multi-Sub Optimizer (Method C).

Single-subwoofer baseline[Top]

We recommend that you establish a baseline using a single subwoofer. This will ensure that you have your miniDSP operating correctly with the rest of your equipment and are able to take measurements and run REW Auto-EQ. It will also provide a useful basis for comparison to assess the improvement obtained with additional subwoofers.

When taking measurements for optimization, ensure that all filtering is turned off. Since AVRs often have a low pass filter on the LFE channel, it's best to send the test signal directly into the miniDSP. In Figure 3, the computer running REW uses the USB input of a 2x4 HD for the test signal. If using a miniDSP 2x4 or 2x4 Balanced, disconnect the AVR from the miniDSP input and connect the output of your soundcard instead.

Example measurement setup (2x4 HD)

Figure 3. Example measurement setup (2x4 HD)

Choose one of the subwoofers to use as the baseline. In the plugin, mute the other subs, then take a number of measurements around the listening area. Graph 1 shows the measurements we obtained for a subwoofer located in the center of the front wall. This is the best location we have found for a single subwoofer in this room.

Example response of single subwoofer showing spatial variation

Graph 1. Uncorrected response of single subwoofer, measured at three locations

We now need to apply global EQ using Room EQ Wizard:

  1. Use REW to take an average of the three measurements. (Use the All SPL tab to select the measurements, then click on the Average the Responses button.)

  2. Use REW Auto-EQ to calculate a correction filter for the average response. Refer to the app note Auto EQ with Room EQ Wizard and be sure to use the correct sample rate setting and number of EQ bands for your hardware/plugin.

  3. Load the correction filter into the PEQ block of input channel 1.

  4. Measure again at the three locations to get the final response.

(For the sake of easier comparison, we use a completely flat target curve in all examples. You can set up a target curve with low bass boost if you wish, or use additional PEQ after applying global EQ.)

Graph 2 shows our measurements at the three locations after applying global EQ. So that's our baseline! Let's see what improvements we can get with more subs.

Example response of single subwoofer after EQ

Graph 2. Corrected response of single subwoofer, measured at three locations

Method A [Top]

Method A uses two or four identical subwoofers positioned symmetrically in the room. It is based on research at Harman International (PDF download). The best subwoofer locations were as shown in Figure 4.

Symmetrical layouts for multiple subwoofers

Figure 4. Symmetrical layouts for multiple subwoofers

To test Method A, we placed two identical sealed subs in our test room as per Layout 1. Graph 3 shows the result after measuring in three locations, applying global EQ to the average, and then remeasuring in the same three locations. The spatial variation is less than for a single subwoofer (Graph 2), except perhaps in the 90 to 105 Hz area, and there are fewer notches.

Corrected response of two subwoofers at front and rear center

Graph 3. Corrected response of two subwoofers (Layout 1), measured at three locations

You may find that you can "tweak" the subs to improve the response. In our case, we found that adding 2 ms of delay to the front subwoofer eliminated a couple of notches in the response and reduced spatial variation above 80 Hz. Overall, compared to Graph 2, this is definitely an improvement!

Method B [Top]

Method A is not always feasible: real rooms are often not rectangular and the needed locations may not be available for practical reasons (furniture, equipment, doors/walkways, etc). Method B is based heavily on (but is not the same as) the technique described by Dr. Earl Geddes in two short papers here. It uses three or more subwoofers with asymmetrical or "random" placement. Figure 5 shows some example layouts with three subs (Geddes also recommends lifting one sub closer to the ceiling if possible).

Note: as part of the optimization process, the delay of individual subs will be changed. If you prefer to initialize the delays to correspond to the distance of each sub from the main listening position, there is no harm in doing so.

Asymmetrical layouts for multiple subwoofers

Figure 5. Sample asymmetrical layouts for multiple subwoofers

To test Method B, we used Layout 4 in our test room with three sealed subwoofers (similar but not all identical). Prior to starting, we measured all three subwoofers at three locations with no adjustment (delay, gain, EQ) applied to any of them. This is shown in Graph 4. While spatial variation is very low up to 50 Hz, it is quite high above 50 Hz. Furthermore, all measurement locations are quite "notchy" and this will be difficult to EQ.

Response of three subwoofers with no optimization

Graph 4. Response of all three subwoofers (Layout 4) with no optimization, measured at three locations

So let's see what improvement we can get! Method B proceeds as follows:

  1. Mute all subs except S1 and run a set of measurements.

  2. Unmute S2 (leave S1 unmuted) and run the measurements again. If the result is better than step 1, proceed to the next step. Otherwise, use the plugin to change the gain and delay (*) of S2 and repeat.

  3. Unmute S3 (leave S1 and S2 also unmuted) and run the measurements. If the result is better than step 2, proceed to the next step. Otherwise, use the plugin to change the gain and delay (*) of S3 and repeat.

  4. If you are using four subs, do the same for S4.

  5. Apply global EQ to the average of the last set of measurements. Then remeasure. Graph 5 shows our final result.

(*) You can also try inverting the sub. If at any step you are unable to produce a better result than previously, you may have to try moving that sub. You can make this method go a little quicker by avoiding parameter settings that cause notches in the response at the center measurement location.

Corrected response of three subwoofers

Graph 5. Optimized and corrected response of all three subwoofers (Layout 4), measured at three locations

This is obviously a big improvement in spatial variation compared to just using multiple subs without this method (Graph 4). There are also no significant notches up to nearly 100 Hz. Compared to Method A (Graph 3), there is more variation in the 20 to 40 Hz range and less above 50 Hz. We re-ran the measurements for each additional subwoofer only a small number of times, so it's possible that more time spent on varying sub parameters and measuring would yield even better results.

Method C [Top]

Method C uses the Multi-Sub Optimizer (MSO) program. This awesome piece of software runs an optimization algorithm on multiple measurements of your subs to generate the best possible response across the listening area.

In a nutshell, the procedure for using MSO is as follows:

  1. Take a measurement of each subwoofer at each measurement location. In our example, with three subwoofers and three measurement locations, that means nine measurements. These measurements will need to use a timing reference.

  2. Load the measurements into MSO. This requires some setup and configuration – for example, telling MSO that you have three subwoofers, assigning the correct measurements to each subwoofer, and so on.

  3. Run the optimization algorithm to minimize spatial variation. MSO will generate gain, delay, and PEQ settings for each subwoofer.

  4. Load the settings generated by MSO into the respective output channels of the miniDSP plugin.

  5. Re-run the measurements to verify your results.

  6. Optionally, apply global EQ to get the flattest response or to apply a target curve.

We have provided a detailed step-by-step guide for the DDRC-88A in the application note Optimizing multiple subwoofers with the DDRC-88BM and Multi-Sub Optimizer. With the exception of plugin configuration in steps 2 and 3, the instructions also apply to the miniDSP 2x4 / HD / Balanced. Take your time and follow all steps carefully. Be sure to set the correct sample rate and number of PEQs available for your hardware/plugin.

To test Method C, we used Layout 4 with the same three subwoofers that we used for Method B. Graph 6 shows our final result. Spatial variation is very low indeed to well above 100 Hz.

Response of three subwoofers optimized with MSO

Graph 6. Response of all three subwoofers (Layout 4) optimized with MSO, measured in three locations

Wrapping up[Top]

It's important to remember that the graphs shown here are not necessarily representative of the results that you will get in your room. As mentioned before, subwoofers in rooms are unpredictable, and the only way to know which method is best for you is to try them.

Finally, if you are using bass management, you can now set it up. To do this, pass your measurement signal through the AVR instead of directly through the miniDSP. With a multisub system, the concept of "distance to subwoofer" can be a bit meaningless so you may find that you need to experiment with the delay on the subwoofer channel to get the smoothest response through the crossover region.

That's it for this app note! Have fun, and please let us know of your multisub and miniDSP results in our forum.

Related Products - Typical multi-sub processors