Living With Low-Q Loudspeakers:
Design Reflex Cabinets for Tannoy Speakers

Before Thiele and Small analysed the bass-reflex speaker system, and published the results in 1961 (sic), the design of such speaker cabinets was empirical.

In conjunction with the low power amplifiers of the day, one emphasis put on speaker drive-unit design was to obtain high efficiency. This lead to speaker designs with large, powerful magnets, particularly if the mass of the cone was not to be lowered to the point where it compromised other aspects of performance (such as an irregular mid-range response).

These historical aspects have left a pool of older-style drive units that have powerful magnet systems. Of course, not all old drive units have powerful magnets, and likewise, some modern drive units also feature powerful magnet designs (e.g. Altec/JBL/Tannoy). The Thiele & Small parameter that reflects the performance of the "motor" system, including the magnet design, is the "Qe" parameter (e for Electrical). In general this parameter varies from around 0.2 through to 2. The speakers with more powerful motor systems have a low Qe. The total Q of a speaker is the combination of the mechanical Q (Qm), and the electrical Q (Qe). In a speaker with a low Q such as 0.25, the contribution from the mechanical Q (say around 3) is quite small, so the total Q may be about 0.2.

Conventional wisdom when designing a loudspeaker, is to tune the system so that the frequency response is as smooth and as extended as possible - no peaks, and no early roll-off. This in turn implies a specific Q (of around 0.5 to 0.7) for the combined speaker-in-box. When a drive unit is placed in a box, the Q is raised. A small box raises the Q more, and a large box raises the Q less.

A: Maximally Flat

With a driver that has a very low Q to start with, a small box is required to achieve an overall Q of 0.5. For example, for a 15-inch Tannoy with Qt=0.2, to raise the Q to achieve a maximally flat response the required box volume is only 1.3 cubic-feet (and the reflex port is tuned to 47Hz)! As can be seen from the graph (see below), this gives a very high cut-off frequency of about 75 Hz - atrocious performance from a 15-inch drive unit! But then again, no one puts 15-inch drive units into 1.3 cubic-foot boxes - do they?

B: Larger Box

The intuitive reaction is to make the box bigger. As Gilbert Briggs (of Wharfedale) used to say: -"smooth runs the brook where the flow is deep". In other words, get a big box, and you get smooth response. If the box is made larger (7 cubic feet), but the port frequency is left unaltered, then this kind of isn't true - see curve B where a peak in response becomes evident from the large volume. But the peak is not really severe and could be acceptable to some tastes.

C: Extended Response

Finally, take the larger 7 cubic-foot box of B, and shift the tuning downwards so the port frequency is 30 Hz instead of 47 Hz. What appears is a gradually drooping response from 200 Hz downwards, propped up by the port resonance at 30Hz, and then rolling off steeply below that. This response is sometimes referred to as a low frequency shelf.

The disadvantage of this alignment are:

The advantages of this alignment are:

Another approach to living with low-Q speakers, is to change the Q. This is done by placing a resistor in series with the driver. This has not been investigated. Apart from the fact that it "goes against the grain", it could affect the mid-range response, and lower the efficiency.

Summary

Living with low-Q speakers requires an un-orthodox approach to the enclosure alignment - one where a larger-than-predicted box should be used, and the port should be tuned lower than predicted. When this is done, practical circumstance such as room placement, or use of tone controls, leads to satisfactory low frequency response.

It is interesting to note that this is more or less exactly how "decent" speakers from the 1950's were operated - in large boxes, placed in the corner (or, "smooth runs the brook where the flow is deep"). This contrasts with the current design approach where the Q of the driver is chosen (higher) to get maximum extension and smoothness from smaller boxes, but placed away from room corners.

 


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